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Our brain is a remarkably complex system. This makes the fact that major aspects of our behavior can be controlled by changes in the production of some small molecules quite remarkable. But here it is: neuromediator serotonin is involved in so many different circuits of our brain that we only recently started to get the picture into focus.
Serotonin (5-HT, 5-hydroxytriptophan) is a monoamine neurotransmitter from the group of biogenic amines. Serotonergic neurons producing this compound are involved in a broad range of physiological and behavioral processes such as cardiovascular regulation, respiration, thermoregulation, mood, circadian cycles, appetite, pain sensitivity, sexual behavior, cognition and learning. They play important part in a range of psychiatric disorders (anxiety disorders, depression, schizophrenia) as well as less structured behavioral impulse-related disorders (violence, substance abuse, obsessive control, gambling addiction, attention deficit disorder etc.).
Structure and components of serotonergic system
There is only small number of cells in our nervous system that produce serotonin. They are scattered in hindbrain and midbrain. Largely they are present in several regions of the brain called raphe nuclei. There are only around 300,000 serotonin-producing neurons in human brain, but they have an extensive system of connections which has huge number of branches. As a result, serotonergic system reaches almost all areas of central nervous system.
Serotonergic system is one of the evolutionary oldest and seems to be involved in various inhibitory responses throughout the central nervous system. It opposes most of other brain regulatory systems in terms of behavioral and sensory output.
Serotonin production and sensitivity is regulated by multiple cellular mechanisms
The outcome of the serotonin release by the nervous cell depends on the type of receptor it binds to on the target cell. Receptor is needed to convey the signal of serotonin to the cell. Depending on the receptor type, the result of signalling can be quite different. At least 17 types of receptors responding to serotonin were discovered so far. They are located on different cell and can be involved in very different functions. Some of the most studied receptors are 5-HT 1A-1D, 5-HT 2 and 5-HT 3.
Lots of proteins and enzymes are involved in the metabolism and re-utilization of serotonin and can influence its level. Mutations and defects in corresponding genes affect the levels of serotonin or sensitivity to serotonin and may cause behavioral changes.
A specific protein called Serotonin Transporter (SERT) is responsible for bringing serotonin back to the cell that released it. This protein is targeted by a number of psychoactive drugs (psychostimulants and antidepressants) that slow down the transport activity and thus inhibit the re-uptake of the neurotransmitter.
The enzyme called MAO-A (monoaminooxidase A) converts serotonin into inactive molecules. Inhibition of this enzyme leads to accumulation of serotonin and has positive effect in releasing the symptoms caused by lowered concentration of serotonin.