Effects of naloxone on appetitive behavior in rats and mice

Naloxone (NX), an opiate receptor antagonist with little intrinsic activity, has been shown to decrease appetitive behavior in rats. This study was undertaken to evaluate further the effects of NX on ingestive behavior in the rat and to extend observations to the mouse. One group of rats and mice was deprived of water for 12 to 48 hr., then allowed access to water for 30 min at 08.00 or 20.00 hr. A second group of non-deprived mice and rats was pretreated with 2M NaCl (i.p.) and given 30 min access to water. A third group was deprived of food for 12 to 48 hr, then given access to food for 60 min at 08.00 or 20.00 hr. NX (0.01-10 mg/kg) was injected s.c. 30 min before each test. Deprivation-induced water intake in rats and mice was significantly reduced by NX at doses of 0.1 and 10 mg/kg respectively. The ED50 for the rats ranged from 1.0-10 mg/kg. NX attenuated water intake induced by 2M NaCl in both rats and mice with an ED50 of about 1.0 mg/kg. Food intake was decreased by 1.0 mg/kg NX in the rat and by 0.1-10 mg/kg in the mouse, but a 50% suppression of food intake could not be attained in either species. It is concluded that NX exerts dose-related suppressive effects on consummatory behavior in mice as well as in rats, and that water intake, particularly that stimulated by hypertonic saline, is more sensitive to the actions of NX than is food intake.