Concentrations of prolactin, LH and FSH after naloxone administration in follicular-phase gilts

Abstract

Studies were initiated to extend previous data on the pituitary hormone secretion after naloxone (NAL) administration to gilts during the follicular phase. Gilts which had exhibited two previous estrous periods were randomly assigned to treatment with 1 mg naloxone (NAL)/kg b.w. on day 16 ( N = 6) or day 20 ( N = 6) of the estrous cycle ( N = 6). Five additional gilts were treated with 2 mg NAL/kg b.w. on day 16. Control animals received saline alone on day 16 ( N = 6) or day 20 ( N = 6). Peripheral blood samples were collected every 15 min from 2 h prior to 6 h after infusion of NAL or saline. Prolactin, LH and FSH concentrations were determined by RIA procedures. On day 20 of the estrous cycle, prolactin concentration after NAL administration was reduced as compared to the control group for the entire post-treatment period ( P < 0.05). The greatest decrease in prolactin concentration was found during the 5th hour after NAL administration ( P < 0.01). On day 16 of the cycle, NAL at both doses did not influence mean concentration of prolactin. The treatment regimen failed to demonstrate significant changes in LH and FSH concentration after NAL administration on days 16 and 20 of the cycle. The results suggest that prolactin secretion is modulated by endogenous opioid peptides during the late follicular phase.