THE EFFECT OF NALOXONE ON THE DURATION OF ESTRUS OVULATION RATE AND ESTRADIOL-17-BETA IN CROSSBRED EWES WITH INDUCED ESTRUS DURING SEASONALANESTRUS

This study was conducted to investigate the effect of high (0.5 mg/kg) and low (1.0 mg) doses of naloxone on sexual behaviour, ovulation rat e and blood plasma concentrations of oestradiol 17 beta in Suffolk x H ampshire ewes induced into oestrus during the non-breeding season (May 1993). For this purpose, 60 ewes were allocated at random to four gro ups of 15 animals. Those animals in group 1 were administered medroxip rogesterone acetate (MAP) per vaginum for 14 days, with eCG (250 IU) i njected on the fourteenth day. Group 2 was treated with MAP + eCG, as well as 0.5 mg/kg of naloxone with the eCG. Group 3 also had MAP + eCG with two injections of 0.5 mg of naloxone i.m. given with the eCG and then 24 h later; the first injection was given with sponge withdrawal . Group 4 was used as a control. Four crossbred rams were used as teas ers. Naloxone (0.5 mg/kg) did not affect ovulation rate but significan tly increased the duration of oestrus and the number of mounts receive d per ewe, when compared with ewes of group 1. When 1.0 mg of naloxone was given in two i.m. injections of 0.5 mg naloxone/12 h, the measure of oestrous behaviour and mounts received per ewe were significantly increased and ovulation rate was not affected. There were no differenc es in plasma oestradiol concentrations between groups 1, 2 and 3. It w as concluded that naloxone as an opioid antagonist modifies sexual beh aviour in the ewe, giving more evidence that endogenous opioids are im portant modulators of reproduction. (C) 1998 Elsevier Science B.V. All rights reserved.