Influence of Stage of the Estrous Cycle on Endogenous Opioid Modulation of Luteinizing Hormone, Prolactin, and Cortisol Secretion in the Gilt1

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Abstract
Fourteen gilts that had displayed one or more estrous cycles of 18-22 days (onset of estrus = Day 0) and four ovariectomized (OVX) gilts were treated with naloxone (NAL), an opiate antagonist, at 1 mg/kg body weight in saline i.v. Intact gilts were treated during either the luteal phase (L, Day 10-11; n = 7), early follicular phase (EF, Day 15-17; n = 3), or late follicular phase (LF, Day 18-19; n = 4) of the estrous cycle. Blood was collected at 15-min intervals for 2 h before and 4 h after NAL treatment. Serum luteinizing hormone (LH) concentrations for L gilts averaged 0.65 .+-. 0.04 ng/ml during the pretreatment period and increased to an average of 1.3 .+-. 0.1 ng/ml (p < 0.05) during the first 60 min after NAL treatment. Serum prolactin (PRL) concentrations for L gilts average 4.8 .+-. 0.2 ng/ml during the pretreatment period and increased to an average of 6.3 .+-. 0.3 ng/ml (p < 0.05) during the first 60 min after NAL treatment. Serum PRL concentration averaged 8.6 .+-. 0.7 ng/ml 7.6 .+-. 0.6 ng/ml in EF and LF gilts, respectively, prior to NAL treatment, and decreased (p < 0.05) to an average of 4.1 .+-. 0.2 ng/ml and 5.6 .+-. 0.4 ng/ml in EF and LF gilts, respectively, during the fourth h after NAL. Naloxone treatment failed to alter serum LH concentrations in EF, LF, or OVX gilts and PRL concentrations in OVX gilts. Serum cortisol concentrations were elevated for 30 min after NAL treatment in OVX gilts (p < 0.05), and EF and LF gilts (p < 0.07), and for 105 min in the L gilts (p < 0.05). These data indicate that endogenous opioids inhibit LH and PRL secretion only during the luteal phase of the estrous cycle. In addition, endogenous opioids inhibit basal secretion of adrenocorticotropin hormone in the gilt.