Relapse to drug-seeking: neural and molecular mechanisms.

A central determinant of addictive disorders in people is increased risk of relapse to drug use even after prolonged periods of abstinence. Recent advances in animal models of relapse indicate that drug-seeking behavior can be triggered by priming injections of the drugs themselves, by drug-associated environmental stimuli, and by footshock stress. The neural mechanisms underlying this relapse can be viewed in general terms as drug-like or proponent processes. Considerable evidence points to the mesolimbic dopamine system, and more specifically to activation of D2-like dopamine receptors in the nucleus accumbens, as a crucial neural substrate utilized by various stimuli that induce relapse. Drug-associated stimuli and stress may activate this system via neural circuits from the prefrontal cortex and amygdala as well as via the hypothalamo-pituitary-adrenal axis. There is also evidence for dopamine-independent mechanisms in relapse as well. A major effort of current research is to identify the long-lasting neuroadaptations within these various brain regions that contribute to relapse in addicted people. One potential neuroadaptation is up-regulation of the cAMP pathway in the nucleus accumbens, which occurs after chronic drug exposure, and represents a drug-opposite or opponent process. Modulation of this system has been related directly to relapse to drug-seeking behavior. Given the long-lasting nature of increased risk of relapse, it is likely that the relevant neuroadaptations are mediated via drug-induced changes in gene expression. A detailed understanding of the neural and molecular basis of relapse will facilitate efforts to develop truly effective treatments and preventive measures.