The effect of B-HT 920, a dopamine D2 agonist, on bar-press feeding in the monkey

Although the dopamine (DA) system has been shown to regulate food intake, the function of the DA receptor subtypes on behavior still remains to be elucidated. In the present study, we examined the effect of B-HT 920, a selective agonist of DA D2 receptors that preferentially affect presynaptic autoreceptors, on both food consumption and execution of a high fixed-ratio bar-press task for food reward in monkeys. Two kinds of bar-press task were used: 1) a cue-triggered bar-press task during the first 40 trials, and 2) a self-paced bar-press task in which the monkeys freely performed bar-press trials until they were satiated. A SC injection of B-HT 920 (25 micrograms/kg) increased food consumption in the home cage. The same facilitatory effect on food consumption was also observed in the operant task condition. During the cue-triggered bar-press task, however, both the latency of the bar-press responses to a cue light and the time required to complete the bar-press trials were prolonged after the injection of B-HT 920. The results suggest that the activation of D2 autoreceptors suppresses the operant food acquisition behavior and increases food consumption through an inhibition of the satiety mechanism rather than an activation of any hunger-related drive.