Different types of opioid receptors involved in the suppression of LH secretion in lactating sheep

To learn the involvement of endogenous opioid peptides (EOP) in the regulation of reproductive activity in ruminants, the effects of different opioid antagonists on luteinizing hormone (LH) secretion were determined in sheep during the early stage of lactation. The opioid receptor antagonists: naloxone (all types of receptors, n = 5), naloxonazine (μ receptor, n = 5), GNTI- (κ receptor, n = 5), naltrindole (δ receptor, n = 5) or the vehicle (control, n = 5) were infused intracerebroventricularly in a series of five 30-min infusions (60 μg/60 μl) at 30-min intervals. The period of the experiment included the non-suckling (10:00–12.30) and suckling (12.30–15.00) periods. Blood samples were collected from 10.00 to 15.00 at 10-min intervals, and plasma LH concentration was assayed by the radioimmunoassay method. The obtained results showed that blocking of the EOP action within the central nervous system in lactating sheep caused a significant (p < 0.001) increase in LH concentration in all treated groups, in comparison to the control. In the naloxone-treated group, a significant (p < 0.05) increase in LH secretion also occurred during suckling. The amplitude of LH pulses increased significantly in the naloxonazine- (p < 0.01) and naltrindole- (p < 0.05) treated ewes compared to the control; there were no significant differences in the frequency of LH pulses among the groups. In conclusion, our study indicates that EOP play a crucial role in the mechanism inhibiting GnRH/LH axis activity in lactating sheep and that the ligands for μ opioid receptor may have the highest inhibitory effect.