Pulsatile hormone secretion

You probably picture hormones flowing like water from an open faucet, steady and nonstop. Several of them do not work that way: they come out in bursts, in pulses. Gonadotropin-releasing hormone, GnRH, is secreted in pulses into the hypophyseal portal circulation, a short vein that carries it from the brain to the pituitary, and growth hormone is released the same way.

And that rhythm is not a whim. For GnRH to do its job, two things are needed: that its neurons have migrated to the right place while the body was forming, and that its secretion be pulsatile.

What makes the pulse useful is that it can be changed in two ways: how often it arrives, that is, the frequency, and how strong each one is, that is, the amplitude. Changing either one changes how much luteinizing hormone and how much follicle-stimulating hormone the pituitary makes and releases. Estrogens make the pulses come more often; testosterone and progesterone space them out.

It shows up very clearly across the menstrual cycle: in the follicular phase there is a pulse every one to two hours, just before ovulation the release becomes continuous, and in the secretory phase it spaces out to one every four hours.