Evaluation of a Role for Prolactin in Growth and the Photoperiod-induced Growth Response in Sheep 1'2

An experiment was conducted to determine whether prolactin is involved in growth or in mediating the photoperiod-induced growth response in sheep. Prolactin was manipulated by im injections of 2-Br-~-ergocryptine (CB154) or sc injections of ovine prolactin (oPRL) "and by two light:dark regimens (16L: 8D and 8L:16D). Fifty-six wether lambs (two/ pen) were allotted to one of four treatments for a 9-wk growth study. Treatments were: 1) 16L: 8D, placebo injections; 2) 16L:8D, CB154 injections; 3) 8L:16D, placebo injections and 4) 8L:16D, oPRL injections. Daily injections of CB154 (.1 mg/kg body weight'Ts), oPRL (.8 mg/kg body weight "Ts) or placebo were in 1 ml volume. Animals were fed ad libitum a complete mixed diet. At wk 8, plasma prolactin concentrations at 3 to 6 h postinjection were 214, 3, 90 and 228 ng/ml for treatments 1, 2, 3 and 4, respectively. Pattern of feed intake, measured at 8-h intervals for a 48-h period, was affected by photoperiod. Animals exposed to the 16L:8D photoperiod consumed 40.0, 42.4 and 17.6% of their total daily feed intake during the first 8 h of light, second 8 h of light and 8 h dark interval, respectively. Those exposed to the 8L:16D regimen consumed 55.2,