ANTINOCICEPTIVE EFFECT OF INTRATHECALLY ADMINISTERED PITUITARY ADENYLATE-CYCLASE ACTIVATING POLYPEPTIDE (PACAP) ON THE RAT FORMALIN TEST

It has been reported that pituitary adenylate cyclase activating polypeptide (PACAP) is a sensory neuropeptide and that it is contained in capsaicin sensitive fibers. PACAP occurs as two variants, PACAP27 and the C-terminally extended PACAP38. We examined the effect of intrathecally administered PACAP27 and PACAP38 on the rat formalin test which is considered to be a model of animal pain induced by inflammation. Fifty microliters of 5% formalin was injected subcutaneously in the plantar surface of the right hind paw. To analyze the effect of PACAP27 and PACAP38, the instances of spontaneous flinching were counted, Administration of either PACAP27 or PACAP38 resulted in a monotonically dose-dependent reduction in the instances of flinching behavior. These results suggested that PACAP may play an important role in inhibiting the transmission of the nociceptive information evoked by inflammation.