title: Repeated exposures intensify rather than diminish the rewarding effects of amphetamine, morphine, and cocaine.
abstract: It is commonly believed that repeated exposures diminish the pleasurable effects of drugs and hence that pleasure must have only a minor role in addiction. In six experiments with rats, repeated exposures to amphetamine, morphine, or cocaine were found to enhance the drug-induced rewarding effect as measured by conditioned place preference. Thus, sensitization to the rewarding effect, rather than tolerance, was obtained. Also, cross-sensitization was obtained; exposures to amphetamine enhanced the rewarding effect of morphine and vice versa; similarly, exposures to morphine enhanced the rewarding effect of cocaine. These findings support a new theory: drugs of abuse are addictive because repeated exposures sensitize the central reward mechanism so that drug taking produces a progressively greater reinforcing effect each time it occurs.
authors.affiliation: Department of Psychology, Memorial University of Newfoundland, St. John's, Canada.
authors.forename: B T
authors.initials: BT
authors.lastname: Lett
mesh_terms: D000661:Amphetamine / Q000494:pharmacology*; D000818:Animals; D002712:Chlorides / Q000494:pharmacology; D003042:Cocaine / Q000494:pharmacology*; D003216:Conditioning, Operant / Q000187:drug effects; D008094:Lithium / Q000494:pharmacology; D018021:Lithium Chloride; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D012201:Reward
chemical_list: D002712:Chlorides; D009020:Morphine; D008094:Lithium; D000661:Amphetamine; D018021:Lithium Chloride; D003042:Cocaine