The effects of analgesics on quickly-learned conditioned suppression in mice

Mice were found to exhibit a marked suppression of motor activity when placed into the same experimental chamber in which they had previously received electrical shocks. This suppression was found to be a stable response when mice, 6-8 weeks of age, received shocks of frequencies of 1/10 or 1/30 Hz, and their motility was examined 24 hr after the delivery of the shock in the same experimental chamber. Control experiments indicated that the mice exhibited similar degrees of locomotor activity, compared to non-shocked mice, that were placed into a different experimental chamber than from the one in which they had previously received electrical shocks. Thus, the suppression of motor activity appeared to be a conditioned emotional response to the environment previously associated with delivery of shock. Morphine, codeine, and pethidine, but not pentazocine and aminopyrine, attenuated the conditioned suppression in a dose-related manner in the shocked mice. The effect of morphine was completely antagonized by pretreatment with naloxone or levallorphan. Thus, it is possible that the opiate-induced reduction of conditioned suppression is mediated by opiate receptor sites.