Calcium Stéphane Peineau

Brigitte Potier

Florence Petit

Pascal Dournaud

Jacques Epelbaum

INTERACTION BETWEEN AMPA-GLUTAMATE AND SST2-SOMATOSTATIN RECEPTORS IN RAT MEDIOBASAL HYPOTHALAMUS REQUIRES ACTIVATION OF NMDA AND/OR METABOTROPIC GLUTAMATE RECEPTORS AND DEPENDS ON INTRACELLULAR

Modulation of glutamatergic transmission by neuropeptides is an essential aspect of neuronal network activity. Activation of the hypothalamic somatostatin sst2 receptor subtype by octreotide decreases AMPA glutamate responses, indicating a central link between a neurohormonal and neuromodulatory peptide and the main hypothalamic fast excitatory

neurotransmitter. In mediobasal hypothalamic slices, sst2 activation inhibits the AMPA component of glutamatergic synaptic responses but is ineffective when AMPA currents are pharmacologically isolated. In mediobasal hypothalamic cultures, the decrease of AMPA currents induced by octreotide requires a concomitant activation of sst2 receptors with either NMDA and/or metabotropic glutamate receptors. This modulation depends on changes in intracellular calcium concentration induced by calcium flux through NMDA receptors or calcium release from intracellular stores following metabotropic glutamate receptor activation. These results highlight an unusual regulatory mechanism in which the simultaneous activation of at least three different types of receptors is necessary to allow somatostatin-induced modulation of fast synaptic glutamatergic transmission in the hypothalamus.

Since the first demonstration of the depressant action of leucine-enkephalin on glutamateevoked responses [START_REF] Barker | Opiate peptide modulation of amino acid responses suggests novel form of neuronal communication[END_REF], interactions between neuropeptides and amino acids have turned out to be a major regulatory mechanism in the control of neuronal excitability and neuronal network activity. Neuropeptides affect particularly GABAergic or glutamatergic synapses by acting either pre-or postsynaptically. In the case of glutamatergic synapses, presynaptic effects lead to a depression or a facilitation of synaptic transmission, as shown recently for Neuropeptide Y [START_REF] Mcquiston | Neuropeptide Y2 receptors inhibit the frequency of spontaneous but not miniature EPSCs in CA3 pyramidal cells of rat hippocampus[END_REF][START_REF] Bijak | Neuropeptide Y reduces epileptiform discharges and excitatory synaptic transmission in rat frontal cortex in vitro[END_REF] or Galanin [START_REF] Mazarati | Modulation of hippocampal excitability and seizures by galanin[END_REF], and for Thyrotropin-releasing hormone [START_REF] Behbehani | Thyrotropin-releasing hormone has profound presynaptic action on cultured spinal cord neurons[END_REF] or oxytocin [START_REF] Jo | Oxytocin modulates glutamatergic synaptic transmission between cultured neonatal spinal cord dorsal horn neurons[END_REF] respectively. Neuropeptides also induce postsynaptic changes in neuronal electrical properties via activation or inhibition of voltage-dependent currents, thus modifying the excitability level of the postsynaptic neurone. However, a direct modulation of postsynaptic glutamate sensitivity has been reported in several cases. For example, depression of glutamate-induced responses has been observed in the presence of cortistatin [START_REF] Vasilaki | Cortistatin affects glutamate sensitivity in mouse hypothalamic neurons through activation of sst2 somatostatin receptor subtype[END_REF] or melanin concentrating hormone [START_REF] Gao | Melanin concentrating hormone depresses synaptic activity of glutamate and GABA neurons from rat lateral hypothalamus[END_REF]. Such neuropeptide/glutamate interactions are especially relevant in the hypothalamus. In particular, amongst many hypothalamic peptides able to efficiently modulate hypothalamic excitatory synaptic transmission [START_REF] Dickson | Systemic administration of growth hormone-releasing peptide activates hypothalamic arcuate neurons[END_REF][START_REF] Feleder | Evidence to suggest that gonadotropin-releasing hormone inhibits its own secretion by affecting hypothalamic amino acid neurotransmitter release[END_REF][START_REF] Van Den Pol | Neuropeptide Ymediated long-term depression of excitatory activity in suprachiasmatic nucleus neurons[END_REF][START_REF] Van Den Pol | Presynaptic and postsynaptic actions and modulation of neuroendocrine neurons by a new hypothalamic peptide, hypocretin/orexin[END_REF][START_REF] Kinney | Galanin-receptor mediated inhibition of glutamate release in the arcuate nucleus of the hypothalamus[END_REF][START_REF] Kombian | Vasopressin preferentially depresses excitatory over inhibitory synaptic transmission in the rat supraoptic nucleus in vitro[END_REF], somatostatin [somatotropin release inhibiting factor, SRIF [START_REF] Brazeau | Hypothalamic polypeptide that inhibits the secretion of immunoreactive pituitary growth hormone[END_REF]] can either increase or decrease glutamate sensitivity [START_REF] Gardette | Modulation by somatostatin of glutamate sensitivity during development of mouse hypothalamic neurons in vitro[END_REF]. This dual effect depends on the activation of either sst1 or sst2 SRIF receptor subtypes, respectively [START_REF] Lanneau | Somatostatin receptor subtypes sst1 and sst2 elicit opposite effects on the response to glutamate of mouse hypothalamic neurones : an electrophysiological and single cell RT-PCR study[END_REF]. However, little is known about the intracellular mechanisms implicated in the SRIF control of glutamate sensitivity besides the involvement of Gi/Go protein pathways [START_REF] Boehm | Somatostatin inhibits excitatory transmission at rat hippocampal synapses via presynaptic receptors[END_REF][START_REF] Lanneau | Somatostatin receptor subtypes sst1 and sst2 elicit opposite effects on the response to glutamate of mouse hypothalamic neurones : an electrophysiological and single cell RT-PCR study[END_REF].

To better understand the mechanisms involved in the inhibitory effects of SRIF on hypothalamic glutamatergic neurotransmission, we focused our attention on sst2 receptor subtype and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) glutamate receptors (GluRs) in mediobasal hypothalamic (MBH) slices. This hypothalamic area was selected because it displays a dense network of somatostatinergic fibres (reviewed in [START_REF] Tannenbaum | Somatostatin. In: Handbook of Physiology Volume 5 : Hormonal Control of Growth[END_REF] and the localization of the sst2 receptor subtype on MBH neurones was thoroughly documented (reviewed in [START_REF] Dournaud | Somatostatin receptors[END_REF]. Moreover, glutamate is the main excitatory neurotransmitter in this area [START_REF] Van Den Pol | Glutamate, the dominant excitatory transmitter in neuroendocrine regulation[END_REF]. In this study, we have analysed the effects of the sst2 agonist octreotide [START_REF] Hoyer | Classification and nomenclature of somatostatin receptors[END_REF] on 1) electrically evoked synaptic responses in slices and, 2) on the currents induced by the rapid application of glutamatergic agonists in cultured neurones. Activation of sst2 receptor subtype decreased AMPA responses in hypothalamic neurones but this occured only with a concomitant activation of either N-methyl-D-aspartate (NMDA) or metabotropic (mGluRs) glutamate receptors and depended on intracellular calcium concentration.

Methods

All experimental procedures involving animals and their care were conducted in conformity with INSERM committee guidelines and according to the principles expressed in the declaration of Helsinki.

HYPOTHALAMIC SLICES Experiments were conducted in 1-2 month-old male Sprague-Dawley rats (Janvier, Genest St Isle, France). Animals were anesthetized with halothane and killed by decapitation using a guillotine.

The brain was rapidly removed from the skull and placed in a chilled (0-3°C) extracellular solution of the following composition (in mM): NaCl,124;KCl,3;MgSO4,1;CaCl2,2;NaHCO3,26;NaH2PO4,1.25;glucose,10 (gassed with 95% O2/5% CO2,pH 7.3,[310][311][312][313][314][315][316][317][318][319][320]. Coronal slices (400 µm-thick) of the hypothalamus, including the median eminence, were cut using a vibratome (Campden Instruments, Leicester, UK). Slices were maintained at 30°C for at least 1 h before recording, transferred to a submersion chamber mounted on the stage of an upright microscope (Zeiss Axioskop, Iena, Germany) and held with a nylon net. The slice was continuously perfused throughout the experiment with gassed (95% O2/5% CO2) extracellular solution at room temperature (20-25°C).

MEDIOBASAL CELL CULTURES Preparation of MBH cultures was adapted from Lanneau et al. (2000b). Sprague-Dawley pregnant rats on the 17th day of gestation were anesthetized with halothane and killed by decapitation using a guillotine. A coronal slice was sampled from each fetus and further dissected in a ventrocaudal fragment containing MBH. Tissues were pooled and disrupted by mechanical trituration in PBS/Foetal Calf Serum (5%) medium. The cell suspension was spun for 10 min at 650 g and the pellet resuspensed in a defined serum-free medium. Cells were then seeded on 35 mm Petri dishes, precoated with gelatin/poly-L-lysine and Foetal Calf Serum, at a density of 3 MBH per dish (650 000 cells per dish). Cells were maintained in a 37°C, 7% CO2 humidified atmosphere, and treated from 5 days in vitro on with 1 µM cytosine arabinoside to block glial proliferation. The medium was changed twice a week. In such culture conditions, neurones represent more than 85% of the cell population.

ELECTROPHYSIOLOGICAL RECORDINGS In hypothalamic slices, patch clamp recording pipettes were made from thick-walled borosilicate glass capillaries (4-6 MΩ when filled with internal solution) using a Brown-Flaming horizontal pipette puller (Sutter Instrument Co, Novato, CA). Electrodes were filled with an internal solution of the following composition (in mM): KCH3SO4, 140; KCl, 6; MgCl2, 2; HEPES, 10; EGTA, 1.1; lidocaine N-ethyl bromide (QX-314) 5, ATP-Na2, 4; GTP0Na, 0.5; pH 7.3; 290-300 mOsm (adjusted with CsOH 0.1M), or for experiments in the presence of 1,2bis(2-aminophenoxy)ethane-tetraacetic acid (BAPTA) (in mM): KCH3SO4, 120; KCl, 6; MgCl2, 2; HEPES, 10; BAPTA, 20; QX-314, 5, ATP-Na2, 4; GTP-Na, 0.5; pH 7.3; 290-300 mOsm. The extracellular solution was perfused at 1-2 ml/min. Patch recording pipettes were connected to the headstage attached to a three-way piezo-electric micromanipulator (Burleigh, BFI Optilas, Evry, France) and neurones were recorded using the whole-cell configuration of the patch-clamp technique [START_REF] Hamill | Improved patchclamp techniques for high-resolution current recording from cells and cell-free membrane patches[END_REF]. After the attainment of cell access, transmembrane voltages and currents were recorded using an Axopatch 1D amplifier (Axon Instruments, Foster City, CA), digitized using a Digidata 1200 interface (Axon Instruments) and stored on a computer. Delivery of command voltages, online analysis and storage of current data into data files were driven by the Acquis 1 software (CNRS, Paris, France). Neurones were voltage-clamped to -70 mV unless otherwise stated and currents recorded at room temperature (20-25°C). Series resistance (10-30 MΩ) was routinely compensated (80%). Cell input resistance and membrane capacitance transients were monitored during the entire recording by applying a -10 mV voltage step (150 msec) before each stimulation. Cells that exhibited changes exceeding 15% during the recording period were rejected. Bipolar tungsten stimulating electrodes were placed in the dorsal area of the MBH region to activate synaptic inputs to the recorded neurones. In all experiments, stimuli (0.2-1 msec duration, 100-500 µA intensity, 0.06 Hz) were applied to construct baseline of postsynaptic responses and to measure the time-dependent effect of drugs. Theoritical reversal potentials of mixed Glutamate/GABA, GABA, Glutamate or AMPA synaptic responses were calculated using the GHK equation. These values were compared to measured mean reversal potentials corrected for junction potentials estimated by measuring the offset potential at the end of each recording. In mediobasal cell cultures, neurones were recorded using the whole-cell configuration of the patch-clamp technique [START_REF] Hamill | Improved patchclamp techniques for high-resolution current recording from cells and cell-free membrane patches[END_REF]. Borosilicate patch-clamp electrodes (4-6 MΩ) were obtained by a two-stage pull on a horizontal electrode puller (BB-CH Mecanex, Geneva, Switzerland) and filled with the internal solution of the following composition (in mM) : KCH3SO4, 140 ; KCl, 6 ; MgCl2, 2 ; HEPES, 10 ; EGTA, 1.1 ; GTP-Na 0.5 ; ATPNa2, 4; pH 7.30, 280-300 mOsm or for, the BAPTA experiments, with the following composition (in mM): KCH3SO4, 120 ; KCl, 6 ; MgCl2, 2 ; HEPES, 10 ; EGTA, 1.1 ; BAPTA, 20 ; GTP-Na 0.5 ; ATPNa2, 4 ; QX-314, 5 ; pH 7.30 ; 280-300 mOsm. QX-314 was omitted from the internal medium during the study of mGluRs. The external recording medium, of the following composition (in mM) : NaCl, 140 ; KCl, 3 ; CaCl2, 2 ; MgCl2, 2 ; HEPES, 10 ; Glucose, 10 ; tetrodotoxin, 0.5.10 -3 ; pH 7.30, 300-320 mOsm, was perfused at 2 ml/min. For the experiment without Ca 2+ in the external medium, the diionic equilibrium of the solution was maintained using BaCl2 2 mM. Patch pipettes were mounted in the headstage attached to a three-way piezo-electric micromanipulator (Burleigh, BFI Optilas, Evry, France). After the attainment of cell access, transmembrane voltages and currents were recorded using an Axopatch 1D (Axon Instruments) amplifier, run on line using an Axon TL-1 DMA interface and stored on a computer. Delivery of command voltages and storage of current data into data files were driven by the Pclamp 6.0.4 software (Axon Instruments). Neurones were voltageclamped at -70 mV unless otherwise stated, and currents were recorded at room temperature (20-25°C). Series resistance (10-30 MΩ) was routinely compensated (80%). Cell input resistance and membrane capacitance transients were monitored as in hypothalamic slices.

DRUG APPLICATIONS Drug aliquots were dissolved in distilled water or DMSO (< 0.1% final concentration in extracellular medium) and kept at -80°C.

In the hypothalamic slice preparation, drugs aliquots were dissolved in the external medium and applied by bath perfusion for at least 10 mn to obtain a steady-state bath concentration.

The following compounds were used : Bicuculline (Sigma, St Quentin-Fallavier, France) ; D-2-amino-5-phosphopentanoic acid (AP5, Tocris Cookson Ltd, Bristol, UK) ; 6-cyano-7nitroquinoxaline-2 ,3-dione (CNQX; Tocris Cookson) ; 1,2,3,4-tetrahydro-6-nitro-2,3-dioxobenzo[f]quinoxaline-7-sulfonamide (NBQX, Sigma) ; 1-(4-aminophenyl)-4-methyl-7,8methyenedioxy-5H-benzodiazepine hydrochloride (GYKI 52466, Sigma). Octreotide 1 µM (SMS 201-995, gift from Novartis, Rueil-Malmaison, France) was used as a potent and long acting sst2 agonist [START_REF] Hoyer | Classification and nomenclature of somatostatin receptors[END_REF]. This compound is also a weak agonist for sst3 and sst5 receptors but sst5 receptor immunoreactivity has not been detected in the rat MBH (reviewed in [START_REF] Dournaud | Somatostatin receptors[END_REF] and sst3 receptors can be excluded from classical pre or postsynaptic sites (reviewed in [START_REF] Schulz | Localization of five somatostatin receptors in the rat central nervous system using subtype-specific antibodies[END_REF]. In mediobasal cell cultures, drug aliquots were diluted in the extracellular medium to the desired concentrations. They were applied with a RSC-160 rapid solution changer (Bio-Logic, Claix, France). Currents generated by the glutamate receptor agonists L-Glutamate (Sigma), AMPA (Tocris Cookson), NMDA (Tocris Cookson) and trans-(+)-1-amino-1.3cyclopentanedicarboxylic acid (trans-(+)-ACPD, Tocris Cookson) were measured for a 2 sec application at 0.05 Hz. After acquisition of a 10 min baseline current amplitudes, octreotide 200 nM was continuously applied for 10 min. To evaluate the effect of the sst2 receptor agonist, a 2 sec application of glutamate agonist(s) plus octreotide was delivered every 20 sec. For the study of NMDA responses, D-serine 20 µM was added to saturate the strychnineinsensitive glycine site of NMDA receptors [START_REF] Mothet | D-serine is an endogenous ligand for the glycine site of the N-methyl-D-aspartate receptor[END_REF].

IMMUNOHISTOCHEMICAL CHARACTERIZATION OF RECORDED NEURONES IN

HYPOTHALAMIC SLICES In some experiments, biocytin (0.5 mg/ml) was added to the recording solution. At the end of the recording period, slices were fixed for 2 h in 4% paraformaldehyde in 0.1 M phosphate buffer (pH 7.4) and cryoprotected at 4° C in 30% buffered sucrose for 24 h. Serial sections (30 µm thick) were cut on a freezing microtome, collected in 0.1 M phosphate buffer and immediately processed for double labeling studies using either somatostatin or sst2A antibodies and biocytin revelation. For SRIF immunohistochemistry, slices were processed using a rabbit polyclonal antiserum directed against SRIF [antisomatostatin1-12 (code IS-7), a gift from Dr. G. Tramu, CNRS URA 339, Bordeaux, France]. To detect the sst2A receptor, which is the only sst2 isoform expressed in MBH neurones [START_REF] Sarret | Preferential expression of sst2A over sst2B somatostatin receptor splice variant in rat brain and pituitary[END_REF], a fully characterized antiserum raised in rabbit against the C-terminal segment 330-369 of the sst2A receptor protein was used [START_REF] Helboe | Development of selective antibodies against the human somatostatin receptor subtypes sst1-sst5[END_REF][START_REF] Helboe | Immunohistochemical localization of the somatostatin receptor subtype 2 (sst2) in the central nervous system of the golden hamster (Mesocricetus auratus)[END_REF]Csaba et al., 2001[START_REF] Csaba | Neurochemical characterization of receptor-expressing cell populations by in vivo agonist-induced internalization: insights from the somatostatin sst2A receptor[END_REF].

Free-floating sections were incubated overnight at room temperature in 1:4000 anti-SRIF antibody or in 1:400 anti-sst2A antibody. SRIF and sst2A antibodies were diluted in 0.1M Tris buffer Saline pH 7.4 (TBS), containing 0.3% Triton X-100 and either 0.5% normal goat serum (NGS) or 0.5% normal donkey serum (NDS). SRIF antibodies were revealed by incubating sections for 45 min in 1:500 goat Cy3-conjugated anti-rabbit IgG (red fluorescent signal ; Jackson Immunoresearch, West Grove, PA) diluted in TBS containing 5% NGS and 0.3% Triton X 100. Sst2A receptor antibodies were revealed using 1:4000 donkey Cy3-conjugated antirabbit IgG (red fluorescent signal ; Jackson Immunoresearch, West Grove, PA) diluted in TBS containing 5% NDS and 0.3% Triton X-100. After several washes, sections were incubated for 45 min in 1:4000 Streptavidin FITC (green fluorescent signal ; Jackson Immunoresearch, West Grove, PA) diluted in TBS to reveal biocytin injected through the recording pipette. Sections were then rinsed in TBS, mounted on glass slides and coverslipped with a Vectashield solution (Vector Laboratories). Sections were analyzed by Confocal Laser Scanning Microscopy using a TCS SP2 confocal imaging system equipped with Ar 488 nm and HeNe 543 nm lasers (Leica Microsystems, Heidelberg, Germany).

Digital images were collected from a single optical plane using a 40x Plan-Apochromat oil-immersion lens (NA 1.25). Pinhole setting was 0.75 Airy unit (AU) for all images. For each optical section, double fluorescence images were acquired in sequential mode to avoid potential contamination by linkage specific fluorescence emission "cross talk".

SINGLE CELL MULTIPLEX RT-PCR OF SST mRNAs

Cells were first patched and cytoplasm harvested by applying a gentle negative pressure to the pipette. The tip of the pipette was then broken in a PCR sterile tube and submitted to retrotranscription for 1h at 37°C as already described (Lanneau et al., 2000b). The resulting 15µl sample was kept at -80°C. Coamplification of the cDNAs encoding the five sst1-5 somatostatinergic receptors was performed simultaneously. Primers were identical to primers used in a previous study (Table I in Lanneau et al., 2000b).

RT reaction products were first amplified in the presence of every primer (25 pmol each) in a 100 µl volume in an automatic thermocycler (Genius Techne, Cambridge, UK) during 20 cycles (94°C for 30 sec, 56°C for 30 sec, 72°C for 30 sec). An extension step was then performed at 72°C for 10 min.

PCR products were purified from primers and salt using Nucleon QC microspin (Amersham, Rainham, UK).

A second round of PCR was then performed using 2 µl of the first purified PCR products as template. In this round, each marker was amplified individually using its specific primer pair for 28-35 cycles. All markers were amplified as described for the first amplification step. Amplification was performed in an automatic thermocycler (Hybaid, Teddington, UK) (94°C for 30 sec, 58-60°C for 30 sec, 72°C for 30 sec).

Fifteen microliters of each individual PCR reaction were then run on 2% agarose gel stained with Gel Star (FMC Bioproducts, Rockland, ME). In each experiment, cytoplasms with nucleus were submitted to the entire protocol without reverse transcriptase. The intronic sequence of SRIF amplicon was never detected, thus ruling out genomic DNA contamination.

DATA ANALYSIS For each cell, amplitudes of evoked or induced currents were normalized relative to responses prior to drug application. Octreotide effect was estimated from amplitude mean decrease under drug in slices and from maximal mean decrease of steady-state currents in cultures. Statistical significance of amplitude variation within each cell was determined using a Student's t test and Bonferonni correction for small samples. Differences were considered significant if p<0.05. This procedure allowed recorded neurones to be classified in two different populations: octreotide-sensitive and octreotide-insensitive. Mean amplitudes of octreotide effect were then compared using ANOVA. Data are expressed as mean ± s.e.m.

Results

SYNAPTIC STIMULATIONS IN HYPOTHALAMIC SLICES Synaptic currents in response to electrical stimulation were recorded in 55 neurones in order to characterize mixed glutamate and γ-aminobutyric acid (GABA) responses (n=12, no antagonist), pure GABA (n=11, addition of CNQX and AP5), glutamate (n=14, addition of bicuculline), AMPA (n=6, addition of bicuculline and AP5) or NMDA (n=6, addition of bicuculline and CNQX) responses, and glutamate responses in presence of intracellular BAPTA (n=6, addition of bicuculline). NBQX (1µM) and GYKI 52466 (20µM) were applied in order to block AMPA receptors and reveal kainate currents [START_REF] Kawai | AMPA receptor activates a G-protein that suppresses a cGMP-gated current[END_REF][START_REF] Chergui | Functional GluR6 kainate receptors in the striatum : indirect down regulation of synaptic transmission[END_REF]. In such conditions, complete blockade of the synaptic response was observed, even after high stimulation intensities or frequencies [START_REF] Vignes | The synaptic activation of kainate receptors[END_REF] NMDA contribution to total glutamate current was estimated by applying AP5 and represented around 5% of the total synaptic current at a holding potential of -70 mV and in the presence of Mg 2+ (1 mM) in the perfusion solution (data not shown). These results suggest that the sst2 agonist modulates mainly the AMPA component of the glutamate synaptic response. However, when the AMPA component was isolated by addition of both bicuculline and AP5, octreotide was ineffective at inhibiting the response in 6 out of 6 recorded neurones (fig 2G ,H). This led us to test whether an influx of calcium through NMDA channels might be required to observe octreotide effects. Indeed, octreotide modulation of glutamate synaptic responses was abolished by intracellular injection of BAPTA in every tested neurone (n=6, Fig 2I,J). To rule out a presynaptic inhibition of glutamate release related to calcium-induced release of a retrograde messenger from the postsynaptic cell, the effect of octreotide was tested on pharmacologically isolated NMDA synaptic currents (n=6). Holding potential was set to -20 mV to remove Mg 2+ blockade and allow a larger calcium influx. Under these conditions, no effect of octreotide was ever observed (data not shown).

As shown in Figure 3, recorded MBH neurones were surrounded by a dense network of somatostatinergic fibers in close apposition to neuronal perykarya and dendrites.

Regional examination of slices immunoreacted with the sst2A receptor antibody revealed that numerous immunopositive neurones were apparent throughout the MBH (data not shown). In accordance with previous studies (reviewed in [START_REF] Dournaud | Somatostatin receptors[END_REF], the largest proportion of receptor-expressing cells was observed in the arcuate nucleus. At the cellular level, receptor immunolabeling was confined to small spherical granules displaying the morphological features of endosomes distributed at the periphery of the cells as well as within intracellular compartments. Such cellular distribution has been recently described in other brain regions and reflect activation and subsequent internalization of cell-surface sst2A receptors in response to agonist stimulation (Csaba et al., 2001[START_REF] Csaba | Neurochemical characterization of receptor-expressing cell populations by in vivo agonist-induced internalization: insights from the somatostatin sst2A receptor[END_REF]. Combined electrophysiological recordings with double-labeling experiments revealed that biocytinfilled neurones displaying no octreotide-induced changes in glutamate synaptic currents never expressed sst2A receptor immunolabeling (Fig 4A -C) while responsive cells displayed sst2A receptor immunoreactivity (Fig 4D -F).

FAST APPLICATIONS OF AGONISTS IN MEDIOBASAL CELL CULTURES

The lack of effect of octreotide on the AMPA component of the synaptic response on the one hand, and the BAPTA blockade of octreotide-induced decrease of the glutamate synaptic response on the other hand, led us to determine whether a coactivation of non-NMDA and NMDA glutamate ionotropic receptor subtypes was necessary to observe the sst2 modulation of the AMPA responses. Indeed, the linearity of synaptic glutamate and AMPA I-V curves (see additional material) strongly suggested that the activated glutamate receptors contained the edited GluR2 subunit and were not permeable to calcium [START_REF] Racca | Differences in Ca 2+ permeability of AMPA-type glutamate receptor channels in neocortical neurons caused by differential GluR-B subunit expression[END_REF]. Experiments were performed on a total number of 78 isolated neurones in primary cultures allowing fast perfusion of the cells with octreotide and various glutamate agonists alone or in combination. In cells held to -70 mV and in the presence of 2 mM Mg 2+ , the AMPA current represented more than 95% of the total recorded inward current, thus indicating a 5% contribution of NMDA activation, similar to that recorded for synaptic responses in slices. As previously reported, at the agonist concentrations used herein, AMPA-mediated responses displayed no fast peak current [START_REF] Swandulla | Functional and molecular characteristics of the glutamate receptor involved in synaptic transmission in the hypothalamus[END_REF]. As shown in Figure 5A and 5B, the fast application of octreotide + glutamate decreased the glutamate-induced inward current in 43% of the tested neurones (3/7) with a maximum average decrease of -15.9 + 4.1 % (p<0.001 vs control values before octreotide in responsive cells, n=3, and values under octreotide in non responsive cells, n=4). As opposed to slice experiments, octreotide effect desensitized rapidly before the end of application. Octreotide-induced decrease was also found when all glutamate receptor subtypes were activated by coapplying AMPA, NMDA and trans(+)-ACPD in 38% of the tested cells (3/8) with a comparable maximum average decrease of -12.5 series of experiments, we tested whether calcium influx through the coactivated NMDA receptors could be responsible for restoring octreotide modulation of the AMPA current. As shown in Figure 5I and 5J, omitting calcium from the extracellular medium totally abolished the decrease of AMPA current during coapplication of AMPA, NMDA and octreotide in all 15 tested neurones. However, octreotide was still able to decrease glutamate response by -14.1 + 1.7 % in 3 out of 7 tested cells even in the absence of extracellular calcium (data not shown) suggesting that mGluRs were involved in sst2/AMPA receptor interactions.

ROLE OF METABOTROPIC GLUTAMATE RECEPTORS Experiments were performed in cultured neurones with no extracellular calcium. Agonist coapplications with octreotide included AMPA and trans-(+)-ACPD in order to activate GluRs and mGluRs, respectively. Under these conditions, octreotide was effective in reducing AMPA current in 43% of the recorded neurones (3/7, Fig 6A,B) with a maximal effect of -15.1 + 3.5 % which desensitized rapidly. After intracellular perfusion of BAPTA, 3 out of 6 tested cells were still responsive to octreotide (Fig 6C ,D), but the magnitude of the effect was significantly diminished (-7.8 + 1.0 % vs -15.1 + 3.5 % in the absence of BAPTA, p<0.001). Furthermore, a 45 minute preincubation with thapsigargin, to deplete intracellular calcium stores, totally abolished the effect of octreotide on AMPA currents elicited during a coapplication of AMPA and trans-(+)-ACPD (Fig 6E ,F,n=6).

DISTRIBUTION OF SST mRNAs IN MBH CELL CULTURES Cytoplasms were harvested from 39 neurones from rat MBH cell cultures and were processed in order to detect expression of the five sst mRNAs at the single cell level. 31% (12/39) of the neurones expressed no sst mRNAs. A same proportion of the cells (12/39) expressed only one subtype whereas the simultaneous expression of 2, 3, 4 or 5 subtypes was detected respectively in 26% (10/39), 8% (3/39), 5% (2/39) and 0% (0/39) of the neurones. The mRNA signal for the sst2 subtype was found in 49% of the neurones, in close agreement with the proportion of octreotide-sensitive cells. The distribution of the other mRNAs subtypes was as follows : sst4 (36%) > sst1 (28%) > sst5 (8%) > sst3 (5%).

Discussion

In this study, we first determined the role of the sst2 receptor subtype on synaptic transmission in rat MBH slices. Electrical stimulation of the dorsal part of MBH elicited mixed glutamatergic and GABAergic synaptic responses in all recorded neurones, in keeping with the major role attributed to these two amino acids in the hypothalamic network (van den Pol et al., 1990 ; Decavel and van den Pol, 1990). Sst2 modulation of the GABA component was never observed, suggesting that octreotide-induced decrease of mixed synaptic responses was entirely due to the effect of the peptide on the glutamate component. Such a lack of SRIF effect on GABAergic transmission has previously been reported in the hippocampus [START_REF] Boehm | Somatostatin inhibits excitatory transmission at rat hippocampal synapses via presynaptic receptors[END_REF][START_REF] Tallent | Somatostatin depresses excitatory but not inhibitory neurotransmission in rat CA1 hippocampus[END_REF]. Glutamate responses were decreased by octreotide in 43% of the recorded neurones and octreotide sensitivity was correlated to the presence of sst2A receptor immunoreactivity. This proportion of octreotide sensitive neurons in MBH slices is in close agreement with the proportion of sst2 receptor-expressing neurones observed in culture by single cell RT-PCR in this study (49%) as well as in previous reports on mouse hypothalamic neurones (40%, [START_REF] Lanneau | Somatostatin receptor subtypes sst1 and sst2 elicit opposite effects on the response to glutamate of mouse hypothalamic neurones : an electrophysiological and single cell RT-PCR study[END_REF]44%, Lanneau et al., 2000a). Octreotide modulation appeared mainly targeted to the AMPA component which represents the main component of fast excitatory transmission within the hypothalamus [START_REF] Wuarin | Patch-clamp analysis of spontaneous synaptic currents in supraoptic neuroendocrine cells of the rat hypothalamus[END_REF].

The effect of octreotide appears unrelated to a depression of neurotransmitter release from presynaptic terminals [START_REF] Boehm | Somatostatin inhibits excitatory transmission at rat hippocampal synapses via presynaptic receptors[END_REF] since neither the postsynaptic NMDA component of the synaptic response nor GABA synaptic responses were affected by the peptide. A decrease in postsynaptic neuronal excitability, mediated by sst2-modulation of postsynaptic voltage-dependent ionic channels (reviewed in [START_REF] Patel | Somatostatin and its receptor family[END_REF], is also unlikely because such a decrease would have equally affected the amplitude of AMPA or GABA responses. Sst2-induced inhibition totally disappeared when the AMPA component was pharmacologically isolated. This indicates that, in such experimental conditions, one essential element between the sst2 receptors and the GluRs was missing. The main difference between non-selective glutamate-and selective AMPA-receptor mediated responses was the pharmacological blockade of NMDA receptors in the latter protocol. Indeed, octreotide inhibition of AMPA responses was restored in cultured neurones when NMDA was coapplied with AMPA.. Based on previously published reports demonstrating the expression of GluR2 mRNA in mediobasal hypothalamic areas [START_REF] Van Den Pol | Ionotropic glutamate-receptor gene expression in hypothalamus : localization of AMPA, Kainate, and NMDA receptor RNA with in situ hybridization[END_REF][START_REF] Eyigor | Distribution of ionotropic glutamate receptor subunit mRNAs in the rat Hypothalamus[END_REF] and given the linearity of AMPA I-V curves observed in both slices and isolated neurones, it appears most likely that native AMPA receptors expressed by hypothalamic neurones contained the edited GluR2 subunit and thus display no calcium permeability [START_REF] Burnashev | Divalent ion permeability of AMPA receptor channels is dominated by the edited form of a single subunit[END_REF][START_REF] Racca | Differences in Ca 2+ permeability of AMPA-type glutamate receptor channels in neocortical neurons caused by differential GluR-B subunit expression[END_REF]. Thus calcium fluxes through NMDA receptors may be necessary for the expression of the sst2-induced modulation of AMPA currents. In the slice experiments, involvement of mGluRs-induced calcium release from intracellular stores appeared unlikely since 1) these receptors are located at a distance from the synaptic zone and 2) they require higher frequencies of stimulation to be activated [START_REF] Ottersen | Organization of glutamate receptors at the synapse[END_REF].

The role of intracellular calcium in the sst2-induced modulation of AMPA current is supported by several arguments. When calcium was omitted from the extracellular medium in cultures, the effect of octreotide was abolished. When intracellular calcium was chelated by BAPTA, sst2-induced inhibition of glutamate synaptic responses was blocked. Moreover, in cultures, octreotide effect was restored by the coactivation of GluRs and mGluRs, strongly depressed by intracellular BAPTA and abolished by thapsigargin preincubation. This last observation suggested that calcium ions involved in the sst2-AMPA modulation pathway may arise indifferently from extracellular or intracellular sources.

The fact that octreotide-induced inhibition of AMPA currents was restored even when coactivated NMDA currents represented, in our experimental conditions, only about 5% of total glutamate-mediated currents is rather surprising. Indeed, the calcium influx mediated by such a small amount of current is probably not very consequent. One hypothesis could be that calcium transients elicited by NMDA receptor activation are confined to a space near the site of entry [START_REF] Yuste | Dendritic spines as basic functional units of neuronal integration[END_REF] leading to a very local rise in calcium-concentration sufficient to trigger intracellular events in a microdomain containing the different receptor subtypes. In keeping with this hypothesis, it has been recently shown that calcium transients restricted to the the immediate vicinity of the site of calcium entry represents the on switch for ERK1/2 kinase signaling independently of global increases in calcium concentration [START_REF] Hardingham | A calcium microdomain near NMDA receptors : on switch for ERK-dependent synapse-to-nucleus communication[END_REF]. Alternatively, this local calcium increase could be responsible for the onset of a calcium-induced calcium release mechanism, as shown in hippocampal dendritic spines [START_REF] Emptage | Single synaptic events evoke NMDA receptor-mediated release of calcium from internal stores in hippocampal dendritic spines[END_REF], leading to substantial calcium release from the internal stores close to glutamate and sst2 receptors and subsequent activation of intracellular messengers. Octreotide induced an equivalent decrease of AMPA currents when mGluRs alone, or mGluRs + NMDA receptors together, are activated. This suggests that a mGluRs-induced increase in intracellular calcium alone can lead to a maximal effect of octreotide on AMPA transmission. Such a result may be highly relevant in the functioning of synaptic networks, especially in view of the putative corelease of glutamate and somatostatin (Lanneau et al., 2000b). During synaptic transmission with a low rate of action potential firing, AMPA and NMDA receptors are activated and only a modest release of neuropeptides occurs [START_REF] Hökfelt | Neuropeptides in perspective : the last ten years[END_REF]. In this case, decrease of excitatory synaptic transmission by sst2 activation would be very low, due to a low level of activation of sst2 receptors by basal somatostatin release. An increase in the rate of action potential discharge, such as in kindling or status epilepticus, will lead to an enhanced release of both somatostatin [START_REF] Manfridi | Functional and histological consequences of quinolinic and acid-induced seizures on hippocampal somatostatin neurons[END_REF][START_REF] Vezzani | Somatostatin release is enhanced in the hippocampus of partially and fully kindled rats[END_REF] and glutamate from nerve terminals. In such conditions, the simultaneous activation of sst2 and mGluRs will result in a higher inhibition of excitatory transmission. Thus, activation of sst2 receptors may represent a physiological brake against overexcitation of synaptic networks. This hypothesis may be relevant to further define the mechanisms of the inhibitory role of somatostatin in seizures and epileptogenesis [START_REF] Vezzani | Brain somatostatin : a candidate inhibitory role in seizures and epileptogenesis[END_REF]. We observed, in slices, a mean octrotide-induced depression of glutamate responses that was twice that observed in cultures neurones (32% versus 16%). This difference may be related to a release of maximal concentrations of glutamate when using evoked potentials in the slice, leading to maximal AMPA responses, whereas the low agonist concentration used in cultured neurones would activate only a subset of available AMPA receptors and induce a smaller excitatory current. This may lead to a less efficient effect of the sst2 agonist on the smaller glutamate responses. Alternatively, glutamate receptors activated during the fast application or the electrical-induced synaptic responses are differentially located on the postsynaptic cell. Thus, only synaptically-based receptors would respond to a single-shock electrical stimulation whereas both synaptically and extrasynaptically-located receptors would be activated when applying drugs. Such a differential receptor distribution at the neuronal surface leads to different regulation of receptor subtypes [START_REF] Morishita | Regulation of synaptic strength by protein phosphatase 1[END_REF]. Thus, sst2 activation may lead to a larger octreotide-induced depression of synaptically-based receptors than of extrasynaptically-based ones, reflected by a more potent effect of the sst2 agonist in slices than in cultures. The octreotide response desensitized rapidly in cultures as opposed to slices. Such a discrepancy between the slice and dispersed neurone protocols has previously been observed. For instance, inhibition of ICa by somatostatin in cultured chick sympathetic neurones desensitizes with a time for half desensitization of approximately 3 minutes [START_REF] Golard | Protein kinase C blocks somatostatininduced modulation of calcium current in chick sympathetic neurons[END_REF], as observed in the present experiments in cultured hypothalamic neurones. On the opposite, [START_REF] Tallent | Somatostatin depresses excitatory but not inhibitory neurotransmission in rat CA1 hippocampus[END_REF] reported that somatostatin-induced depression of AMPA/kainate EPSCs in rat hippocampal slices did not desensitize in their recording conditions.

Since somatostatin receptor desensitization was hypothetized to be dependent on receptor internalization rather than on phosphorylation events [START_REF] Beaumont | Somatostatin receptor densensitization in NG 108-15 cells. A consequence of receptor sequestration[END_REF], the differences observed in desensitization kinetics between cultures and slices may be related to different internalization processes. Indeed, it has been shown in different culture systems that the maximal rate of rat sst2 internalization ranged between 50% and 95% (reviewed in Csaba and Dournaud, 2001) whereas it reaches only 20-30% in rat brain slices even after a 40 minute exposure to sst2 agonist [START_REF] Boudin | Somatostatininduced regulation of SST2A receptor expression and cell surface availability in central neurons : role of receptor internalization[END_REF]. Therefore, it is likely that the available number of membrane sst2 receptors is largely decreased in isolated cultured cells as compared to slices. This may explain the persistance of octreotide effect in slices as opposed to its rapid disappearance in isolated hypothalamic neurones. Previously reported effects of SRIF on neurotransmitter-activated receptors dealt with SRIFinduced changes of postsynaptic sensitivity and only involved two classes of postsynaptic receptors (reviewed in [START_REF] Vezzani | Brain somatostatin : a candidate inhibitory role in seizures and epileptogenesis[END_REF]. To our knowledge, only one example showed a true cooperativity between receptors : in SH-SY5Y human neuroblastoma cells, SRIF applied alone is ineffective on intracellular calcium levels whereas coapplication of SRIF and carbachol evoked an elevation above that caused by carbachol alone [START_REF] Connor | Neuropeptide Y Y2 receptor and somatostatin sst2 receptor coupling to mobilization of intracellular calcium in SH-SY5Y human neuroblastoma cells[END_REF]. However, cooperativity between several transmitters has already been shown, such as for serotonin, adenosine and SRIF in the coupling of their respective receptors to an inwardly rectifying potassium current [START_REF] Sodickson | Neurotransmitter activation of inwardly rectifying potassium current in dissociated hippocampal CA3 neurons : interactions among multiple receptors[END_REF]. Such observations are in keeping with the present results where coactivation of three receptors is required to produce a biological effect (Figure 7). A link between intracellular calcium and the different ionotropic glutamate receptor subtypes has recently been evidenced, such as the control of the targeting of GluR2-containing GluRs by Ca2+-permeable GluRs [START_REF] Liu | Synaptic activity at calcium-permeable AMPA receptors induces a switch in receptor subtype[END_REF], or a transient depression of the kainate receptor current induced by Ca 2+ influx through NRs [START_REF] Ghetti | NMDA-dependent modulation of hippocampal kainate receptors by calcineurin and Ca 2+ /calmodulin-dependent protein kinase[END_REF]. Interactions between receptors belonging to different families such as ionotropic and G protein-coupled receptors also exist. Thus, a rise of postsynaptic calcium is required for the induction of D1/D5-induced sustained enhancement of GluRs-and NRs-mediated currents [START_REF] Yang | Sustained enhancement of AMPA receptor-and NMDA receptormediated currents induced by D1/D5 receptor activation in the hippocampus : an essential role of postsynaptic Ca 2+[END_REF]. Given the broad spectrum of the intracellular targets modified by the increase in calcium ions, several hypotheses can be considered in the interaction between sst2 and glutamate receptors depending on intracellular calcium. Common intracellular targets to both intracellular calcium (reviewed in [START_REF] Berridge | Neuronal calcium signaling[END_REF] and sst2 (reviewed in Csaba and Dournaud, 2001) mostly involve kinases or phosphatases such as the adenylate cyclase/cAMP/PKA (reviewed in [START_REF] Schindler | Somatostatin receptors in the central nervous system[END_REF] or the MAP kinase (reviewed in [START_REF] Cole | Characterisation of somatostatin sst2 receptor splice variants[END_REF] pathways. An interaction between calcineurin phosphatase and sst receptors activated by SRIF-28 has also been described [START_REF] Zhu | Calcineurin modulates G-protein-mediated inhibition of N-type calcium channels in rat sympathetic neurons[END_REF].

Activation of these pathways in response to both sst2 and intracellular calcium may lead to phosphorylation or dephosphorylation of GluRs (reviewed in [START_REF] Greengard | The neurobiology of slow synaptic transmission[END_REF], a major mechanism in the regulation of neurotransmitter receptors (reviewed in [START_REF] Swope | Phosphorylation of ligand-gated ion channels : a possible mode of synaptic plasticity[END_REF]. For example, dephosphorylation of GluRs is observed during long term depression in the hippocampus, a phenomenon associated with a decrease in GluRs sensitivity [START_REF] Lee | NMDA induces long-term synaptic depression and dephosphorylation of the GluR1 subunit of AMPA receptors in hippocampus[END_REF]. Interestingly, this NMDA-induced long term depression associated with dephosphorylation of GluRs can be observed after only a 3 minute application of NMDA, a delay compatible with our present observations [START_REF] Lee | NMDA induces long-term synaptic depression and dephosphorylation of the GluR1 subunit of AMPA receptors in hippocampus[END_REF]. Alternatively, the sst2 receptor itself could be the target for phosphorylation or dephosphorylation following intracellular calcium rise (reviewed in Csaba and Dournaud, 2001), a mechanism that could account for changes in sst2 receptor properties allowing AMPA modulation.

Finally, a direct interaction between sst2 receptor and GluRs, induced by intracellular calcium rise, could also account for the modifications of glutamate responses. Such a physical association has recently been demonstrated between dopamine D5 receptor subtype, a G protein-coupled receptor as the sst2 receptor, and GABAA receptor subtype, a ligand-gated channel mediating fast interneuronal synaptic transmission analogous to GluRs (Liu et al., 2000). It has been demonstrated that sst2 can physically bind to proteins with anchoring and scaffolding functions [START_REF] Zitzer | Somatostatin receptor interacting protein defines a novel family of multidomain proteins in human and rodent brain[END_REF]. One of these proteins, named SSTRIP (sst receptor interacting protein, or SHANK1), also binds, directly or by other binding proteins, to NRs, mGluRs and GluRs (reviewed in [START_REF] Craig | Molecular heterogeneity of central synapses : afferent and target regulation[END_REF]. Therefore, this physical link may, under calcium control, modify AMPA sensitivity by a mechanism comparable to that of Homer proteins controlling constitutive activity of mGluR1a or mGluR5 receptors [START_REF] Ango | Agonist-independent activation of metabotropic glutamate receptors by the intracellular protein Homer[END_REF]. In summary, the modulation of GluRs sensitivity by activation of the SRIF sst2 receptor subtype represents a complex mechanism under the control of intracellular calcium levels. The calcium rise is induced by Ca (A) Time plot of normalized evoked synaptic responses. Five neurones displayed a highly significant decreased synaptic response during octreotide application () and 7 neurones were unresponsive (■).

(B) Average recording of 3 synaptic current traces before (CTRL) and during octreotide application (OCT) in one responsive neurone.

(C) Time plot of normalized evoked GABAergic synaptic responses. Synaptic responses were unaffected by octreotide in 11 tested neurone (■).

(D) Average recording of 3 GABAergic synaptic current traces in one neurone before (CTRL) and during octreotide application (OCT).

(E) Time plot of normalized evoked glutamatergic synaptic responses. Synaptic responses were highly significantly depressed under octreotide application in 6 neurones () and unaffected in 8 neurones (■). (F) Average recording of 3 glutamatergic synaptic current traces before (CTRL) and during octreotide application (OCT) in one responsive neurone.

(G) Time plot of normalized evoked AMPA synaptic responses. None of the 6 tested neurones showed a depressed synaptic response during octreotide application (■).

(H) Average recording of 3 AMPA synaptic current traces in one neurone before (CTRL) and during octreotide application (OCT).

(I) Time plot of normalized evoked glutamatergic synaptic responses in the presence of the calcium chelator BAPTA in the recording pipette. None of the 6 tested neurones showed depression of synaptic responses during octreotide application (■).

(J) Average recordings of 3 glutamatergic synaptic current traces in one neurone before (CTRL) and during octreotide application (OCT) in the presence of intracellular BAPTA. Scale bars : 50 pA, 10 ms (A) Time plot of normalized glutamate-induced responses. 3 neurones presented a highly significant depression of the response during octreotide application () and 4 neurones were unresponsive (■).

(B) Recordings of glutamate-induced currents before (CTRL) and during octreotide application (OCT) in one responsive neurone.

(C) Time plot of normalized responses to coapplication of AMPA, NMDA and trans(+)-ACPD. Three neurones showed a highly significant depression of the responses during octreotide application () and five neurones were unaffected (■).

(D) Recordings of AMPA/NMDA/trans(+)-ACPD currents before (CTRL) and during octreotide application (OCT) in one responsive neurone.

(E) Time plot of normalized AMPA-induced responses. No change in the response was observed under octreotide in 9 tested neurones (■).

(F) Recordings of AMPA-induced currents in one neurone before (CTRL) and during octreotide application (OCT).

(G) Time plot of normalized responses to coapplication of AMPA and NMDA. Five neurones showed a highly significant depression of the responses during octreotide application () and 8 neurones were unaffected (■).

(H) Recordings of AMPA/NMDA currents before (CTRL) and during octreotide application (OCT) in one responsive neurone.

(I) Time plot of normalized responses to coapplication of AMPA and NMDA in the absence of external calcium. No depression of AMPA/NMDA responses was observed in the presence of octreotide in any of 15 tested neurones (■).

(J) Recordings of AMPA/NMDA currents in one neurone before (CTRL) and during octreotide application (OCT) in the absence of external calcium. Each trace was fitted by a spline curve with a uniform local average smoother of span 100. (C) Time plot of normalized responses to coapplication of AMPA and trans-(+)-ACPD with BAPTA in the recording pipette. The inhibitory effect of octreotide was highly significantly decreased in 3 neurones () as compared to its effect in the absence of BAPTA (a).

(D) Recordings of AMPA/trans-(+)-ACPD currents in the presence of BAPTA before (CTRL) and during octreotide application (OCT) in one responsive neurone.

(E) Time plot of normalized responses to coapplication of AMPA and trans-(+)-ACPD following a thapsigargin pretreatment. No inhibitory effect of octreotide could be detected in 6 tested neurones (■).

(F) Recordings of AMPA/trans-(+)-ACPD currents in one neurone after thapsigargin pretreatment before (CTRL) and during octreotide application (OCT). Each trace was fitted by a spline curve with a uniform local average smoother of span 100. Scale bars : 20 pA, 1 s Figure 7 : Schematic representation of sst2/AMPA interactions through intracellular calcium modifications. During synaptic transmission, release of glutamate (Glu) from nerve endings activate AMPA (GluRs) and NMDA (NMDARs) receptors, as well as, when synaptic activity increases, metabotropic glutamate receptors (mGluRs). The activation of NMDA receptors and/or mGluRs induces an increase in intracellular calcium concentration (Ca 2+ ). Calcium sources can be either extracellular, ions flowing through NMDA receptors, or intracellular, ions being released from endoplasmic reticulum (ER) following activation of the inositol phosphate pathway (IP) by mGluRs. Simultaneously, somatostatin (SRIF), released from identical or different nerve terminals, activates sst2 receptors (sst2). The activation of Gi/Go protein coupled-sst2 receptors induces a decrease in the amplitude of glutamate-induced AMPA currents. If intracellular calcium concentration does not increase, sst2-modulation of GluRs does not occur. Thus, the amplitude of the sst2-induced inhibitory modulation depends on intracellular calcium levels. Putative links between calcium, sst2 and GluRs, are shown in the inset.

(P) = phosphorylation/dephosphorylation sites.

  , thereby ruling out the presence of a kainate synaptic component. Current voltage (I-V) curves were also obtained from 20 neurones in order to characterize the I-V relationship for each type of synaptic response. The observed reversal potentials were : -17 + 4 mV for mixed glutamate/GABA responses (Fig 1A, n=4), -52 + 2 mV for pure GABA responses (Fig 1B, n=8), +13 + 5 mV for pure glutamate responses (Fig 1C, n=5), and 12 + 6 mV for AMPA responses (Fig 1D, n=3), in agreement with calculated GHK reversal potentials in our experimental conditions (GABA : -50 mV, glutamate : +13 mV, AMPA : +13 mV). As shown in Figure 2A and 2B, mixed glutamate/GABA synaptic responses were decreased by -28.8 + 3.8 % under octreotide (p<0.001 vs control values before octreotide in responsive cells, n=5, and values under octreotide in non responsive cells, n=7). The octreotide effect took place about three minutes after the beginning of the perfusion and slowly increased during its application. No recovery was observed. To test whether octreotide affected specifically glutamate and/or GABA components of the synaptic responses, experiments were reproduced in the presence of either CNQX and AP5 (n=11) to isolate the GABA component, or bicuculline (n=14) to isolate the glutamate response. Octreotide had no effect on the GABA component (Fig 2C,D) whereas octreotide inhibited the glutamate component in 43% of the recorded neurones (6/14, fig 2E,F) with an average decrease of -32.3 + 5.9 % (p<0.001 vs control values before octreotide in responsive cells, n=6, and values under octreotide in non responsive cells, n=8). The amplitude and kinetics of octreotide effects on glutamate responses were not significantly different from those observed for mixed glutamate/GABA responses.

  + 0.4 % (p<0.001 vs control values before octreotide in responsive cells, n=3, and values under octreotide in non responsive cells, n=5 ; Fig 5C,D). Fast application of AMPA with octreotide on 9 cells never led to a decreased AMPA current (Fig 5E,F) while octreotide coapplied with AMPA and NMDA decreased the AMPA current by -8.4 + 1.6 % (p<0.001 vs control values before octreotide in responsive cells, n=5, and values under octreotide in non responsive cells, n=8, Fig 5G,H). In a second

  2+influx through NMDA receptors and/or by calcium release from internal stores after mGluRs activation. These two pathways are most likely implicated in the modulation of glutamate sensitivity in different physiological conditions, either during normal synaptic transmission for NMDA receptors or in the case of excitatory hyperactivity for mGluRs.These results suggest an unsuspected regulatory mechanism involving at least three different receptor subtypes and bring a new level of interaction in the modulatory effects of neuropeptides on fast synaptic transmission.

Figure 1 :

 1 Figure 1 : Electrophysiological and pharmacological characterization of evoked synaptic

Figure 3 :

 3 Dense somatostatinergic innervation of recorded mediobasal hypothalamus neurones. (a) The recording area (*, bleaching due to image acquisition) is included in a dense network of somatostatinergic fibres. (b) Biocytin revelation by FITC-labelled streptavidin of a recorded cell. (c) Somatostatinergic fibres in the vicinity of the recorded neurone. (d) Merged images of b and c illustrates the dense network of somatostatinergic fibres surrounding the recorded neurone. (e) single focal acquisition showing the presence of close appositions between somatostatinergic fibres and the neuronal perikaryon or dendrites (arrows). Scale bars : 200 µm in a, 10 µm in b (applies to b,c,d) and e ; ARC, arcuate nucleus ; ME, median eminence ; VMH, ventromedial hypothalamic nucleus ; 3V, third ventricle.

Figure 4 :

 4 Figure 4 : Correlation between octreotide sensitivity and sst2A receptor expression (A-D) A :

Figure 5 :

 5 Figure 5 : Effect of the sst2 receptor agonist octreotide on the activation of glutamate

  Scale bars : 20 pA, 1 s Figure 6 : Role of group I mGluRs in sst2-evoked AMPA depression in mediobasal hypothalamic cultures. (All experiments were performed in the absence of extracellular calcium) (A) Time plot of normalized responses to coapplication of AMPA and trans-(+)-ACPD. 3 neurones displayed a highly significantly decreased response during octreotide application () and 4 neurones were unresponsive (■).(B) Recordings of AMPA/trans-(+)-ACPD currents before (CTRL) and during octreotide application (OCT) in one responsive neurone.

Acknowlegdements : We thank Dr Faivre-Bauman and Dr Loudes for the microdissection technique, Dr Csaba for helpful assistance during immunohistochemical processing, Dr Tramu for providing the somatostatin antibody, Dr Helboe for gift of the sst2A antibody, and Dr Davis for careful reading of the manuscript. This work was supported by INSERM, CNRS and QLG-1999-0908 european grant to JE. SP was a recipient of a fellowship from the French Research Ministery.