Morphine attenuation of conditioned autoanalgesia: implications for theories of situation-specific tolerance to morphine analgesia.

The effect of morphine administration on the development of conditioned autoanalgesia was investigated in four experiments. Animals were administered either morphine or saline and then either exposed or not exposed to nociceptive stimulation. In Experiments 1, 2, and 4 the nociceptive stimulus to which animals were exposed was electric footshock, and in Experiment 3 it was thermal stimulation produced by exposure to a hot plate. It was found that morphine administration attenuated the development of conditioned autoanalgesia produced by exposure to 1-mA shock for 45 s when tests for conditioned autoanalgesia were conducted when animals were under the influence of saline or morphine (Experiments 1 and 2). Morphine also attenuated the conditioned autoanalgesia arising from exposure to 1-mA shock for 15 s, but only when the conditions for the development and expression of conditioned autoanalgesia were made optimal (Experiment 4). Morphine failed to block conditioning when animals were exposed to 2.5-mA shock for 180 s (Experiment 1). Morphine also attenuated conditioned autoanalgesia when animals were exposed to thermal stimulation (Experiment 3), with the degree of attenuation increasing as a function of the intensity of the nociceptive stimulus. The results are discussed in terms of their implications for theories of situation-specific tolerance to the analgesic effect of morphine.