The integrating center of the reproductive hormonal axis is the hypothalamus (Figure
1). The hypothalamus is the site of production of the peptide hormone
gonadotropin-releasing hormone (GnRH) which is transported to the adenohypophysis
of the pituitary gland by a short portal venous system where it stimulates the synthesis
and release of gonadotropic hormones (luteinizing hormone-LH and follicle stimulating
hormone-FSH). Both neural input from the central nervous system and humoral
factors from the testis modulate the secretion of GnRH. The GnRH neurons receive
input from neurons in other parts of the brain including the amygdala and both the
olfactory and the visual cortex. The release of GnRH is seasonal (peaks in the spring),
circadian (highest testosterone levels are in the a.m.) and pulsatile (peaks occur every
90-120 minutes). GnRH has a very short half-life in the blood (approximately 2 to 5
minutes). The pituitary gland is therefore exposed to high levels of GnRH in
hypophyseal-portal blood for brief periods of time. This pulsatile pattern of GnRH
release appears to be essential for stimulatory effects on LH and FSH release whereas
constant exposure to GnRH results in paradoxical inhibitory effects on LH and FSH
release.