Studies have shown that
5-hydroxytryptamine (5-HT) plays an important role in the descending pathway of
pain modulation from brainstem to the spinal cord. Using selective
5-HT receptor antagonists, the present study investigated which type of
5-HT receptor(s) in the spinal cord was involved in the
morphine-induced anti-nociception in intact rats, in rats with nerve injury and in rats with
inflammation. The hindpaw withdrawal latencies decreased significantly after sciatic nerve injury and hindpaw
inflammation compared with intact rats. Intrathecal administration of 25 or 10 microg of the selective 5-HT(1A) recepter antagonist
spiroxatrine, but not 1 microg of
spiroxatrine, significantly blocked the increased hindpaw withdrawal latencies to thermal and mechanical stimulation induced by intra-periaqueductal gray injection of 1 microg of
morphine in intact rats.
Intrathecal injection of the 5-HT(2) receptor antagonist
RS 102221 and the 5-HT(3) receptor antagonist
MDL 72222 had no significant effects on the increased hindpaw withdrawal latencies to both noxious stimulations induced by intra-periaqueductal gray injection of
morphine. Furthermore, intrathecal administration of
spiroxatrine, but not
RS 102221 nor
MDL 72222, significantly attenuated the increased hindpaw withdrawal latencies induced by intra-periaqueductal gray administration of
morphine in rats with nerve injury and in rats with
inflammation. The results demonstrate that the
5-HT(1A) receptor, not 5-HT(2) nor 5-HT(3) receptor, plays an important role in the descending pathway of anti-nociception from the brainstem to the spinal cord in intact rats, in rats with nerve injury and in rats with
inflammation.