GnRH.

Gonadotropin-releasing hormone, the top of the reproductive hormone axis.

Gonadotropin-releasing hormone is a short peptide released by the hypothalamus that instructs the pituitary to secrete luteinizing hormone and follicle-stimulating hormone, which in turn drive the gonads to produce sex hormones and gametes. It sits at the top of the reproductive endocrine axis, and it is released in pulses rather than continuously. That pulsatility is not a detail, it is the mechanism, and it produces one of the most instructive results in endocrinology. Delivered in a pulsatile pattern, GnRH stimulates the axis as intended. Delivered continuously, the same molecule desensitizes the pituitary and shuts the axis down, which is why long-acting GnRH agonists are used clinically to suppress sex hormone production. The same compound therefore produces opposite outcomes depending purely on the timing of delivery, with an initial flare of stimulation preceding the suppression. GnRH antagonists achieve suppression directly without that flare. This example is worth carrying into any reading about peptides, because it demonstrates concretely that a receptor's response depends on the pattern of stimulation and not only on the molecule and the amount. Continuous strong activation of a pulsatile system is a different intervention from mimicking its natural rhythm.