Serotonin and the regulation of pituitary hormone secretion and of neuroendocrine rhythms.

The importance of 5-HT synaptic transmission for induced or cyclic activation of pituitary secretion now seems widely agreed upon. Raphe structures, the suprachiasmatic nucleus, and, hypothetically, identified mesencephalic 5-HT containing neurons can be assumed to represent parts of the neuronal circuitry of a central clock which synchronizes neuroendocrine rhythms. Cyclic input from the pineal gland may also be involved, though the meaning of this has not yet been clearly established. The primary role of the transmitter may be to modulate transmission to neurosecretory neurons at the output of this clock. This modulation is not hormone-specific and affects, simultaneously, several endocrine functions. 5-HT has also been shown to facilitate the pituitary response to discrete neuroendocrine reflexes, such as the suckling-induced release of prolactin. 5-HT interactions with hormone control provide a good illustration of the neuromodulatory role of the amine.