Repeated exposures intensify rather than diminish the rewarding effects of amphetamine, morphine, and cocaine

Psychopharmacology (Berl). 1989;98(3):357-62. doi: 10.1007/BF00451687.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amphetamine / pharmacology*
  • Animals
  • Chlorides / pharmacology
  • Cocaine / pharmacology*
  • Conditioning, Operant / drug effects
  • Lithium / pharmacology
  • Lithium Chloride
  • Male
  • Morphine / pharmacology*
  • Rats
  • Rats, Inbred Strains
  • Reward*


  • Chlorides
  • Morphine
  • Lithium
  • Amphetamine
  • Lithium Chloride
  • Cocaine