Gabapentin is a novel anti-epileptic drug, which is used in clinical practice to treat epilepsy. This drug is also used as an analgesic in pain patients. The antinociceptive effect of this drug was assessed using the formalin test in the rat.

In order to investigate the effects of gabapentin on the trigeminal nerve territory, we injected 0.5 % formalin into the upper lip. Adult, male, Sprague-Dawley rats received a 50mul subcutaneous injection of 5% formalin into one vibrissal pad and the consequent, facial grooming behavior was monitored. Consistent with previous investigations using the formalin model, animals exhibited biphasic nocifensive grooming (phase 1, 0-12 min; phase 2, 12-60 min).

The intraperitoneal administration gabapentin 5 minutes prior to the formalin injection led to a significant, dose-dependent reduction in grooming time during phase 2. In high doses, gabapentin also reduced the time of grooming during phase 1.

The Intraperitoneal injection of gabapentin has an analgesic effect in the facial formalin rat model and this analgesic effect increases dose-dependently.