LH-SECRETION IN OVARIECTOMIZED EWES - EFFECTS OF MORPHINE AND OVARIAN-STEROID INTERACTIONS WITH NALOXONE DURING THE BREEDING-SEASON AND ANESTRUM

Morphine (MOR) infusion (i.v.) was used to mimic effects of endogenous opioid peptides (EOPs) on LH secretion in ovariectomized (OVX) ewes during the breeding season and anestrum. MOR decreased luteinizing hormone (LH) pulse frequency and/or amplitude at higher doses (0.5 and 1.0 mg kg-1 h-1), and increased basal serum LH concentrations at a lower dose (0.01 mg kg-1 h-1). Simultaneous MOR and naloxone (NAL) infusion (0.5 mg kg-1 h-1, each) did not affect LH secretion, suggesting that effects of MOR were EOP receptor-specific. EOP involvement in ovarian steroid feedback was examined by NAL infusion and ovarian steroid replacement in OVX ewes. NAL infusion increased LH pulse frequency in OVX ewes during the breeding season, suggesting ovarian steroid-independent, but EOP-dependent suppression of LH secretion. NAL did not alleviate seasonal suppression of LH pulse frequency during the anestrum, suggesting ovarian steroid-independent, EOP-independent suppression of LH secretion. NAL eliminated progesterone (P4) suppression of LH pulse frequency during the breeding season, suggesting that a major portion of P4 feedback on tonic LH secretion is EOP-dependent. P4 replacement in the anestrum did not suppress LH pulse frequency. NAL infusion, following P4 replacement in the anestrum, increased LH pulse frequency to levels observed in untreated OVX ewes during the breeding season. It was concluded that EOP-dependent P4 feedback experimentally over-rode EOP-independent suppression of LH pulse frequency during the anestrum. EOP activity was not required for estradiol (E2) to suppress LH secretion from the pituitary during the breeding season.