Neuroendocrine control of ovulation

Abstract

Modern methods of diagnosis have made the distinction between hypothalamic failure and ovarian failure routine. Failure of the orderly progression of hypothalamic gonadotrophin-releasing hormone (GnRH) → pituitary gonadotrophins → ovarian steroids and inhibin → hypothalamus/pituitary results in anovulation/amenorrhea. The hypothalamic connections that regulate the pattern and amplitude of GnRH pulses are plastic and respond to external/psychological conditions and internal/metabolic factors that may affect the hypothalamic substrate on which estrogen levels can act. We trace the neuroendocrine regulation of the ovarian cycle, concentrating on hypothalamic connections that underlie negative and positive feedback control of GnRH and the complementary role of the adenohypophysis. The main hormone regulating this "central axis" and the development of the endometrium is estradiol which is exported from the developing ovarian follicles and thereby closes the feedback loop with follicle development. Progesterone and inhibin are also involved. Neuroendocrine responses to internal and external factors can cause anovulation and amenorrhea. Generally, these are accompanied by abnormal negative feedback between estradiol and the gonadotrophins; coexistence of low estradiol and luteinizing hormone/follicle-stimulating hormone. There are three main causes: (1) genetic diseases that interfere with the migration of GnRH cells into the brain or result in misfolding of GnRH; (2) input from the brain that interrupts normal feedback (e.g. stress and weight loss amenorrhea); and (3) the effect of agents which alter central neurotransmission and hypothalamic function (e.g. elevated prolactin and psychotropic medications). All types of hypothalamic insufficiency result in insufficient stimulation of the ovaries. In addition to amenorrhea, this central alteration also results in other complications (downstream disease) that make hypothalamic amenorrhea of greater consequence than simply reproductive failure. Thus, there may be more at stake in the diagnosis and treatment of hypothalamic failure than brings the patient to her caregiver.