See also: serotonin and food intake
What is Serotonin
Serotonin - also known as "good mood hormone", 5-hydroxytryptamine or 5-HT - is a neurotransmitter synthesized in the brain and other tissues from the essential amino acid tryptophan.
Functions
As a precursor to melatonin, serotonin regulates circadian rhythms, synchronizing the sleep-wake cycle with daily endocrine fluctuations.
Food and serotonin
Serotonin intervenes in the control of appetite and eating behavior, leading to an early appearance of the sense of satiety, a lower intake of carbohydrates in favor of proteins and a reduction, generally, of the quantity of food ingested. Not surprisingly, many people who complain of a decrease in mood (for example a pre-menstrual depression, see pre-menstrual syndrome) feel an important need for sweets (rich in simple carbohydrates) and chocolate (contains and promotes the production of serotonin, because it is rich in simple sugars, as well as psychoactive substances). Not surprisingly, therefore, some anorectic drugs useful in the treatment of obesity, such as fenfluramine, work by increasing the serotonin signal.
The ingestion of many carbohydrates stimulates the secretion of insulin, a hormone that facilitates the entry of nutrients into cells, including amino acids with the exception of tryptophan. Consequently, after massive insulin secretion in response to hyperglycemia, the relative levels of tryptophan in the blood increase (because those of other amino acids decrease). The relative increase in tryptophan facilitates its passage into the central nervous system, where it increases the production of serotonin; this triggers a classic negative feed-back mechanism that decreases the desire to eat carbohydrates. With a similar mechanism, serotonin levels also increase during physical exertion (which partly explains the antidepressant effects of motor activity); the excessive increase of this substance during a strenuous and prolonged effort is involved in the perception of fatigue.
After a meal rich in proteins, and therefore in tryptophan, the concentration of this amino acid in the blood increases, without however varying the brain levels of Serotonin. This lack of effect is due to the fact that, in parallel, the concentration of other amino acids also increases in the blood which, so to speak, prevent the passage of tryptophan to the brain. For this reason, the intake of food containing tryptophan or a specific supplement does not significantly increase the level of serotonin; even its administration is not possible because it is decomposed before it can produce its effect.
Serotonin and Intestine
Serotonin regulates motility and intestinal secretions, where the presence of enterochromaffin cells containing serotonin is conspicuous; determines diarrhea if present in excess and constipation if present in defect. This action, in particular, is sensitive to the interrelation between the "enteric nervous system" and the brain (Central Nervous System - CNS) and explains how important psychophysical stresses very often have repercussions on intestinal motility.
Serotonin and Cardiovascular System
In the cardiovascular system, serotonin acts on the contraction of the arteries, contributing to the control of blood pressure; it also stimulates the contraction of the smooth muscles of the bronchi, bladder and large intracranial vessels (a massive vasoconstriction of the cerebral arteries seems to trigger the migraine attack like an excessive vasodilation).
Serotonin is also present in platelets, of which it stimulates aggregation by exerting a vasoconstrictor and thrombogenic activity in response to lesion of the vasal endothelium (for example in response to a trauma).
Sexuality and Social Behavior
The serotinonergic system is also involved in the control of sexual behavior and social relationships (low levels of serotonin seem to be linked to hypersexuality and aggressive antisocial behavior). No coincidence that some drugs that increase the release of serotonin and / or the activity of its receptors, such as ecstasy, induce euphoria, a sense of increased sociability and self-esteem. In addition to sexual behavior, serotonin has inhibitory effects on pain sensitivity, appetite and body temperature.
Drugs and serotonin
At the level of the CNS, after being released by the axonic terminal, a part of serotonin interacts with the postsynaptic receptors, while the excess is degrated by MAO (monoamine oxidase) or re-absorbed (reuptake) by the presynaptic terminal, where it is stored in particular vesicles. MAO inhibitor drugs cause an irreversible block of monoamine oxidases, increasing the concentration of serotonin and other cerebral monoamines at the CNS level; they are therefore useful in the therapy of depression, even if their use is today reduced due to the important side effects. At the level of the central nervous system, the deficient serotonin is in fact due to pathological mood declines; a lack of serotonin can therefore cause depressions, but also states of anxiety and aggression. Many antidepressants (such as prozac) work by blocking the reabsorption of serotonin, thereby restoring and enhancing its signal, which is particularly poor in depressed people; the same action is covered by hypericum (or Erba di San Giovanni). Some of these drugs simultaneously increase the signal of serotonin and that of noradrenaline (serotoninergic and noradrenergic effect, typical of duloxetine and venlafaxine). Also some drugs with anti-migraine properties increase the serotonin signal (they are serotinonergic receptor agonists, like sumatriptan), while other medicines taken with the same purposes have an opposite effect (pizotifene and methysergide)
The existence of many drugs able to interfere with the metabolism of serotonin carrying out partly diversified effects depends, as mentioned, on the presence of different receptors (there are at least 7 types), distributed in the various tissues of the body and with which their interact active principles.
Excess of Serotonin
An excess of serotonin causes nausea and vomiting and it is no coincidence that this is one of the main side effects of various antidepressant drugs, such as prozac (the nausea arises in the first week of therapy and then regresses); ondansetron, a drug that acts as an antagonist of serotonin receptors, is instead a powerful antiemetic (it prevents the vomiting reflex, particularly strong during chemotherapy cycles).
