O que é a serotonina?


A serotonina ou 5-hidroxitriptamina (5-HT) é um neurotransmissor presente em várias áreas do sistema nervoso central. Regula uma multiplicidade de funções as mais importantes das quais estão relacionadas com a depressão.
Em regra os indivíduos deprimidos têm níveis baixos de serotonina no sistema nervoso central. Para obviar a esta situação administram-se inibidores da recaptação da serotonina pelos neurónios, como seja o caso de medicamentos como fluxetina ou PROZAC, como é conhecido comercialmente. Um certo número de alimentos, como bananas, tomate, são ricos no precursor da serotonina, o triptofano. Sem este precursor não é possível sintetizar serotonina suficiente para as suas várias funções.

A serotonina, também chamada 5-hidroxitriptamina é um neurotransmissor (substância envolvida na comunicação de informação entre células nervosas ou entre células nervosas e células efectoras). Também existe em algumas células não nervosas (incluindo alguns tumores intestinais). É fabricada, no nosso organismo, a partir do triptofano (um amino-ácido).
Tem funções de controlo da motilidade e secreção gastrointestinal. No sistema nervoso central está envolvida no controlo do estado de sono ou vigília, consciência, agressividade e humor. Por isso algumas perturbações destes sistemas (por exemplo ansiedade, depressão), podem ser tratadas com medicamentos que interferem com os sistemas da serotonina.

A serotonina é, quimicamente, a 5-hidroxitriptamina. É sintetizada a partir do amino-ácido triptofano, por hidroxilação no anel aromático e descarboxilação, sucessivas. É um mediador do sistema nervoso, abundante em certas áreas como o hipotálamo. Exibe funções vasoconstritoras.

A serotonina é um neurotransmissor, isto é uma molécula envolvida na comunicação entre as células do nosso cérebro (neurónios). Esta comunicação é fundamental para a nossa percepção e avaliação do meio que nos rodeia, e para a nossa capacidade de resposta aos estímulos ambientais. Apesar de serem poucos os neurónios no nosso cérebro com capacidade para produzir e libertar serotonina, existe um grande número de células que detectam esse neurotransmissor. Desse modo, a serotonina desempenha um importante papel no funcionamento do nosso sistema nervoso e existem numerosas patologias relacionadas com alterações na actividade deste neurotransmissor. A serotonina parece ter funções tão diversas como o controlo da libertação de algumas hormonas e a regulação do ritmo circadiano, do sono e do apetite, entre outras. Diversos fármacos que controlam a acção da serotonina como neurotransmissor são actualmente utilizados, ou estão a ser ensaiados, em patologias como a ansiedade, depressão, obesidade, enxaqueca e esquizofrenia, entre outras. Drogas como o "ecstasy" e o LSD mimetizam alguns dos efeitos da serotonina em algumas células alvo.

For most people who are not in touch with science, the word serotonin is
virtually unknown to them. If they have heard of serotonin, they probably
associate it with depression or as a term used in one of their past psychology courses. Serotonin is involved in the state of depression, but it is also involved in human behavior, appetite, hunger, the cardiovascular system, and most notably, the central nervous system. Without the natural chemical serotonin, messages from the brain would not be properly
transmitted throughout the nervous system. This basically means that without
normal levels of serotonin, the simple acts of eating, sleeping, living, and
walking could not occur normally.

The year 1948 marked the first time that serotonin was isolated in blood.
Since it was isolated in blood serum, the name serotonin came about
(Woolley, 1962). Serotonin is a neurotransmitter which helps relay
information throughout the nervous system. Serotonin is stored in the nerve
cells where it sits until the brain relays a message. The nerve cell then
releases the serotonin into the space between it and the next nerve cell.
Acting as a bridge, the serotonin carry the message across to the next cell
(OCD Resource Center, 1997). After the information has been relayed, the
serotonin returns to the nerve cell until the next message comes along.
Serotonin is found in three major areas of the human body: blood vessels,
the central nervous system, and in the intestinal wall. Serotonin is also
believed to be related to mood, appetite, and emotion. According to Dr. P.J.
Cowen of the University of Oxford, serotonin is the controller of mood and
the vital functions of the brain (Cowen, 1998). It is essential that the
human body has a normal level of serotonin between the nerve cells and in
the brain. When serotonin levels are lower than normal, people may suffer
from depression or other psychological and physical problems.

There seems to be a relationship between low levels of serotonin and
depression. People with low levels of serotonin usually suffer from
depression. Likewise, patients who suffer from Obsessive Compulsive Disorder
(OCD), also show lower than normal levels of serotonin between nerve cells
(OCD Resource Center, 1997). OCD patients are characterized as overly
compulsive, very obsessive, and have tremendous anxiety.

Low levels of serotonin are also involved in the acts of aggression, such as
violent crimes or even suicide (CSU, Chico Department of Psychology, 1997).
However, scientists are contiuing to search for a definite correlation
between low levels of serotonin and suicide. For example, scientists have
conducted experiments on monkeys to observe the effects that low levels of
serotonin would have on them. The results showed that the monkeys with lower
than normal serotonin levels acted more aggressively and had a tendency
towards risky behavior.

Lower than normal levels of serotonin may also be associated with the
control of hunger and appetite (CSU, Chico Department of Psychology, 1997).
For the treatment of obesity, drugs are taken to increase the level of
serotonin in the brain, helping to reduce appetite. The drugs used to treat
obesity include fenfluramine/Pondimin and dexfenfluramine/Redux. In recent
years the anti-obesity Œfen-phen,' was developed in order to increase
serotonin levels. On the downside, some users of the drug ultimately died.
According to the International Coalition for Drug Awareness and a
Massachussetts Institue of Technology press release, "too much serotonin
damages blood vessels in lungs, and can harm heart valves" (MIT Press
Release, 1998). To treat depression, drugs such as Prozac, Zoloft, and Paxil
are used to block the reuptake of serotonin in the axon terminals to keep
sufficent levels of serotnin between nerve cells.

Although increasing serotonin by artificial means has proven at times to be
unhealthful, serotonin levels may be increased by natural means. Tryptophan,
an amino acid which is found in the blood stream, can help increase sertonin
levels (Cowen, 1998). Foods such as walnuts, pineapple, turkey, chicken,
yogurt, and bananas contain large amounts of tryptophan. By incorporating
these foods into a normal diet, serotonin levels may increase to normal
levels. Also, by combining these foods with carbohydrates such as pastas,
the brain is able to absorb tryptophan more effectively, which plays a key
role in deciding how much serotonin will be produced in the brain.

Serotonin is one of a group of chemical messengers known as
neurotransmitters that carry out communication in the brain and body. The
message molecules flow from a nerve cell or neuron onto other neurons that
act as receivers. There, they attach to a distinctly shaped area on the
neuron called a receptor site. This union, which is like a key fitting into
a lock, triggers signals that either allow the message to be passed on to
other cells or prevent the message from being forwarded. Since the discovery
of serotonin in the 1950s, researchers are finding evidence that one of its
roles is to mediate emotions and judgment.
For example, in animal studies, scientists discovered that low
serotonin levels may be associated with impulsive or risky behavior.
Researchers observed monkeys and found that the ones who took more dangerous
leaps traveling from tree to tree had lower serotonin levels and more
injuries from falling. Other scientists examined rats and found the ones
with low serotonin levels chose a small immediate reward instead of waiting
for a bigger prize.
Scientists also have compiled studies that show serotonin is
implicated in aggressive acts. One example involves mice who lack one type
of receptor that responds to serotonin. These defective mice attack
intruders faster and more intensely. Other researchers examined the spinal
fluid of murderers in Finland. Their results indicate that these individuals
have abnormally low levels of serotonin.
Some researchers now believe that suicide may be the ultimate act of
inwardly directed impulsive aggression.
In one new area of research, scientists are examining humans with
brain imaging techniques and believe defects in serotonin processing are
partially responsible for suicidal behavior. Preliminary results show that
depressed people who have attempted suicide exhibit lower serotonin activity
(see brain images) in response to a drug that boosts the amount of the
chemical in the brain. In addition, the depressed had a history of comitting
impulsive acts including overeating and aggressive behaviors such as
lighting fires or fighting.
Scientists plan to uncover the brain areas where serotonin's
inactivity causes the most havoc and identify all the components that play a
role in the defective processing. Researchers view the brain in action with
a special imaging technique called positron emission tomography (pet). These
pet scans compare the brain activity of a severely depressed patient (left)
to a healthy volunteer (right). Both people received a drug called
fenfluramine that is known to increase the availability of serotonin in the
brain. The scans demonstrate that only the healthy patient has an increase
in serotonin activity, which is represented by the orange and yellow
shading. This suggests that the severely depressed have a very blunted
response to the neurotransmitter or very low serotonin levels. Researchers
plan to use the technology to devise imaging guidelines that would identify
patients with this deficiency.