Changes in Cannabinoid Receptor Subtype 1 Activity and Interaction with Metabotropic Glutamate Subtype 5 Receptors in the Periaqueductal Gray- Rostral Ventromedial Medulla Pathway in a Rodent Neuropathic Pain Model

Enza      Palazzo; Livio      Luongo; Giulia      Bellini; Francesca      Guida; Ida      Marabese; Serena      Boccella; Francesca      Rossi; Sabatino      Maione; Vito      de Novellis

Abstract

This study analyzed the effect of intra-ventrolateral periaqueductal grey (VL PAG) cannabinoid receptor (CB) stimulation on pain responses and rostral ventromedial medulla (RVM) neural activity in the chronic constriction injury (CCI) model of neuropathic pain in rats. Interaction between CB₁ and metabotropic glutamate 1 and 5 (mGlu₁/mGlu_{5) receptors was also investigated together with the expression of the CB₁ receptor associated Gai3 and cannabinoid receptor interacting 1a (CRIP 1a) proteins and the endocannabinoid synthesising and hydrolysing enzymes.}

In rats not subjected to CCI-induced pain, intra-VL PAG (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl) pyrrolo[1,2,3-de]-1,4-benzoxazin-6-yl]-1-naphthalenylmethanone mesylate (WIN 55,212-2) (2-4-8 nmol), a CB receptor agonist, increased the tail flick latency and changed the ongoing activity of RVM OFF and the tail flick-related activity of the ON and OFF cells, accordingly. These effects were prevented by SR141716A and MPEP, selective CB₁ and mGlu₅ receptor antagonists, respectively, though not by CPCCOEt, a selective mGlu₁ receptor antagonist. A higher dose up to 16 nmol of WIN 55,212-2 was necessary to increase tail flick latency and change ON and OFF cell activity in CCI rats. Consistently, CCI rats showed a decrease in the expression of CB₁ receptors, NAPE-PLD, Gαi3 and CRIP 1a proteins;the expression of diacylglycerol lipase A (DAGLA) was increased while fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MGL) did not change. As in control rats, MPEP and SR141716A also blocked WIN 55,212-2- induced effects in CCI rats. These data demonstrate a down regulation of the endocannabinoid system and a functional interaction between mGlu5 and CB₁ receptors for cannabinoid-mediated effect in the PAG-RVM pain circuitry in neuropathic pain inflicted rats.

Keywords: Cronic constriction injury, cannabinoid recepors, metabotropic glutamate receptors, periaqueductal grey, ON and OFF cell, neuropathic pain, gray-rostral ventromedical medulla pathway, RVM, microinjections, RT-PCR, Western blot, Immunochemistry