Dopamine, neuroleptics and reinforced behavior.

Animals working for any one of a variety of positive reinforcers (food, water, brain-stimulation...) produce dramatic decrements in operant performance when challenged with dopamine antagonist neuroleptic drugs. This well-established fact has generated considerable research aimed at identifying the precise nature of the drug-induced behavioral impairment. The two most oft-cited hypotheses suggest that much of the reduction in operant responding can be accounted for by either an "anhedonic" or "motoric" consequence of dopamine antagonism. Several novel behavioral paradigms are described which were devised to more clearly elucidate both the motor and reward impairing qualities of neuroleptic agents. Motor performance was assessed in a food-reinforced task in which a computer-operated force-transducer was used to obtain detailed analyses of the biophysical properties of operant responding. These studies suggested that neuroleptics impair the temporal nature (i.e., "timing") of normal operant behavior and not the physical capacity of the animals (i.e., the ability to emit "force"). This identification of a robust "slowing" effect of neuroleptic challenge, suggested that the investigation of putative "reward" impairments should best be conducted with behavioral paradigms in which the test data are collected from undrugged animals. Three such paradigms are described: a partial reinforcement extinction test, an incentive motivation test, and a conditioned place preference test. To date, our results suggest that when motor confounds are avoided, dopamine antagonist drugs can still produce patterns of operant behavior that very closely resemble those observed with actual reductions in reward magnitude. Such data provide support for the contention that central dopaminergic substrates play a role in the neurobiology of positive reinforcement.