Role of dopamine D-1 and D-2 receptor subtypes in mediating dopamine agonist effects on food consumption in rats.

The effects of selective D-2 and D-1 dopamine (DA) receptor agonists on food consumption were investigated in free-feeding rats. A selective D-2 receptor agonist, (+)-4-propyl-9-hydroxynaphthoxazine (PHNO), increased the consumption of standard food pellets in the dose range of 7.5-120 micrograms/kg, while SKF 38393 (5.0 mg/kg), a selective D-1 receptor agonist, decreased food pellet intake. The increase in food pellet intake produced by PHNO was blocked by haloperidol (an antagonist relatively selective for the D-2 receptor at the dose used, 0.05 mg/kg) and SCH 23390 (20 micrograms/kg, a D-1 receptor selective antagonist). Increasing "arousal" by disturbance associated with repeated food weighing also increased food pellet consumption, but did not diminish PHNO-elicited feeding. However, the same range of doses of PHNO (7.5-120 micrograms/kg) which increased food pellet intake decreased consumption of a liquid diet, and had no overall effect on a highly palatable liquid diet. The increase in consumption of solid food induced by PHNO appears to be secondary to enhancement of chewing behaviors. In contrast, the decrease in food intake induced by SKF 39393 may be due to a direct action of the drug on neural feeding mechanisms.