Effect of emotions on nociceptive threshold in rats

Tail-flick latency was measured in male rats exposed to inescapable footshock for 2 min (0.8 mA, 1 sec every 5 sec) in rats that witnessed effects of footshock on other rats and in rats exposed to a strong auditory stimulus (8 kHz, 96 db SPL) either intermittently (1 sec every 5 sec) or continuously for 2 min. A significant increase in tail-flick latency occurred in rats exposed to footshock as well as in rats that only witnessed footshock. Continuous auditory stimulation also had an analgesic effect, while intermittent stimulation similar to the sound produced by rats given footshock failed to affect tail-flick latency significantly. The analgesic effect of witnessing footshock habituated rapidly and was counteracted by naloxone (10 mg/kg IP), while naloxone failed to affect audiogenic analgesia. Analgesia produced by witnessing a noxious treatment is probably mediated by an emotional reaction. It is suggested that the phenomenon be termed "emotiogenic analgesia"