Melatonin and hypothalamic-pituitary-gonadal axis.

Melatonin (N-acetyl-5-methoxy-tryptamine), a principal product of the pineal gland, is produced mainly during the dark phase of the circadian cycle. This hormone plays a crucial role in the regulation of circadian and seasonal changes in various aspects of physiology and neuroendocrine functions. In mammals, melatonin can influence sexual maturation and reproductive functions via activation of its receptors and binding sites in the hypothalamic-pituitary-gonadal (HPG) axis. This review summarizes current knowledge of melatonin on the hypothalamus, pituitary gland, and gonads. We also review recent progress in clinical applications of melatonin or potentials of using melatonin, as a reducer of oxidative stress, to improve reproductive functions for the diseases such as women infertility.