Acute thermal hyperalgesia in the rat is produced by activation of N-methyl-D-aspartate receptors and protein kinase C and production of nitric oxide.

There is general agreement that activation of the N-methyl-D-aspartate receptor is involved in thermal hyperalgesia. However, there is less agreement on the specific intracellular events subsequent to receptor activation and the involvement of other excitatory amino acid receptors in thermal hyperalgesia. In the present study, we found that the intrathecal administration of N-methyl-D-aspartate produced a dose- (1 fmol-1 pmol) and time-dependent thermal hyperalgesia. In contrast, over the dose range tested, intrathecal administration of either alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionate (AMPA; 10 fmol-100 pmol), 1,3-trans-1-aminocyclopentyl-1,3-dicarboxylate (10 fmol-100 pmol), quisqualate (10 pmol-5 nmol) or a 1:1 combination of AMPA and 1,3-trans-1-aminocyclopentyl-1,3-dicarboxylate (total dose 20 fmol-200 pmol) did not produce any evidence of thermal hyperalgesia; greater doses produced a caudally-directed biting and scratching behavior that precluded testing in the paradigm used. A fixed dose of 1,3-trans-1-aminocyclopentyl-1,3-dicarboxylate (100 pmol) did, however, potentiate the effects of N-methyl-D-aspartate (1-100 fmol). Thermal hyperalgesia produced by N-methyl-D-aspartate (1 pmol) was attenuated by intrathecal administration of the N-methyl-D-aspartate receptor-selective antagonist 2-amino-5-phosphonopentanoate (100 pmol), but not by the AMPA receptor-selective antagonist 6,7-dinitroquinoxaline-2,3-dione (1 nmol) or the metabotropic receptor antagonist 2-amino-3-phosphonoproprionate (10 nmol). In a second series of experiments, we examined the role of different signal transduction systems in acute N-methyl-D-aspartate-produced thermal hyperalgesia. N-Methyl-D-aspartate-produced thermal hyperalgesia (1 pmol) was attenuated by intrathecal hemoglobin (1-100 pmol) and dose-dependently by intrathecal N(G)-nitro-L-arginine methyl ester (10 pmol-l nmol), Methylene Blue (10 pmol-l nmol) and chelerythrine (1-100 pmol), suggesting that acute N-methyl-D-aspartate-mediated thermal hyperalgesia involves activation of nitric oxide synthase and protein kinase C. In contrast, N-methyl-D-aspartate-produced thermal hyperalgesia was unaffected by intrathecal administration of the phospholipase A2 inhibitor mepacrine (10 nmol) or the phospholipase C inhibitor neomycin (10 nmol). While prostaglandins and leukotrienes have been suggested to play a role in hyperalgesia, N-methyl-D-aspartate-produced thermal hyperalgesia (1 pmol) was unaffected by the non-selective eicosanoid inhibitor nordihydroguaiarate (1 nmol), the cyclo-oxygenase selective inhibitor indomethacin (10 nmol) or the lipoxygenase selective inhibitor baicalein (1 nmol). The results of the present study suggest that acute thermal hyperalgesia can be produced by activation of N-methyl-D-aspartate receptors. Activation of AMPA, metabotropic or co-activation of AMPA and metabotropic glutamate receptors, at the doses tested, did not produce an acute thermal hyperalgesia. The thermal hyperalgesia produced by N-methyl-D-aspartate is mediated by activation of nitric oxide synthase and protein kinase C, but not by phospholipase C, phospholipase A2, cyclo-oxygenase or lipoxygenase. Collectively, the results are consistent with a role for spinal N-methyl-D-aspartate receptors, nitric oxide and protein kinase C in thermal hyperalgesia.