Feeding behavior and its responsiveness to naloxone differ in lean and obese sheep.

Feeding and its regulation by opioids were studied in lean sheep and sheep in the static phase of dietary obesity. Sheep were fasted 16 h and on separate days were injected IV with 0 (saline), 0.01, 0.1, 1, or 3 mg/kg naloxone 5 min before they were allowed ad lib intake for the ensuing 32 h. All sheep were in chronic zero energy balance when not fed ad lib during naloxone treatment. After 0 mg/kg naloxone, intake rate was at least twice as fast (p < 0.05) in lean than obese sheep through the first 4 h of ad lib feeding, but was similar (approximately 0.5 g/min) in both groups of sheep after 8 h of ad lib feeding. Dose-dependent inhibitory effects of naloxone on intake were observed in lean and obese sheep through the first 4 h of ad lib feeding with maximum inhibition at +2 h. Dose-response curve for naloxone inhibition of intake was shifted leftward in obese compared with lean sheep. Dose of naloxone needed to inhibit intake by 25% was less (p < 0.05) in obese (0.13 mg/kg) than lean (0.57 mg/kg) sheep when both groups experienced similar plasma concentrations of injected naloxone. Basal concentrations of immunoreactive beta-endorphin in fasted plasma were similar in lean (33 +/- 4 pg/ml) and obese (48 +/- 9 pg/ml) sheep. Dietary obesity in sheep was associated with reduced appetite and with enhanced responsiveness to the intake-inhibitory effects of naloxone.