Effect of naloxone on intake and growth hormone concentration in lactating and non-lactating ewes

Abstract

Thirty-two ewes (16 Suffolk and 16 Targhee) were used in two experiments to investigate effects of energy requirements, naloxone, breed, and number of lambs/ewe on feed intake and growth hormone (GH) concentrations. These experiments tested the hypothesis that the setpoint for intake, mediated through endogenous opioid tone, would be greater in lactating ewes than non-lactating ewes. Experiment 1 was conducted during lactation (43 d postpartum) with ewes confined individually with their lambs. Experiment 2 was conducted when ewes were non-pregnant and non-lactating. Naloxone treatments were 0, 0.5, 1.5, or 3 mg of naloxone/kg of ewe BW. After 3.5 h of fasting, ewe DMI was measured every 20 min for 5.5 h and expressed as g/kg of BW. Blood samples were taken at 15 min intervals for 5 h after naloxone was administered. Although level of naloxone and experiment influenced the repeated measures analysis of DMI ( P=0.01), no experiment × level of naloxone interaction was detected ( P=0.86). Increasing level of naloxone had a negative quadratic ( P<0.05) effect on DMI during the first 30 min in Experiments 1 and 2. Total DMI for the 5.5 h collection period was not influenced ( P>0.23) by naloxone level. An experiment by breed interaction was detected for DMI ( P=0.04). In Experiment 1, DMI was greater ( P=0.02) for Targhee than Suffolk ewes but breeds did not differ ( P=0.68) in DMI during Experiment 2. Number and frequency of GH pulses responded quadratically ( P=0.02) to increasing level of naloxone. Number, frequency, and height of GH pulses and mean GH concentration were greater in Experiment 1 than Experiment 2, and greater for Targhee than for Suffolk ewes ( P<0.05). Mean GH concentration ( P<0.05) and GH pulse height ( P<0.10) were greater for ewes with twin lambs than single lambs. An experiment × number of lambs born/ewe interaction was detected ( P=0.05) for mean GH concentration. Mean GH concentration was greater ( P=0.01) in Experiment 1 for ewes rearing twin lambs than single lambs. Mean GH concentration did not differ ( P=0.78) between ewes rearing single and twin lambs in Experiment 2. Although level of naloxone influenced number and frequency of GH peaks, the does of naloxone that would inhibit feeding did not differ between forage-fed ewes at high and low energy requirements.