EFFECT OF NALOXONE AND PULSATILE LUTEINIZING-HORMONE-RELEASING HORMONE INFUSIONS ON ESTRADIOL-INDUCED LUTEINIZING-HORMONE SURGES IN IMMATURE GILTS

The aim of the study was to understand why immature 60-day-old gilts p roduce delayed low amplitude luteinizing hormone (LH) surges in respon se to oestradiol benzoate. In Expt 1, gilts (n = 36) were challenged w ith oestradiol benzoate and subsequently received either no further tr eatment or were infused with saline or various doses of the opioid ant agonist, naloxone, for 6-48 h during the expected LH surge (48-96 h af ter oestradiol benzoate). No differences were observed among groups in the magnitude or duration of the LH surge. In contrast to the other g roups, LH concentrations in gilts infused for 48 h with naloxone did n ot decrease after the surge period. In Expt 2, gilts (n = 34) were cha llenged with oestradiol benzoate or sesame oil and subsequently receiv ed pulses of luteinizing-hormone-releasing hormone (LHRH) or saline so lution during the expected surge period. Two other groups were fed met hallibure to pharmacologically suppress the oestradiol benzoate-induce d LH surge. In addition, one of these groups was given pulses of an LH RH agonist (LHRH-A) during the surge period. Within 2 h of the start o f pulsatile LHRH infusion, LH increased in sesame oil-treated gilts, b ut not in oestradiol benzoate-treated gilts, suggesting that the pitui tary responsiveness to LHRH in immature gilts is decreased by oestroge n before the onset of the LH surge. Pulsatile LHRH infusion did not en hance the amplitude of oestradiol benzoate-induced LH surges nor did i t advance its onset. Feeding methallibure suppressed the oestradiol be nzoate-induced LH surge. LHRH-A infusion of methallibure-treated gilts produced LH surge amplitudes similar to those of controls, but much b elow those previously observed by us in pubertal gilts or by others du ring a similar LHRH-A treatment regimen in mature gilts. In addition, the biphasic surge response to oestradiol benzoate, typical for the im mature gilt, was abolished in LHRH-A-treated gilts. We suggest that oe stradiol fails to generate mature LH surges because the gonadotrophs o f the immature gilt cannot respond to enhanced LHRH secretion during t he surge period in an adult-like manner.