Enhanced acetylcholine release in hippocampus and cortex during the anticipation and consumption of a palatable meal

In rats trained for 14 days to consume a palatable liquid chocolate meal (Sustacal), in vivo brain microdialysis was used to measure release of acetylcholine in the frontal cortex and hippocampus during anticipation and consumption of the meal. Rats were trained in an experimental chamber in which they were separated from the Sustacal by a screen for 20 min (trained, rewarded group). The screen was then removed and the rats were allowed 20 min of access to the meal. Two control groups were run concurrently: these groups consisted of rats (i) that were trained over 14 days but only had access to water in the experimental chamber (trained, non-rewarded), or (ii) that were introduced into the experimental chamber for the first time on the final test (i.e. dialysis) session, and presented with Sustacal (naive). Different results were obtained in the hippocampus and frontal cortex. In the hippocampus there were no group differences with respect to acetylcholine release. Thus, in all three groups acetylcholine release increased to about 220% of basal values when animals were placed in the experimental chamber. In the frontal cortex, acetylcholine release also increased significantly in all three groups. However, the extent of this increase was significantly greater in the trained, rewarded group, reaching approximately 300% of basal values during the anticipatory and consummatory components of the task. The significant increases in acetylcholine release which occurred in both the hippocampus and frontal cortex of each of the three groups are consistent with an involvement of cholinergic basal forebrain neurons in the regulation of arousal or attention. In addition, however, acetylcholine release in the frontal cortex can be further selectively enhanced by the animal's past training experience, perhaps being associated with the anticipation of reward.