Photoperiodic regulation of the timing of puberty in the female sheep.

Abstract

Photoperiod is an environmental cue that has a major influence on the timing of puberty in many seasonal breeders, such as the female sheep. This dissertation aims to: (a) determine how early in development photoperiod modulates the timing of reproductive maturation in the female lamb, and (b) determine sex-specific photoperiod requirements for puberty in sheep. The bases for sex differences in photoperiodism in terms of prenatal organization of the brain are also examined. The age at puberty in lambs maintained under various artificial photoperiods was determined from the timing of the increase in luteinizing hormone (LH) secretion. The model used was the gonadectomized lamb treated chronically with estradiol to provide a constant steroid feedback on LH secretion throughout development. Before puberty, LH secretion is low because of hypersensitivity of the neuronal elements controlling LH secretion to steroid inhibition. At puberty, sensitivity to steroid feedback is diminished, and LH increases. In the female sheep, photoperiod experienced early in postnatal life influences the timing of reproductive maturation. Maintenance of females under a gradually decreasing photoperiod beginning at birth decreased the age at puberty by half compared to lambs raised under natural conditions. However, the timing of puberty under this artificial photoperiod was not influenced by varying the prenatal photoperiod, despite previous studies which suggest that photoperiod experienced before birth modulates the secretion of prolactin postnatally. The age at puberty in females is determined by when decreasing photoperiods are experienced. In contrast, puberty in males occurs regardless of photoperiod cues. Therefore, maturation in males always proceeds at the same age ($\sim$10 weeks) under either increasing, decreasing, or constant photoperiod. Exposure of female fetuses to exogenous testosterone modifies the brain and decreases photoresponsiveness; these androgenized females become capable of attaining puberty under photoperiods that inhibit sexual maturation in normal females. Hence, the findings of this dissertation indicate that photoresponsiveness is fundamentally a female trait which is diminished by the action of androgens during development.