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 vocalization in response to tail shock (STS), were applied to the tail before 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), intromissions (I; P < 0.001), and ejaculation (E; P < 0.01) than before the initiation of copulation. In order to assess the importance of vaginal stimulation (VS) by penile insertion during mating, in Experiment 2 30% STS were applied 300-400 ms after the initiation of mounting to ensure that the stimuli fell within the period of penile insertion occurring during I and E. M failed to significantly inhibit vocalizations to 30% STS. By contrast, both I and E markedly inhibited vocalizations in response to STS. This effect was transitory since subjects (Ss) vocalized to nearly all 30% STS when delivered 15 s after I or E. Copulatory analgesia (CA) was abolished by the bilateral transection of the pelvic and hypogastric nerves but not by the transection of the pudendal nerve (Experiment 3). The magnitude of CA was calibrated by determining the doses of morphine sulfate (MS) required to produce similar decrements 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 (Experiment 4). Pelvic-hypogastric neurectomy, but not pudendal neurectomy, also significantly reduced the effect of VS on facilitating lordosis, inducing immobilization and hind leg extension, and blocking the withdrawal reflex to foot pinch (Experiment 5).(ABSTRACT TRUNCATED AT 250 WORDS)