Matzel L D, Miller R R
Physiol Behav. 1987;39(1):135-40. doi: 10.1016/0031-9384(87)90410-0.
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.
三项实验研究了暴露于先前与足部电击配对的刺激下的大鼠条件性镇痛的发展情况。在实验1中，如果在进行甩尾镇痛测试之前，先让大鼠暴露于一个先前曾给予无信号电击的环境中90秒，那么甩尾潜伏期会增加。在测试当天，在暴露于该环境之前给予阿片类拮抗剂纳洛酮，这种镇痛作用就会被阻断。实验2确定，在测试前立即让大鼠暴露于条件环境中90秒和300秒，会引发类似的镇痛效果，这可通过对热刺激（热板）做出舔爪反应的潜伏期增加来衡量。然而，相对于未暴露于该环境的动物，在热板测试前立即让动物暴露于厌恶环境5秒，未观察到明显的镇痛效果。在实验3中，如果在5秒暴露与热板测试之间有85秒的延迟，那么让动物暴露于厌恶环境5秒所产生的镇痛效果与暴露90秒相当。这些结果表明，短暂暴露于先前与电击配对的刺激可以激活内源性阿片系统，但这些阿片类药物的镇痛作用会延迟。文中还讨论了内源性阿片类药物在学习中的作用。
