SHORT-TERM STIMULATION OF LUTEINIZING-HORMONE (LH) SECRETION BY NALOXONE TREATMENT IN THE PREGNANT GILT

An experiment was initiated to determine the involvement of endogenous opioids (EOP) in the modulation of luteinizing hormone (LH) secretion during two stages of pregnancy at 40 and 70 days in the pig. Twenty-f our crossbred pregnant gilts (150 +/- 10 kg) were given 1 mg kg-1 body weight (BW) of naloxone (NAL), or 3 ml saline i.v. at 40 days (NAL, n =6; saline, n=6) or at 70 days (NAL, n=6; saline, n=6) of pregnancy. B lood plasma was collected at 15 min intervals for 1 h before and 3 h a fter treatment with NAL or saline. Following this period, all NAL-trea ted gilts were given 1 mug kg-1 BW of gonadotropin releasing hormone ( GnRH) in 3 ml saline i.v. and blood samples were collected every 15 mi n for 3 h. At 40 days of pregnancy, mean plasma LH concentrations were 0.22 +/- 0.1 ng ml-1 for 1 h before NAL treatment. In five of the six treated pigs, the infusion of NAL increased LH concentration to 1.33 +/- 0.5 ng ml-1 (P<0.05), 1.58 +/- 0.42 ng ml-1 (P<0.01), and 1.28 +/- 0.39 ng ml-1 (P<0.01) at 15 min, 30 min and 45 min, respectively. Con centrations of LH returned to control values by 90 min post-treatment. The control group values ranged from 0.15 +/- 0.01 ng ml-1 to 0.37 +/ - 0.34 ng ml-1 1 h before to 3 h after saline administration. At 70 da ys of pregnancy, NAL did not alter plasma LH concentrations, although some increases were detected in individual gilts. Mean plasma LH conce ntrations were 0.16 +/- 0.04 ng ml-1 for 1 h before NAL, and 0.61 +/- 0.35 ng ml-1, 0.46 +/- 0.27 ng ml-1 and 0.77 +/- 0.29 ng ml-1 during t he 1 h, 2 h and 3 h periods after NAL treatment, respectively. The inf usion of GnRH increased (P<0.001) LH concentrations in both pregnancy periods. Significant changes in plasma progesterone concentrations wer e not observed. These results indicate that the opioids are stronger m odulators of LH secretion at 40 days of pregnancy than later in pregna ncy in the pig. It is possible that during the second half of pregnanc y, an alternative control of the EOP system may exist.