Short-term inhibition of prolactin secretion by naloxone treatment in the pregnant gilt

The involvement of endogenous opioids in modulation of prolactin (PRL) secretion during pregnancy in the pig was studied. Twenty-four crossbred pregnant gilts (150 ± 10 kg) were cannulated via the cephalic vein 24–48 h before treatment with 1 mg kg−1 body weight of naloxone (NAL) or 3 ml of saline (CONT) i.v. at Day 40 (NAL, n = 6; CONT, n = 6) or Day 70 (NAL, n = 6; CONT, n = 6) of pregnancy. Blood plasma was collected at 15 min intervals from 1 h before to 3 h after treatment with NAL or saline. At Day 40 of pregnancy, administration of NAL caused a decrease in mean plasma PRL concentrations at 60 min, 120 min and 180 min post-treatment (NAL, 19.1 ± 1.3 ng ml−1, P < 0.05; 15.8 ± 0.6 ng ml−1, P < 0.001; 14.6 ± 0.7 ng ml−1, P < 0.001, respectively) when compared with the CONT group (22.9 ± 0.7 ng ml−1, 21.6 ± 0.6 ng ml−1 and 22.4 ± 0.5 ng ml−1, respectively). Mean plasma estradiol concentration was higher (P < 0.01) in the NAL group during the second and third hour post-treatment than in the CONT group. At Day 70 of pregnancy, infusion of NAL also decreased (P < 0.001) plasma PRL concentrations at 60 min, 120 min and 180 min after treatment (20.1 ± 1.6 ng ml−1, 16.2 ± 1.5 ng ml−1 and 14.8 ± 0.4 ng ml−1, respectively) compared with the CONT group (33.4 ± 1.7 ng ml−1, 34.1 ± 1.3 ng ml−1 and 29.1 ± 0.9 ng ml−1, respectively). Estradiol concentrations were not different (P > 0.05) between groups in this stage of gestation. Mean concentrations of progesterone were similar during the pre- and post-treatment periods in both stages of pregnancy. data would suggest a possible role of the opioids in modulation of PRL secretion at these stages of pregnancy in the pig.