The influence of amphetamine on preference for lateral hypothalamic versus prefrontal cortex or ventral tegmental...

Rats were trained to bar-press for intermittent reinforcement on a concurrent schedule offering self-stimulation (SS) at the animal's choice of one of two different brain loci. On the concurrent schedule, the relative reward value of the two reinforcers is evaluated by the way the subject divides its session time responding for these reinforcers, thus yielding a rate-free measure of reward in addition to response rate data. In animals with electrodes in the lateral hypothalamus (LH) and prefrontal cortex (PFC), amphetamine dose-dependently increased response rates as well as the proportion of time allotted to LH stimulation, demonstrating that the reward value of LH stimulation was increased relative to PFC stimulation. This finding supports the hypothesis that DA systems modulate the rewarding value of LH but not PFC SS, and it suggests that differing neural mechanisms underlie these two behaviors. In animals with LH/ventral tegmental area (VTA) implants, amphetamine had no effect on preference, although it produced an overall increase in rate. This suggests that the drug elevates the rewarding value of LH and VTA stimulation to a similar degree, and that the two regions may have a common DA-related reward substrate. Finally, it was found that when the two reinforcers were equally preferred (50% session time allotted towards each reinforcer), response rates for the two rewards were not necessarily equal. This confirms that SS response rate is not a simple function of reward magnitude.