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The opioid antagonist naloxone was administered to prepubertal heifers, and its effects on progesterone, cortisol, and LH secretions were studied. Pooled pretreatment values in peripheral plasma for progesterone averaged 0.26 +/- 0.08 ng/ml (+/- SE), and those for cortisol averaged 13 +/- 1.2 ng/ml. Naloxone treatment at either 1.1 (Nal-1) or 2.0 (Nal-2) mg/kg BW significantly increased circulating progesterone within 15 min of i.v. injection, which was sustained for 45 and 75 min, respectively. Maximal progesterone concentrations were 1.8 +/- 0.16 ng/ml and 1.8 +/- 0.19 ng/ml at the low and high naloxone dosages; progesterone values were basal by 120 min. Naloxone caused a dose-dependent increase in plasma cortisol averaging 20 +/- 3.1, 28 +/- 4.2, and 49 +/- 5.7 ng/ml in the immediate 120 min posttreatment in control, Nal-1, and Nal-2 heifers, respectively; but naloxone injection, at either the low or the high dosage, had no effect on plasma LH concentration. The effect of naloxone on progesterone concentration was dependent on the age and body weight of the animal; it was first observed at 28 mo of age and between 130 and 160 kg BW. None of the heifers that showed transient elevations in progesterone exhibited estrous behavior or had a palpable CL throughout the study. These coincident transient increases in circulating progesterone and cortisol demonstrate that an endogenous inhibitory opioidergic system is involved in the control of steroid hormone secretion in prepubertal zebu heifers. Further, an adrenal source of this progesterone is indicated, but whether the progesterone participates in the induction of an LH increase in the prepubertal heifer is not clear.

Opioids Modulate Progesterone Production in Prepubertal Bunaji Heifers1