DOPAMINERGIC CORRELATES OF MOTIVATED BEHAVIOR - IMPORTANCE OF DRIVE

In vivo brain microdialysis was used to monitor changes in dopamine (D A) release in the nucleus accumbens (NAc) during anticipatory and cons ummatory components of feeding behavior, During 10 daily training sess ions, rats were first confined to one compartment of a testing chamber for 10 minutes, During this period (anticipatory phase) they were pre vented from gaining access to a highly palatable liquid meal by a wire mesh screen, The screen was then removed and the animals were permitt ed to consume the meal for 20 min (consummatory phase), On removal of the screen, the latency to begin drinking decreased and the amount con sumed increased as a function of days of training, both measures reach ing asymptotic levels by day 7, Trained animals were implanted with di alysis probes in the NAc on day 10, and on day 12 DA release was monit ored during the feeding session, Compared to controls, trained animals failed to show significantly greater increases in accumbal DA release during the anticipatory phase, all groups showing small (similar to 1 0%) increases on being placed in the test chamber, In contrast, compar ed to controls, DA release increased significantly in the NAc during c onsumption of the palatable meal, The magnitude of this increase was s ignificantly enhanced (30% vs 71% peak increase) in animals that were 20 hr food deprived at the time of testing, The latter animals also sh owed a statistically significant increase (24%) in DA release during t he anticipatory phase, A subsequent experiment in which consumption of the palatable liquid was limited to 5 mi in deprived and nondeprived animals indicated that only part of the deprivation-induced potentiati on of accumbal DA release could be attributed to the larger volume con sumed by the deprived animals, That is, the same volume and rate of co nsumption of a small amount of the liquid diet produced a significantl y greater increase in accumbal DA release in deprived than in nondepri ved animals (42% vs 23% peak increase), Feeding-induced increases in a ccumbal DA release were not due to postingestional factors as direct i njections of the liquid diet into the stomach by gavage failed to prod uce this effect, The results of these experiments indicate (1) that co nsummatory rather than anticipatory aspects of feeding are robustly as sociated with increases in DA release in the NAc, and (2) that motivat ional state can influence the magnitude of the neurochemical events th at are associated with goal-directed behaviors.