Ovariectomized ewes were treated with 100 IU insulin, iv, which caused reductions in blood sugar and plasma LH concentrations. The effect was prevented by the infusion (iv) of glucose, suggesting that neuroglycopenia and not a direct action of insulin was the cause of reduced LH secretion. An iv infusion of naloxone (40 mg/h for 2 h), which commenced 25 min before the insulin injection, blocked the inhibitory effect of insulin on LH secretion, but it did not prevent the decrease in plasma glucose concentrations. In this treatment group and in a group treated only with naloxone, the opioid antagonist significantly stimulated LH secretion. To determine whether CRF might be involved in the insulin-induced decrease in LH secretion, 50 micrograms CRF were injected into ovariectomized sheep. Despite producing very high circulating concentrations of CRF within 2 min of injection and the stimulation of cortisol secretion during most of the 4-h posttreatment period, plasma LH levels were not affected. In addition, the intracerebroventricular administration of 10 micrograms CRF or 10 micrograms CRF plus 10 micrograms arginine vasopressin (AVP) did not affect LH secretion. These observations suggest that insulin-induced hypoglycemia decreased LH secretion by neuroglycopenia. This may involve an opioidergic mechanism, but does not involve activation of the hypothalamo-pituitary-adrenocortical axis.

Summary. Ewes were sampled during the mid-late luteal phase of the oestrous cycle. Hypophysial portal and jugular venous blood samples were collected at 5\p=n-\10min 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\p=n-\8-hportal blood sampling period. In contrast, large amplitude GnRH/LH pulses were observed during naloxone treatment in 5/6 ewes. The mean ( \ m=+-\ s.e.m.) amplitude of the LH secretory episodes during the naloxone infusion (1\m=.\07\ m=+-\ 0\m=.\11ng/ml) was significantly (P < 0\m=.\05) greater than that before the infusion in the same sheep (0\m=.\54 \ m=+-\0\m=.\15 ng/ml). Naloxone significantly (P < 0\m=.\005) increased the mean GnRH pulse amplitude in the 5/6 responding ewes from a pre-infusion value of 0\m=.\99\ m=+-\ 0\m=.\22pg/min to 4\m=.\39\ m=+-\1\m=.\10pg/min during infusion. This episodic GnRH secretory rate during naloxone treatment was also significantly (P < 0\m=.\05)greater than in the saline-infused sheep (1\m=.\53 \m=+-\0\m=.\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.

The natural opioid ligand, beta-endorphin, and the opioid antagonist, naloxone, were administered intracerebroventricularly (i.c.v.) to evaluate effects on LH secretion in ovariectomized ewes and in ovariectomized ewes treated with oestradiol-17 beta plus progesterone either during the breeding season or the anoestrous season. Ovary-intact ewes were also studied during the follicular phase of the oestrous cycle. Jugular blood samples were taken at 10-min intervals for 8 h and either saline (20-50 microliters), 100 micrograms naloxone or 10 micrograms beta-endorphin were injected i.c.v. after 4 h. In addition, luteal phase ewes were injected i.c.v. with 25 micrograms beta-endorphin(1-27), a purported endogenous opioid antagonist. In ovariectomized ewes, irrespective of season, saline and naloxone did not affect LH secretion, but beta-endorphin decreased the plasma LH concentrations, by reducing LH pulse frequency. The effect of beta-endorphin was blocked by administering naloxone 30 min beforehand. Treating ovariectomized ewes with oestradiol-17 beta plus progesterone during the breeding season reduced plasma LH concentrations from 6-8 micrograms/l to less than 1 microgram/l. In these ewes, saline did not alter LH secretion, but naloxone increased LH pulse frequency and the plasma concentrations of LH within 15-20 min. During anoestrus, the combination of oestradiol-17 beta plus progesterone to ovariectomized ewes reduced the plasma LH concentrations from 3-5 micrograms/l to undetectable levels, and neither saline nor naloxone affected LH secretion. During the follicular phase of the oestrous cycle, naloxone enhanced LH pulse frequency, which resulted in increased plasma LH concentrations; saline had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Surgical disconnection of the ovine hypothalamus from the pituitary gland (hypothalamo-pituitary disconnection; HPD) has provided a useful experimental model for studying the control of gonadotrophin secretion. The objective of the present study was to define the characteristics of prolactin secretion using stimuli acting through the hypothalamus or directly on the pituitary gland in HPD ewes. Prolactin responses to either a stressful stimulus or the dopaminergic antagonists metoclopramide (20 mg i.v.) or chlorpromazine (50 mg i.v.) seen in intact animals (sham-HPD) were completely abolished by HPD. Injection of TRH (100 micrograms i.v.) caused an immediate release of prolactin in both groups of ewes. In the HPD ewes plasma prolactin concentrations remained raised for at least 3 h after TRH injection, whereas in sham-HPD ewes prolactin concentrations began to decline after 20 min. Administration of bromocriptine (1 mg i.v.) 10 min after TRH inhibited the prolonged response to TRH in HPD ewes. The results support the hypothesis that prolactin exerts a short-loop feedback effect on its own secretion at the hypothalamic level.

To determine whether opioid mechanisms modulate the positive feedback effect of oestrogen on LH secretion, anoestrous ewes were given a single injection of 50 micrograms oestradiol benzoate (OB), followed by infusions of morphine or naloxone. All sheep were injected i.m. with 50 micrograms OB at 00.00 h. In experiment 1, sheep were given i.v. infusions of the following: group 1, 12 ml saline/h from 09.00 to 15.00 h (n = 12); group 2, 40 mg naloxone/h from 09.00 to 12.00 h (n = 5); group 3, 40 mg naloxone/h from 10.00 to 14.00 h (n = 5); group 4, 10 mg morphine/h from 09.00 to 15.00 h (n = 5); and group 5, 20 mg morphine/h from 09.00 to 15.00 h (n = 5). Jugular blood samples were taken at 30-min intervals to monitor LH surges, which commenced 13.0 +/- 0.6 h after injection of OB in control (OB plus saline) ewes. The infusions of naloxone or morphine did not affect the timing or magnitude of the oestrogen-induced LH surge. To examine the possibility that opioidergic regulation of the LH surge occurred earlier than the infusion regimens in experiment 1, sheep were infused from the time of the OB injection (00.00 h) until 15.00 h. In this experiment (experiment 2), sheep were given i.v. infusions of the following: group 1, 4.2 ml saline/h (n = 5); group 2, 20 mg naloxone/h (n = 5); and group 3, 20 mg morphine/h (n = 5). As in experiment 1, treatment with neither the opioid agonist or antagonist was able to alter the positive feedback response of OB.(ABSTRACT TRUNCATED AT 250 WORDS)