Periaqueductal gray metabotropic glutamate receptor subtype 7 and 8 mediate opposite effects on amino acid release, rostral ventromedial medulla cell activities, and thermal nociception
ABSTRACT The current study has investigated the involvement of periaqueductal gray (PAG) metabotropic glutamate subtype 7 and 8 receptors (mGluR(7) and mGluR(8)) in modulating rostral ventromedial medulla (RVM) ongoing and tail flick-related on and off cell activities. Our study has also investigated the role of PAG mGluR(7) on thermoceptive threshold and PAG glutamate and GABA release. Intra-ventrolateral PAG (S)-3,4-dicarboxyphenylglycine [(S)-3,4-DCPG (2 and 4 nmol/rat)] or N,N(I)-dibenzhydrylethane-1,2-diamin dihydrochloride (AMN082, (1 and 2 nmol/rat), selective mGluR(8) and mGluR(7) agonists, respectively, caused opposite effects on the ongoing RVM on and off cell activities. Tail flick latency was increased or decreased by (S)-3,4-DCPG or AMN082 (2 nmol/rat), respectively. (S)-3,4-DCPG reduced the pause and delayed the onset of the off cell pause. Conversely, AMN082 increased the pause and shortened the onset of off cell pause. (S)-3,4-DCPG or AMN082 did not change the tail flick-induced onset of on-cell peak firing. The tail flick latency and its related electrophysiological effects induced by (S)-3,4-DCPG or AMN082 were prevented by (RS)-alpha-methylserine-o-phosphate (100 nmol/rat), a group III mGluR antagonist. Intra-ventrolateral PAG perfusion with AMN082 (10 and 25 microM), decreased thermoceptive thresholds and glutamate extracellular levels. A decrease in GABA release was also observed. These results show that stimulation of PAG mGluR(8) or mGluR(7) could either relieve or worsen pain perception. The opposite effects on pain behavior correlate with the opposite roles played by mGluR(7) and mGluR(8) on glutamate and GABA release and the ongoing and tail flick-related activities of the RVM on and off cells.
- SourceAvailable from: Sherry Shu-Jung Hu
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- "Whether group II and III mGluRs play an antinociceptive or pronociceptive role might dependent on the activated subtype of mGluRs within each group and its localization. For example, in the rat tail-flick test, activation of mGluR7 and mGluR8 is pronociceptive and antinociceptive, respectively (Marabese et al., 2007). However, both in vivo and in vitro studies suggested that group I mGluR activation leads to antinociception. "
ABSTRACT: Marijuana has been used to relieve pain for centuries. The analgesic mechanism of its constituents, the cannabinoids, was only revealed after the discovery of cannabinoid receptors (CB1 and CB2 ) two decades ago. The subsequent identification of the endocannabinoids, anandamide and 2-arachidonoylglycerol (2-AG), and their biosynthetic and degradation enzymes discloses the therapeutic potential of compounds targeting the endocannabinoid system for pain control. Inhibitors of the anandamide and 2-AG degradation enzymes, fatty acid amide hydrolase and monoacylglycerol lipase, respectively, may be superior to direct cannabinoid receptor ligands as endocannabinoids are synthesized on demand and rapidly degraded, focusing action at generating sites. Recently, a promising strategy for pain relief was revealed in the periaqueductal gray (PAG). It is initiated by Gq -protein-coupled receptor (Gq PCR) activation of the phospholipase C-diacylglycerol lipase enzymatic cascade, generating 2-AG that produces inhibition of GABAergic transmission (disinhibition) in the PAG, thereby leading to analgesia. Here, we introduce the antinociceptive properties of exogenous cannabinoids and endocannabinoids, involving their biosynthesis and degradation processes, particularly in the PAG. We also review recent studies disclosing the Gq PCR-phospholipase C-diacylglycerol lipase-2-AG retrograde disinhibition mechanism in the PAG, induced by activating several Gq PCRs, including metabotropic glutamatergic (type 5 metabotropic glutamate receptor), muscarinic acetylcholine (M1/M3), and orexin 1 receptors. Disinhibition mediated by type 5 metabotropic glutamate receptor can be initiated by glutamate transporter inhibitors or indirectly by substance P, neurotensin, cholecystokinin and capsaicin. Finally, the putative role of 2-AG generated after activating the above neurotransmitter receptors in stress-induced analgesia is discussed.European Journal of Neuroscience 02/2014; 39(3):467-84. DOI:10.1111/ejn.12475 · 3.67 Impact Factor
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- "ed fear , as well as LTP in the amygdala has been observed ( Schmid & Fendt , 2006 ) . The mGlu 8 receptor PAM AZ12216052 also showed an ability to reduce anxiety - like behavior in mice ( Duvoisin et al . , 2011 ) . Administration of DCPG in mice with carrageenan - injected hind paws ( arthritis pain model ) produced anti - hyperalgesic effects ( Marabese et al . , 2007 ) . In other studies , DCPG was found to exert dose - dependent anticonvulsant activity ( Chapman et al . , 2001 ; Moldrich et al . , 2001a ; Watanabe et al . , 2011 ) . 2 . 3 . 6 . 5 . mGlu 8 receptors and drugs of abuse . There is a report showing that the mGlu 8 receptor agonist DCPG suppressed ethanol self - administration and cue -"
ABSTRACT: Glutamatergic excitatory transmission is implicated in physiological and pathological conditions like learning, memory, neuronal plasticity and emotions, while glutamatergic abnormalities are reported in numerous neurological and psychiatric disorders, including neurodegenerative diseases, epilepsy, stroke, traumatic brain injury, depression, anxiety, schizophrenia and pain. Also, several lines of evidence have accumulated indicating a pivotal role for glutamatergic neurotransmission in mediating addictive behaviors. Among the proteins regulating glutamatergic transmission, the metabotropic glutamate receptors (mGluR) are being developed as pharmacological targets for treating many neuropsychiatric disorders, including drug addiction. In this review we describe the molecular structure of mGluRs and their distribution, physiology and pharmacology in the central nervous system, as well as their use as targets in preclinical studies of drug addiction.Pharmacology [?] Therapeutics 12/2013; 142(3). DOI:10.1016/j.pharmthera.2013.12.012 · 7.75 Impact Factor
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- "mGluR7 stimulation by AMN082, a selective mGluR7 agonist , locally microinjected into the VL PAG and CeA, reduced the nociceptive threshold [19,20,23]. AMN082 also decreased extracellular glutamate release within the VL PAG . Glutamate into the PAG relieves pain [24-26] thus due to the autoreceptor role of mGluR7 [6,27-30], AMN082 may lead to a decrease in glutamate level and subsequent pain facilitation. "
ABSTRACT: The metabotropic glutamate receptor 7 (mGluR7) negative allosteric modulator, 6-(4-methoxyphenyl)-5-methyl-3-pyridin-4-ylisoxazolo[4,5-c]pyridin-4(5H)-one (MMPIP), was locally microinjected into the ventrolateral periaqueductal gray (VL PAG) and the effect on pain responses in formalin and spare nerve injury (SNI) -induced neuropathic pain models was monitored in the rat. The activity of rostral ventromedial medulla (RVM) "pronociceptive" ON and "antinociceptive" OFF cells was also evaluated. Intra--VL PAG MMPIP blocked the first and second phase of nocifensive behaviour in the formalin pain model. MMPIP increased the tail flick latency and simultaneously increased the activity of the OFF cells while inhibiting that of ON cells in rats with SNI of the sciatic nerve. MMPIP failed to modify nociceptive responses and associated RVM ON and OFF cell activity in sham rats. An increase in mGluR7 gene, protein and staining, the latter being associated with vesicular glutamate transporter-positive profiles, has been found in the VL PAG in SNI rats. Blockade of mGluR7 within the VL PAG has an antinociceptive effect in formalin and neuropathic pain models. VL PAG mGluR7 blockade offers a target for dis-inhibiting the VL PAG-RVM pathway and silencing pain in inflammatory and neuropathic pain models.Molecular Pain 09/2013; 9(1):44. DOI:10.1186/1744-8069-9-44 · 3.53 Impact Factor