Research techniques based upon the behavioral principles of operant conditioning have provided a complementary approach to the standard pharmacologic analysis of physical dependence upon opiates (SEEvE~S 1936(SEEvE~S , 1958)). With these techniques, a simple arbitrary response is conditioned by following it with a reinforcement. For example, a food deprived rhesus monkey learns to press a lever because pressing a ]ever leads to a pellet of food. By the same token, if a lever pressing response is followed by the injection of an opiate to a physically dependent animal, that response will be learned.
Using the general principle outlined above, several investigators have recently demonstrated that it is possible to condition physically dependent rats to emit operant responses for morphine reinforcement. NichoLs and co-workers (1956, 1959), have shown that dependent rats will learn to drink water containing morphine rather than the initially preferred morphine-free tap water. W~Ks (1960, ]962) has recently shown that unrestrained rats can be conditioned to emit a lever-pressing response to receive intravenously infused morphine on a fixed ratio reinforcement schedule. In addition, he has shown that pretreatment with nalorphine produces an increase in rate of responding for the drug, presumably produced by the induced abstinence syndrome.