PURPOSE: In a previous study using the tail-flick test, we found that intracerebroventricular administration of D-serine, an endogenous co-agonist at the glycine sites of N-methyl-D-aspartate (NMDA) receptors, elicited an antinociceptive effect on thermal nociception. The purpose of the present study was to evaluate the effect of intracerebroventricular administration of D-serine on nociception induced by tissue damage or inflammation using the formalin test. METHODS: Infusion of drugs into the third ventricle in rat was performed via indwelling cannulae. Drugs were infused at a volume of 10 μl over 2 min, and the infusion cannula was left in place for 2 min before removal. The formalin test was performed 10 min after drug administration. RESULTS: Intracerebroventricular administration of D-serine significantly and dose-dependently decreased the number of flinches in both the early and late phases in the formalin test. This antinociceptive effect was antagonized by intracerebroventricular administration of L-701,324, a selective antagonist at the glycine sites of NMDA receptors. CONCLUSION: The present data suggest that activation of NMDA receptors via glycine sites at the supraspinal level induces an antinociceptive effect on both acute and tonic pain.
PURPOSE: In a previous study using the tail-flick test, we found that intracerebroventricular administration of D-serine, an endogenous co-agonist at the glycine sites of N-methyl-D-aspartate (NMDA) receptors, elicited an antinociceptive effect on thermal nociception. The purpose of the present study was to evaluate the effect of intracerebroventricular administration of D-serine on nociception induced by tissue damage or inflammation using the formalin test. METHODS: Infusion of drugs into the third ventricle in rat was performed via indwelling cannulae. Drugs were infused at a volume of 10 μl over 2 min, and the infusion cannula was left in place for 2 min before removal. The formalin test was performed 10 min after drug administration. RESULTS: Intracerebroventricular administration of D-serine significantly and dose-dependently decreased the number of flinches in both the early and late phases in the formalin test. This antinociceptive effect was antagonized by intracerebroventricular administration of L-701,324, a selective antagonist at the glycine sites of NMDA receptors. CONCLUSION: The present data suggest that activation of NMDA receptors via glycine sites at the supraspinal level induces an antinociceptive effect on both acute and tonic pain.