Functional interaction between N-methyl-D-aspartate receptor and ascorbic acid during neuropathic pain induced by chronic constriction injury of the sciatic nerve.

BACKGROUND: Neuropathic pain is a chronic pain condition, which is resistant to therapy. Ascorbate was released because of the activation of glutaminergic neurons. Due to the important role of N-methyl-D-aspartate (NMDA) receptors in the pathophysiology of neuropathic pain, this study investigated the analgesic efficacy of ascorbic acid (AA) in neuropathic pain condition and the role of NMDA receptors in this effect.

METHODS: For this purpose, adult male rats were randomly allocated to experimental groups (n=8 in each group). Neuropathic pain was induced by chronic constriction injury (CCI) of the sciatic nerve. During the second week after CCI, animals received a single injection of 1, 3, 5, or 10 mg/kg of AA intraperitoneally and pain threshold was determined 15 and 60 min later. The antinociceptive effect of chronic administration was also evaluated by intraperitoneal injection (IP) of 3 mg/kg AA for 3 weeks. To determine the role of NMDA receptors, separate groups of animals 30 min after single injection of AA (1 mg/kg) animals received i.p. injection of ketamine (5 mg/kg), MK-801 (0.01 mg/kg), or glutamate (1000 nmol) and were tested 20 min afterwards. Data analyzed by ANOVA and Newman-Keuls tests and p<0.05 were considered as significant.

RESULTS: IP of 3, 5 and 10 mg/kg increased the pain threshold during the second week after CCI (p<0.05, F=3 in tactile allodynia and p<0.01, F=3.2 in thermal and mechanical hyperalgeisa). Chronic administration of AA also produced antinociceptive effect. Ascorbic acid (1 mg/kg, i.p.) inhibited MK-801 and ketamine-induced antinociception response significantly (p<0.001, F=2). It also prevented the analgesic effect of glutamate administration (p<0.001, F=2).

CONCLUSIONS: The results indicated that AA produced a dose-dependent antinociceptive effect that seems to mediate through its interaction with NMDA receptors.