Activation of group III metabotropic glutamate receptors regulates stress-induced analgesia through an endocannabinoid mechanism
Gregg, Laura Corrine
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The endocannabinoid 2-arachidonoylglycerol (2-AG) produces adaptive changes in pain responses following exposure to environmental stressors. This phenomenon, termed stress-induced analgesia (SIA), is dependent upon mobilization and accumulation of 2-AG in the periaqueductal gray (PAG). 2-AG within the PAG may activate CB1 receptors on GABAergic neurons to reduce inhibition of output neurons that form part of a descending antinociceptive pathway. Alternatively, 2-AG may activate CB1 receptors on glutamatergic neurons to inhibit pro-nociceptive pathways that facilitate pain. 2-AG acts as a retrograde signal that binds to CB1 receptors to produce antinociception. However, the mechanisms contributing to the mobilization of 2-AG are only beginning to be discovered. We examined the role of presynaptic group III metabotropic glutamate receptors (mGluRs), which are negatively coupled to adenylyl cyclase and reduce GABAergic inhibition, on the phenomenon of SIA in the PAG. Microinjection into the dorsolateral PAG (dlPAG) of the group III mGluR agonst L-AP4 produced a dose-dependent enhancement of SIA through a CB1-dependent mechanism. By contrast, off-site injections of L-AP4 failed to enhance SIA. The L-AP4-induced enhancement of SIA was blocked by the group III mGluR antagonist UBP1112 at a dose that was found to be ineffective in modulating SIA when administered alone. Microinjection of the group III mGluR antagonist UBP1112 into the dlPAG produced a dose-dependent suppression of SIA and also blocked the enhancement of SIA induced by L-AP4. This effect involved the dorsolateral PAG because off-site injections failed to alter SIA. Our findings suggest a previously unrecognized role for group III mGluRs in controlling endocannabinoid-dependent stress-induced analgesia, presumably by controlling the mobilization of endocannabinoids, likely 2-AG, in the PAG.