Participation of opiatergic, GABAergic, and serotonergic systems in the expression of copulatory analgesia in male rats.

Copulation in the male rat provoked an abrupt and significant rise in the threshold to induce vocalization by electrical shock to the tail (copulatory analgesia, CA). The possible effect on CA of the intrathecal (IT) administration of receptor antagonists to neurotransmitters participating in nociception was ascertained in this study. CA was significantly reduced, though not abolished, by IT injections of either naloxone, picrotoxin, or methysergide, but not by strychnine or yohimbine. This analgesic effect was achieved without significantly altering copulatory behavior. Results suggest that both brain and spinal systems participate in the development of CA. Brain effects would be mediated by descending serotonergic fibers, although intrinsic spinal systems would involve both opiate and GABA interneurons.