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

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 exp eriments tested the hypothesis that the setpoint for intake, mediated throu gh endogenous opioid tone, would be greater in lactating ewes than non-lact ating ewes. Experiment 1 was conducted during lactation (43 d postpartum) w ith 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 DM I was measured every 20 min for 5.5 h and expressed as g/kg of BW. Blood sa mples were taken at 15 min intervals for 5 h after naloxone was administere d. Although level of naloxone and experiment influenced the repeated measur es analysis of DMI (P=0.01), no experiment x level of naloxone interaction was detected (P=0.86). Increasing level of naloxone had a negative quadrati c (P<0.05) effect on DMI during the first 30 min in Experiments 1 and 2. To tal DMI for the 5.5 h collection period was not influenced (P>0.23) by nalo xone 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 bur 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 concen tration were greater in Experiment I than Experiment 2, and greater for Tar ghee 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 lam bs. An experiment x number of lambs born/ewe interaction was detected (P=0. 05) for mean GH concentration. Mean GH concentration was greater (P=0.01) i n Experiment 1 for ewes rearing twin lambs than single lambs. Mean GH conce ntration did not differ (P=0.78) between ewes rearing single and twin lambs in Experiment 2, Although level of naloxone influenced number and frequenc y of GH peaks, the does of naloxone that would inhibit feeding did not diff er between forage-fed ewes at high and low energy requirements. (C) 2000 Pu blished by Elsevier Science B.V. All rights reserved.