Recruitment time of conditioned opioid analgesia.

Three experiments examined the development of conditioned analgesia in rats exposed to stimuli that had previously been paired with footshock. In Experiment 1, tailflick latencies increased if the tailflick test for analgesia was immediately preceded by 90 sec of exposure to a context in which unsignaled shock had previously been administered. This analgesia was blocked by the opiate antagonist naloxone administered prior to exposure to the context on the test day. Experiment 2 determined that 90 and 300 sec of exposure to the conditioning context immediately prior to testing evoked comparable analgesia as indexed by increased latencies to pawlick in response to thermal stimulation (hotplate). However, no analgesia was evident in animals exposed to the aversive context for 5 sec immediately prior to the hotplate test relative to animals not exposed to that context. In Experiment 3, a 5-sec exposure to the aversive context produced analgesia comparable to a 90-sec exposure if an 85-sec delay intervened between the 5-sec exposure and the hotplate test. These results suggest that brief exposure to stimuli previously paired with shock can activate the endogenous opioid system, but the analgesic action of these opioids is delayed. Implications for the role of endogenous opioids in learning are discussed.