Mice, melatonin and the circadian system.
Melatonin effects are discussed by reviewing results from mice with intact or disrupted melatonin signaling. Melatonin, the neuroendocrine hand of the clock produced in the pineal gland during night, acts upon two receptor subtypes. Melatonin receptors are found in the suprachiasmatic nuclei (SCN), hypophysial pars tuberalis (PT) and adrenal gland. In SCN, melatonin interacts with PACAP, a neuropeptide of the retinohypothalamic tract. Moreover, melatonin acts on the SCN to modulate the activity of the sympathetic nervous system. Melatonin is not required to maintain rhythmic clock gene expression in SCN. By contrast, the rhythmic clock gene expression in PT depends on a melatonin signal interacting with adenosine. Melatonin may also affect clock gene protein levels in the adrenal cortex and influence adrenal functions. In conclusion, melatonin may serve the synchronization of peripheral oscillators by interacting with other neuroactive substances. A stress-reducing potency of melatonin needs to be explored in further studies.