Comparison of the effects of morphine and immobilization stress on discrimination performance of rats

The effects of morphine and those of immobilization stress (IMS) on performance of rats in discrete-trial, two-choice discrimination tasks were compared. In Experiments 1 and 2, two shocks of different intensities were discriminative stimuli (SD) for choices in a T-maze. In Experiments 3 and 4, responses were right and left lever presses in an operant chamber, where correct responses were signaled by either shocks of different intensities (shock discrimination task) or by the position of a brief light flash (light discrimination task). In Experiment 1, both 70-min IMS and 5 mg/kg morphine induced greater disruptions in trials in which the higher shock was the SD, and there were no significant differences between treatments. However, in Experiment 2, only the effects of morphine were blocked by 10 mg/kg naltrexone. In Experiment 3, the effects of IMS and morphine were not selective with respect to SD shock level in the shock discrimination task, nor were they task (shock vs. light) selective. Also, the effect of morphine was significantly greater than the effect of IMS in both tasks. The results of Experiment 4 indicated that effects induced by actual alterations in the intensities of the SD shocks did not mimic those induced by either morphine or IMS but that abrupt decreases in the duration of the SD lights induced effects similar to those of morphine. These studies indicate that different mechanisms mediate the effects of IMS and morphine and much of their behavioral effects in shock discrimination tasks are due to dissociative processes rather than alterations in perceived shock intensity.