Evidence from the Hoebel laboratory has suggested that a particular balance between DA and ACh may indeed exist within the NAc and that changes in this balance may contribute to alterations in ingestive behavior (for a perspective, see [11]). In rats, presentation of an aversively conditioned taste results in an increase of extracellular ACh in the NAc [60], in contrast to the resulting decrease in DA mentioned above. Moreover, it is becoming increasing apparent that discrete manipulations in the level of ACh activity in NAc can have profound effects on the motivation to eat. For example, hungry rats stop feeding if the DA/ACh balance in the NAc is tilted in favor of excess cholinergic tone (Fig. 2). In this study, rats were implanted with bilateral microdialysis probes in the NAc and allowed to feed ad libitum over the course of their night cycle. Probes implanted in control animals were perfused throughout the night with a standard perfusion medium (Ringer), whereas experimental subjects had the perfusion solution switched to one containing the indirect cholinergic agonist neostigmine after 3 hr. Results showed
