THE EFFECTS OF INTRAVENOUS-INFUSION OF NALOXONE AND PROGESTERONE ON LUTEINIZING-HORMONE SECRETION IN OVARIECTOMIZED EWES

This study was designed to see if endogenous opioids suppressed lutein ising hormone (LH) secretion in the absence of ovarian steroids and if they mediated negative feedback suppression of LH secretion by proges terone in the ewe. Twelve ovariectomised ewes received each of four tr eatments at 1 day intervals: (1) intravenous saline infusion, (2) nalo xone infusion (0.5 mg kg-1 h-1), (3) progesterone infusion (42 mug kg- 1 h-1), (4) naloxone and progesterone infusion for 12 h. Progesterone (5.07 +/- 0.27 ng ml-1) decreased LH pulse frequency and mean and basa l serum concentration of LH. Naloxone increased mean and basal serum c oncentrations of LH. The administration of progesterone. and naloxone together resulted in LH pulse frequency (1.62 +/- 0.07 pulses h-1) and mean (1.89 +/- 0.13 ng ml-1) and basal serum concentrations (1.33 +/- 0.11 ng ml-1) of LH that were similar to the control ewes (1.80 +/- 0 .06 pulses h-1, 2.26 +/- 0.28 ng ml-1, 1.67 +/- 0.20 ng ml-1, respecti vely) but lower than in ewes receiving naloxone alone (1.94 +/- 0.04 p ulses h-1, 3.00 +/- 0.37 ng ml-1, 2.09 +/- 0.22 ng ml respectively). T he combined treatment also resulted in LH pulse frequency (1.62 +/- 0. 07 pulses h-1) that was greater than in ewes only receiving progestero ne (1.39 +/- 0.09 pulses h-1). We concluded that there are steroid-ind ependent effects of opioids on LH secretion in long-term ovariectomise d ewes, that opioids probably do not entirely mediate the negative fee dback effects of progesterone on LH secretion and that opioids largely affect LH secretion at the hypothalamus but pituitary effects cannot be ruled out.