Involvement of group Ⅲ metabotropic glutamate receptors in the modulation of spinal nociceptive signals

BACKGROUND:Previous morphological studies have demonstrated that groupⅢmetabotropic glutamate receptors(mGluRs) are commonly found in nociceptive pathways,particularly in the terminals of primary afferent fibers in the spinal dorsal horn. OBJECTIVE:To investigate the role of groupⅢmGluRs in a rat model of spinal nociception by intrathecal administration of a selective agonist,L-Serine-O-phosphate(L-SOP). DESIGN,TIME AND SETTING:A randomized,controlled,animal experiment.The study was performed at the Department of Physiology and Neurobiology,Shanxi Medical University,between March 2007 and May 2008. MATERIALS:L-SOP of groupⅢmGluRs(Tocris Cookson Ltd,UK),formalin(Sigma,USA),rabbit anti-c-Fos polyclonal antibody and biotin-labeled goat anti-rabbit IgG(Cell Signaling Technology, USA) were used in this study. METHODS:A total of 26 healthy Wistar rats,aged 1 month and weighing 100-120 g,were subjected to intrathecal catheter implantation.After 5-8 days,10 rats were selected according to experimental requirements.L-SOP 250 nmol in 10μL,or the equivalent volume of normal saline,was administered by intrathecal injection into the Lregion of the spinal cord in the experimental and control groups,respectively.After 15 minutes,formalin(5%,50μL) was subcutaneously injected into the plantar of the left hindpaw of each rat to establish formalin-induced pain models. MAIN OUTCOME MEASURES:Nociceptive behavioral responses and immunohistochemical examination of Fos expression. RESULTS:Intrathecal injection of L-SOP significantly attenuated the second phase nociceptive response compared with the control group(P<0.05),and Fos expression in the spinal dorsal horn was significantly decreased along with the number of Fos-like immunoreactive neurons(P<0.05). CONCLUSION:GroupⅢmGluRs are involved in the modulation of nociceptive signals,and their activation suppresses the transmission of nociceptive signals.