Evoked potential conditioning using morphine as the unconditioned stimulus in rats

It is believed that conditioning processes play an important role in the development of narcotic dependence and may be particularly responsible for relapse after treatment. In addition, it was suggested that this conditioning is reflected by altered neural responses to stimuli which were associated with drug administration. To test these hypotheses, evoked potentials were recorded from four regions of rats' brains during a differential classical conditioning procedure. The conditioned and unconditioned effects of morphine on neural responses in each of these regions were studied during both addiction and withdrawal. The conditioning procedure resulted in alterations in the cortical response to the stimulus which was associated with morphine, but no alterations occurred in the response to the differential stimulus. Furthermore, the conditioned cortical response was found to persist after withdrawal from morphine. However, possibly due to state-dependency effects, the conditioned response was expressed only when the subjects were morphine intoxicated. Thus, the conditioned neural response found in this experiment did not reflect the processes postulated to lead to relapse of drug-free addicts.