Introduction
Serotonin (5-hydroxytryptamine) is a tissue hormone and a neurotransmitter (transmitter of nerve cells).
Definition
Serotonin is a hormone and neurotransmitter, i.e. messenger substance of the nervous system. Its biochemical name is 5-hydroxy-tryptophan, which means that serotonin is a derivative, i.e. derivative of the amino acid tryptophan. The effect of a hormone and neurotransmitter always depends on its receptors on the target cells.
Since serotonin can bind to several receptors, it has a very broad spectrum of action, although it is found primarily in the brain stem. Formation of serotonin: The hormone serotonin is synthesized from the amino acid tryptophan via the intermediate product 5-hydroxy-tryptophan, which is either produced in nerve cells of the brain or in specialized cells such as the enterochromaffin cells of the intestine. The serotonin in the intestinal cells is broken down by monoaminooxidase (MAO) and other enzymes, which form the final product 5-hydroxyindoleacetic acid.
This breakdown product is ultimately excreted with the urine. In its function as a neurotransmitter, serotonin is reabsorbed into the releasing nerve cell and thus recycled. Serotonin is also the starting point for the synthesis of the hormone melatonin, which is produced in the pineal gland (epiphysis). The receptors matching serotonin are cell surface receptors or ion channels.
Tasks
Serotonin acts as a mediator between nerve cells and in this way transmits information. It is probably best known for its mood-lifting effect, which is why it is often called the “happiness hormone”. In fact, it plays a major role in our limbic system.
This is the system in which our emotions are processed. If a lot of serotonin is produced and released, we feel happy. But it can do even more.
It works on the nerves that transmit pain, inhibits and controls the human sleep-wake rhythm. Serotonin is also a hormone, i.e. a messenger substance that takes over tasks outside the nervous system. In its function as a hormone, it is involved in the regulation of the blood flow to organs and promotes the movements of the intestine.
Regulation: The stimulation of serotonin release is tissue-specific; for example, it is released when the blood platelets (thrombocytes) are activated. The effect is terminated when the hormone is broken down or resumed in the nerve cells. Serotonin is characterized by many effects.
These partially contradictory (antagonistic) effects of the hormone are made possible by the many different serotonin receptors. Serotonin influences the cardiovascular system, the gastrointestinal tract, blood clotting, the central nervous system, intraocular pressure and cell growth. Depending on the organ, the hormone enables either vasoconstriction (constriction) or dilation (dilatation) of blood vessels.
In the muscles, vasodilatation takes place after exposure to serotonin, so that blood circulation is increased. In the lungs or kidneys, on the other hand, the effect of the hormone causes vasoconstriction. Overall, the influence of serotonin on systemic blood pressure is complex.
Effects are achieved both directly on the vessels and via the central nervous system, which interact with each other in the level of blood pressure. In the gastrointestinal tract, serotonin acts on the one hand directly as a hormone and on the other hand as a nerve transmitter of the enteric nervous system (intestinal nervous system). In its function as a neurotransmitter, serotonin promotes intestinal motility and the transport of food (peristalsis), which occurs through alternating tension and slackening of the intestine.
The stimulus for nausea and vomiting and the information about pain in the intestinal area are also transmitted by serotonin. The second mode of action as a hormone begins with the release of serotonin from the intestinal cells with an affinity for enterochromaffin. After ingestion of food, the hormone is released through the food pulp due to the increased pressure of the intestinal lumen, so that the resulting increase in peristalsis enables digestion and food passage.
With regard to blood clotting, serotonin stimulates blood clotting by increasing platelet aggregation (thrombocyte aggregation). When a clot (thrombus) forms, the hormone is released from the blood platelets (thrombocytes) that bind to it, causing vasoconstriction and promoting clotting. Serotonin also acts as an enhancer of other blood-clotting-promoting substances.
In the central nervous system there is the so-called serotonergic system. The origin of this system can be found in special nerve nuclei, the raphe nuclei of the brain.These nerve nuclei are distributed throughout the brainstem. Serotonin is involved in the regulation of sleep, mood, temperature, pain processing, appetite and sexual behavior.
In particular, the hormone promotes alertness. It is secreted more when awake, but hardly at all during sleep. The hormone melatonin, which is produced in the pineal gland (epiphysis), is involved in the regulation of the sleep-wake rhythm.
Serotonin also reduces appetite, which is controlled by the tryptophan concentration in the blood. When it rises, more insulin is released, so that the absorption of tryptophane into the cerebral circulation (via the blood-brain barrier) is stimulated. The excess supply of tryptophan increases the production of serotonin, which has an appetite-suppressing effect.
Regarding mood, serotonin euphorizes, can cause hallucinations and inhibits impulsive or aggressive behavior. The feeling of anxiety and depressive moods are reduced by serotonin. Serotonin also regulates the processing of pain and the body temperature; sexual behaviour and sexual functions are inhibited.
Serotonin also promotes wound healing by stimulating the growth of certain cells. This effect as a growth hormone is also found in heart cells (myocytes), which are also stimulated to proliferate by serotonin. Furthermore, serotonin has certain functions in the human eye.
It is responsible for the intraocular pressure, which is probably regulated by the pupil width and the amount of aqueous humor. When the formation of aqueous humor is increased, the pressure inside the eye rises, as well as when the pupil dilates, since this causes the outflow path of the aqueous humor to be blocked. Increased serotonin levels after eating chocolate are discussed.
This could be explained by the tryptophan contained in chocolate, which is converted into serotonin by the body, so that the serotonin concentration increases. This is used to explain the mood-lifting effect of chocolate. Another opinion says however that not the tryptophane of chocolate, but the high amount of carbohydrates is responsible for the mood-lifting effect.
Serotonin is associated with the development of depression and migraine, among other things. Depression is an affective disorder and describes the state of joylessness and dejection. It includes inhibition of drive, thinking disorders and insomnia.
A lack of serotonin is discussed as one of the causes of depression, although this is not fully understood. It has been observed that the uptake of serotonin into the brain and blood platelets is reduced, which has been attributed to a genetically modified serotonin transporter. Migraine is a disease with recurring unilateral pulsating headaches.
In addition, other symptoms such as nausea, vomiting, sensitivity to light and noise (photophobia, phonophobia) can accompany the pain. Prior to this, the occurrence of a so-called aura is possible, which is characterized by visual or hearing disorders, sensitive or motor deficits. Before and after a migraine attack, varying levels of serotonin have been observed in patients with migraine headaches, whereby a low level probably promotes the spread of the headache.
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