Phenylpropanolamine and amphetamine disrupt postprandial satiety in rats.

Two tests of the behavioral specificity of the anorectic effects of amphetamine (AM) and phenylpropanolamine (PPA) were done. Intraperitoneal injections of each drug reduced the size of condensed milk test meals in 30-min pellet-deprived rats. The dose-response relations in semi-log coordinates were linear and parallel, but AM (ED50, 2.0 +/- 0.1 mumol/kg) was about ten times more potent than PPA (ED50, 24.6 +/- 0.1 mumol/kg). Periprandial behaviors were observed using a time-sampling technique. Both AM and PPA disrupted the normal behavioral sequence of postprandial satiety throughout their anorectic ranges, but they did so differently. AM increased postprandial exploratory behavior, decreased or eliminated resting, and, at larger doses, elicited stereotypy. In contrast, PPA inhibited both grooming and exploration, and increased resting. The drugs' effects on water intake were tested in 17-hr water-deprived rats. AM's adipsic effect (ED50, 2.3 +/- 0.1 mumol/kg) was similar to its anorectic effect. PPA also inhibited drinking, although slightly less potently (ED50, 56.6 +/- 0.1 mumol/kg) than it did feeding. Thus, under conditions maximizing the anorectic potencies of systemically administered AM and PPA in rats, both drugs inhibited feeding nonspecifically rather than by eliciting normal postprandial satiety.