Gonadotropin-releasing hormone (GnRH) is a peptide hormone secreted by hypothalamic neurons that regulates release of luteinizing hormone and follicle-stimulating hormone from gonadotrophs in the anterior pituitary. The hypophyseal portal system carries this upstream signal directly to its pituitary target.
The releasing factor and the released products are separate hormones. That distinction was made experimentally by testing hypothalamic material for its ability to stimulate pituitary output. A 1971 report by Andrew Schally and collaborators described a purified releasing factor that stimulated both luteinizing hormone and follicle-stimulating hormone, helping establish a common hypothalamic input to the two responses.[ref]
Hypothalamic follicle-stimulating hormone (FSH) and luteinizing hormone (LH)-regulating hormone: structure, physiology, and clinical studies by Andrew Schally and others. Abstract: a single purified hypothalamic releasing hormone stimulates both LH and FSH
Timing is part of the signal. In the 1982 sheep study by Iain J. Clarke and James T. Cummins, GnRH pulses measured in portal blood plasma were synchronous with LH pulses measured downstream in peripheral blood plasma.[ref]
The temporal relationship between gonadotropin releasing hormone (GnRH) and luteinizing hormone (LH) secretion in ovariectomized ewes by Iain J. Clarke and James T. Cummins. Abstract: the two sampling locations and pulse synchrony
The hypophyseal portal system article describes that measurement geometry in more detail.A time-averaged concentration therefore discards information about the pattern of hormone delivery. The experiment measured both the upstream signal and its pituitary output, linking a chemical identity to a physiological rhythm.
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