Non-opiate analgesia following stressful acoustic stimulation.

The change in the nociceptive reactions of rats was characterized after stressful acoustic (115 dB) stimulation. Acoustic loading for five minutes resulted in considerable analgesia in the hot-plate test, whereas a significant analgesic response was not observed in the tail-flick test. The analgesic reaction after acoustic stimulation was resistant to naloxone pretreatment and was also found in morphine-tolerant rats, but the acute thermoregulatory and analgesic effects of morphine were greatly potentiated by simultaneous acoustic loading. Substance P or cholecystokinin treatment likewise failed to prevent the analgesic effect of auditory stimulation. No tolerance developed to the analgesic effect on repeated stressing. Diltiazem, a slow calcium channel blocker, facilitated the analgesia. The data suggest a stress-induced analgesia with obviously non-opiate properties, although an indirect involvement of opiate effects could not be excluded.