MOMENTARY ANALGESIA PRODUCED BY COPULATION IN FEMALE RATS

To assess possible changes in nociception during copulation in estrous rats, electric shocks that were 20% suprathreshold for eliciting voca lization in response to tail shock (STS), were applied to the tail bef ore the initiation of copulation and, thereafter, coincident with the onset of mounting bouts by the male (Experiment 1). Females vocalized significantly less during non-intromittive mounts (M; P < 0.001), intr omissions (I; P < 0.001), and ejaculation (E; P < 0.01) than before th e initiation of copulation. In order to assess the importance of vagin al stimulation (VS) by penile insertion during mating, in Experiment 2 30% STS were applied 300-400 ms after the initiation of mounting to e nsure that the stimuli fell within the period of penile insertion ocur ring during I and E. M failed to significantly inhibit vocalizations t o 30% STS. By contrast, both I and E markedly inhibited vocalizations in response to STS. This effect was transitory since subjects (Ss) voc alized to nearly all 30% STS when delivered 15 s after I or E. Copulat ory analgesia (CA) was abolished by the bilateral transection of the p elvic and hypogastric nerves but not by the transection of the pudenda l nerve (Experiment 3). The magnitude of CA was calibrated by determin ing the doses of morphine sulfate (MS) required to produce similar dec rements in vocalization to STS. The analgesic effects of I and E were equivalent to more than 10 mg/kg and 15 mg/kg, respectively, of MS (Ex periment 4). Pelvic-hypogastric neurectomy, but not pudendal neurectom y, also significantly reduced the effect of VS on facilitating lordosi s, inducing immobilization and hind leg extension, and blocking the wi thdrawal reflex to foot pinch (Experiment 5). Pelvic-hypogastric neure ctomy also significantly reduced sexual receptivity, as indicated by a reduction in the number of I that the females in this group received.