Gonadotropin-releasing hormone (GnRH) cells of the terminal nerve as a model neuromodulator system.

Modulation of ionic channel properties by neurotransmitters and hormones is called neuromodulation and may be the basis for many long-lasting changes in animal behavior, e.g. changes in the arousal or motivational states. Gonadotropin-releasing hormone (GnRH), originally identified as a hypophysiotropic hormone, is now believed to act also as a neuromodulator. From studies of electrical activities and morphology of terminal nerve cells (major source of GnRH) of a fish brain, a general hypothesis regarding modulator neurons is proposed; modulator neurons have endogenous oscillatory activities which vary according to the animal's hormonal or environmental conditions. These modulator neurons, in turn, regulate neuronal excitabilities in a wide variety of brain regions simultaneously via multiple axonal branches.