Seasonal changes in the secretion of gonadotropic hormones and in the endocrine response of the pituitary of male sheep in the absence of gonadal influence

Abstract

Circulating levels of luteinizing hormone (LH), follicle stimulating hormone (FSH), and prolactin (PRL) were assessed in four long-term castrate rams at monthly intervals for 1 year beginning in March. Each month, rams were bled from the jugular vein at 10-min intervals for an 8-h period; 10 micrograms synthetic gonadotropin releasing hormone (GnRH) was injected into the jugular vein following 6 h of sampling. Changes in natural daylength were associated with seasonal variations in mean serum levels of LH (r = 0.57, P less than 0.05) and PRL (r = 0.94, P less than 0.01). LH levels declined 55% between June and October as LH peaks became more frequent and progressively smaller in magnitude, and base-line levels decreased; these trends were reversed by December. In comparison, FSH levels increased 40% between June and September and then returned to relatively low values in October. Seasonal variations in the magnitude of endogenous LH pulses were not associated with definitive changes in the LH response of the pituitary to exogenous GnRH. Although LH and FSH levels in the castrate rams were typically high, many of the seasonal-directional changes in the secretory characteristics of these hormones (i.e., LH peak frequency and magnitude, and mean FSH level) were similar to those reported for the intact ram. Data indicate that photoperiodic regulation of hypothalamic function of rams is in part via direct or steroid-independent means.