A brief review about melatonin, a pineal hormone

Abstract

Melatonin is a ubiquitous molecule in nature, being locally synthesized in several cells and tissues, besides being a hormone that is centrally produced in the pineal gland of vertebrates, particularly in mammals. Its pineal synthesis is timed by the suprachiasmatic nucleus, that is synchronized to the light-dark cycle via the retinohypothalamic tract, placing melatonin synthesis at night, provided its dark. This unique trait turns melatonin into an internal synchronizer that adequately times the organism's physiology to the daily and seasonal demands. Besides being amphiphilic, melatonin presents specific mechanisms and ways of action devoted to its role as a time-giving agent, being widely spread in the organism. The present review aims to focus on melatonin as a pineal hormone with specific mechanisms and ways of action, besides presenting the clinical syndromes related to its synthesis and/or function disruptions.

Figure 1. Melatonin molecule (232,2 molecular weight).

Figure 2. Neural control of pineal melatonin synthesis. RHT: retinohypothalamic tract. SCN: suprachiasmatic nucleus. PVH: paraventricular nucleus. SCG: superior cervical ganglion.

Figure 3. Melatonin synthesis pathway and hepatic metabolization. The enzymes are written in italic and the derived molecules are underlined.

Figure 4. Melatonin mechanisms of action. Classical receptor mediated and non-mediated pathways and the involvement of nuclear, cytosolic and membrane receptors.

Figure 5. Melatonin ways of action. The upper boxes represent the different ways of action. The second-line boxes explain how melatonin causes the correspondent effects. The third-line boxes show examples of the correspondent effects. Note that the Prospective effects are further classified in Proximal ou consecutive and Distal or prolonged effects.