ANIMAL-MODELS OF DRUG CRAVING

Drug craving, the desire to experience the effect(s) of a previously e xperienced psychoactive substance, has been hypothesized to contribute significantly to continued drug use and relapse after a period of abs tinence in humans. In more theoretical terms, drug craving can be conc eptualized within the framework of incentive motivational theories of behavior and be defined as the incentive motivation to self-administer a psychoactive substance. The incentive-motivational value of drugs i s hypothesized to be determined by a continuous interaction between th e hedonic rewarding properties of drugs (incentive) and the motivation al state of the organism (organismic state). In drug-dependent individ uals, the incentive-motivational value of drugs (i.e., drug craving) i s greater compared to non-drug-dependent individuals due to the motiva tional state (i.e., withdrawal) developed with repeated drug administr ation. In this conceptual framework, animal models of drug craving wou ld reflect two aspects of the incentive motivation to self-administer a psychoactive substance. One aspect would be the unconditioned incent ive (reinforcing) value of the drug itself. The other aspect would be relatively independent of the direct (unconditioned) incentive value o f the drug itself and could be reflected in the ability of previously neutral stimuli to acquire conditioned incentive properties that could elicit drug-seeking and drug-taking behavior. Animal models of drug c raving that permit the investigation of the behavioral and neurobiolog ical components of these two aspects of drug craving are reviewed and evaluated. The models reviewed are the progressive ratio, choice, exti nction, conditioned reinforcement and second-order schedule paradigms. These animal models are evaluated according to two criteria that are established herein as necessary and sufficient criteria for the evalua tion of animal models of human psychopathology: reliability and predic tive validity. The development of animal models of drug craving will h ave heuristic value and allow a systematic investigation of the neurob iological mechanisms of craving.