Melatonin has multiple mechanisms through which it can achieve different results in the organism. Among these, one differentiates between receptor-mediated effects, i.e. those that function according to the key-keyhole principle, and those mechanisms that take place even without the use of docking sites.
Melatonin receptors can be divided into different classes. A differentiation is made between melatonin receptor 1 and 2 (MTL1 and MTL2), which each have their own specific distribution and partially specific function. Both are so-called membrane receptors, on the outside of which melatonin is able to dock. If this type of bond occurs, then a variety of processes are triggered through specific G proteins inside the cell, which lead to subsequent effects of melatonin on the cell. Additionally, there is a third class of receptors, which is not anchored within the cell membrane, but is located inside the cell. However, this class of receptors is only found in certain species of animals and does not play a significant role for humans.
The melatonin receptors 1 and 2 mentioned above are distributed among a number of organs, where one finds them on specific cell structures. The time information, which is based on the oscillating (swinging) melatonin concentration, is transferred to tissues and the entire body.
The highest density of melatonin receptors can be found in certain areas of the brain. It is interesting to note that the distribution can be very different among various species. For example, rodents and mammals mainly possess receptors in the areas that control the inner clock and gonad functions (organs of the body which are needed to produce sex hormones and serve for reproduction), while in humans the other area (besides the inner clock that contains the most receptors) is in the cerebellum and cerebral cortex, but not in the areas that control gonad functions. This makes it clear that the function of melatonin has changed in the course of evolution, and that not all the results that can be collected from animal models can be transferred to humans one-to-one. Besides the receptors in the brain, many of these binding sites have been discovered in other peripheral organs. A few of these organs that contain melatonin receptors include the pancreas, liver, eyes, skin, certain blood vessels, the intestinal tract and parts of the gonads. The exact function of these receptors is still being heavily researched. It is certain that these connection sites play a part in connecting the day/night rhythm with specific organ functions. On the other hand, it is also postulated that melatonin has additional functions which are most easily described as fine-tuning and synchronizing the interactions between organs.
In addition to the receptor-transmitting function of melatonin described above, it also has an anti-oxidative effect. In this case it does not require specific binding spots, but instead acts as a potent radical scavenger. Results from experiments on animals can be directly transferred to humans, as the function is solely a result of the molecular structure of melatonin. Since melatonin can enter almost all cells, it is able to protect all organs as soon as it is present. In consideration of the fact that the hormone is mostly produced at night and can very easily pierce through the blood-brain-barrier, it can play a preventative role against oxidative disturbances in both the brain’s nerve cells, as well as in other organs during sleep at night. This also helps to explain a number of positive effects of melatonin which are not directly related to its interaction with specific receptors, but rely on its radical-scavenging characteristic, whether it be in animals or humans.