Robert L. Goodman

Neuroendocrine basis of seasonal reproduction

Publisher Summary This chapter discusses the strategy of seasonal breeding, the role of photoperiod in timing the annual reproductive cycle, the hypothalamo-pituitary mechanisms that mediate photoperiodic regulation of estrous cyclicity, and the photoperiodic pathway to luteinizing hormone (LH) pulse generator. To understand how photic input to the LH pulse generator leads to seasonal changes in gonadal activity, the sequence of endocrine events that normally leads to ovulation during the estrous cycle of the ewe must be considered. These preovulatory events occur during a 2–3 day follicular phase and include a precipitous drop in progesterone, a progressive rise in tonic LH secretion, a sustained increase in estradiol secretion, and the LH surge. The pivotal step in this sequence is the sustained increase in tonic LH secretion. A great deal of insight has been gained into the complex interplay between the neural and endocrine response systems that underlie the seasonal reproductive process in the short-day breeding ewe. Specifically, light cues activate retinal photoreceptors and are transmitted via a monosynaptic tract to the suprachiasmatic nuclei of the hypothalamus. After interacting with the circadian system, the photic information is relayed to the pineal gland that transduces the neural message into a hormonal signal in the form of a circadian rhythm of melatonin secretion. The pattern of this melatonin signal, which is interpreted as inductive or suppressive, sets the frequency of the LH pulse generator and determines the capacity of this neural oscillator to respond to the negative feedback action of estradiol. The resulting changes in the episodic pattern of gonadotropin secretion, in turn, dictate whether or not estrous cycles can occur.