Inducing a Preference for Morphine in Rats without Premedication

Abstract

“ADDICTION” to morphine and morphine-type drugs can be shown in laboratory animals in several ways. Among the most convincing¹ are experiments in which rats and monkeys are induced to administer the drug to themselves either by drinking solutions of it^2,3 or by pressing levers which release intravenous injections through an implanted catheter^4,5. Standard experimental procedures involve a period of premedication: daily injections of increasing doses of morphine are given for 2–3 weeks in order to make the animals “passively” dependent on the drug. Injections are then stopped and the animals have withdrawal symptoms^6,7 which are only relieved by more morphine; so they learn to seek and self-administer the drug. Monkeys will, however, learn to press levers for intravenous injections even without premedication, though such learning is much slower. Some monkeys will also voluntarily drink morphine solutions, which are bitter, but others persistently reject them unless pre-medicated (personal communication from Seevers). In rats, premedication has usually been considered essential for learning either kind of self-administration, but even premedicated rats will initially reject morphine solutions if given a choice between them and water, probably because of the bitter taste.