Intrathecal serotonin in mice: analgesia and inhibition of a spinal action of substance P.

Serotonin, administered intrathecally in mice, produced dose-related analgesia in the tail flick test and the subcutaneous hypertonic saline assay. Low doses (2.5-5 ng) of serotonin blocked the biting and scratching response elicited by intrathecal substance P. However, higher doses of serotonin itself elicited a behavioral syndrome characterized by scratching of the torso with the hindlimbs. Both the analgesic response and the scratching response due to serotonin were blocked by specific serotonin antagonists and the analgesia is likely mediated by a postsynaptic action on dorsal horn nociceptive neurons.