Attenuation of formalin-induced nociceptive behaviors following local peripheral injection of gabapentin.

Gabapentin (GP) has been shown to have antihyperalgesic properties and the site of drug action is reported to be the central nervous system. The goal of the present study was to determine whether GP also has a peripheral site of action. Rats received intraplantar 20-microl injections of 6, 60 or 600 microg GP + 2% formalin, 300 or 600 microg S-(+)-3-isobutylgaba + 2% formalin, 600 microg R-(-)-3-isobutylgaba + 2% formalin or formalin alone. The two lower doses of GP significantly reduced flinching and lifting/licking behavior during phase 2; however, phase 1 behaviors were unaffected, 600 microg GP significantly reduced these nociceptive behaviors during both phases. 600 microg S-(+)-3-isobutylgaba also reduced formalin-induced nociceptive behaviors; however, 600 microg of the isomer R-(-)-3-isobutylgaba had no effect. The antihyperalgesic effect of GP (1) was not due to a systemic effect since animals injected with 600 microg GP in one hindpaw and 2% formalin into the contralateral hindpaw developed nociceptive behaviors which were no different than those seen in animals injected with formalin alone; (2) was not due to a local anesthetic effect since needle sticks within the drug-injected region evoked paw withdrawal behavior which was not different from pre-drug levels; (3) was blocked by 20 microl D-serine but not by L-serine. Although the mechanism of action of GP has yet to be elucidated, these results indicate that GP has a peripheral site of action and thus may offer a novel therapeutic agent for topical or local treatment of pain of peripheral origin.