GnRH Pulsatility

GnRH is not secreted continuously. A network of hypothalamic neurons, coordinated by arcuate KNDy cells that co-express kisspeptin, neurokinin B and dynorphin, fires in synchronised bursts that release GnRH into the portal circulation roughly once an hour in the follicular phase and more slowly at other times. The pituitary reads the interval as well as the amount: faster pulses favour transcription of the LH beta subunit, slower pulses favour FSH beta. Sustained occupancy of the receptor, by contrast, desensitises the gonadotrope entirely.

The demonstration is one of the cleanest in endocrinology. In rhesus monkeys with hypothalamic lesions, Knobil's group in the late 1970s restored normal gonadotropin secretion with an infusion delivering one pulse per hour, then abolished it again by giving the identical total quantity as a continuous infusion. That result is now therapy: portable pumps delivering pulsatile GnRH restore gonadotropin secretion and fertility in congenital hypogonadotropic hypogonadism, including Kallmann syndrome. Persistently rapid pulse frequency, favouring LH over FSH, is a recognised feature of polycystic ovary syndrome.

What this changes is the assumption that receptor activation is a quantity. Here it is a waveform, and the same ligand at the same total exposure produces either a functioning reproductive axis or a chemically castrated one depending only on timing.

The practical error appears at the blood draw: a single LH sample captures an arbitrary point on a pulse and can differ several-fold from one taken twenty minutes later, so characterising secretion requires frequent sampling over hours, and a low isolated LH is weak evidence on its own. The other is commercial, a listing quoting receptor potency as though potency alone predicted the direction of the endocrine response, ignoring that continuous exposure inverts the outcome.