The Hypothalamushypophysis- gonadal (HPG) axis is subject to both positive feed-forward and negative feedback regulation at several levels. At the level of the hypothalamus, early recognition of the pulsatile nature of gonadotropin releasing hormone secretion led to the notion of a central ‘‘pulse generator”, the inherent oscillatory activity of which controls the secretory rhythm of GnRH neurons (Knobil, 1980). Hypothalamic pulse generator is extensively modulated by a multitude of higher level inputs including photoperiod, environmental stress, metabolic state and nutritional status, as well as various endocrine mediators. .
