Joël Bockaert
email: joel.bockaert@igf.cnrs.fr

Sylvie Claeysen

Carine Becamel

Aline Dumuis

Philippe Marin

Carine Bécamel

Neuronal 5-HT metabotropic receptors: fine-tuning of their structure, signaling, and roles in synaptic modulation

Keywords: Serotonin, Serotonin receptors, G-protein-coupled receptors, Receptosome, Signaling Receptor subtype Intracellular loop C-terminus Function of the identified interaction

come

Introduction

Serotonin is certainly one of the neurotransmitters that the general public knows the best but is also the most intriguing for researchers. The name "serotonin" retained in the literature indicates its presence in the serum (released from platelets; [START_REF] Rapport | Serum vasoconstrictor (serotonin). IV. Isolation and characterization[END_REF]; however, 5-HT has also been called "enteramine" because of its early recognized presence, at high concentrations, in enterochromaffin cells (95% of 5-HT is found in platelets and the gastro-intestinal tract; [START_REF] Erspamer | Identification of enteramine, the specific hormone of the enterochromaffin cell system, as 5-hydroxytryptamine[END_REF]. Serotonin is one of the oldest signaling substances to appear during evolution in coelenterates, arthropods, molluscs, tunicates, and vertebrates [START_REF] Collier | The occurrence of 5-hydroxytryptamine (5-HT) in nature[END_REF]. Unsurprisingly, therefore, serotonergic neurons are also among the earliest neurons to be generated during mammalian development, with 5-HT being released before conventional synapses are established [START_REF] Gaspar | The developmental role of serotonin: news from mouse molecular genetics[END_REF]. 5-HT is well known by the public, thanks to its roles in normal physiology including sleep, appetite, thermoregulation, and pain perception, and more importantly in human pathologies such as migraine, depression, fear and anxiety, obsessive compulsive disorders, schizophrenia, and addiction.

A provocative role for serotonin systems in spiritual experiences has also been proposed in a recent article, published in a respected journal, by [START_REF] Borg | The serotonin system and spiritual experiences[END_REF], although whether this can be taken seriously remains debatable. 5-HT, like a few other neurotransmitters such as acetylcholine, glutamate, and γ-aminobutyric acid (GABA), acts via two categories of receptors: ionotropic and metabotropic receptors. Ionotropic receptors (channel receptors) have a low affinity for their neurotransmitter ligand but a rapid activation constant (a few milliseconds). In contrast, metabotropic receptors (receptors acting through G protein activation and second messenger production) exhibit both a high affinity for their neurotransmitter and a slow activation constant (in seconds or longer). Fifteen genes encoding 5-HT receptors have been cloned in the mammalian brain [START_REF] Hoyer | 5-HT receptor classification and nomenclature: towards a harmonization with human genome[END_REF]. Recently, three novel 5-HT3-like genes (5-HT3C, 5-HT3D, and 5-HT3E) have been identified [START_REF] Niesler | Cloning, physical mapping and expression analysis of the human 5-HT3 serotonin receptor-like genes HTR3C, HTR3D and HTR3E[END_REF]), but to date, no functional analysis has been published. The 5-HT3 receptors are ionotropic receptors [START_REF] Van Hooft | 5-HT3 receptors in the CNS: 3B or not 3B?[END_REF] and will not be discussed in this review. The others are G-protein-coupled receptors (GPCRs) with seven transmembrane domains. They can be categorized into four groups according to their main second messenger system: the 5-HT1 receptors coupled to Gαi/Gαo proteins (5-HT1A, 5-HT1B, 5-HT1D, 5-HT1E, 5-HT1F); the 5-HT2 receptors coupled to Gαq proteins (5-HT2A, 5-HT2B, 5-HT2C); the 5-HT4, 5-HT6, and 5-HT7 receptors coupled to Gs proteins; and the 5-HT5 receptors for which the coupling is still uncertain [START_REF] Raymond | Multiplicity of mechanisms of serotonin receptor signal transduction[END_REF]. The diversity of metabotropic 5-HTR proteins is further increased by post-genomic modifications, such as alternative splicing and mRNA editing. This will be the subject of the first part of this review. Their neuronal localization, including the synaptic localization when known, and their signaling pathways will be discussed in the second section.

The third part deals with the nature and functions of a variety of GPCR-interacting proteins (GIPs; [START_REF] Bockaert | The "magic tail" of G protein-coupled receptors: an anchorage for functional protein networks[END_REF]Bockaert et al. , 2004b) that form, together with 5-HTRs, "receptosomes" implicated in their targeting and trafficking and in the fine tuning of their signaling properties. For lack of space, this review will often take illustrative examples rather than being exhaustive.

Genomic and post-genomic complexity of 5-HT metabotropic receptor structure

2.1. Gene structure Metabotropic 5-HTRs are generated from 12 different genes in the human genome (NCBI, Map Viewer at http://www.ncbi.nlm.nih.gov/mapview/). These genes diverged from a common ancestor, and their structure reflects their evolutionary relationship [START_REF] Barnes | A review of central 5-HT receptors and their function[END_REF]. 5-HT1Rs are coded by intronless genes and clearly constitute a subgroup of genes that were derived from a duplicated ancestor (Fig. 1). 5-HT2R genes are also closely related to two similar introns located at the end of transmembrane domain 2 (TM2) and within TM4-coding sequences. The 5-HT2CR gene presents an additional intron in the extracellular domain of the coding sequences and extends over more than 183kb. Alternative splicing of the 5-HT2R genes does not generate any functional splice variant [START_REF] Xie | The human serotonin 5-HT2C receptor: complete cDNA, genomic structure, and alternatively spliced variant[END_REF]. The 5-HT4R gene is far more complex. The coding sequence extends over more than 185 kb and includes at least 14 exons leading to multiple splicing combinations. To date, ten functional splice variants have been described that can be generated by alternative splicing of the human transcript [START_REF] Bender | Structure of the human serotonin 5-HT4 receptor gene and cloning of a novel 5-HT4 splice variant[END_REF][START_REF] Hiroi | Identification and characterization of the human serotonin-4 receptor gene promoter[END_REF]. For nine of these variants, splicing gives rise to alternate C-terminal domains (Fig. 1). As some exons (such as the h or the i exons), in the 5-HT4R gene, do not present any in-frame stop codons, many additional combinations probably remain to be discovered [START_REF] Brattelid | Cloning, pharmacological characterisation and tissue distribution of a novel 5-HT4 receptor splice variant, 5-HT4(i)[END_REF]). The relevance and the functional role of such a multiplicity of isoforms also remain to be elucidated. The finding that the lesswell represented 5-HT4R splice variants (e.g., 5-HT4(a)R and 5-HT4(e)R) interact with specific intracellular proteins supports the view that these isoforms are in charge of the fine regulation of signal transduction and are not the result of transcriptional leakage [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF]). Two 5-HT5R genes have been described in the rodent, but only the 5-HT5AR can be generated in humans [START_REF] Rees | Cloning and characterisation of the human 5-HT5A serotonin receptor[END_REF].

A 5-HT5BR pseudogene had been localized on chromosome 2 in Build 35.1 of the human genome. However, this annotation has been suppressed in the actual build (36.1). The 5-HT6R gene presents a splicing site that lies in the middle of the intracellular loop 3 and that is similar to the splicing site found in the 5-HT5AR gene. Two separate exons encode TM6 and TM7 of the 5-HT6R. A non-functional splice variant has been described for this receptor [START_REF] Olsen | Identification of a human 5-HT6 receptor variant produced by alternative splicing[END_REF]. The human 5-HT7R gene is more complex: different acceptor and donor sites can be used to generate three receptors that differ in the composition and length of their C-terminal domains (5-HT7(a), 5-HT7(b), and 5-HT7(d)). Because of its altered intron-exon organization, the rat gene generates the 5-HT7(c) but not the 5-HT7(d)) isoform [START_REF] Heidmann | Four 5-hydroxytryptamine7 (5-HT7) receptor isoforms in human and rat produced by alternative splicing: species differences due to altered intron-exon organization[END_REF]. This splicing position giving rise to alternate Cterminus is similar to that found in the 5-HT4R gene. The different C-terminal domains of the 5-HT7R variants probably recruit specific sets of intracellular proteins that might modulate their function as demonstrated for the 5-HT4R isoforms.

Editing

RNA editing is a dynamic mechanism that generates molecular and functional diversity. The most common type of RNA editing involves nucleotide substitution that consists of either cytidine to uridine (C to U) or, most frequently, adenosine to inosine (A to I). Inosine is read as guanosine by the translational machinery of the cell [START_REF] Higuchi | RNA editing of AMPA receptor subunit GluR-B: a base-paired intron-exon structure determines position and efficiency[END_REF]). The 5-HT2CR is the only known GPCR to be regulated by editing. Editing takes place at four major positions on exon V (corresponding to the second exon in the coding sequence; Fig. 1), termed sites A, B, C, and D, and one minor site, termed E (Fig. 2a). 5-HT2CR mRNA editing has been identified as A to I discrepancies between genomic and cDNA sequences in the region encoding the second intracellular loop of the receptor. This A to I substitution alters the coding potentials of three codons. Theoretically, 5-HT2CR editing can generate up to 32 different mRNAs that encode 24 receptor isoforms from the non-edited form (INI) to the fully edited form (VGV; [START_REF] Fitzgerald | Messenger RNA editing of the human serotonin 5-HT2C receptor[END_REF]Fig. 2). The editing of 5-HT2C transcripts is tissue-specific. Seven major 5-HT2CR isoforms, encoded by 11 distinct RNA species, are expressed in rat brain compared with 14 receptor isoforms encoded by 23 trancripts in human brain. The most prevalent isoform in human brain is 5-HT2C-VSV, whereas 5-HT2C-VNV is the most abundant in the rat (Fig. 2b).

Neuronal and synaptic localizations and signaling functions

The classical view of pre-synaptic 5-HT-containing terminals forming synaptic contacts has been challenged by the team of L. Descarries [START_REF] Descarries | Serotonin nerve terminals in adult rat neocortex[END_REF]. This does not mean that classical 5-HT synapses do not occur, but they are not largely present. Nerve endings often lack the junction membrane specialization that characterizes the synapse. The released 5-HT may diffuse in the extracellular space and reach relatively distant targets. Moreover, the pleiotropic functions of serotonin are not only exerted by the concerted actions of multiple serotonin receptor subtypes but also by the multiple coupling of each receptor subtype. We will focus, in this review, on signaling in native brain tissues.

5-HT1ARs

The highest density of 5-HT1ARs is found in the limbic areas [START_REF] Lanfumey | 5-HT1 receptors[END_REF]. 5-HT1A auto-receptors are localized in cell bodies and dendrites of 5-HT neurons of the dorsal and median raphe nuclei [START_REF] Sotelo | Direct immunohistochemical evidence of the existence of 5-HT1A autoreceptors on serotoninergic neurons in the midbrain raphe nuclei[END_REF][START_REF] Riad | Somatodendritic localization of 5-HT1A and preterminal axonal localization of 5-HT1B serotonin receptors in adult rat brain[END_REF]Fig. 3a). They are chiefly expressed along extra-synaptic sites, as previously seen for many metabotropic GPCRs [START_REF] Riad | Somatodendritic localization of 5-HT1A and preterminal axonal localization of 5-HT1B serotonin receptors in adult rat brain[END_REF]. At the post-synaptic level, such as the hippocampal formation, they are localized on dendrites of pyramidal and granule cells (Fig. 3a; [START_REF] Riad | Somatodendritic localization of 5-HT1A and preterminal axonal localization of 5-HT1B serotonin receptors in adult rat brain[END_REF]. 5-HT1ARs are negatively coupled to adenylyl cyclase (AC) via pertussis-toxinsensitive Gαi and/or Gαo proteins [START_REF] Vivo | Characterization of the 5-hydroxytryptamine1A receptor-mediated inhibition of forskolin-stimulated adenylate cyclase activity in guinea pig and rat hippocampal membranes[END_REF][START_REF] Weiss | Synaptogenesis of cultured striatal neurones in serum-free medium: a morphological and biochemical study[END_REF]Fig. 4). However, 5-HT1ARs do not seem to be coupled to AC inhibition in dorsal raphe nuclei, in spite of their high density within these nuclei. Not only 5-HT1A auto-receptors, but also post-synaptic 5-HT1ARs inhibit neuronal firing. This inhibition is attributable to the activation of G-protein-gated inwardly rectifying potassium (GIRK) currents and the inhibition of Ca 2+ channels [START_REF] Andrade | A G protein couples serotonin and GABAB receptors to the same channels in hippocampus[END_REF][START_REF] Oleskevich | G alpha o1 decapeptide modulates the hippocampal 5-HT1A potassium current[END_REF][START_REF] Sodickson | Neurotransmitter activation of inwardly rectifying potassium current in dissociated hippocampal CA3 neurons: interactions among multiple receptors[END_REF]Fig. 4). Activation of 5-HT1A auto-receptors delays the action of selective serotonin reuptake inhibitors (SSRIs). Indeed, SSRIs are efficient at the post-synaptic level only after desensitization of 5-HT1A auto-receptors [START_REF] Adell | Strategies for producing faster acting antidepressants[END_REF]. For unknown reasons, 5-HT1A autoreceptors desensitize much faster and to a much greater extent than post-synaptic receptors [START_REF] Karschin | Heterologously expressed serotonin 1A receptors couple to muscarinic K+ channels in heart[END_REF]. In serotonin transporter (SERT) knock-out mice, 5-HT1A auto-receptors are desensitized and down-regulated in the dorsal raphe nuclei, but not in the hippocampus [START_REF] Fabre | Altered expression and functions of serotonin 5-HT1A and 5-HT1B receptors in knock-out mice lacking the 5-HT transporter[END_REF]. Interestingly, 5-HT1ARs inhibit N-methyl-D-aspartate (NMDA) function in prefrontal pyramidal neurons by decreasing the transport and expression of NR2B subunits into dendrites [START_REF] Yuen | Serotonin 5-HT1A receptors regulate NMDA receptor channels through a microtubule-dependent mechanism[END_REF]. This effect may explain the inhibition of long-term potentiation (LTP) by 5-HT1ARs [START_REF] Edagawa | 5-HT1A receptor-mediated inhibition of long-term potentiation in rat visual cortex[END_REF]. 5-HT1AR activation stimulates neurogenesis in the dentate gyrus and the subventricular zone [START_REF] Brezun | Depletion in serotonin decreases neurogenesis in the dentate gyrus and the subventricular zone of adult rats[END_REF], an effect that seems to be required for antidepressant effects [START_REF] Santarelli | Requirement of hippocampal neurogenesis for the behavioral effects of antidepressants[END_REF].

They also reduce the length of dendrites and the number of spines [START_REF] Yan | 5-HT1a receptors mediate the neurotrophic effect of serotonin on developing dentate granule cells[END_REF]. 5-HT1AR agonists have been developed for anxiety. 5-HT1AR invalidation consistently increases anxiety in mice [START_REF] Gross | The developmental origins of anxiety[END_REF][START_REF] Lanfumey | 5-HT1 receptors[END_REF]. Positive coupling of 5-HT1ARs to AC, phospholipase C (PLC) and extracellular signal-regulated kinases (ERK) pathways have only been seen in heterologous systems [START_REF] Raymond | Multiplicity of mechanisms of serotonin receptor signal transduction[END_REF]) (Fig. 4).

5-HT1BRs and 5-HT1DRs

5-HT1BRs and 5-HT1DRs are not expressed on cell bodies of 5-HT neurons but on terminals of 5-HT neurons and on terminals of other neurons, such as dopaminergic, GABAergic, and glutamatergic neurons, where they inhibit neurotransmitter release [START_REF] Starke | Modulation of neurotransmitter release by presynaptic autoreceptors[END_REF][START_REF] Pauwels | 5-HT 1B/D receptor antagonists[END_REF][START_REF] Sari | Light and electron microscopic immunocytochemical visualization of 5-HT1B receptors in the rat brain[END_REF][START_REF] Riad | Somatodendritic localization of 5-HT1A and preterminal axonal localization of 5-HT1B serotonin receptors in adult rat brain[END_REF]. They are highly expressed in the substantia nigra and the globus pallidus (probably on striato-pallidal and striato-nigral GABAergic neurons). 5-HT1BRs and 5-HT1DRs are also present in the cortex. 5-HT1B/1D agonists (triptans) are active with respect to migraine, probably by inhibiting trigeminal neurotransmission. 5-HT1BRs are also present on thalamo-cortical axon terminals carrying sensory afferents from one whisker. These terminals form clusters, called barrels [START_REF] Gaspar | The developmental role of serotonin: news from mouse molecular genetics[END_REF], in layer IV (Fig. 3b). 5-HT1BRs, like the other 5-HT1Rs, are negatively coupled to AC via Gαi/Gαo in native brain tissues, such as the substantia nigra [START_REF] Bouhelal | 5-HT1B receptors are negatively coupled with adenylate cyclase in rat substantia nigra[END_REF]Fig. 4). In transfected cells, 5-HT1BRs regulate PLC, PLD, Akt kinase, neuronal nitric oxide synthase (nNOS), and ERK pathways, activate K + channels, and inhibit Ca 2+ channels [START_REF] Raymond | Multiplicity of mechanisms of serotonin receptor signal transduction[END_REF]Fig. 4). Pharmacological and genetic studies suggest a role for 5-HT1BRs in the pathophysiology of obsessive-compulsive disorders, drug addiction, depression, and sleep. 5-HT1BR knock-out mice are aggressive [START_REF] Gingrich | Dissecting the role of the serotonin system in neuropsychiatric disorders using knockout mice[END_REF].

Both in vitro and in vivo experiments indicate that 5-HT1BRs control the development of axonal harbors and axon terminal clustering [START_REF] Gaspar | The developmental role of serotonin: news from mouse molecular genetics[END_REF]. 5-HT1ERs and 5-HT1FRs These receptor subtypes are present in the brain, but little data concerning their neuronal localization are available. 5-HT1FR has been found on pre-synaptic trigeminal nerve terminals and mediates some of the effect of a new generation of triptans (rizatriptan, naratriptan, zolmitriptan; [START_REF] Lanfumey | 5-HT1 receptors[END_REF]. Negative coupling to AC has been demonstrated in heterologous cells, but not neurons [START_REF] Raymond | Multiplicity of mechanisms of serotonin receptor signal transduction[END_REF]Fig. 4).

5-HT2ARs

5-HT innervation of the prefrontal cortex has received extensive attention from neuroanatomists, physiologists, and pharmacologists [START_REF] Leysen | 5-HT2 receptors[END_REF]. 5-HT2ARs are particularly abundant in the pre-frontal cortex (Fig. 3c; [START_REF] Jakab | 5-Hydroxytryptamine2A serotonin receptors in the primate cerebral cortex: possible site of action of hallucinogenic and antipsychotic drugs in pyramidal cell apical dendrites[END_REF][START_REF] Cornea-Hebert | Cellular and subcellular distribution of the serotonin 5-HT2A receptor in the central nervous system of adult rat[END_REF], 2002;[START_REF] Miner | Ultrastructural localization of serotonin2A receptors in the middle layers of the rat prelimbic prefrontal cortex[END_REF] and are certainly the site of action of hallucinogens, such as LSD (Aghajanian and Marek 1999b), and of some atypical anti-psychotics, such as clozapine and risperidone [START_REF] Adell | Strategies for producing faster acting antidepressants[END_REF]. Alterations in 5-HT2AR function may be a contributing factor to the cognitive impairment in mental disorders such as schizophrenia and depression [START_REF] Kroeze | The molecular biology of serotonin receptors: therapeutic implications for the interface of mood and psychosis[END_REF]. Light-and electron-microscopic immunohistochemistry has been used to determine the cellular and subcellular localization of 5-HT2ARs in the pre-frontal cortex [START_REF] Jakab | 5-Hydroxytryptamine2A serotonin receptors in the primate cerebral cortex: possible site of action of hallucinogenic and antipsychotic drugs in pyramidal cell apical dendrites[END_REF][START_REF] Cornea-Hebert | Cellular and subcellular distribution of the serotonin 5-HT2A receptor in the central nervous system of adult rat[END_REF], 2002;[START_REF] Miner | Ultrastructural localization of serotonin2A receptors in the middle layers of the rat prelimbic prefrontal cortex[END_REF]represented in Fig 3c). A dense localization has been found in apical dendrite shafts, proximal to the pyramidal cell soma [START_REF] Jakab | 5-Hydroxytryptamine2A serotonin receptors in the primate cerebral cortex: possible site of action of hallucinogenic and antipsychotic drugs in pyramidal cell apical dendrites[END_REF]. Some receptors are present in the soma of pyramidal cells and in the dendrites of local circuited neurons [START_REF] Jakab | 5-Hydroxytryptamine2A serotonin receptors in the primate cerebral cortex: possible site of action of hallucinogenic and antipsychotic drugs in pyramidal cell apical dendrites[END_REF]. In one study [START_REF] Miner | Ultrastructural localization of serotonin2A receptors in the middle layers of the rat prelimbic prefrontal cortex[END_REF], but not in another one [START_REF] Jakab | 5-Hydroxytryptamine2A serotonin receptors in the primate cerebral cortex: possible site of action of hallucinogenic and antipsychotic drugs in pyramidal cell apical dendrites[END_REF], a dense localization has been found on the spine heads and necks of pyramidal neurons. A marked intracellular density of 5-HT2ARs, which may serve as a receptor reserve for trafficking to the plasmamembrane, has also been described [START_REF] Cornea-Hebert | Cellular and subcellular distribution of the serotonin 5-HT2A receptor in the central nervous system of adult rat[END_REF], 2002). A minority of 5-HT2ARs is also found on axon terminals [START_REF] Jakab | 5-Hydroxytryptamine2A serotonin receptors in the primate cerebral cortex: possible site of action of hallucinogenic and antipsychotic drugs in pyramidal cell apical dendrites[END_REF][START_REF] Miner | Ultrastructural localization of serotonin2A receptors in the middle layers of the rat prelimbic prefrontal cortex[END_REF]. Whether 5-HT2ARs are localized on pre-synaptic glutamatergic terminals remains a matter of debate. In the primate cortex, a pre-synaptic localization on glutamatergic terminals has also been reported [START_REF] Jakab | 5-Hydroxytryptamine2A serotonin receptors in the primate cerebral cortex: possible site of action of hallucinogenic and antipsychotic drugs in pyramidal cell apical dendrites[END_REF] although at low levels, whereas in the rat cortex, such a localization remains to be established [START_REF] Miner | Ultrastructural localization of serotonin2A receptors in the middle layers of the rat prelimbic prefrontal cortex[END_REF]. Some of these discrepancies may be related to the nature of the antibodies used. In the rat prefrontal cortex, 5-HT2ARs have also been detected on terminals of aminergic fibers [START_REF] Miner | Ultrastructural localization of serotonin2A receptors in the middle layers of the rat prelimbic prefrontal cortex[END_REF], which may explain the stimulation of this neurotransmitter release following 5-HT2AR activation [START_REF] Pehek | M100,907, a selective 5-HT(2A) antagonist, attenuates dopamine release in the rat medial prefrontal cortex[END_REF]. In vitro electrophysiological studies indicate that 5-HT2ARs enhance spontaneous excitatory post-synaptic currents (EPSCs; Aghajanian and Marek 1999a;[START_REF] Marek | 5-HT2A receptor or alpha1-adrenoceptor activation induces excitatory postsynaptic currents in layer V pyramidal cells of the medial prefrontal cortex[END_REF][START_REF] Zhou | Activation of serotonin receptors modulates synaptic transmission in rat cerebral cortex[END_REF].

Activation of 5-HT2ARs also increases late EPSPs evoked by the electrical stimulation of afferent fibers [START_REF] Adell | Strategies for producing faster acting antidepressants[END_REF]. These effects are probably mediated by a release of glutamate and subsequent activation of α-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) receptors, since they are inhibited by AMPA antagonists (Aghajanian and Marek 1999a) and by pre-synaptic group II metabotropic glutamate agonists, which are known to inhibit glutamate release [START_REF] Marek | Physiological antagonism between 5-hydroxytryptamine (2A) and group II metabotropic glutamate receptors in prefrontal cortex[END_REF]. A presynaptic localization of 5-HT2ARs on glutamatergic terminals may explain these results. However, if 5-HT2ARs are primarily localized on spine heads of pyramidal cells at asymmetric synapses receiving glutamate innervation, one has to speculate a post-synaptic influence on pre-synaptic glutamatergic release. This could be achieved via the postsynaptic production of a retrograde messenger. An increase in neuronal excitability may also be explained by a 5-HT2AR-mediated inhibition of afterhyperpolarizations (IsAHP) in the prefrontal cortex [START_REF] Villalobos | Serotonergic regulation of calcium-activated potassium currents in rodent prefrontal cortex[END_REF]. However, the situation may be more complex because 5-HT inhibits (rather than stimulates) cortical pyramidal neuron activity in vivo [START_REF] Zhou | Activation of serotonin receptors modulates synaptic transmission in rat cerebral cortex[END_REF]. This could be the result of several synaptic effects. 5-HT2ARs induce a massive enhancement of spontaneous inhibitory post-synaptic currents (IPSCs) in pyramidal neurons via the activation of interneurons [START_REF] Zhou | Activation of serotonin receptors modulates synaptic transmission in rat cerebral cortex[END_REF], an inhibition of the Na + current, which reduces dendritic excitability [START_REF] Carr | Serotonin receptor activation inhibits sodium current and dendritic excitability in prefrontal cortex via a protein kinase C-dependent mechanism[END_REF], an inhibition of the L-type Ca 2+ current [START_REF] Day | Stimulation of 5-HT(2) receptors in prefrontal pyramidal neurons inhibits Ca(v)1.2 L type Ca(2+) currents via a PLCbeta/IP3/ calcineurin signaling cascade[END_REF], and an inhibition of NMDA receptor-mediated transmission [START_REF] Arvanov | LSD and DOB: interaction with 5-HT2A receptors to inhibit NMDA receptormediated transmission in the rat prefrontal cortex[END_REF]. Indeed, depending on the kinetics of all these effects and on the kinetics of their desensitization, 5-HT2ARs may inhibit or excite pyramidal neuron activity. Normal cognitive functions may depend on a fine-tuning of these opposing types of regulation [START_REF] Williams | The physiological role of 5-HT2A receptors in working memory[END_REF]. Interestingly, both the supra-normal level of 5-HT and the depletion of 5-HT result in altered spatial working memory and decision making, respectively [START_REF] Williams | The physiological role of 5-HT2A receptors in working memory[END_REF]. Other electrophysiological studies suggest pre-and postsynaptic localizations of 5-HT2A/2CRs in the spinal cord. In superficial dorsal horn synapses, pre-synaptic 5-HT2A/2CRs induce a long-lasting facilitation of miniature EPSC frequency [START_REF] Hori | Long-lasting synaptic facilitation induced by serotonin in superficial dorsal horn neurones of the rat spinal cord[END_REF]. At the same synapses, postsynaptic 5-HT2A/2CRs transforms silent glutamatergic synapses into active synapses, a mechanism that requires the interaction of AMPA receptors with PDZ-domain-containing proteins (post-synaptic density/disc large/zona occludens-1 proteins; [START_REF] Li | Silent glutamatergic synapses and nociception in mammalian spinal cord[END_REF][START_REF] Li | AMPA receptor-PDZ interactions in facilitation of spinal sensory synapses[END_REF]. With regard to second messenger production, 5-HT2ARs activate PLC, PLA2, and the ERK pathway in neurons [START_REF] Raymond | Multiplicity of mechanisms of serotonin receptor signal transduction[END_REF]. They also activate the Janus kinase (JAK)/transcription (STAT) pathway in skeletal and smooth muscle [START_REF] Guillet-Deniau | Identification and localization of a skeletal muscle secrotonin 5-HT2A receptor coupled to the Jak/STAT pathway[END_REF][START_REF] Banes | Activation of the JAK/STAT pathway in vascular smooth muscle by serotonin[END_REF] and PLD in nonneuronal cells [START_REF] Raymond | Multiplicity of mechanisms of serotonin receptor signal transduction[END_REF]Fig. 4). Unexpectedly, some antagonists (mianserin, spiperone, mesulergine) induce desensitization and internalization of 5-HT2ARs both in vivo and in vitro [START_REF] Sanders-Bush | Adaptive regulation of central serotonin receptors linked to phosphoinositide hydrolysis[END_REF][START_REF] Roth | Chronic mianserin treatment decreases 5-HT2 receptor binding without altering 5-HT2 receptor mRNA levels[END_REF]. This process may contribute to the redistribution of 5-HT2ARs from the membrane of apical dendrites to the cytoplasm of neuronal cell bodies following chronic treatment of rats with atypical antipsychotics [START_REF] Willins | Clozapine and other 5-hydroxytryptamine-2A receptor antagonists alter the subcellular distribution of 5-hydroxytryptamine-2A receptors in vitro and in vivo[END_REF].

5-HT2BRs

In the adult brain, 5-HT2BRs are found only in discrete nuclei of the cerebellum, the lateral septum, the dorsal hypothalamus, and the medial amygdala, contrasting with its wide expression in peripheral tissues, including the heart and lung. As 5-HT2ARs, 5-HT2BRs are coupled to Gαq proteins and activate PLC. However, to date, no study has reported such a coupling within neurons [START_REF] Wainscott | Pharmacological characteristics of the newly cloned rat 5-hydroxytryptamine2F receptor[END_REF].

5-HT2BRs have also been reported to stimulate Ca 2+ mobilization in astrocytes derived from various rat cerebral regions [START_REF] Sanden | 5-Hydroxytryptamine2B receptors stimulate Ca2+ increases in cultured astrocytes from three different brain regions[END_REF]. Additional 5-HT2BR-mediated signaling pathways have been described in non-neuronal cells expressing endogenous 5-HT2BRs. Stimulation of the 5-HT2BR is associated with an increase in cyclic GMP through the dual activation of constitutive and inducible NOS. Gαq/11-coupled 5-HT2BRs also activate PI3K/Akt and ERK1/2 signaling pathways [START_REF] Raymond | Multiplicity of mechanisms of serotonin receptor signal transduction[END_REF]) (Fig. 4).

5-HT2CRs

A comparison of the distribution of 5-HT2CR mRNA with that of 5-HT2CR protein labeled with 3 Hmesulergine in monkey and human brains has revealed both pre-and postsynaptic localizations of 5-HT2CR (Lopez-Gimenez et al. 2001;[START_REF] Leysen | 5-HT2 receptors[END_REF]). These pre-and post-synaptic localizations of 5-HT2CRs have been further confirmed by electron-microscopic studies performed in the mouse [START_REF] Becamel | The serotonin 5-HT2A and 5-HT2C receptors interact with specific sets of PDZ proteins[END_REF]Fig. 3d). Data on the localization of 5-HT2CRs in the ventral tegmental area (VTA) are not consensual. Some suggest that 5-HT2CRs are primarily localized on non-dopaminergic neurons, probably GABAergic neurons (Di [START_REF] Matteo | Role of 5-HT(2C) receptors in the control of central dopamine function[END_REF]. These receptors may be constitutively active [START_REF] Berg | Physiological relevance of constitutive activity of 5-HT2A and 5-HT2C receptors[END_REF], yielding tonic GABAergic inhibition of dopaminergic neurons. Other data suggest that 5-HT2CRs are localized on dopaminergic neurons [START_REF] Ji | Disruption of PTEN coupling with 5-HT2C receptors suppresses behavioral responses induced by drugs of abuse[END_REF]Fig. 3d). Indeed, all studies agree that the activation of 5-HT2CRs inhibits dopaminergic neurons [START_REF] Gobert | Serotonin(2C) receptors tonically suppress the activity of mesocortical dopaminergic and adrenergic, but not serotonergic, pathways: a combined dialysis and electrophysiological analysis in the rat[END_REF][START_REF] Ji | Disruption of PTEN coupling with 5-HT2C receptors suppresses behavioral responses induced by drugs of abuse[END_REF].

Atypical anti-psychotics and particularly those with inverse agonism release this inhibition, providing a way to activate dopaminergic systems [START_REF] Gobert | Serotonin(2C) receptors tonically suppress the activity of mesocortical dopaminergic and adrenergic, but not serotonergic, pathways: a combined dialysis and electrophysiological analysis in the rat[END_REF][START_REF] De Deurwaerdere | Constitutive activity of the serotonin2C receptor inhibits in vivo dopamine release in the rat striatum and nucleus accumbens[END_REF][START_REF] Berg | Physiological relevance of constitutive activity of 5-HT2A and 5-HT2C receptors[END_REF]. This activation may be relevant for the treatment of depression and the negative symptoms of schizophrenia. In kittens, 5-HT2CRs have a characteristic columnar expression in layer IV of the visual cortex during the critical period for ocular dominance plasticity. 5-HT2C agonists modulate synaptic plasticity during this period [START_REF] Gaspar | The developmental role of serotonin: news from mouse molecular genetics[END_REF]. 5-HT2CRs are also present in hypothalamic proopiomelanocortin (POMC)/cocaine amphetamine-regulated transcript (CART) neurons within the arcuate nucleus [START_REF] Miller | Serotonin 5-ht2c receptor agonists: potential for the treatment of obesity[END_REF]. Activation of POMC/CART neurons inhibits feeding. This is certainly the main reason that 5-HT2C -/-mice are obese [START_REF] Miller | Serotonin 5-ht2c receptor agonists: potential for the treatment of obesity[END_REF]). The highest density of 5-HT2CRs is found in epithelial cells of the choroid plexus, where these receptors control spinal fluid production [START_REF] Leysen | 5-HT2 receptors[END_REF]). 5-HT2CR has been reported to activate PLC via Gαq/11 in many brain regions, including the choroid plexus [START_REF] Sanders-Bush | Putative selective 5-HT-2 antagonists block serotonin 5-HT-1c receptors in the choroid plexus[END_REF][START_REF] Chang | Dissecting G protein-coupled receptor signaling pathways with membranepermeable blocking peptides. Endogenous 5-HT(2C) receptors in choroid plexus epithelial cells[END_REF]Fig. 4). These receptors also activate PLD in choroid plexus epithelial cells through a mechanism involving both Gα13 and Gβγ subunits [START_REF] Mcgrew | Phospholipase D activation by endogenous 5hydroxytryptamine 2C receptors is mediated by Galpha13 and pertussis toxin-insensitive Gbetagamma subunits[END_REF]. 5-HT2CR-mediated stimulation of PLA2 and AC have been exclusively described in transfected cells [START_REF] Raymond | Multiplicity of mechanisms of serotonin receptor signal transduction[END_REF]Fig. 4). As observed for 5-HT2ARs, 5-HT2CRs are down-regulated following chronic antagonist exposure [START_REF] Van Oekelen | 5-HT2A and 5-HT2C receptors and their atypical regulation properties[END_REF]. However, the effects of such treatments are still controversial. Indeed, Schlag and coworkers (2004) have recently demonstrated that only 5-HT2CR agonists and partial agonists promote receptor desensitization and internalization, whereas neither inverse agonists nor neutral antagonists have these effects [START_REF] Schlag | Ligand dependency of 5-hydroxytryptamine 2C receptor internalization[END_REF]).

Further discussion of 5-HT2CR signaling, in particular with regard to editing, can be found below (Functions associated with editing).

3.6. 5-HT4Rs 5-HT4Rs are mainly present in limbic structures (hypothalamus, nucleus accumbens, amygdala), islands of Calleja, olfactory tubercle, basal ganglia (striatum, globus pallidus), hippocampus, substantia nigra, and septum. Although the immunolocalization of 5-HT4Rs is still not possible in brain synapses because of a lack of satisfactory antibodies, indirect evidence suggests that most of 5-HT4Rs are localized postsynaptically. However, an heterologous presynaptic localization on terminals of cholinergic, GABAergic (dentate gyrus), dopaminergic, and serotonergic neurons is also likely because of the increase in the release of these neurotransmitters by 5-HT4 agonists. Results from lesion studies and electrophysiological studies are also consistent with a pre-synaptic localization of 5-HT4Rs [START_REF] Bockaert | 5-HT4 receptors: an update[END_REF](Bockaert et al. , 2004a;;[START_REF] Bockaert | Localization of 5-HT4 receptors in vertebrate brain and their potential roles[END_REF]. Two studies have reported the immunolocalization of 5-HT4Rs. One of them shows the localization of 5-HT4Rs in respiratory neurons in the pre-Boetzinger complex [START_REF] Manzke | 5-HT4(a) receptors avert opioid-induced breathing depression without loss of analgesia[END_REF]. Half of these neurons co-express µ-receptors and Substance-P-reactive neurokinin-1 (NK1) receptors. Reverse transcription/polymerase chain reaction experiments have confirmed these findings [START_REF] Manzke | 5-HT4(a) receptors avert opioid-induced breathing depression without loss of analgesia[END_REF]. In these neurons, 5-HT4R activation overcomes fentanylinduced respiratory depression (by reversing the inhibition of fentanyl-induced cAMP production) and reestablishes stable respiratory rhythms. The second study based on a different antibody has shown the immunostaining of myenteric and submucosal plexuses [START_REF] Liu | Expression and function of 5-HT4 receptors in the mouse enteric nervous system[END_REF]. Electron-microscopic analysis has indicated an axonal localization, especially in varicosities that contain large dense-cored and small synaptic vesicles. Electrophysiological data have revealed a pre-synaptic action of 5-HT4Rs on cholinergic neurons leading to the increase in post-synaptic nicotinic EPSCs in mouse myenteric neurons (Bockaert et al. 2004a;[START_REF] Liu | Expression and function of 5-HT4 receptors in the mouse enteric nervous system[END_REF]. They are also consistent with observations that 5-HT4Rs stimulate acetylcholine release in the myenteric plexus [START_REF] Kilbinger | Effects of 5-HT4 receptor stimulation on basal and electrically evoked release of acetylcholine from guinea-pig myenteric plexus[END_REF][START_REF] Kilbinger | Benzimidazolones and renzapride facilitate acetylcholine release from guinea-pig myenteric plexus via 5-HT4 receptors[END_REF].

5-HT4Rs are positively coupled to AC in mice colliculi neurons and in guinea pig hippocampal membranes [START_REF] Dumuis | A nonclassical 5-hydroxytryptamine receptor positively coupled with adenylate cyclase in the central nervous system[END_REF][START_REF] Bockaert | Pharmacological characterization of 5-HT4 receptors positively coupled to adenylate cyclase in adult guinea pig hippocampal membranes: effect of substituted benzamide derivatives[END_REF]Fig. 4). In colliculi neurons, the 5-HT4Rsmediated increase in cAMP concentration seen after a short application of agonist leads to the inhibition of voltage-gated K + channels, including Ca 2+ -activated K + channels, and their closure [START_REF] Fagni | The 5-HT4 receptor subtype inhibits K+ current in colliculi neurons via activation of a cyclic AMP-dependent protein kinase[END_REF][START_REF] Ansanay | A cyclic AMP-dependent, long-lasting inhibition of a K+ current in mammalian neurons[END_REF]. Moreover, the activation of 5-HT4Rs produces a long-lasting inhibition of K+ channels because of the long-lasting blockade of phosphatase [START_REF] Ansanay | A cyclic AMP-dependent, long-lasting inhibition of a K+ current in mammalian neurons[END_REF]. In CA1 pyramidal neurons of hippocampus, 5-HT4Rs reduce the AHP that follows action potentials. Moreover, 5-HT4R activation induces a reduction in spike accommodation [START_REF] Andrade | 5-Hydroxytryptamine4 receptors mediate the slow excitatory response to serotonin in the rat hippocampus[END_REF][START_REF] Torres | 5-Hydroxytryptamine4 receptors reduce afterhyperpolarization in hippocampus by inhibiting calcium-induced calcium release[END_REF].

These effects are mediated by cAMP and protein kinase A (PKA) activation. 5-HT1ARs, 5-HT4Rs, and 5-HT7Rs differentially expressed in CA1 neurons allow space-specific modulation of the hyperpolarization-activated current (Ih) current, which is known to adjust synaptic integration and the repetitive discharge behavior of CA1 neurons [START_REF] Bickmeyer | Differential modulation of I(h) by 5-HT receptors in mouse CA1 hippocampal neurons[END_REF]. In prefrontal cortex pyramidal neurons, 5-HT4Rs can activate or inhibit GABAergic synaptic transmission. This regulation of the GABAA current is also mediated by cAMP-and PKA-dependent signaling pathways [START_REF] Cai | Activity-dependent bidirectional regulation of GABA(A) receptor channels by the 5-HT(4) receptor-mediated signalling in rat prefrontal cortical pyramidal neurons[END_REF].

In hippocampal neurons, activation of the endogenous 5-HT4Rs leads to a decrease in the length and number of neurites through the Gα13/RhoA signaling pathway (Fig. 4; [START_REF] Ponimaskin | 5-Hydroxytryptamine 4(a) receptor is coupled to the Galpha subunit of heterotrimeric G13 protein[END_REF].

Moreover, of note, 5-HT4R activation, at least in dorsal root ganglion (DRG) cells, is associated with an increase in the tetrodotoxin-insensitive Na + current, but the messenger involved in this effect does not appear to be cAMP [START_REF] Cardenas | Two parallel signaling pathways couple 5HT1A receptors to N-and L-type calcium channels in C-like rat dorsal root ganglion cells[END_REF]. 5-HT4Rs increase, via the production of cAMP, the activated exchange factor, Epac. Epac in turn activates the small G protein Rap1, which increases Ras GTP loading. This Epac-dependent signaling pathway stimulates sAPPα secretion [START_REF] Lezoualc'h F | The serotonin 5-HT4 receptor and the amyloid precursor protein processing[END_REF]. In colliculi neurons, the desensitization of 5-HT4Rs is homologous and results in the activation of G-proteincoupled receptor kinase type 2 (GRK2; [START_REF] Ansanay | Characterization of homologous 5hydroxytryptamine4 receptor desensitization in colliculi neurons[END_REF][START_REF] Barthet | Uncoupling and endocytosis of 5-hydroxytryptamine 4 receptors. Distinct molecular events with different GRK2 requirements[END_REF]. In heterologous cells, the GRK2 concentration is too low, and homologous desensitization of 5-HT4Rs does not occur [START_REF] Barthet | Uncoupling and endocytosis of 5-hydroxytryptamine 4 receptors. Distinct molecular events with different GRK2 requirements[END_REF]. Several reports clearly show the modulation of LTP and long-term depression (LTD) by 5-HT4 agonists (Kemp andManahan-Vaughan 2004, 2005). Interestingly, 5-HT4Rs exert both a tonic and a phasic stimulatory control over dorsal 5-HT raphe neurons [START_REF] Lucas | 5-HT4 receptors exert a frequencyrelated facilitatory control on dorsal raphe nucleus 5-HT neuronal activity[END_REF]. This effect might be enhanced by the activation of 5-HT4Rs expressed in the medial prefrontal cortex [START_REF] Lucas | Frontocortical 5-HT4 receptors exert positive feedback on serotonergic activity: viral transfections, subacute and chronic treatments with 5-HT4 agonists[END_REF]. 5-HT4Rs also positively modulate learning and memory, reward, sensitivity to novelty, and stress (Bockaert et al. 2004a;[START_REF] Compan | Attenuated response to stress and novelty and hypersensitivity to seizures in 5-HT4 receptor knock-out mice[END_REF]).

5-HT5Rs

Studies on the cellular and subcellular localization of 5-HT5Rs by using immunohistochemical methods have led to contradictory results. Both astrocytes and neuronal localizations have been reported. One study reports a localization on 5-HT neurons, suggesting a possible role as autoreceptors. However, the quality of the antibodies used and the lack of specific pharmacological tools preclude any definitive conclusion regarding the functional role of these receptors [START_REF] Nelson | 5-HT5 receptors[END_REF]. Surprisingly, no study to date has identified a functional coupling of 5-HT5Rs in native neuronal tissue. Rat 5-HT5R transfected into C6 glioma cells has been shown to couple to the inhibition of cAMP production. In the same C6 glioma cells, one study has reported that the human 5-HT5AR couples to multiple signal transduction pathways, e.g., the inhibition of cAMP accumulation and of ADP-ribosyl cyclase and the activation of outward currents. This study has indicated that human 5-HT5AR is coupled to Gαi/Gαo; it also raises the possibility of a cross-coupling of the receptor to Gαq/11 [START_REF] Noda | Recombinant human serotonin 5A receptors stably expressed in C6 glioma cells couple to multiple signal transduction pathways[END_REF]Fig. 4).

5-HT6Rs

5-HT6Rs are almost exclusively expressed in the central nervous system. They are found at high concentrations in limbic and cortical regions [START_REF] Woolley | 5-HT6 receptors[END_REF]) but are not expressed in 5-HT neurons. Immunohistochemical studies including electron-microscopic investigations have shown prominent dendritic localization. Some non-motile cilia of the nucleus accumbens, the caudate-putamen, the olfactory tubercles, and the islands of Calleja cells also express 5-HT6Rs [START_REF] Brailov | Localization of 5-HT(6) receptors at the plasma membrane of neuronal cilia in the rat brain[END_REF]. In the striatum, 5-HT6Rs are expressed by medium-sized spiny neurons projecting to the substantia nigra or globus pallidus [START_REF] Gerard | Immuno-localization of serotonin 5-HT6 receptor-like material in the rat central nervous system[END_REF][START_REF] Sebben | 5-HT6 receptors positively coupled to adenylyl cyclase in striatal neurones in culture[END_REF]. Preferential GABAergic localization is suggested in most brain localizations. No electrophysiological data are available. Selective antagonists have been synthesized and can now be used to investigate the physiological roles of 5-HT6Rs. For example, the selective antagonist SB-271046 has been shown to increase glutamate release probably via deinhibition of GABAergic inhibition [START_REF] Woolley | 5-HT6 receptors[END_REF]). Many studies have tried to correlate 5-HT6R polymorphisms with psychiatric disorders [START_REF] Woolley | 5-HT6 receptors[END_REF]). Contradictory results have been reported. Pharmacological studies and localization studies indicate possible roles in psychotic and affective disorders, anxiety, epilepsy, and memory retention but not the regulation of feeding [START_REF] Woolley | 5-HT6 receptors[END_REF]). Native 5-HT6Rs, like 5-HT4Rs and 5-HT7Rs, are positively coupled to AC via the Gαs protein in striatal tissue or primary cultures of striatal neurons [START_REF] Schoeffter | 5-Hydroxytryptamine receptors with a 5-HT6 receptor-like profile stimulating adenylyl cyclase activity in pig caudate membranes[END_REF][START_REF] Sebben | 5-HT6 receptors positively coupled to adenylyl cyclase in striatal neurones in culture[END_REF]). This positive coupling has been confirmed by 5-HT6Rs being expressed in various heterologous expression models [START_REF] Ruat | A novel rat serotonin (5-HT6) receptor: molecular cloning, localization and stimulation of cAMP accumulation[END_REF]Fig. 4).

5-HT7Rs

In the brain, 5-HT7Rs are mainly found in the thalamus, hypothalamus (suprachiasmatic nucleus), cerebral cortex (both on pyramidal and GABAergic neurons), hippocampus (all CA regions), and amygdala [START_REF] Hedlund | Functional, molecular and pharmacological advances in 5-HT7 receptor research[END_REF][START_REF] Thomas | 5-HT7 receptors[END_REF]. Most 5-HT7Rs are localized on both the soma and axon terminals (outside the synapses) of GABAergic neurons. Within the cerebellum, 5-HT7Rs are exclusively located in Purkinje neurons. 5-HT7Rs are also found in intra-and extra-cranial blood vessels and meningeal tissues [START_REF] Hedlund | Functional, molecular and pharmacological advances in 5-HT7 receptor research[END_REF]). In the guinea pig hippocampus, 5-HT7Rs have been recognized as being the receptor with a high affinity for 5-carboxamidotryptamine (5-CT) coupled to AC [START_REF] Bockaert | Pharmacological characterization of 5-HT4 receptors positively coupled to adenylate cyclase in adult guinea pig hippocampal membranes: effect of substituted benzamide derivatives[END_REF][START_REF] Bockaert | 5-HT4 receptors: an update[END_REF]. In heterologous cells, the four 5-HT7R splice variants activate cAMP production via the Gαs protein [START_REF] Ruat | A novel rat serotonin (5-HT6) receptor: molecular cloning, localization and stimulation of cAMP accumulation[END_REF]. 5-HT7Rs also activate the ERK1/2 pathway in hippocampal neurons. However, the signaling pathway used by the receptor to activate ERK remains unclear. Most of the 5-HT7R-mediated activation of ERK has been found to be mediated via Gαs. Both PKA-dependent and -independent pathways might contribute to ERK activation. Until recently, the pathway for ERK activation was shown to require cAMP, PKA, and MAP kinase (MEK) in neurons endogenously expressing 5-HT7Rs and in 5-HT7R-transfected cells (including PC12 and HEK 293cells; [START_REF] Norum | Ras-dependent ERK activation by the human G(s)-coupled serotonin receptors 5-HT4(b) and 5-HT7(a)[END_REF]. However, another report indicates that 5-HT7Rs activate ERK in hippocampal neurons and transfected PC12 cells through a pathway independent of PKA but requiring a cAMP-GEF, such as Epac [START_REF] Lin | Coupling of neuronal 5-HT7 receptors to activation of extracellular-regulated kinase through a protein kinase A-independent pathway that can utilize Epac[END_REF]. 5-HT7Rs inhibit slow AHP in thalamic and trigeminal motor neurons [START_REF] Goaillard | Serotonin suppresses the slow afterhyperpolarization in rat intralaminar and midline thalamic neurones by activating 5-HT(7) receptors[END_REF]. In vivo, SSRIs increase the population spike amplitude in CA1 and CA3 areas. This effect is blocked by SB-269970-A, a specific 5-HT7R antagonist [START_REF] Matsumoto | Differential characteristics of endogenous serotonin-mediated synaptic transmission in the hippocampal CA1 and CA3 fields of anaesthetized rats[END_REF]. In the CA1 region and in thalamic and rat DRG neurons, 5-HT7Rs increases excitability through a mechanism involving a cAMP-mediated Ih current [START_REF] Bickmeyer | Differential modulation of I(h) by 5-HT receptors in mouse CA1 hippocampal neurons[END_REF]. All these effects suggest an important role for 5-HT7Rs in the control of neuronal excitability and may contribute to greater information transfer within a neuronal network. 5-HT7Rs endogenously expressed in hippocampal neurons significantly increase neurite length through the Gα12, RhoA, and Cdc42 GTPasesdependent signaling pathway (Kvachnina et al. 2005) (Fig 4). Functional studies have also implicated 5-HT7Rs in thermoregulation and the activation of rapid-eye-movement (REM) sleep via the modulation of suprachiasmatic nucleus neurons [START_REF] Thomas | 5-HT7 receptors[END_REF]. In this regard, 5-HT7R antagonists inhibit REM sleep and increase the latency to onset of REM sleep. This effect suggests that 5-HT7R antagonists are candidatesfor treating depression, a situation in which the REM sleep is disturbed. A role of 5-HT7Rs in learning and memory is suggested by the observed decrease in synaptic plasticity in CA1 and contextual fear conditioning in 5-HT7R -/-mice [START_REF] Thomas | 5-HT7 receptors[END_REF]. 5-HT7Rs are recognized for their important role in controlling hormonal release not only at the hypothalamic, but also at the peripheral level. A role in pain and migraine (via their action on blood vessels but also on neurogenic inflammation) is under investigation. Finally, a role in schizophrenia remains to be found in animal models [START_REF] Hedlund | Functional, molecular and pharmacological advances in 5-HT7 receptor research[END_REF][START_REF] Thomas | 5-HT7 receptors[END_REF].

Fine-tuning of signaling functions

Constitutive activity

Studies performed mainly in heterologous cells transfected with both native and mutant forms of 5-HT1Rs [START_REF] Albert | Constitutive G(i2)-dependent activation of adenylyl cyclase type II by the 5-HT1A receptor. Inhibition by anxiolytic partial agonists[END_REF][START_REF] Kiel | Pharmacological properties of the naturally occurring Phe-124-Cys variant of the human 5-HT1B receptor: changes in ligand binding, G-protein coupling and second messenger formation[END_REF], 5-HT2Rs [START_REF] Egan | Creation of a constitutively activated state of the 5hydroxytryptamine2A receptor by site-directed mutagenesis: inverse agonist activity of antipsychotic drugs[END_REF], 5-HT4Rs [START_REF] Claeysen | Novel brain-specific 5-HT4 receptor splice variants show marked constitutive activity: role of the C-terminal intracellular domain[END_REF], 5-HT6Rs, and 5-HT7Rs have demonstrated that 5-HTRs can exist in a conformation that mimics an active state of the receptor in the absence of agonist, thus revealing that these receptors display constitutive activity. All these studies highlight the importance of i2 and/or i3 loops in the regulation of constitutive activity of the majority of 5-HTRs and of the role of the C-terminus in the constitutive activity of 5-HT4R and 5-HT7R splice variants, which differ in the length of their C-terminal tail (for a review, see [START_REF] Bockaert | Molecular and cellular determinants of GPCR splice variant constitutive activity[END_REF]. However, to date, only 5-HT2ARs and 5-HT2CRs have been shown to exhibit constitutive activity in vivo [START_REF] Berg | Physiological relevance of constitutive activity of 5-HT2A and 5-HT2C receptors[END_REF]. [START_REF] Harvey | Role of the serotonin 5-HT(2A) receptor in learning[END_REF] has shown that 5-HT2AR inverse agonists, but not neutral antagonists, retard the learning of the conditioned eye link response in rabbits stimulated by LSD. This effect is not attributable to the blockade of endogenous 5-HT by inverse agonists, but rather to an inhibition of endogenous constitutive activity, because it occurs after lesions.

Evidence of an in vivo constitutive activity of 5-HT2CRs has come from experiments by De

Deurwaerdere and co-workers (2004); by using microdialysis, they have demonstrated that SB 206553, an inverse agonist, but not the neutral antagonist SB 242084, enhances basal dopamine release in dopamine-innervated areas, such as the frontal cortex, the accumbens nucleus, and the striatum. The effects of the inverse agonist are insensitive to the lesion of 5-HT neurons and are blocked by the neutral antagonist.

Functions associated with editing

As seen above, 5-HT2CR exhibits a unique mechanism of generating multiple functional receptor variants through a process called mRNA editing [START_REF] Burns | Regulation of serotonin-2C receptor G-protein coupling by RNA editing[END_REF]. RNA transcripts encoding the 5-HT2CR undergo A to I RNA editing events at up to five sites (see above). In heterologous cells, RNA editing appears to regulate the pattern of intracellular signaling [START_REF] Burns | Regulation of serotonin-2C receptor G-protein coupling by RNA editing[END_REF]. The non-edited form (5-HT2C-INIR) is more potent in stimulating PLC and exhibits more constitutive activity than the edited forms [START_REF] Niswender | RNA editing of the human serotonin 5-hydroxytryptamine 2C receptor silences constitutive activity[END_REF]. Moreover, non-edited 5-HT2C-INIR is capable of coupling to the Gα13 protein (activating the PLD signaling pathway and promoting Rho GTPase activity; [START_REF] Mcgrew | RNA editing of the human serotonin 5-HT2C receptor disrupts transactivation of the small G-protein RhoA[END_REF]), whereas the fully edited isoform 5-HT2C-VGVR fails to activate Gα13 [START_REF] Price | RNA editing of the human serotonin 5-HT2C receptor alters receptormediated activation of G13 protein[END_REF]. RNA editing has an important implication for serotonergic signal transduction in vivo. Editing is regulated by 5-HT itself. Depletion of 5-HT increases the expression of edited forms having a high affinity for 5-HT, whereas stimulation of the 5-HT2A/2CRs has the opposite effect (Gurevich et al. 2002a). Moreover, altered RNA editing of the 5-HT2CR may play a role in depression. 5-HT2C mRNA isoforms expressed in human prefrontal cortex differ in tissues obtained from depressed suicide victims compared with healthy controls. In suicides, three abundant isoforms, resulting from an increase at the E and C sites and a decrease at the D site and encoding receptors, are expressed. These isoforms exhibit decreased basal activity and decreased agonist affinity and potency (Gurevich et al. 2002b). Recently, [START_REF] Kishore | The snoRNA HBII-52 regulates alternative splicing of the serotonin receptor 2C[END_REF] have reported that the small nucleolar RNA (snoRNA) HBII-52, which exhibits sequence complementarity to the alternative spliced exon V encoding for a functional 5-HT2CR, regulates receptor mRNA splicing. Notably, Prader-Willy syndrome patients (characterized by neonatal muscular hypotonia, early childhood obesity, hypogonadism, and mental retardation) who do not express HBII-52 snoRNA exhibit abnormally low levels of the non-edited 5-HT2CR isoform, suggesting a role of serotonergic systems in Prader-Willi syndrome [START_REF] Kishore | The snoRNA HBII-52 regulates alternative splicing of the serotonin receptor 2C[END_REF].

5-HTR interacting proteins and modulation of signaling

The first proteins found to have functional interactions with metabotropic 5-HTRs were, of course, G proteins (see above). Although coupling to G proteins remains a central event in 5-HTR-mediated signaling, an increasing body of evidence indicates that the regulation of their functional status also depends on interactions with a variety of additional protein partners. Like other GPCRs, 5-HTRs associate with β-arrestins in a manner that is dependent on receptor phosphorylation by GRKs. The binding of arrestins to the receptors promotes their internalization. This process requires the recruitment, by arrestins, of binding partners, including clathrin, the central component of the endocytic machinery, AP2, another endocytic protein, and intracellular trafficking proteins such as NSF, ARF6, and ARNO [START_REF] Lefkowitz | Transduction of receptor signals by beta-arrestins[END_REF]. β-Arrestins also serve as scaffolds for signaling enzymes such as c-Src, members of the MAP kinase pathways, and the c-Jun Nterminal kinase 3, which participate in 5-HTR signaling [START_REF] Lefkowitz | Transduction of receptor signals by beta-arrestins[END_REF]. In addition to the binding to β-arrestins, which seems to be a general mechanism, 5-HTR subtypes can recruit specific protein partners. This section will describe our current knowledge concerning these subtype-specific interactions. Metabotropic 5-HTRs are certainly the GPCRs for which the highest number of specific accessory proteins have been identified, thanks to methodologies such as the yeast twohydrid screen and proteomic approaches combining affinity chromatography experiments and the identification of receptor partners by mass spectrometry. However, our present cognizance of 5-HTR protein partners fluctuates from one receptor to another.

For some 5-HTRs (e.g., 5-HT2AR, 5-HT2CR, and several 5-HT4R splice variants), interacting proteins have been extensively characterized by using, in part, proteomic approaches, whereas no binding partner has so far been identified for a number of 5-HTRs (Table 1). As for other GPCRs, most of the hitherto identified 5-HTR interacting proteins associate with their C-terminus, even though some of them have been shown to bind to motifs located on intracellular loops (i2 and i3 loops).

Proteins interacting with the 5-HTR C-terminal domain

PDZ-domain-containing proteins

The acronym PDZ derives from the three first proteins (PSD-95/Disc large/Zonula occludens-1) in which these domains have been described [START_REF] Nourry | PDZ domain proteins: plug and play![END_REF]. To date, PDZ proteins are certainly the most numerous proteins involved in the scaffolding of multiprotein complexes that contribute to the targeting, the trafficking, and the fine tuning of signaling properties of membrane-bound receptors, including GPCRs. Genome sequence analysis predicts the presence of more than 300 PDZ proteins in humans. PDZ proteins generally bind to a conserved consensus sequence (PDZ ligand) located at the extreme C-terminus of target receptors. PDZ domains have been classified according to their specificity for these C-terminal motifs. Class I PDZ domains preferentially bind to S/TXφ, class II to φXφ and class III to E/DXφ motifs, where φ represents a hydrophobic residue and X any residue [START_REF] Nourry | PDZ domain proteins: plug and play![END_REF]. Several 5-HTRs express class I or II canonical PDZ ligands, suggesting that they may interact with PDZ-based protein scaffolds. These include the three 5-HT2R subtypes (SCV, SYV/I, and SSV, for 5-HT2AR, the 5-HT2BR, and 5-HT2CR, respectively), several 5-HT4R splice variants (SCF-5-HT4(a)R, VPV-5-HT4(e)R and 5-HT4(f)R), and one 5-HT7R variant (FVL, 5-HT7(b)R). The first evidence of an interaction between one 5-HTR and one PDZ protein was obtained by [START_REF] Ullmer | Cloning and characterization of MUPP1, a novel PDZ domain protein[END_REF]. Screening a human brain cDNA library with the yeast two-hybrid system, this group isolated and cloned a multi-PDZ protein (the protein called MUPP1), which interacts with the C-terminal domain of the human 5-HT2C receptor. MUPP1, which contains thirteen PDZ domains and no obvious catalytic domains, is the PDZ protein that contains the highest number of PDZ domains identified to date. The 5-HT2CR specifically interacts with the 10 th PDZ domain of MUPP1, leaving the other PDZ domains free to interact with additional partners within a larger complex [START_REF] Becamel | Interaction of serotonin 5-hydroxytryptamine type 2C receptors with PDZ10 of the multi-PDZ domain protein MUPP1[END_REF][START_REF] Parker | Agonistinduced phosphorylation of the serotonin 5-HT2C receptor regulates its interaction with multiple PDZ protein 1[END_REF]. This interaction is dynamically regulated by agonist-dependent receptor phosphorylation at Ser458, located in the PDZ-binding motif (position-2; [START_REF] Parker | Agonistinduced phosphorylation of the serotonin 5-HT2C receptor regulates its interaction with multiple PDZ protein 1[END_REF]. The receptor/MUPP1 interaction induces both conformational changes in the MUPP1 protein [START_REF] Parker | Agonistinduced phosphorylation of the serotonin 5-HT2C receptor regulates its interaction with multiple PDZ protein 1[END_REF] and receptor clustering at the cell surface [START_REF] Becamel | Interaction of serotonin 5-hydroxytryptamine type 2C receptors with PDZ10 of the multi-PDZ domain protein MUPP1[END_REF]. MUPP1 also associates with the 5-HT2AR and 5-HT2BR, but the functional significance of these interactions remains to be demonstrated [START_REF] Becamel | Interaction of serotonin 5-hydroxytryptamine type 2C receptors with PDZ10 of the multi-PDZ domain protein MUPP1[END_REF]. A more comprehensive description of PDZ proteins that interact with 5-HT2AR and 5-HT2CR C-termini has been performed by using proteomic approaches comparing proteins "fished out" by using the receptor Ctermini as bait and mutant baits in which the C-terminal valine residue, which is critical for interaction with target PDZ domains, has been mutated into an alanine [START_REF] Becamel | Synaptic multiprotein complexes associated with 5-HT(2C) receptors: a proteomic approach[END_REF][START_REF] Becamel | The serotonin 5-HT2A and 5-HT2C receptors interact with specific sets of PDZ proteins[END_REF]).

These studies have demonstrated that each 5-HT2R subtype interacts with specific sets of PDZ proteins, even though they share an identical PDZ ligand (SXV), thereby underpinning the role of residues located outside this canonical sequence in the specificity for PDZ domains [START_REF] Becamel | The serotonin 5-HT2A and 5-HT2C receptors interact with specific sets of PDZ proteins[END_REF]. They have also revealed that 5-HT2ARs only interact with PDZ proteins located at the post-synapse (e.g., PSD-95), whereas 5-HT2CRs associate with PDZ proteins exhibiting both presynaptic and post-synaptic localizations (e.g., the Veli3/ CASK/Mint1 complex), consistent with the differential distribution of the receptors at the synaptic junction. The association of 5-HT2ARs with PSD-95 has been shown to enhance receptor-mediated signal transduction in heterologous cells, a process that can be related to the ability of PSD-95 to inhibit agonist-induced 5-HT2AR internalization (Xia et al. 2003a). The functional consequences of the other interactions remain to be demonstrated. However, several studies with receptors mutated at the PDZ ligand indicate that 5-HT2R/PDZ protein interactions play a critical role in governing receptor targeting in neurons and/or modulating their signal transduction properties. The 5-HT2AR PDZ ligand seems to be essential for preferential receptor targeting to the dendritic compartment of pyramidal cortical neurons (Xia et al. 2003b). The deletion of the 5-HT2CR PDZ ligand, which prevents agonist-induced receptor phosphorylation (see above), modulates the desensitization/ resensitization of the receptor Ca 2+ effector pathway in NIH-3T3 fibroblasts [START_REF] Backstrom | Deletion of the serotonin 5-HT2C receptor PDZ recognition motif prevents receptor phosphorylation and delays resensitization of receptor responses[END_REF]. A similar approach has indicated the 5-HT2BR PDZ ligand is essential for receptor-mediated activation of constitutive and inducible NOS activities in a murine cell line [START_REF] Manivet | PDZ-dependent activation of nitric-oxide synthases by the serotonin 2B receptor[END_REF]. The interaction of 5-HT2Rs (probably 5-HT2ARs) with PDZ proteins may also be involved in the persistent facilitation of sensory synaptic transmission in the spinal cord dorsal horn, a process underlying chronic pain, following tissue injury [START_REF] Li | Silent glutamatergic synapses and nociception in mammalian spinal cord[END_REF]. This facilitation results from the mobilization of AMPA receptors into the synapses, transforming silent synapses into functional ones [START_REF] Li | Silent glutamatergic synapses and nociception in mammalian spinal cord[END_REF]. The recruitment of synaptic AMPA receptors induced by 5-HT2 agonists requires PDZ proteins that physically interact with GluR2/3 AMPA receptor subunits [START_REF] Li | AMPA receptor-PDZ interactions in facilitation of spinal sensory synapses[END_REF]. Thus, 5-HT2 and AMPA receptors appear to be functionally linked by a PDZ-based protein scaffold, contributing to the facilitation of nociceptive transmission in the spinal cord. The molecular composition of this PDZ scaffold is still unknown. Specific sets of PDZ proteins interacting with 5-HT4(a)R and 5-HT4(e)R have also been characterized by proteomic screens [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF]). Among the 5-HT4(a)R PDZbinding partners, NHERF-1 (also designated as Ezrinradixin-moesin-binding phosphoprotein 50, EBP50) and SNX27, a recently cloned sorting nexin, have attracted particular attention. Interaction of the 5-HT4(a)R with NHERF-1 targets the receptor to specialized subcellular microdomains and seems to be involved in receptormediated cytoskeleton remodeling and the modulation of cell shape [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF].

SNX27 belongs to a large protein family involved in the targeting of cell surface receptors to the endocytic compartment. The 5-HT4(a)R/ SNX27 interaction has been consistently found to target part of the receptor pool to early endosomes in heterologous cells [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF]. Two forms of SNX27 that differ in their C-termini (designated as SNX27a and b) have been identified. Both proteins interact with the 5-HT4(a)R C-terminus and are enriched in the brain. SNX27b, but not SNX27a, has been found to be up-regulated in the neocortex of rats following acute exposure to psychostimulants, such as amphetamines and cocaine [START_REF] Kajii | A developmentally regulated and psychostimulant-inducible novel rat gene mrt1 encoding PDZ-PX proteins isolated in the neocortex[END_REF]. This up-regulation, blocked by D1 receptor antagonists, is reminiscent of stimulant-induced behavioral sensitization and therefore may be involved in the initiation and/or maintenance of long-term neuronal adaptation in response to psychostimulant compounds. As 5-HT4Rs have been involved in drug sensitization and addiction [START_REF] Compan | Attenuated response to stress and novelty and hypersensitivity to seizures in 5-HT4 receptor knock-out mice[END_REF]), the 5-HT4R/SNX27 interaction may contribute to neuronal adaptation mechanisms underlying these processes.

Other proteins interacting with 5-HTR C-termini

Several 5-HTR subtypes have been shown to interact physically with calmodulin (CaM) via their Cterminus (5-HT2AR and 5-HT2CR;[START_REF] Becamel | Synaptic multiprotein complexes associated with 5-HT(2C) receptors: a proteomic approach[END_REF][START_REF] Turner | Interaction of calmodulin with the serotonin 5-hydroxytryptamine2A receptor. A putative regulator of G protein coupling and receptor phosphorylation by protein kinase C[END_REF] and/or their intracellular loops (5-HT1AR and 5-HT2AR;[START_REF] Turner | Calmodulin interacts with the third intracellular loop of the serotonin 5-hydroxytryptamine1A receptor at two distinct sites: putative role in receptor phosphorylation by protein kinase C[END_REF][START_REF] Turner | Interaction of calmodulin with the serotonin 5-hydroxytryptamine2A receptor. A putative regulator of G protein coupling and receptor phosphorylation by protein kinase C[END_REF]. Putative CaM-binding sites have also been identified by using a CaM-binding site search engine (http://calcium. uhnres.utoronto.ca/ctdb/pub_pages/search/index.htm) for the intracellular loops of 5-HT1B, 5-HT1D, 5-HT1E, 5-HT1F, 5-HT2C, 5-HT6, and 5-HT7 receptors and in the C-terminus of 5-HT4 and 5-HT7 receptors [START_REF] Turner | Interaction of calmodulin with the serotonin 5-hydroxytryptamine2A receptor. A putative regulator of G protein coupling and receptor phosphorylation by protein kinase C[END_REF], but these interactions remain to be established. The 5-HT2AR C-terminal CaM-binding site, located at the juxtamembrane region, overlaps with a putative PKC phosphorylation site. CaM binding and receptor phosphorylation by PKC in vitro have been found to be antagonistic. This suggests that the interaction of CaM with the 5-HT2AR C-terminus can modulate receptor phosphorylation and desensitization [START_REF] Turner | Interaction of calmodulin with the serotonin 5-hydroxytryptamine2A receptor. A putative regulator of G protein coupling and receptor phosphorylation by protein kinase C[END_REF]. The 5-HT2AR C-terminus also interacts with ARF1 and, to a lesser extent, ARF6 [START_REF] Robertson | Selective interaction of ARF1 with the carboxy-terminal tail domain of the 5-HT2A receptor[END_REF]. ARF proteins bind to a conserved NPxxY motif that has been found in a number of rhodopsin family GPCRs and that is located at the junction of the transmembrane domain 7 and the C-terminal domain [START_REF] Johnson | Role of the conserved NPxxY motif of the 5-HT(2A) receptor in determining selective interaction with isoforms of ADP-Ribosylation Factor (ARF)[END_REF]). This motif is critical in determining selective interaction with isoforms of ARF [START_REF] Johnson | Role of the conserved NPxxY motif of the 5-HT(2A) receptor in determining selective interaction with isoforms of ADP-Ribosylation Factor (ARF)[END_REF]). The interaction between 5-HT2AR and ARF proteins appears to be essential for 5-HT2AR-mediated PLD activation, which is independent of Gαq/11 [START_REF] Robertson | Selective interaction of ARF1 with the carboxy-terminal tail domain of the 5-HT2A receptor[END_REF]. A weaker interaction between ARF1-6 and the i3 loop of the receptor has also been demonstrated. In addition to PDZ proteins and CaM, the 5-HT2CR C-terminal domain recruits two other proteins with unknown functions: PTD4, a putative GTPbinding protein, and PICOT, a novel PKC theta-interacting protein with a thioredoxin homology domain that is involved in the regulation of the thioredoxin system [START_REF] Becamel | Synaptic multiprotein complexes associated with 5-HT(2C) receptors: a proteomic approach[END_REF]. The 5-HT4(a)R associates with CRMP2, a member of the collapsin response mediator protein (CRMP) family, essential components of the molecular machinery leading to growth cone collapse in response to semaphorins.

CRMP2 induces the outgrowth of multiple axons with a collapse of all dendrites in hippocampal neurons in vitro, thereby promoting neuronal polarity [START_REF] Inagaki | CRMP-2 induces axons in cultured hippocampal neurons[END_REF]. 5-HT4(a)Rs have been shown to cause Gα13-and RhoA-dependent neurite retraction and cell rounding in neuroblastoma x glioma NIE-115 cells [START_REF] Ponimaskin | 5-Hydroxytryptamine 4(a) receptor is coupled to the Galpha subunit of heterotrimeric G13 protein[END_REF]. Whether the physical interaction between 5-HT4(a)R and CRMP2 contributes to the receptor regulation of neuronal architecture remains to be elucidated. Sec23A, a component of the COPII complex involved in the transport of secretory and plasma membrane proteins from the endoplasmic reticulum to the Golgi apparatus, has been found to associate with the 5-HT4(e)R C-terminus [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF]). This interaction is inhibited by the deletion of the four C-terminal residues of the receptor, indicating a critical role of the extreme Cterminal sequence in the trafficking of short 5-HT4R variants from the endoplasmic reticulum to the plasma membrane.

Proteins interacting with the intracellular loops of 5-HTRs

As previously mentioned, CaM-binding sites have been identified in the intracellular loops of several 5-HTR subtypes. CaM has been shown to interact with two distinct motifs located in the N-and C-termini juxtamembrane regions of the 5-HT1AR i3 loop [START_REF] Turner | Calmodulin interacts with the third intracellular loop of the serotonin 5-hydroxytryptamine1A receptor at two distinct sites: putative role in receptor phosphorylation by protein kinase C[END_REF]. Binding of CaM to 5-HT1AR has been found to be antagonistic with PKC-mediated receptor phosphorylation, suggesting a role in receptor desensitization. This interaction may also be critical for the 5-HT1AR-mediated activation of the ERK signaling pathway, a process involving agonist-induced receptor internalization [START_REF] Rocca | Serotonin 5-HT1A receptor-mediated Erk activation requires calcium/calmodulin-dependent receptor endocytosis[END_REF]. Interaction of CaM with the i2 loop of 5-HT2ARs decreases receptor-mediated [ 35 S]GTPγs binding to NIH-3T3 cell membranes, suggesting a inhibitory role of CaM in 5-HT2AR-mediated signal transduction by dampening the interaction with the G protein [START_REF] Turner | Interaction of calmodulin with the serotonin 5-hydroxytryptamine2A receptor. A putative regulator of G protein coupling and receptor phosphorylation by protein kinase C[END_REF]. 5-HT2ARs interact (via their intracellular loops) with other proteins that influence receptor subcellular distribution within neurons or modulate their signal transduction properties. These include the cytoskeletal microtubuleassociated protein MAP1A, caveolin-1, and the p90 ribosomal S6 kinase 2 (RSK2). The association of 5-HT2ARs (i3 loop) with MAP1A may be responsible for the intracellular distribution of 5-HT2ARs (in an inactive state) in the dendritic compartment of pyramidal neurons of the cerebral cortex [START_REF] Cornea-Hebert | Similar ultrastructural distribution of the 5-HT(2A) serotonin receptor and microtubuleassociated protein MAP1A in cortical dendrites of adult rat[END_REF]. The 5-HT2AR/ caveolin-1 interaction, which has been observed in both C6 glioma cells and rat brain synaptic membrane preparations, appears to be essential for targeting the receptor to lipid rafts. This interaction facilitates 5-HT2AR coupling to Gαq proteins [START_REF] Bhatnagar | Caveolin-1 interacts with 5-HT2A serotonin receptors and profoundly modulates the signaling of selected Galphaq-coupled protein receptors[END_REF]) and appears to be essential for the serotonin-induced inhibition of voltage-gated K + channels (Kv1.5), which form a ternary complex with 5-HT2ARs and caveolin-1 upon 5-HT2AR activation in smooth muscle cells of the pulmonary artery [START_REF] Cogolludo | Serotonin inhibits voltage-gated K+ currents in pulmonary artery smooth muscle cells. Role of 5-HT2A receptors, caveolin-1, and KV1.5 channel internalization[END_REF]. In contrast, the association of 5-HT2ARs (i3 loop) with RSK2 seems to be required for the tonic inhibition of receptor signaling exerted by this kinase [START_REF] Sheffler | p90 ribosomal S6 kinase 2 exerts a tonic brake on G protein-coupled receptor signaling[END_REF]). This tonic inhibition may result from constitutive receptor phosphorylation leading to a constitutively desensitized state. RSK2 expresses a C-terminal class I PDZ ligand (TAL), and RSK2/PDZ protein interactions have been demonstrated to be important for RSK2mediated modulation of excitatory synaptic transmission. This offers the possibility that 5-HT2AR, RSK2, and PDZ proteins form a unique signaling complex modulating 5-HT2AR function. In fetal myoblasts, serotonin induces a physical association of the 5-HT2A receptor with Jak2, providing a way of recruiting STAT3 by the receptor-kinase complex and the subsequent phosphorylation and nuclear translocation of STAT3 [START_REF] Guillet-Deniau | Identification and localization of a skeletal muscle secrotonin 5-HT2A receptor coupled to the Jak/STAT pathway[END_REF]. The binding site of Jak2/STAT3 on the receptor (intracellular loops versus the C-terminus) is still uncertain. Activation of members the JAK/STAT pathway upon 5-HT2AR activation has also been reported in vascular smooth muscle [START_REF] Banes | Activation of the JAK/STAT pathway in vascular smooth muscle by serotonin[END_REF]), but whether their physical association with the receptor is required for this activation remains to be demonstrated. A recent study has revealed that 5-HTRs are not only capable of interacting with protein kinases, but that they can also associate with phosphatases, modulating their phosphorylation state. This study has demonstrated a physical interaction between the 5-HT2CR i3 loop and the tumor suppressor PTEN, an enzyme exhibiting both lipid and protein phosphatase activities [START_REF] Ji | Disruption of PTEN coupling with 5-HT2C receptors suppresses behavioral responses induced by drugs of abuse[END_REF]. This interaction takes place in dopaminergic neurons of the VTA innervating the accumbens nucleus, these are tonically inhibited by active 5-HT2CRs. PTEN coupling with 5-HT2CRs prevents agonist-induced receptor phosphorylation at C-terminal serine residues. Disruption of the 5-HT2CR/PTEN interaction with a membrane-permeant peptidyl mimetic of the PTEN-binding site consistently reproduces the inhibitory effect of 5-HT2C agonists on VTA dopamine neuron firing. Such a peptide also prevents the rewarding effects of cannabinoids, effects that are mediated by the increased activity of VTA dopamine neurons, without the apparent side-effects observed following agonist activation of 5-HT2CR. These experiments indicate that the regulation of 5-HT2CR functional activity mediated by the interaction with PTEN plays an important role in the reinforcing role of drugs [START_REF] Ji | Disruption of PTEN coupling with 5-HT2C receptors suppresses behavioral responses induced by drugs of abuse[END_REF]. Another recent study has reported the interaction of the third intracellular loop of 5-HT1BRs with p11 (also called S100A10 or calpactin I light chain or annexin II light chain), a member of the S100 EF-hand protein family [START_REF] Svenningsson | Alterations in 5-HT1B receptor function by p11 in depression-like states[END_REF]. This interaction enhances 5-HT1BR signaling efficacy by promoting its expression at the cell surface. Several pieces of evidence suggest that the dysfunction of 5-HT1BR regulation by p11 might be involved in depressive states: (1) p11 brain expression is increased by antidepressant treatments, whereas the level of the protein is reduced in an animal model of depression and in the brain of depressed patients; (2) p11 overexpression reproduces some behavior triggered by antidepressants in mice, and (3) p11 knock-out mice exhibit a depression-like syndrome and reduced responsiveness to antidepressant treatments [START_REF] Svenningsson | Alterations in 5-HT1B receptor function by p11 in depression-like states[END_REF]. These last-mentioned examples highlight the essential role of 5-HTR/accessory protein interactions in physiology and pathology and the potential of therapeutic strategies aimed at developing drugs capable of disrupting or reinforcing these interactions and exhibiting fewer side-effects than classical agonist-or antagonist-based treatments.

Concluding remarks

Metabotropic 5-HTRs are directly or indirectly the site of action of the most important drugs used to treat widely expressed psychiatric diseases such as depression, schizophrenia, and obsessive-compulsive behaviors. They are also the direct or indirect site of action of several important drugs of abuse such as cocaine and LSD. Finally, they play an essential role in cognition. Therefore, we need increase our knowledge of the 5-HTRs themselves, their signal transduction properties in neuronal cells, their role in synaptic plasticity, and the functional consequences of their interaction with accessory proteins. We have seen in this review that many questions remain to be addressed. With respect to 5-HTR-associated proteins, recent studies have clearly established their important functional and perhaps physiological roles (Bockaert et al. 2004b;[START_REF] Ji | Disruption of PTEN coupling with 5-HT2C receptors suppresses behavioral responses induced by drugs of abuse[END_REF][START_REF] Svenningsson | Alterations in 5-HT1B receptor function by p11 in depression-like states[END_REF]. We are confident that pharmacological agents and perhaps drug targeting will clarify these interactions.

Table 1: 5-HTR accessory proteins and their role in receptor functionality.

Only receptor subtypes for which interacting proteins have been unambiuously identified are mentioned. * Refers to experiments in which the receptor PDZ binding motif has been deleted.

Figure 3

Examples of well-documented regional and cellular localizations (pink) of 5-HT1ARs, 5-HT1B:1DRs, 5-HT2ARs, and 5-HT2CRs. They can be localized on cell bodies (either on 5-HT neurons or other neurotransmitter-containing neurons) or on axon terminals (of 5-HT neurons or other neurotransmittercontaining neurons). 5-HT2CRs are also localized on the choroid plexus (POMC pro-opiomelanocortin, CART cocaine-and amphetamine-regulated transcript).

Figure legends Figure 1

 1 Figure legends

Figure 2 RNA editing of 5 -

 25 Figure 2 RNA editing of 5-HT2CR transcripts. a Position of the editing sites (A-E) within the human 5-HT2CR mRNA and the predicted amino acid sequences are shown for the non-edited INI isoform and for the fully edited VGV isoform. b Comparison of 5-HT2CR editing efficiency at each editing site for the human brain (dark blue bars) and rat brain (pale blue bars).

Figure 4 5 -

 5 Figure 4 5-HTR signaling pathways and effectors (blue 5-HTR signal transduction in neurons, gray signaling linkages only in transfected cell lines, PL phospholipase, ERK extracellular signal-regulated kinase, PK protein kinase, IP3 inositol triphosphate, DAG diacylglycerol, MAPK mitogen-activated protein kinase, NOS nitric oxide synthase, AHP after-hyperpolarization, JAK Janus kinase, STAT transcription pathway, ih hyperpolarization-activated current, Epa activated exchange factor).

Figure 1 Figure 2 Figure 3 Figure 4

 1234 Figure 1

5-HT2A

PDZ proteins* Targeting to dendrites, activation of silent synapses in the spinal cord [START_REF] Li | AMPA receptor-PDZ interactions in facilitation of spinal sensory synapses[END_REF]Xia et al., 2003b) PSD-95

Trafficking, signaling (Xia et al., 2003a) MUPP1 [START_REF] Becamel | Interaction of serotonin 5-hydroxytryptamine type 2C receptors with PDZ10 of the multi-PDZ domain protein MUPP1[END_REF] SAP97 [START_REF] Becamel | The serotonin 5-HT2A and 5-HT2C receptors interact with specific sets of PDZ proteins[END_REF] MAGI2 [START_REF] Becamel | The serotonin 5-HT2A and 5-HT2C receptors interact with specific sets of PDZ proteins[END_REF] MPP3 [START_REF] Becamel | The serotonin 5-HT2A and 5-HT2C receptors interact with specific sets of PDZ proteins[END_REF] CIPP [START_REF] Becamel | The serotonin 5-HT2A and 5-HT2C receptors interact with specific sets of PDZ proteins[END_REF] Caveolin Internalization, signaling [START_REF] Bhatnagar | Caveolin-1 interacts with 5-HT2A serotonin receptors and profoundly modulates the signaling of selected Galphaq-coupled protein receptors[END_REF] ARF1-6 ARF1-6

Signaling [START_REF] Robertson | Selective interaction of ARF1 with the carboxy-terminal tail domain of the 5-HT2A receptor[END_REF] MAP1A

Ribosomal S6 kinase

Signaling [START_REF] Sheffler | p90 ribosomal S6 kinase 2 exerts a tonic brake on G protein-coupled receptor signaling[END_REF] Calmodulin Calmodulin

Receptor phosphorylation / desensitization [START_REF] Turner | Interaction of calmodulin with the serotonin 5-hydroxytryptamine2A receptor. A putative regulator of G protein coupling and receptor phosphorylation by protein kinase C[END_REF])

5-HT2B

PDZ proteins* NO signaling [START_REF] Manivet | PDZ-dependent activation of nitric-oxide synthases by the serotonin 2B receptor[END_REF] MUPP1 [START_REF] Becamel | Interaction of serotonin 5-hydroxytryptamine type 2C receptors with PDZ10 of the multi-PDZ domain protein MUPP1[END_REF])

5-HT2C

PDZ proteins* Desensitization/resensitization [START_REF] Backstrom | Deletion of the serotonin 5-HT2C receptor PDZ recognition motif prevents receptor phosphorylation and delays resensitization of receptor responses[END_REF] MUPP1

Receptor clustering, phosphorylation ? [START_REF] Ullmer | Cloning and characterization of MUPP1, a novel PDZ domain protein[END_REF]) [START_REF] Becamel | Interaction of serotonin 5-hydroxytryptamine type 2C receptors with PDZ10 of the multi-PDZ domain protein MUPP1[END_REF]) [START_REF] Parker | Agonistinduced phosphorylation of the serotonin 5-HT2C receptor regulates its interaction with multiple PDZ protein 1[END_REF] PSD-95 [START_REF] Becamel | Synaptic multiprotein complexes associated with 5-HT(2C) receptors: a proteomic approach[END_REF] SAP97 [START_REF] Becamel | The serotonin 5-HT2A and 5-HT2C receptors interact with specific sets of PDZ proteins[END_REF] SAP102 [START_REF] Becamel | The serotonin 5-HT2A and 5-HT2C receptors interact with specific sets of PDZ proteins[END_REF] MAGI-2 [START_REF] Becamel | The serotonin 5-HT2A and 5-HT2C receptors interact with specific sets of PDZ proteins[END_REF] MPP3 [START_REF] Becamel | Synaptic multiprotein complexes associated with 5-HT(2C) receptors: a proteomic approach[END_REF] Veli3/CASK/Mint1 [START_REF] Becamel | Synaptic multiprotein complexes associated with 5-HT(2C) receptors: a proteomic approach[END_REF][START_REF] Becamel | The serotonin 5-HT2A and 5-HT2C receptors interact with specific sets of PDZ proteins[END_REF] Calmodulin [START_REF] Becamel | Synaptic multiprotein complexes associated with 5-HT(2C) receptors: a proteomic approach[END_REF] PICOT [START_REF] Becamel | Synaptic multiprotein complexes associated with 5-HT(2C) receptors: a proteomic approach[END_REF] PTD4 [START_REF] Becamel | Synaptic multiprotein complexes associated with 5-HT(2C) receptors: a proteomic approach[END_REF] PTEN

Receptor dephosphorylation, behavioral responses to drugs [START_REF] Ji | Disruption of PTEN coupling with 5-HT2C receptors suppresses behavioral responses induced by drugs of abuse[END_REF])

5-HT4(a) SNX27

Targeting, trafficking [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF] NHERF-1(EBP50)

Targeting [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF] MAGI2 [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF] MPP3 [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF] Veli 1-3 [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF]

CRMP2

Regulation of axon growth ? [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF] 5-HT4(e) CIPP [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF] nNOS [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF] Sec23

Trafficking to plasma membrane ? [START_REF] Joubert | New sorting nexin (SNX27) and NHERF specifically interact with the 5-HT4a receptor splice variant: roles in receptor targeting[END_REF]