Effects of intrathecal naloxone and atropine on the nociceptive suppression induced by norepinephrine and serotonin at the spinal level in rats.

The interrelations among norepinephrine (NE), serotonin (5-HT), opiate-like substances (OLS), and acetylcholine (ACh) were investigated by using electrophysiological method combining with intrathecal (i.t.) injection. The results show that: (1) pretreatment with i.t. naloxone (Nal) completely reversed the NE-induced suppression of nociceptive discharges in parafascicular (PF) neurons, but partially reversed that of induced by i.t. 5-HT; (2) pretreatment with i.t. atropine (Atr) completely reversed the suppression induced by either NE or 5-HT. The results suggest that OLS may act as a necessary mediator for NE-induced suppression on the spinal transmission of nociceptive inputs, while it is only partially involved in the 5-HT-induced suppression, and moreover, that endogenous ACh is necessary for the performance of nociceptive suppression induced by either spinal NE or 5-HT administration.