Unconditioned stress-induced analgesia following exposure to brief footshock.

Four experiments examined the properties of unconditioned analgesia elicited by electric footshock stimuli using unconditioned stimulus (US) parameters typical of aversive conditioning paradigms. In all experiments, analgesia was inferred from the latency to paw lick in response to painful thermal stimulation in the hot-plate assay. In Experiment 1, rats exposed to a 1-s, 2-mA electric shock US showed significantly longer latencies to respond to painful thermal stimulation than nonshocked controls, whereas nonsignificant increases in response latencies were observed with 1-s shock USs of either 0.5 or 1.25 mA. In Experiment 2, rats exposed to a 2-mA electric shock US showed systematic increases in latencies to respond to painful thermal stimulation as the duration of the shock was varied between 0.5 and 2 s. Experiment 3 showed that this form of shock-induced analgesia was of short temporal duration. Specifically, significant increases in latencies to respond to painful thermal stimulation occurred 30 s but not 90 or 300 s following exposure to a 1-s, 2-mA shock US. Experiment 4 demonstrated that this form of analgesia was unaffected by pretreatment with the opiate receptor antagonist naloxone in dosages of 1, 5, 10, or 20 mg/kg. Finally, there was no evidence showing that environmental stimuli paired with shock presentations acquired the capacity to evoke analgesia as a conditioned response. The implications of shock-induced analgesia for the study of aversive conditioning and behavior are discussed.