The endocrinology of the menstrual cycle: the interaction of folliculogenesis and neuroendocrine mechanisms.

(1) The gonadotropins are secreted in a pulsatile fashion in response to the similar pulsatile release of GnRH from neurosecretory neurons centered in the arcuate nucleus of the medial basal hypothalamus. (2) The gonadal steroids appear to exert their feedback effects both directly on the pituitary and through modulation of the pulsatile pattern of GnRH secretion. They may also influence the degree of sialylation and subsequent biologic activity of the gonadotropins. (3) GnRH release is under the control of catecholaminergic neurotransmitters. Norepinephrine appears to act as an excitatory agent, whereas dopamine inhibits GnRH secretion. (4) Dopamine also directly inhibits PRL release and may be the prolactin-inhibiting factor. (5) The endorphins are endogeneous opiate peptides and are derived from a common ACTH/ beta-lipotropin precursor. Through modulation of neurotransmitter mechanisms, the endorphins may affect both PRL and gonadotropin secretion. (6) The catecholestrogens, by virtue of their structural similarity to the neurotransmitters, may mediate the central feedback actions of the gonadal steroids.