Characterization of nociceptive responses and spinal releases of nitric oxide metabolites and glutamate evoked by different concentrations of formalin in rats.

A comparison was made of spontaneous nociceptive behaviors elicited by subcutaneous injection of formalin (0.5-10.0%) into the plantar or dorsal surface of the right hindpaw in rats. In the present study, we also examined the effect of paw formalin injection on the release of nitric oxide (NO) metabolites (nitrite/nitrate) and glutamate from the spinal cord in anesthetized rats using a dialysis probe placed in the lumbar subarachnoid space. Two distinct quantifiable behaviors indicative of pain were identified by formalin injected into both regions of the paw. There were no significant alterations in the number of flinches during the early and late phases induced by different regions of formalin injection. However, the early phase licking/biting activity evoked by formalin injection into the plantar surface of the paw was significantly higher than that evoked by formalin injected into the dorsal region. The maximum effect in the early and late phases was produced by 5.0% formalin injection into the dorsal and plantar paw. At a higher concentration (10.0%) of formalin, nociceptive behavioral responses were decreased except for the late phase flinching when injected into the dorsal paw. Injections of formalin (5.0%) into both regions of the paw evoked a biphasic spinal release of nitrite/nitrate with a significant increase during the early phase (0-10 min) and the late phase (30-80 or 90 min). A higher concentration of formalin (10.0%) failed to produce a clear-cut release of nitrite/nitrate. A significant increase of glutamate was observed in the 0-10 min samples obtained after injection of formalin (5.0%) into the plantar and dorsal surface of the paw, whereas 0.5 and 10.0% formalin induced no substantial release. These results suggest that 5.0% formalin should be used when studying antinociceptive activity of NO- and N-methyl-D-aspartate-related compounds in the formalin test in rats. Formalin injection into the plantar surface of the paw might prove to be useful for evoking the licking/biting response, particularly in the early phase.