Francine Acher
email: francine.acher@parisdescartes.fr

Cyril Goudet
email: cyril.goudet@igf.cnrs.fr

Therapeutic potential of group III metabotropic glutamate receptor ligands in pain

Metabotropic glutamate receptors (mGluRs) modulate neurotransmission all along the pain neuraxis.

While the involvement of group I and group II mGluRs in pain is well documented, information has only just started to emerge concerning the role and contribution of group III mGluRs subtypes to pain modulation. Recent data suggests that these receptors reduce symptoms in animal models of chronic pain, as well as regulate neurotransmission at different levels of ascending and descending pain pathway, suggesting that group III mGluRs may be interesting therapeutic targets for the development of analgesics.

INTRODUCTION

Pain is one of the most common symptoms in clinical medicine and represents a permanent medical problem, being an essential component in the therapeutic management of many diseases. Pain can be classified as acute when it is short lasting or chronic when it persists for a long time after the original affection. Acute pain serves the important function of protecting the integrity of the body by detecting actual or potential tissue damage. Chronic pain is among the most debilitating and costly afflictions in North America and Europe, seriously affecting the quality of life of more than 19% of adult Europeans [START_REF] Bouhassira | Prevalence of chronic pain with neuropathic characteristics in the general population[END_REF][START_REF] Ospina | Prevalence of Chronic Pain: an Overview[END_REF][START_REF] Verhaak | Prevalence of chronic benign pain disorder among adults: a review of the literature[END_REF]. Unfortunately, while acute pain can be correctly managed, chronic pain is not efficiently alleviated by current treatments [START_REF] Breivik | Survey of chronic pain in Europe: prevalence, impact on daily life, and treatment[END_REF][START_REF] Finnerup | The evidence for pharmacological treatment of neuropathic pain[END_REF][START_REF] Nightingale | The neuropathic pain market[END_REF]. Therefore, a better understanding of cellular and molecular pathophysiological mechanisms is essential for identifying new pharmacological targets.

Glutamate and pain

Glutamate is the main excitatory neurotransmitter of the mammalian central nervous system and is implicated in many physiological and pathological processes. Glutamate is notably the main neurotransmitter involved in pain transmission. At the synaptic level, glutamate activates two classes of receptors: ionotropic and metabotropic glutamate receptors (mGluRs). Central sensitization of the pain neuraxis is associated with hyperexcitability of the glutamatergic system and leads to the development of the evoked pain symptoms, allodynia (pain due to a stimulus that does not normally provoke pain) and hyperalgesia (increased pain from a stimulus that normally provokes pain) observed in patients with chronic pain [START_REF] Latremoliere | Central sensitization: a generator of pain hypersensitivity by central neural plasticity[END_REF]. Both iGluRs and mGluRs are involved in the induction and the maintenance of this sensitization. The blockade of increased glutamatergic activity may represent a pivotal mean to reduce chronic pain but awaits a clearer identification of adequate targets.

Metabotropic glutamate receptors

mGluRs are G-protein coupled receptors activated by glutamate, the major excitatory neurotransmitter of the central nervous system (CNS). They are involved in the modulation of synaptic activity. They are thus considered as potential therapeutic targets since less side effects are anticipated compared to essential actors of synaptic transmission.

The 8 members of this family are classified in 3 groups: Group I receptors (mGlu1 and 5) are postsynaptic and positively modulate glutamatergic transmission while group II (mGlu2 and 3) and group III receptors (mGlu4, 6, 7 and 8) are predominantly presynaptic and play an inhibitory role on neurotransmission (except for mGlu6, a post-synaptic receptor which is expressed solely in bipolar ON cells in the retina). Group III mGluRs can act either as autoreceptors on glutamatergic terminals or heteroreceptors on GABAergic terminals. mGluRs form constitutive dimers composed of two subunits cross-linked by a disulphide bridge.

Dimer formation is mandatory for the function of these receptors [START_REF] Kniazeff | Dimers and beyond: The functional puzzles of class C GPCRs[END_REF]. It has long been believed that mGluRs strictly assemble into homodimers but a recent study has shown that certain mGluR subtypes can heterodimerize in vitro [START_REF] Doumazane | A new approach to analyze cell surface protein complexes reveals specific heterodimeric metabotropic glutamate receptors[END_REF]. Heterodimerization could have consequences notably in terms of pharmacological profile, signalling response and protein partners. To date, there is no clear evidence of heterodimerization of mGluRs in vivo. However, recently, mGlu2/4 heterodimers have been suspected of existing at corticostriatal synapses, based on the detection of a unique pharmacological profile as compared to the mGlu2 or mGlu4 homodimers [START_REF] Yin | Selective actions of novel allosteric modulators reveal functional heteromers of metabotropic glutamate receptors in the CNS[END_REF]. Since compatible mGluR subtypes coexist in several regions of the CNS, our comprehension of the regulation of CNS function by mGluRs, including pain, may evolve rapidly in the light of heterodimerization.

The different subtypes of mGluRs are expressed all along the pain neuraxis where they modulate the perception of pain (Figure 1). In general, blocking group-I mGluRs or activating group-II mGluRs alleviates pain (see [START_REF] Chiechio | Metabotropic glutamate receptors and the control of chronic pain[END_REF][START_REF] Montana | Metabotropic glutamate receptors as targets for analgesia: antagonism, activation, and allosteric modulation[END_REF][START_REF] Palazzo | Supraspinal metabotropic glutamate receptors: a target for pain relief and beyond[END_REF] for recent reviews).

The present review will focus on group III mGluRs for which less information is available, mainly due to the lack of selective pharmacological tools. However, the recent progress in the development of subtype selective ligands are opening the way to a better understanding of their modulatory function and therapeutic potential in pain.

GROUP III MGLUR PHARMACOLOGY

As described in Figure 1, mGlu4, 7, 8 receptors of group III are localized all along the pain neuraxis. In order to investigate if these receptors may be novel therapeutic targets to reduce pain symptoms, various ligands were used (Figure 2). However, interpretation of these experiments should take into consideration the limitations of these compounds. Most of the drugs that were evaluated are nonselective among the group III subtypes or display metabolic instability. Moreover, a 100 ratio of EC50's in cell-based assays does not ensure a selective effect in vivo since other factors such as bioavailability, receptor localization and drug concentration may also have an influence.

Both orthosteric ligands and allosteric modulators have been employed. These ligands are listed in this chapter, together with their effects and, when known, their limitations.

Orthosteric ligands

Since mGlu4, 7, 8 receptors are mostly presynaptic, agonists were used to evaluate if downregulation of neurotransmitter release mediated by their activation may provide any benefit to treat pain symptoms (Figure 2). The most widely applied drug is L-AP4, a selective agonist of group III mGlu receptors, which does not discriminate between mGlu4 and 8 receptors, that has been used in inflammatory and neuropathic pain models [START_REF] Fisher | The contribution of metabotropic glutamate receptors (mGluRs) to formalin-induced nociception[END_REF][START_REF] Fisher | Antinociceptive effects following intrathecal pretreatment with selective metabotropic glutamate receptor compounds in a rat model of neuropathic pain[END_REF][START_REF] Chen | Distinct roles of group III metabotropic glutamate receptors in control of nociception and dorsal horn neurons in normal and nerve-injured Rats[END_REF][START_REF] Jung | Central metabotropic glutamate receptors differentially participate in interleukin-1beta-induced mechanical allodynia in the orofacial area of conscious rats[END_REF][START_REF] Lee | Low doses of cannabinoids enhance the antinociceptive effects of intracisternally administered mGluRs groups II and III agonists in formalin-induced TMJ nociception in rats[END_REF][START_REF] Zhang | Effects of activation of group III metabotropic glutamate receptors on spinal synaptic transmission in a rat model of neuropathic pain[END_REF][START_REF] Chen | Differential roles of peripheral metabotropic glutamate receptors in bee venom-induced nociception and inflammation in conscious rats[END_REF][START_REF] Dolan | Activation of metabotropic glutamate receptor 7 in spinal cord inhibits pain and hyperalgesia in a novel formalin model in sheep[END_REF][START_REF] Carlton | Group II/III metabotropic glutamate receptors exert endogenous activity-dependent modulation of TRPV1 receptors on peripheral nociceptors[END_REF][START_REF] Liu | Metabotropic glutamate subtype 7 and 8 receptors oppositely modulate cardiac nociception in the rat nucleus tractus solitarius[END_REF][START_REF] Lee | Peripheral group II and III metabotropic glutamate receptors in the knee joint attenuate carrageenan-induced nociceptive behavior in rats[END_REF]. L-AP4 is about 10 times more potent than glutamate, however, it does not cross the blood brain barrier and needs to be injected in situ. L-SOP, a phosphate analog of L-AP4 with similar properties, has been used in early studies about the contribution of group III mGluRs in the periaqueductal gray to the regulation of nociception [START_REF] Maione | Characterisation of mGluRs which modulate nociception in the PAG of the mouse[END_REF][START_REF] Maione | Periaqueductal gray matter metabotropic glutamate receptors modulate formalin-induced nociception[END_REF].

ACPT-I is a group III mGluR agonist demonstrating similar selectivity to L-AP4 but is able to be administered systemically [START_REF] Acher | Synthesis and pharmacological characterization of aminocyclopentane tricarboxylic acids (ACPT): new tools to discriminate between metabotropic glutamate receptor subtypes[END_REF][START_REF] Palucha-Poniewiera | Peripheral administration of group III mGlu receptor agonist ACPT-I exerts potential antipsychotic effects in rodents[END_REF]. The use of ACPT-I [START_REF] Acher | Synthesis and pharmacological characterization of aminocyclopentane tricarboxylic acids (ACPT): new tools to discriminate between metabotropic glutamate receptor subtypes[END_REF] provided successful results in rat models of inflammatory and neuropathic pain, while leaving acute pain perception unchanged in healthy rats [START_REF] Goudet | Group III metabotropic glutamate receptors inhibit hyperalgesia in animal models of inflammation and neuropathic pain[END_REF]. The weaker agonist, ACPT-III, was also used and shown to enhance the antiallodynic action of morphine in a neuropathic pain model in rats [START_REF] Lee | Potentiation of morphine antiallodynic efficacy by ACPT-III, a group III metabotropic glutamate receptor agonist, in rat spinal nerve ligation-induced neuropathic pain[END_REF]. Two compounds (S)-DCPG [START_REF] Thomas | S)-3,4-DCPG, a potent and selective mGlu8a receptor agonist, activates metabotropic glutamate receptors on primary afferent terminals in the neonatal rat spinal cord[END_REF] and LSP4-2022 [START_REF] Goudet | A novel selective metabotropic glutamate receptor 4 agonist reveals new possibilities for developing subtype selective ligands with therapeutic potential[END_REF] have been tested as mGlu8 and mGlu4 receptor subtype selective agonists, respectively, in inflammatory or neuropathic pain models [START_REF] Liu | Metabotropic glutamate subtype 7 and 8 receptors oppositely modulate cardiac nociception in the rat nucleus tractus solitarius[END_REF][START_REF] Marabese | Periaqueductal gray metabotropic glutamate receptor subtype 7 and 8 mediate opposite effects on amino acid release, rostral ventromedial medulla cell activities, and thermal nociception[END_REF][START_REF] Marabese | Effects of (S)-3,4-DCPG, an mGlu8 receptor agonist, on inflammatory and neuropathic pain in mice[END_REF][START_REF] Ren | Differential effects of mGluR7 and mGluR8 activation on pain-related synaptic activity in the amygdala[END_REF][START_REF] Vilar | Alleviating pain hypersensitivity through activation of type 4 metabotropic glutamate receptor[END_REF][37]. However, a recent study suggests that care must be taken when employing higher concentrations of DCPG since non-selective effects in slices from mice lacking mGlu8 receptors have been observed with DCPG concentrations above 1 µM, primarily mediated by mGlu2 receptors [START_REF] Mercier | Characterisation of an mGlu8 receptor-selective agonist and antagonist in the lateral and medial perforant path inputs to the dentate gyrus[END_REF].

Competitive group III mGlu receptor antagonists have been used to assess the validity of agonist effects but also to verify the interaction between glutamatergic and GABAergic synapses in the spinal dorsal horn after stimulation of the nociceptive primary afferents [START_REF] Zhou | Increased nociceptive input rapidly modulates spinal GABAergic transmission through endogenously released glutamate[END_REF]. The most potent group III antagonist is LY341495, however, it is even more potent at group II mGlu receptors [START_REF] Kingston | LY341495 is a nanomolar potent and selective antagonist of group II metabotropic glutamate receptors[END_REF].

Nevertheless, it has been used in several studies [START_REF] Jung | Central metabotropic glutamate receptors differentially participate in interleukin-1beta-induced mechanical allodynia in the orofacial area of conscious rats[END_REF][START_REF] Lee | Low doses of cannabinoids enhance the antinociceptive effects of intracisternally administered mGluRs groups II and III agonists in formalin-induced TMJ nociception in rats[END_REF][START_REF] Chen | Differential roles of peripheral metabotropic glutamate receptors in bee venom-induced nociception and inflammation in conscious rats[END_REF][START_REF] Carlton | Group II/III metabotropic glutamate receptors exert endogenous activity-dependent modulation of TRPV1 receptors on peripheral nociceptors[END_REF][START_REF] Goudet | Group III metabotropic glutamate receptors inhibit hyperalgesia in animal models of inflammation and neuropathic pain[END_REF][START_REF] Zhou | Increased nociceptive input rapidly modulates spinal GABAergic transmission through endogenously released glutamate[END_REF][START_REF] Liang | Characterization of long-term potentiation of primary afferent transmission at trigeminal synapses of juvenile rats: essential role of subtype 5 metabotropic glutamate receptors[END_REF]. Other antagonists, such as MAP4, MSOP and CPPG, are more specific to group III but are weaker. Yet they were still useful tools in most pain studies (MAP4 [START_REF] Chen | Distinct roles of group III metabotropic glutamate receptors in control of nociception and dorsal horn neurons in normal and nerve-injured Rats[END_REF][START_REF] Goudet | Group III metabotropic glutamate receptors inhibit hyperalgesia in animal models of inflammation and neuropathic pain[END_REF][START_REF] Ren | Differential effects of mGluR7 and mGluR8 activation on pain-related synaptic activity in the amygdala[END_REF][START_REF] Liang | Characterization of long-term potentiation of primary afferent transmission at trigeminal synapses of juvenile rats: essential role of subtype 5 metabotropic glutamate receptors[END_REF][START_REF] Fundytus | Attenuation of morphine withdrawal symptoms by subtype-selective metabotropic glutamate receptor antagonists[END_REF], MSOP [START_REF] Chen | Differential roles of peripheral metabotropic glutamate receptors in bee venom-induced nociception and inflammation in conscious rats[END_REF][START_REF] Liu | Metabotropic glutamate subtype 7 and 8 receptors oppositely modulate cardiac nociception in the rat nucleus tractus solitarius[END_REF][START_REF] Maione | Characterisation of mGluRs which modulate nociception in the PAG of the mouse[END_REF][START_REF] Marabese | Effects of (S)-3,4-DCPG, an mGlu8 receptor agonist, on inflammatory and neuropathic pain in mice[END_REF][START_REF] Palazzo | Metabotropic and NMDA glutamate receptors participate in the cannabinoid-induced antinociception[END_REF][START_REF] Palazzo | Interaction between vanilloid and glutamate receptors in the central modulation of nociception[END_REF][START_REF] Jin | Peripheral glutamate receptors are required for hyperalgesia induced by capsaicin[END_REF][START_REF] Dolan | The selective metabotropic glutamate receptor 7 allosteric agonist AMN082 inhibits inflammatory pain-induced and incisioninduced hypersensitivity in rat[END_REF], CPPG [START_REF] Jung | Central metabotropic glutamate receptors differentially participate in interleukin-1beta-induced mechanical allodynia in the orofacial area of conscious rats[END_REF][START_REF] Lee | Low doses of cannabinoids enhance the antinociceptive effects of intracisternally administered mGluRs groups II and III agonists in formalin-induced TMJ nociception in rats[END_REF][START_REF] Zhang | Effects of activation of group III metabotropic glutamate receptors on spinal synaptic transmission in a rat model of neuropathic pain[END_REF][START_REF] Zhou | Increased nociceptive input rapidly modulates spinal GABAergic transmission through endogenously released glutamate[END_REF])

Allosteric modulators

Positive allosteric modulators (PAMs) have been assayed for pain relief (Figure 2). PHCCC, the first identified mGlu4 selective PAM [START_REF] Maj | PHCCC, a positive allosteric modulator of mGluR4: characterization, mechanism of action, and neuroprotection[END_REF][START_REF] Marino | Allosteric modulation of group III metabotropic glutamate receptor 4: a potential approach to Parkinson's disease treatment[END_REF], revealed similar effects to ACPT-I in inflammatory or neuropathic pain models [START_REF] Goudet | Group III metabotropic glutamate receptors inhibit hyperalgesia in animal models of inflammation and neuropathic pain[END_REF]. Another mGlu4 PAM, VU155041 [START_REF] Niswender | Discovery, characterization, and antiparkinsonian effect of novel positive allosteric modulators of metabotropic glutamate receptor 4[END_REF], attenuated mechanical hyperalgesia in a neuropathic pain rat model when injected intrathecally [START_REF] Osikowicz | Glutamate receptor ligands attenuate allodynia and hyperalgesia and potentiate morphine effects in a mouse model of neuropathic pain[END_REF], but was ineffective in changing thermal nociception and ON and OFF cell activity when injected in the dorsal striatum of naive or neuropathic rats [37]. The discovery of new mGlu4 receptor PAMs [START_REF] Celanire | Recent advances in the drug discovery of metabotropic glutamate receptor 4 (mGluR4) activators for the treatment of CNS and non-CNS disorders[END_REF] should allow further investigations. An mGlu8 receptor PAM, AZ12216052 [START_REF] Duvoisin | Acute pharmacological modulation of mGluR8 reduces measures of anxiety[END_REF], behaved like (S)-DCPG, the orthosteric agonist with similar selectivity, increasing tail flick latency and OFF cell activity and decreasing ON cell activity in neuropathic rats after injection into the dorsal striatum [37]. However, AZ12216052 reduced anxiety in mGlu8 KO mice, suggesting that this PAM may also act on other receptors mGlu8 [START_REF] Duvoisin | Opposing roles of mGluR8 in measures of anxiety involving non-social and social challenges[END_REF]. The mGlu7 agonist-PAM AMN082 [START_REF] Mitsukawa | A selective metabotropic glutamate receptor 7 agonist: activation of receptor signaling via an allosteric site modulates stress parameters in vivo[END_REF] has attracted much interest and was introduced in the largest evaluation of group-III PAMs' [START_REF] Dolan | Activation of metabotropic glutamate receptor 7 in spinal cord inhibits pain and hyperalgesia in a novel formalin model in sheep[END_REF][START_REF] Liu | Metabotropic glutamate subtype 7 and 8 receptors oppositely modulate cardiac nociception in the rat nucleus tractus solitarius[END_REF][START_REF] Marabese | Periaqueductal gray metabotropic glutamate receptor subtype 7 and 8 mediate opposite effects on amino acid release, rostral ventromedial medulla cell activities, and thermal nociception[END_REF][START_REF] Marabese | Effects of (S)-3,4-DCPG, an mGlu8 receptor agonist, on inflammatory and neuropathic pain in mice[END_REF][START_REF] Ren | Differential effects of mGluR7 and mGluR8 activation on pain-related synaptic activity in the amygdala[END_REF][START_REF] Dolan | The selective metabotropic glutamate receptor 7 allosteric agonist AMN082 inhibits inflammatory pain-induced and incisioninduced hypersensitivity in rat[END_REF][START_REF] Osikowicz | Glutamate receptor ligands attenuate allodynia and hyperalgesia and potentiate morphine effects in a mouse model of neuropathic pain[END_REF][START_REF] Wang | Spinal metabotropic glutamate receptor 4 is involved in neuropathic pain[END_REF]. However, contradictory results were described and should be taken with precaution as AMN082 is rapidly metabolized into a monoamine transporter inhibitor [START_REF] Rizzo | The metabotropic glutamate receptor 7 allosteric modulator AMN082: a monoaminergic agent in disguise?[END_REF]. A negative allosteric modulator (NAM) of mGlu7 receptor, MMPIP [START_REF] Suzuki | In vitro pharmacological characterization of novel isoxazolopyridone derivatives as allosteric metabotropic glutamate receptor 7 antagonists[END_REF],

was recently used in series of experiments, showing the antinociceptive effect of the blockade of this particular receptor in the ventrolateral periaqueductal gray in formalin and neuropathic pain models [START_REF] Palazzo | Effects of a metabotropic glutamate receptor subtype 7 negative allosteric modulator in the periaqueductal grey on pain responses and rostral ventromedial medulla cell activity in rat[END_REF]. Interestingly, the inhibiting activity of MMPIP has been reported to be context dependent, meaning that MMPIP may not antagonize the coupling of mGlu7 to its native signaling pathways in all cellular contexts, as exemplified by its inability to block agonist-mediated responses at the Schaffer collateral-CA1 synapse, where mGlu7 has been previously shown to modulate neurotransmission [START_REF] Niswender | Context-dependent pharmacology exhibited by negative allosteric modulators of metabotropic glutamate receptor 7[END_REF]. This context-dependent signaling bias has to be taken into consideration when using this compound and its derivatives. In addition, the new mGlu7 NAM's ADX71743 [START_REF] Kalinichev | ADX71743, a potent and selective negative allosteric modulator of metabotropic glutamate receptor 7 (mGlu7): in vitro and in vivo characterization[END_REF] and XAP044 [61] should allow future investigations.

PHENOTYPES OF GROUP III MGLURS KNOCK-OUT MICE

Mice lacking mGlu4 receptors display normal spontaneous motor activity but present an impaired ability to learn and memorize [START_REF] Pekhletski | Impaired cerebellar synaptic plasticity and motor performance in mice lacking the mGluR4 subtype of metabotropic glutamate receptor[END_REF][START_REF] Gerlai | Altered spatial learning and memory in mice lacking the mGluR4 subtype of metabotropic glutamate receptor[END_REF]. The sensitivity to strong noxious mechanical compression is altered and the onset of the nociceptive behavior in the inflammatory phase of the formalin test is accelerated in mGlu4 KO mice as compared to their wild type littermates, whereas responses to punctate mechanical stimulation and nocifensive responses to thermal noxious stimuli are not modified [START_REF] Vilar | Alleviating pain hypersensitivity through activation of type 4 metabotropic glutamate receptor[END_REF]. To our knowledge, the sensory perception phenotypes of mGlu7 and mGlu8 KO mice have not been determined. Mice lacking mGlu7 present increased seizure susceptibility [START_REF] Sansig | Increased seizure susceptibility in mice lacking metabotropic glutamate receptor 7[END_REF] together with an antidepressant and anxiolytic-like phenotype [START_REF] Cryan | Antidepressant and anxiolytic-like effects in mice lacking the group III metabotropic glutamate receptor mGluR7[END_REF] while mGlu8 KO mice exhibit an anxiogenic phenotype [START_REF] Duvoisin | Acute pharmacological modulation of mGluR8 reduces measures of anxiety[END_REF][START_REF] Linden | Increased anxiety-related behavior in mice deficient for metabotropic glutamate 8 (mGlu8) receptor[END_REF][START_REF] Duvoisin | Increased measures of anxiety and weight gain in mice lacking the group III metabotropic glutamate receptor mGluR8[END_REF]. Interestingly, subtle differences in amygdala-dependent behavior and physiology between mGlu7 and mGlu8 KO mice suggest differential roles of these receptors. Indeed, while mGlu7 KO mice display a general deficit in conditioned fear, mGlu8 KO mice show a dramatic reduction in contextual fear [START_REF] Fendt | Combining pharmacological and electrophysiological approaches with mGlu7 and mGlu8 KO mice, the authors investigated the roles of mGlu7 and mGlu8 in amygdala-dependent behavior and synaptic plasticity. It revealed the different roles for mGlu7 and mGlu8 in amygdala synaptic transmission, fear learning and its expression[END_REF]. These results are well correlated with studies that used a pharmacological approach (see below). Since there is a strong comorbidity between chronic pain, depression and anxiety [START_REF] Breivik | Survey of chronic pain in Europe: prevalence, impact on daily life, and treatment[END_REF], targeting these receptors may be of interest in chronic pain.

LOCALIZATION AND ROLE OF GROUP III MGLURS IN THE PAIN NEURAXIS

The presence of group III mGluRs has been identified in different areas involved in pain processing (Figure 1), with the exception of mGlu6 which is exclusively expressed in the retina.

Periphery

Few studies have addressed the role of peripheral group III mGluRs in pain. Intraplantar group III mGluR agonist L-AP4 delivery attenuates the increased nociception induced by capsaicin [START_REF] Carlton | Group II/III metabotropic glutamate receptors exert endogenous activity-dependent modulation of TRPV1 receptors on peripheral nociceptors[END_REF][START_REF] Govea | Group III metabotropic glutamate receptors and transient receptor potential vanilloid 1 co-localize and interact on nociceptors[END_REF] or bee venom [START_REF] Chen | Differential roles of peripheral metabotropic glutamate receptors in bee venom-induced nociception and inflammation in conscious rats[END_REF], while administration of L-AP4 into the knee joint reduces hyperalgesia induced by carrageenan-induced arthritic pain [START_REF] Lee | Peripheral group II and III metabotropic glutamate receptors in the knee joint attenuate carrageenan-induced nociceptive behavior in rats[END_REF]. mGlu8 has been identified in the unmyelinated axons of digital nerves [START_REF] Govea | Group III metabotropic glutamate receptors and transient receptor potential vanilloid 1 co-localize and interact on nociceptors[END_REF]. To our knowledge, the expression of other subtypes of group III mGluRs that may also mediate L-AP4 action in the periphery has not been documented.

Spinal cord

The superficial laminae of the dorsal horn of the spinal cord receives sensory information from primary afferents responding to noxious and non-noxious stimuli (see [START_REF] Todd | Neuronal circuitry for pain processing in the dorsal horn[END_REF] for a comprehensive review on pain processing in the dorsal horn). mGlu4 is present in the inner lamina II of the dorsal horn of spinal cord [START_REF] Azkue | Immunoreactivity for the group III metabotropic glutamate receptor subtype mGluR4a in the superficial laminae of the rat spinal dorsal horn[END_REF][START_REF] Ohishi | Distributions of the mRNAs for L-2-amino-4-phosphonobutyrate-sensitive metabotropic glutamate receptors, mGluR4 and mGluR7, in the rat brain[END_REF][START_REF] Ohishi | Presynaptic localization of a metabotropic glutamate receptor, mGluR7, in the primary afferent neurons: an immunohistochemical study in the rat[END_REF], more precisely, both in the presynaptic terminal of unmyelinated C-fibers and spinal neurons terminals [START_REF] Vilar | Alleviating pain hypersensitivity through activation of type 4 metabotropic glutamate receptor[END_REF]. mGlu7 is located in presynaptic terminal of sensory neurons in the laminae I and II in the dorsal horn [START_REF] Ohishi | Presynaptic localization of a metabotropic glutamate receptor, mGluR7, in the primary afferent neurons: an immunohistochemical study in the rat[END_REF] but the nature of sensory neurons expressing mGlu7 remains to be determined. mGlu8 is located in the soma of TRPV1 expressing-unmyelinated DRG neurons [START_REF] Govea | Group III metabotropic glutamate receptors and transient receptor potential vanilloid 1 co-localize and interact on nociceptors[END_REF][START_REF] Carlton | Colocalization of metabotropic glutamate receptors in rat dorsal root ganglion cells[END_REF], but doesn't seem to be present in the spinal cord [START_REF] Valerio | Metabotropic glutamate receptor mRNA expression in rat spinal cord[END_REF][START_REF] Duvoisin | A novel metabotropic glutamate receptor expressed in the retina and olfactory bulb[END_REF].

Interestingly, spinal group III mGluRs, and mGlu4 in particular, seem to be devoted to the modulation of hypersensitivity to applied stimuli observed in animal models of persistent or chronic pain, whilst leaving reactions to acute noxious stimuli unchanged in naive animals. Indeed, spinal group III mGluR activation by intrathecal injection of selective group III agonists (L-AP4 or ACPT-I) reduces hypersensitivity to pain in animal models of inflammatory or neuropathic pain without affecting acute pain perception in healthy animals [START_REF] Fisher | Antinociceptive effects following intrathecal pretreatment with selective metabotropic glutamate receptor compounds in a rat model of neuropathic pain[END_REF][START_REF] Chen | Distinct roles of group III metabotropic glutamate receptors in control of nociception and dorsal horn neurons in normal and nerve-injured Rats[END_REF][START_REF] Goudet | Group III metabotropic glutamate receptors inhibit hyperalgesia in animal models of inflammation and neuropathic pain[END_REF][START_REF] Mills | Role of group II and group III metabotropic glutamate receptors in spinal cord injury[END_REF]. Moreover, activation of spinal group III mGluRs decreases firing of spinal cord dorsal horn projection neurons [START_REF] Chen | Distinct roles of group III metabotropic glutamate receptors in control of nociception and dorsal horn neurons in normal and nerve-injured Rats[END_REF] and their ability to control the excess of excitatory transmission is reinforced in spinal cord of inflamed or neuropathic animals [START_REF] Zhang | Effects of activation of group III metabotropic glutamate receptors on spinal synaptic transmission in a rat model of neuropathic pain[END_REF][START_REF] Vilar | Alleviating pain hypersensitivity through activation of type 4 metabotropic glutamate receptor[END_REF].

Similar effects are obtained by the selective activation of spinal mGlu4. Indeed, intrathecal application of the selective mGlu4 agonist LSP4-2022 [START_REF] Vilar | Alleviating pain hypersensitivity through activation of type 4 metabotropic glutamate receptor[END_REF], or the selective mGlu4 PAMs PHCCC [START_REF] Goudet | Group III metabotropic glutamate receptors inhibit hyperalgesia in animal models of inflammation and neuropathic pain[END_REF] and VU0155041 [START_REF] Wang | Spinal metabotropic glutamate receptor 4 is involved in neuropathic pain[END_REF], alleviates pain hypersensitivity in animal models of chronic pain without affecting acute pain in healthy animals. Interestingly, the systemic injection of mGlu4 agonist LSP4-2022 abolishes mechanical hypersensitivity in inflammation [START_REF] Vilar | Alleviating pain hypersensitivity through activation of type 4 metabotropic glutamate receptor[END_REF]. On the other hand, intrathecal administration of AMN082, an mGlu7 PAM, does not reduce hyperalgesia in neuropathic pain [START_REF] Wang | Spinal metabotropic glutamate receptor 4 is involved in neuropathic pain[END_REF].

Supraspinal

At the supraspinal level, the various subtypes of group III mGluRs can be found in the different areas of the pain ascending and descending pathways (Figure 1).

The main ascending projections from dorsal horn neurons are directed toward the thalamus and the midbrain periaqueductal grey region (PAG). The thalamus is notably an essential relay and processing point of nociceptive inputs from the spinal cord to the cerebral cortex. The thalamus shows an intense labeling for almost all mGluRs, as revealed by in situ hybridization [START_REF] Neto | Differential distribution of metabotropic glutamate receptor subtype mRNAs in the thalamus of the rat[END_REF]. However, the potential role of group III mGluRs in the processing of sensory information in the thalamus remains to be clarified.

The amygdala is an important center for the processing of the emotional component associated with pain [START_REF] Neugebauer | Chapter 11 Subcortical processing of nociceptive information: basal ganglia and amygdala[END_REF]. 'In arthritic rats, the group III agonist, L-AP4, inhibits the pain-related synaptic plasticity of the central nucleus of the amygdala (CeA), the output structure that projects to the hypothalamus and several brainstem regions such the PAG [START_REF] Han | Enhanced group III mGluR-mediated inhibition of painrelated synaptic plasticity in the amygdala[END_REF].

An important pain descending pathway originates within PAG and projects to the spinal dorsal horn via the rostral ventromedial medulla (RVM) [START_REF] Millan | Descending control of pain[END_REF]. Activation of this descending system elicits analgesia by inhibiting ascending nociceptive transmission at the spinal cord level. Stimulation of group III mGluRs in the PAG facilitates pain behavior in naïve animals and in the formalin test [START_REF] Maione | Periaqueductal gray matter metabotropic glutamate receptors modulate formalin-induced nociception[END_REF][START_REF] Maione | Metabotropic glutamate receptors modulate serotonin release in the rat periaqueductal gray matter[END_REF].

So far, preclinical studies aimed to decipher the individual roles of group III mGluRs in the supraspinal structures involved in pain processing have mainly focused on the opposing effects of mGlu7 and mGlu8 (See [START_REF] Palazzo | Supraspinal metabotropic glutamate receptors: a target for pain relief and beyond[END_REF][START_REF] Palazzo | Supraspinal metabotropic glutamate receptor subtype 8: a switch to turn off pain[END_REF] for extensive and recent reviews on the subject). Indeed, while systemic administration of the mGlu7 PAM AMN082 decreases allodynia in an animal model of neuropathic pain [START_REF] Osikowicz | Glutamate receptor ligands attenuate allodynia and hyperalgesia and potentiate morphine effects in a mouse model of neuropathic pain[END_REF], and systemic administration of the mGlu8 selective agonist (S)-3,4-DCPG reduces hyperalgesia induced by inflammatory or neuropathic pain model [START_REF] Marabese | Effects of (S)-3,4-DCPG, an mGlu8 receptor agonist, on inflammatory and neuropathic pain in mice[END_REF], opposing effects are observed these two receptors are observed at the level of PAG, RVM or amygdala. Blockade of mGlu7 by microinjection of the selective mGlu7 NAM MMPIP in the PAG reduces the pain related behaviors in formalin and neuropathic pain models and differentially modulates RVM ON and OFF cell activity [START_REF] Palazzo | Effects of a metabotropic glutamate receptor subtype 7 negative allosteric modulator in the periaqueductal grey on pain responses and rostral ventromedial medulla cell activity in rat[END_REF], while its activation increases pain [START_REF] Marabese | Periaqueductal gray metabotropic glutamate receptor subtype 7 and 8 mediate opposite effects on amino acid release, rostral ventromedial medulla cell activities, and thermal nociception[END_REF]. On the other hand, injection of the mGlu8 selective agonist (S)-3,4-DCPG into the PAG alleviates pain in mice models of inflammatory and neuropathic pain [START_REF] Marabese | Periaqueductal gray metabotropic glutamate receptor subtype 7 and 8 mediate opposite effects on amino acid release, rostral ventromedial medulla cell activities, and thermal nociception[END_REF][START_REF] Marabese | Effects of (S)-3,4-DCPG, an mGlu8 receptor agonist, on inflammatory and neuropathic pain in mice[END_REF]. Moreover, activation of mGlu8 in the dorsal striatum by local injection of (S)-3,4-DCPG or AZ12216052, a selective mGlu8 PAM, inhibits ON cells and stimulates OFF cells within RVM and reduces thermoceptive responses and mechanical allodynia in neuropathic pain [37]. At the level of the amygdala, activation of mGlu7 in the CeA by AMN082 aggravates sensory and emotional components in a model of arthritic inflammatory pain, while mGlu8 activation by (S)-3,4-DCPG reduces them [START_REF] Palazzo | Group III mGluR7 and mGluR8 in the amygdala differentially modulate nocifensive and affective pain behaviors[END_REF][START_REF] Palazzo | Metabotropic glutamate receptor subtype 8 in the amygdala modulates thermal threshold, neurotransmitter release, and rostral ventromedial medulla cell activity in inflammatory pain[END_REF]. Interestingly, the opposite effects of mGlu7 and mGlu8 activation might be correlated with their respective autoreceptor or heteroreceptor role on glutamatergic or GABAergic terminals in PAG [START_REF] Marabese | Periaqueductal gray metabotropic glutamate receptor subtype 7 and 8 mediate opposite effects on amino acid release, rostral ventromedial medulla cell activities, and thermal nociception[END_REF] and their segregation in different neuronal pathways within the amygdala [START_REF] Fendt | Combining pharmacological and electrophysiological approaches with mGlu7 and mGlu8 KO mice, the authors investigated the roles of mGlu7 and mGlu8 in amygdala-dependent behavior and synaptic plasticity. It revealed the different roles for mGlu7 and mGlu8 in amygdala synaptic transmission, fear learning and its expression[END_REF][START_REF] Dobi | Neural substrates for the distinct effects of presynaptic group III metabotropic glutamate receptors on extinction of contextual fear conditioning in mice[END_REF].

CONCLUSION

Taken together, these data suggest that group III mGluRs could be interesting therapeutic targets to alleviate chronic pain. Indeed, these receptors seem to be dedicated to the modulation of pain hypersensitivity and cognitive and emotional impairments observed in preclinical models of chronic pain. However, with the help of the novel selective pharmacological tools, further studies will be needed in order to clarify the precise roles of each subtype and their therapeutic potential. **Demonstration that activation of spinal group III mGluRs by the group III agonist L-AP4 attenuates allodynia and inhibits the evoked responses of ascending dorsal horn neurons in nerve-injured rats while it produces no effect on nociception or spinal transmission in naive rats.

*In this study, the authors describe a very complete characterization of a potent, brain penetrant and selective mGlu7 NAM ADX71743 both in vitro and in vivo in many behavioural models. * This study describes XAP044, a selective and brain penetrant mGlu7 NAM with anti-stress, antidepressant-and anxiolytic-like efficacy in vivo, able to modulate synaptic plasticity within amygdala and which likely targets an allosteric pocket located within the extracellular domain of the receptor contrary to other mGluRs NAMs described so far.

  Peterlik D, Neuhauser C, Bouhelal R, Kaupmann K, Laue G, Uschold-Schmidt N, Feuerbach D, Zimmermann K, Ofner S, Cryan JF et al: Blocking metabotropic glutamate receptor subtype 7 (mGlu7) via the Venus flytrap domain (VFTD) inhibits amygdala plasticity, stress, and anxiety-related behavior. J Biol Chem (2014) 289:10975-10987.

Figure 1 .

 1 Figure 1. Localization and function of mGluRs in the pain neuraxis.The different subtypes of mGluRs are expressed all along the pain neuraxis where they modulate pain perception. Ascending pain pathway is in red and descending pathway in green. Up arrows mean that activation of a particular subtype is proalgesic while down arrows mean that activation is analgesic. PAG: Periaqueducal Gray; RVM: Rostral ventromedial medulla; DRG: Dorsal Root Ganglion.

Figure 2 .Figure 1 .

 21 Figure 2. Chemical structures of orthosteric and allosteric group III mGlu receptor ligands.

Figure 2 .

 2 Figure 2. Chemical structures of orthosteric and allosteric group III mGlu receptor ligands.

Table 1 . Potencies of ligands used to study group III mGluR function in preclinical models of pain.

 1 EC50 or IC50 values are taken from the literature, except for * and ** which stands for EC50 values determined either by intracellular Ca release measurement* or inositol phosphate accumulation** on rat clone of group III mGluRs transiently expressed in HEK293 cells (personal data).

		mGlu4	mGlu6	mGlu7	mGlu8	Ligand issues
	Agonist (EC50 µM)
	Glu (**)	3.4	24.2	>500	2.4
	L-AP4 *	0.2	1	249	0.3	not selective among mGlu4/6/8 [87]
	L-SOP [88] [87]	1-4	3	160-1200 2	not selective among mGlu4/6/8 [87]
	ACPT-I (**)	1.7	10.6	280	5	not selective among mGlu4/6/8 [29]
	(S)-DCPG [31](*)	8.8-3.2	3.6-3,2 >100	0.03-0.04	not selective at concentration >1µM
						[38]
	LSP4-2022 (*)[32]	0.1	4.4	11.6	29.2	rapid clearance [32]
	Antagonists (IC50 µM)
	LY341495 [88]	6.8-9.7 1.1-1.8 0.99	0.17	not selective among mGluRs [87]
	MAP4 [88] [87]	90-190			25-105	weak potency [87]
	CPPG [88]	12	4	17	11	weak potency [88]
	PAM (EC50 µM)
	PHCCC (*)	5.1				lack of selectivity, relatively low
						potency and poor solubility [49]
	VU155041 (*)	1.0				poor brain exposure [50]
	AZ12216052 [51]	1.0				lack of selectivity described in a study
						[52]
	AMN082 [53]					metabolic instability [56] (55)
	NAM (IC50 µM)
	MMPIP [57]			0.22		context dependent inhibitor [59]
	ADX71743[60]			0.06		poor solubility [60]
	XAP044 [61]			3.5		recent disclosure, no hindsight [61]

ACKNOWLEDGEMENTS

This work was supported by grants from the Fondation Recherche Médicale (FRM team DEQ20130326522), the Fundació La Marató de TV3 (Ref 110232), Eranet Neuron and the Agence Nationale de la Recherche (ANR-12-NEUR-0003; ANR-13-BSV1-006).FA and CG were supported by the Centre National de la Recherche Scientifique (CNRS).

CONFLICT OF INTEREST


Sommeria

T urbulence denotes complex fluid motion, as opposed to smooth laminar flows. It evokes agitation, disorder and chaos. The definition of chaos restricts it to dynamic systems of which only the time behaviour is complex -deterministic but sensitive to the initial condition. Turbulence involves spatial as well as temporal complexity, with 'fractal' multi-scale velocity fields.

Although physics is successful in grasping many remote and extreme phenomena, it is often of little help with common problems encountered in life. In De rerum natura, Lucretius describes the random motion of dust seen in a ray of sunlight, an early hint at the motion of atoms in a vacuum. Yet nowadays, we know more about atoms than about the turbulent air motion revealed by dust.

The relevant laws of mechanics have been known since Newton's time; the resulting partial-differential equations for a fluid were derived by Leonhard Euler, with a viscous term introduced by Claude-Louis Navier in 1823. Turbulence is supposedly included in these equations as complex, unsteady flow solutions. Modern computers have begun to grasp this complexity, but the available spatial resolution is still too low for a full model of the air flow in a room. We need to resolve the scale on which viscous smoothing occurs -just a few millimetres -and we also need to determine the initial condition. But measuring such three-dimensional velocity fields is beyond the ability of current technology.

A direct numerical simulation would provide gigabytes of raw data, but this would just be the starting point. Statistical properties, such as mean transport of air mass, of temperature and of chemicals, must be considered. Wall friction and noise emission (pressure fluctuations) are relevant in aerodynamics. The gradients of wind and concentration of transported chemicals have intriguing, intermittent fluctuations, which can control chemical reactions. The emergence of coherent flow structures is also remarkable, but is often difficult to characterize unambiguously.

'Predictability' expresses the sensitivity of predictions to a small initial perturbation, or error -thereby imposing, for example, limitations on meteorological forecasts. Exponential error growth (chaos) is observed in simplified models that retain only the largest scales of motion. (This has been popularized as the 'butterfly' effect, from the legend about a butterfly wingbeat eventually modifying the weather.) This is unlikely, however, as such unpredictability is overestimated in models that are simplified to a few dynamic variables describing only large scales. In reality, the global effects of many small-scale perturbations tend to be predictable. Likewise, colliding molecules have highly unpredictable motion, but collectively give rise to the very predictable properties of gases.

Sensitivity to physical perturbations (for instance, vibration at a wall) is also a key issue, about which laboratory experiments provide valuable information. Turbulence is therefore a genuine scientific field, involving experiments, theory and numerical simulations. Although it relies on classical mechanics equations, the solutions of which are approached by computation, conceptual understanding is needed. This is essential not only in engineering design -for instance in improving aerodynamics by active control of boundary layers -but also for modelling the evolution of our natural environment.

In the early Universe, turbulence controlled the development of density fluctuations that eventually led to the formation of galaxies. Similarly, it governs the development of fluctuations in molecular clouds at the origins of stars, as well as the accretion of protoplanetary nebulae and the agglomeration of dust that leads to planetary formation. Climate changes are governed by turbulent fluctuations in the Sun, the atmosphere and the oceans, and the melting of polar ice caps is controlled by turbulent heat transfer.

The concept of eddy viscosity (and diffu-sivity) is widely used to model the effects of the small, unresolved scales of turbulence in 'large eddy simulations' . Navier derived his expression of viscosity as an effect of unspecified random transport; such arguments were later formalized by James Clerk Maxwell and Ludwig Boltzmann in the kinetic theory of gases.

The statistics of a continuum fluid motion pose a quite different problem: unlike molecules, fluid elements interact over long ranges (by pressure force) and deform irreversibly.

In two dimensions, the vorticity (curl of velocity) of each fluid element is conserved. The system is therefore like a set of elementary vortices (spinning-tops) with a long-range interaction. Statistical equilibrium states involve clusters of aligned elementary vortices. This explains the persistence of large, coherent vortices, such as Jupiter's Great Red Spot, in turbulent atmospheres and oceans.

But the most common form of turbulence is three-dimensional and far from equilibrium, with an irreversible cascade of energy towards small scales. Andrei Kolmogorov described this cascade in 1941, arguing that the dynamics are scale-independent. However, this is only approximately true. Deviations have been obtained, in the form of intermittency exponents, for transport of a pollutant by a random gaussian velocity (simplified synthetic turbulence). Extension of these descriptions to turbulent flow itself would be a great achievement, but not the whole story.

Real turbulent flows are diverse, depending on boundary effects, rotation and external forces (such as buoyancy). Randomness is not externally imposed, but rather spontaneously arises. There will be no single breakthrough -we face deep questions about information processing and probabilities, and about the very meaning of scientific understanding and prediction. Turbulence allows us to venture towards this new frontier with the safeguard of rigorous classical mechanics.

Unweaving the whirls

Turbulence is a genuine scientific field, involving experiments, theory and numerical simulations.

Up in smoke : the erratic motion of the particles reveals the turbulence of the surrounding air.

  FURTHER READING: Frisch, U. Turbulence: The Legacy of A. N. Kolmogorov (Cambridge Univ. Press, Cambridge, 1995). Lesieur, M. Turbulence in Fluids (Kluwer Academic, Dordrecht, The Netherlands, 1997). Lesieur, M., Yaglom, A. & David, F. New Trends in Turbulence (Springer, Heidelberg, 2001). Shraiman, B. I. & Siggia, E. D. Nature 405, 639-646 (2000).