Neural mechanisms of drug reinforcement.

The brain substrates involved in the effect of cocaine on brain stimulation reward, in the psychomotor activation associated with cocaine, and in cocaine self-administration appear to be focused on the medial forebrain bundle and its connections with the basal forebrain, notably the nucleus accumbens. Chronic access to cocaine produces a withdrawal state as reflected in increases in brain stimulation reward thresholds, and this change in reward threshold appears to be opposite to the actions of the drug administered acutely. These effects are thought to reflect a change in the activity of reward elements in the medial forebrain bundle and may be responsible for the negative reinforcing state associated with the anhedonia of cocaine withdrawal. Opiate receptors particularly sensitive to the reinforcing effects of heroin also appear to be located in the region of the nucleus accumbens and the ventral tegmental area. There is good evidence for both dopamine-dependent and dopamine-independent opioid interactions in the ventral tegmental-nucleus accumbens connection. In addition, the opiate receptors in the region of the nucleus accumbens may become sensitized during the course of opiate withdrawal and thus become responsible for the aversive stimulus effects of opiate dependence. Reliable measures of the acute reinforcing effects of ethanol have been established in rat models, and substantial evidence exists to show that non-deprived rats will orally self-administer pharmacologically relevant amounts of ethanol in lever-press choice situations. Neuropharmacological studies of ethanol reinforcement in non-dependent rats suggest important roles for serotonin, GABA and dopamine. A role for opioid peptides in ethanol reinforcement may reflect more general actions of opioid peptides in consummatory behavior. Studies of ethanol dependence have implicated brain GABAergic and CRF systems in the more motivational aspects of withdrawal. Future studies will need to focus on the common neurobiologic changes associated with all these drugs, particularly regarding their hedonic and motivational properties.