Naloxone evokes large-amplitude GnRH pulses in luteal-phase ewes

Ewes were sampled during the mid-late luteal phase of the oestrous cycle. Hypophysial portal and jugular venous blood samples were collected at 5-10 min intervals for a minimum of 3 h, before i.v. infusions of saline (12 ml/h; N = 6) or naloxone (40 mg/h; N = 6) for 2 h. During the 2-h saline infusion 2/6 sheep exhibited a GnRH/LH pulse; 3/6 saline infused ewes did not show a pulse during the 6-8-h portal blood sampling period. In contrast, large amplitude GnRH/LH pulses were observed during naloxone treatment in 5/6 ewes. The mean (± s.e.m.) amplitude of the LH secretory episodes during the naloxone infusion (1.07 ± 0.11 ng/ml) was significantly (P < 0.05) greater than that before the infusion in the same sheep (0.54 ± 0.15 ng/ml). Naloxone significantly (P < 0.005) increased the mean GnRH pulse amplitude in the 5/6 responding ewes from a pre-infusion value of 0.99 ± 0.22 pg/min to 4.39 ± 1.10 pg/min during infusion. This episodic GnRH secretory rate during naloxone treatment was also significantly (P < 0.05) greater than in the saline-infused sheep (1.53 ± 0.28 pg/min). Plasma FSH and prolactin concentrations did not change in response to the opiate antagonist. Perturbation of the endogenous opioid peptide system in the ewe by naloxone therefore increases the secretion of hypothalamic GnRH into the hypophysial portal vasculature. The response is characterized by a large-amplitude GnRH pulse which, in turn, causes a large-amplitude pulse of LH to be released by the pituitary gland.