DIFFERENTIAL ANTINOCICEPTIVE EFFECTS OF YOHIMBINE IN THE RAT FORMALIN TEST

Although many pharmacological studies indicate that yohimbine antagonize the antinociceptive effects ofα2-adrenoceptor agonists, there are evidences that yohimbine by itself produces antinociceptive effects inthe formalin test. However, its site of action on nociceptive processing is not fully understood. In thisinvestigation, a series of experiments were designed to study the antinociceptive effects of intraperitoneal(i.p.), intraplantar (i.pl.) and intrathecal (i.t.) administration of yohimbine in the nociceptive processing.Yohimbine (2 and 4 mg/kg, i.p.) induced antinociception in the early phase (0-5 min) as well as in the latephase (10-60 min) of formalin test. While i.pl. yohimbine (5-100 μg) decreased the response in the earlyphase, i.t. yohimbine (30 μg) decreased pain behavior in the late phase of formalin test in rats.In conclusion, our findings show that yohimbine induces antinociception in both phases of formalin testand its effects are produced at least in part through actions at the peripheral terminal of primary afferentsor at the spinal level.