Figure 1.

The connection between melatonin secretion and circadian rhythm. The suprachiasmatic nuclei (SCN), which are located immediately above the optic chiasm in the anterior-ventral area of the hypothalamus, are believed to include the pacemaker of this clock. This circadian clock reset comes about through the retinohypothalamic tract (RHT), which sends light information directly from the retina to a subset of SCN neurons. A pineal hormone known as melatonin is most abundant in the blood at night and least prevalent during the day. Its secretion is governed by a rhythm-generating process in the SCN, which is regulated by light. Melatonin is not only regulated by the circadian oscillator but also provides the oscillator with a feedback signal for darkness. Bilateral structure of the SCN and its “core” and “shell” subregions with vasoactive intestinal peptide (VIP) and gastrin-releasing peptide (GRP) in the light-responsive core and arginine vasopressin (AVP)-expressing cells in the shell; optic chiasm (OC); the 3rd cerebral ventricle (V3).