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The biphasic effect of oestradiol (E2) on gonadotrope responsiveness is clearly evident in orchidectomized sheep (wethers) receiving E2 and hourly pulses of GnRH. We hypothesized that the duration of E2-induced reduction in gonadotrope responsiveness differed between the breeding (November) and anoestrous (May) seasons in sheep. To test this hypothesis wethers (n = 6/group) were infused (i.v.) with E2 (2 micrograms/50 kg per h) and received hourly pulses of GnRH (200 ng/50 kg per pulse) or saline in May or November. The pattern of LH secretion during the 72 h infusion period was determined. Serum concentrations of LH did not differ with season in control wethers receiving vehicle alone. Similarly, continuous infusion of E2 resulted in a 3-fold reduction in serum LH, irrespective of season. This E2-induced suppression of serum LH was reversed by concurrent episodic delivery of GnRH. The interval between initiation of infusion and return of pretreatment concentrations of LH was taken as a measure of the duration of E2-induced suppression of gonadotrope responsiveness. The duration of this E2-dependent response varied with season, with suppression of gonadotrope responsiveness more prolonged (P &lt; 0.05) in May (36.7 +/- 2.9 h) than in November (14.3 +/- 1.1 h). In a companion study we examined the effect of melatonin on the duration of E2-induced suppression of gonadotrope responsiveness. Wethers received blank or melatonin-containing implants in March. Sixty days after implant insertion (mid-May) wethers received E2 (2 micrograms/50 kg per h) and hourly pulses of GnRH (200 ng/50 kg per pulse) or saline for 72 h. Continuous delivery of E2 alone resulted in a 3-fold decrease in serum concentrations of LH in both control and melatonin-treated wethers. The duration of E2-induced suppression of gonadotrope responsiveness in animals receiving E2 and GnRH was extended (P &lt; 0.05) in wethers with blank implants (48.0 +/- 0.7 h), relative to the duration of suppression in melatonin-treated wethers (14.5 +/- 1.0 h). Taken together these data indicate that E2-induced suppression of gonadotrope responsiveness is more extended during the anoestrous season. However, this seasonal effect can be reversed by continuous administration of melatonin.

The effect of season and melatonin on GnRH-induced LH secretion in oestradiol-treated orchidectomized sheep