Serotonin contributes to the spinal antinociceptive effects of morphine.

This study was designed to determine if morphine administered intrathecally (IT) interacts with serotonergic or noradrenergic nerve terminals in the spinal cord to produce analgesia on the spinally mediated tail-flick test. Male Sprague-Dawley rats were fitted with IT catheters. One week later, animals were spinally pretreated with receptor antagonists selective for opioid, serotonin or alpha-adrenoceptors, and the ability of these agents to alter spinal morphine-induced antinociception was assessed. Morphine dose-dependently elevated tail-flick latency in a naltrexone-reversible manner. The serotonin receptor antagonists spiroxatrine (5-HT1A), pindolol (5-HT1B), ritanserin (5-HT2) and ICS 205-930 (5-HT3) attenuated the spinal analgesic effects of morphine. In contrast, the alpha 1 and alpha 2-adrenoceptor antagonists prazosin and yohimbine, respectively, did not alter morphine-induced elevations in tail-flick latency. These data substantiate earlier reports that spinal morphine-induced antinociception relies on an opioid receptor-mediated component in addition to a local serotonergic component. The finding that the alpha-adrenoceptor antagonists did not alter the antinociceptive effects of IT morphine suggests that spinal norepinephrine does not contribute to the analgesic effects of the opiate.