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

BACKGROUND： Previous morphological studies have demonstrated that group Ⅲ metabotropic 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 group Ⅲ mGluRs 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 group Ⅲ mGluRs （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 10 μL, or the equivalent volume of normal saline, was administered by intrathecal injection into the L3-5 region of the spinal cord in the experimental and control groups, respectively. After 15 minutes, formalin （5%, 50 μL） 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： Group Ⅲ mGluRs are involved in the modulation of nociceptive signals, and their activation suppresses the transmission of nociceptive signals.

Intrathecal MK-801 inhibits formalin-induced activation of spinal p38-MAPK in rats

BACKGROUND： p38 mitogen-activated protein kinase （MAPK） plays an instrumental role in signal transduction from the cell surface to the nucleus, while subcutaneous injection of formalin can induce increased activation of spinal p38 MAPK. However, the mechanisms underlying the formalin-induced activation of spinal p38 MAPK in rats are unclear. OBJECTIVE： To observe the effects of N-methyl-D-aspartic acid （NMDA） receptor antagonist MK-801 on the formalin-induced activation of spinal p38 MAPK in rats. DESIGN, TIME AND SETTING： This randomized grouping, controlled animal experiment was performed at the Department of Physiology and Neurobiology, Shanxi Medical University between May and November 2007. MATERIALS： Forty eight healthy, adult Wistar rats were randomly divided into two groups： formalin ＋ normal saline （n = 12） and formalin ＋ MK-801 （n = 36）. The formalin ＋ MK-801 group was further divided into three subgroups according to the dosage of MK-801 （10, 50, and 100 nmol/L, 12 rats for each subgroup） METHODS： Following anesthesia, polyethylene tubing filled with sterile normal saline was implanted into the subarachnoid cavity. On postoperative days 5-8, rats received a 15 minute perfusion of normal saline or MK-801 （10, 50, and 100 nmol/L） in the formalin ＋ normal saline and formalin ＋ MK-801 groups, respectively, followed by formalin injection for the induction of nociceptive behavior. MAIN OUTCOME MEASURES： Detection of total p38 MAPK and of phosphorylated p38 MAPK by western Blot analysis; observation of nociceptive behaviors in the 1 hour after formalin injection. RESULTS： Western Blot analysis revealed that injection of formalin had no effect on total p38 MAPK expression but resulted in increased phosphorylation of p38 MAPK in the spinal cord. This increase was apparent after 5 minutes, peaked at 20 minutes, and thereafter descended and reached control levels after 45 minutes. Pretreatment with MK-801 （10, 50, 100 nmol/L） resulted in a dose-dependent reduction of p38 MAPK phosphorylation in the spinal cord, 20 minutes after formalin injection. Injection of 50 and 100 nmol/L MK-801 produced a suppression of the first phase of nociceptive behaviors, and all three doses of MK-801 resulted in dose-dependent inhibition of the second phase of nociceptive behaviors. CONCLUSION： The NMDA receptor participates in formalin-induced activation of p38 MAPK in the rat spinal cord.