Opiate-like substances mediate norepinephrine-induced but not serotonin-induced antinociception at spinal level - Reevaluation by an electrophysiological model of formalin test in rats

After subcutaneous injection of formalin (5%, 50 mul) into a hindpaw of rat s, biphasic excitatory nociceptive discharges were recorded extracellularly in thalamic parafascicular neurons. Intrathecal (i.t.) administration of e ither norepinephrine (NE, 6 nmol, 10 mul) or serotonin (5-HT, 120 nmol, 10 mul) prior to the second phase significantly inhibited the second phase of the formalin-induced parafascicular nociceptive discharges. Intrathecal nal oxone (Nal, 50 nmol, 10 mul) did not show any effect on the parafascicular nociceptive discharges. However, when i.t. Nal was given 5 min before NE, N al prevented the NE antinociceptive effect. Pre-administration of Nal befor e 5-HT did not affect the antinociceptive effects of 5-HT on the second pha se of nociceptive discharges. These results indicate that opiate-like subst ances are involved in the mediation of NE-induced antinociception. It is su ggested that endogenous NE and 5-HT released from brainstem descending term inals at the spinal level carry out their antinociceptive actions different ly. (C) 2001 Elsevier Science Inc. All rights reserved.