Serotonin – Important for Physical and Mental Health

Dear BLOG readers, dear PEAK customers,

We’ve already covered several hormones in our blog, particularly those that directly influence muscle growth.

Blog post on testosterone

Blog post on insulin

Blog post on IGF-1

Blog about the growth hormone HGH

Today, I’d like to focus on a hormone that plays important roles in both physical and mental health—namely, serotonin.

Serotonin functions in the body as both a hormone and a neurotransmitter.

Serotonin is commonly referred to as the “happiness hormone.” In Part 1, I’ll explain why this is the case and the wide-ranging functions serotonin performs throughout the body.

In Part 2, I’ll explore serotonin-related conditions, particularly depression, and explain how we can positively influence our serotonin levels through exercise and diet.

What distinguishes hormones from neurotransmitters?

Although it is usually referred to as a hormone, serotonin is both a hormone and a neurotransmitter.

Neurotransmitters are chemical messengers found at nerve synapses. They are produced in nerve endings and in the brain. Some of these chemical messengers are also produced in the stomach lining (secretin), the muscles, the skin (dermis), or other parts of our body.

Hormones are signaling substances produced in specific organs. They are distributed to all parts of the body via the bloodstream and lymphatic system. In cooperation with the nervous system (neurotransmitters), hormones regulate crucial functions such as metabolism, growth, and sexuality.

All signaling substances (neurotransmitters, neurohormones, or hormones) are viewed as one large, interconnected system. Most signaling substances are present throughout the body. Activity, temperament, and so on depend on factors such as how neurotransmitters like norepinephrine and dopamine energize us, the extent to which serotonin calms us, or the influence of performance-enhancing but energy-consuming thyroid hormones, and so on.

Properties of Serotonin

Serotonin belongs to the group of biogenic amines. Biogenic amines are produced from amino acids through the process of enzymatic decarboxylation. In the case of serotonin, it is formed from the amino acid tryptophan via the intermediate 5-hydroxytryptophan (5-HTP). Vitamin B6 is also required for the formation of serotonin.

Serotonin induces a certain degree of indifference. A state of alertness caused by norepinephrine is dampened by serotonin. Surges in activity caused by catecholamines are curbed by serotonin. An excess of serotonin supports the analgesic (pain-inhibiting) action of the body’s own morphine by raising the pain threshold (the intensity of stimulation required to trigger a sensation of pain).

Serotonin—Occurrence

Nature

Serotonin occurs naturally in the plant kingdom. Walnuts, for example, contain up to 300 µg/g of serotonin. Small amounts of serotonin are also found in pineapples, kiwis, bananas, plums, tomatoes, and cocoa. Serotonin also occasionally appears as a minor component in psychoactive herbal drugs.

Walnuts contain exceptionally high amounts of serotonin!

Body Fluids

Serotonin can be detected in several bodily fluids in the human body:

• Brain and spinal cord
• Blood plasma
• Urine

In the blood, serotonin occurs exclusively as a component of platelets. The enterochromaffin cells of the gastrointestinal tract produce serotonin and release it into the blood, where it is taken up by the platelets.

Gastrointestinal tract

The largest amount of serotonin is found in the gastrointestinal tract. About 95%, or approximately 9.5 mg, is stored there. 90% of the 95% in the gastrointestinal tract is stored by the so-called

enterochromaffin cells. These are located in the epithelial tissue and independently produce tissue hormones. These hormones regulate the gastrointestinal tract. The remaining 5–10% are found in the nerve cells (neurons) of the enteric nervous system.

Central Nervous System

In the central nervous system, serotonin is found primarily in the neurons of the raphe nuclei, a group of nuclei distributed throughout the entire brainstem.

Biosynthesis and Degradation of Serotonin

Absorption

When taken orally, approximately 75% of serotonin is absorbed (into the bloodstream).

Biosynthesis

Serotonin is synthesized in two steps from the amino acid tryptophan. Tryptophan is the precursor to serotonin. The enzymes tryptophan hydroxylase and aromatic L-amino acid decarboxylase are required for this conversion.

In step 1, tryptophan is converted into the non-proteinogenic amino acid 5-hydroxytryptophan (5-HTP). In step 2, 5-HTP is converted into the end product, serotonin.

The main site of serotonin production is the intestinal mucosa. From there, serotonin is distributed throughout the body via platelets. Since serotonin cannot cross the blood-brain barrier, it must be produced separately in the central nervous system.

Breakdown

Serotonin produced in nerve cells is reabsorbed from the synaptic cleft via a serotonin transporter (SERT) and recycled.

Serotonin released from the enterochromaffin cells of the intestinal mucosa is also bound via SERT in the intestinal epithelial cells and to thrombocytes.

Urine

The primary enzyme responsible for the breakdown of serotonin is monoamine oxidase (MAO) type A. Through a synthetic step, serotonin is converted to 5-hydroxyindolylacetaldehyde. A further synthetic step converts it to aldehyde dehydrogenase with the help of the enzyme aldehyde dehydrogenase.

Aldehyde dehydrogenase can be detected in urine and is considered the main excretion product of serotonin.

Melatonin

A small portion of the serotonin found in the brain is converted, through several synthetic steps, into the pineal gland hormone melatonin. The enzymes involved are serotonin N-acetyltransferase, acetyl-coenzyme A, acetylserotonin O-methyltransferase, and S-adenosylmethionine.

The accompanying diagram shows the biosynthesis and degradation of serotonin in the synaptic cleft.

Serotonin—Functions

General

In general, active serotonin in the brain has mood-enhancing, relaxing, sleep-promoting, antidepressant, and motivational effects. It is involved in regulating mood, sleep patterns, and sex drive.

It is involved in regulating appetite and satiety and also acts as a pain inhibitor.

In the gastrointestinal tract, it is involved in regulating motility, motor function, and absorption. In blood vessels, serotonin causes vasoconstriction.

There are 14 different serotonin receptors. These transmit their signals partly via ion channels and partly via G-protein-coupled mechanisms. Serotonin receptors are distributed depending on tissue, cell type, and physiological state, allowing the body to respond to specific serotonin concentrations through different signal transduction pathways (further signal mediation).

Cardiovascular System

In the cardiovascular system, serotonin causes both contraction and relaxation of vascular smooth muscle. Serotonin-mediated vasoconstriction occurs primarily in the kidneys and lungs. Serotonin has a vasodilatory effect in skeletal muscle.

When serotonin is injected, a three-phase reaction occurs within a few seconds:

• Initial drop in blood pressure
• Rise in blood pressure
• Prolonged hypotension (low blood pressure)

Serotonin can also regulate blood pressure and vascular tone via the central nervous system.

Blood clotting

Platelets are directly involved in blood clotting. The clumping of platelets causes the closure of wounds, etc. Platelets store serotonin but also have their own serotonin receptors.

Platelet aggregation occurs via serotonin receptors in conjunction with ADP and thrombin. Serotonin reduces blood flow through vasoconstriction (narrowing of blood vessels) and thus further contributes to wound healing.

Gastrointestinal Tract

In the gastrointestinal tract, serotonin exerts both motor and sensory effects. The gastrointestinal tract responds to serotonin produced in the enterochromaffin cells of the intestine, as well as to serotonin-mediated signals from the enteric nervous system.

Motor

The serotonin produced in the intestinal mucosa activates nerve cells in the enteric nervous system. The transmission of these signals occurs partly via the neurotransmitter acetylcholine and partly via inhibitory motor neurons using the transmitter nitric oxide (NO).

Sensory

As a sensory neurotransmitter, serotonin—after signals are transmitted via the “vagus nerve”—leads to nausea and vomiting in the brain. Gastrointestinal symptoms are also transmitted to the brain via serotonin

Digestion

The main cells responsible for producing serotonin are the enterochromaffin cells. Increased intra-intestinal pressure triggers the release of serotonin, thereby activating peristalsis. Aromatic substances also stimulate serotonin production and, consequently, intestinal movement (motility).

Eye

In the eye, serotonin acts as a regulator of intraocular pressure via receptors located there. Certain nerves in the eye use serotonin as a neurotransmitter.

For this reason, taking selective serotonin reuptake inhibitors (SSRIs) can lead to the development of glaucoma.

Central Nervous System

Diagram of the CNS Serotonin System

From the raphe nuclei (nuclei of the CNS found throughout the brainstem), serotonergic neurons project to all regions of the brain, thereby influencing nearly all brain functions both directly and indirectly.

Since serotonin cannot cross the blood-brain barrier, it must be produced locally. In the CNS, serotonin influences perception, sleep, temperature regulation, sensory function, pain perception, appetite, sexual behavior, and hormone secretion.

Serotonin acts both at the synaptic cleft and through release over exposed nerve endings.

Mood

Excessively high serotonin levels in the brain—for example, due to the use of selective serotonin reuptake inhibitors—can cause restlessness and hallucinations. Depression, anxiety, or aggression can often be attributed neurochemically to a serotonin or tryptophan deficiency.

Serotonin generally has an inhibitory effect on impulsivity and aggressive behavior. This occurs through stimulation of brain regions responsible for emotional regulation. Specific serotonin receptors are present in these regions for this purpose.

The euphoric and hallucinogenic effects, such as those caused by taking LSD, are due to the activation of a different type of receptor.

Sleep-Wake Cycle

Here, serotonin’s role is to regulate the waking state. This occurs by influencing the hypothalamus, the seat of the human body’s internal clock. Serotonin-containing neurons are active during wakefulness, inhibited during deep sleep, and virtually completely inactive during REM sleep (paradoxical sleep). At the same time, the timing of sleep is also regulated by the release of melatonin.

Prolonged psychological or psychosomatic stress leads to a depletion of serotonin (and norepinephrine) in the body. The consequences of this reduced serotonin availability and activity include sleep disturbances, a lack of motivation to move, and depression.

Appetite

Serotonin and Carbohydrates

The release of serotonin in the brain is directly linked to food intake. The consumption of carbohydrates leads, via the release of insulin, to an increase in the uptake of tryptophan across the blood-brain barrier, which results in the release of serotonin in the brain.

Appetite Suppressant

High serotonin levels are associated with appetite suppression. Overweight individuals often have reduced tryptophan levels in the blood and low serotonin levels in the brain.

A side effect of selective serotonin reuptake inhibitors is a loss of appetite. This effect is mediated primarily through two specific serotonin receptors.

Pain

Serotonin is released when nerve cells are injured, thereby triggering a pain response. Serotonin stored in the spinal cord also serves to either intensify or reduce pain responses.

Sexual Behavior

Upon ejaculation, serotonin is immediately released into the hypothalamus. This primarily results in a suppression of libido and sexual function. Here, serotonin acts as an antagonist to dopamine.

Here, too, elevated serotonin levels caused by selective serotonin reuptake inhibitors (SSRIs) can lead to impaired erectile function, a reduction in sexual desire, or delayed ejaculation.

Temperature Regulation

Depending on the area of the brain where it is released, serotonin can lead to an increase (hyperthermia) or a decrease (hypothermia) in body temperature. Here, too, only certain receptors are involved in mediating this effect.

Serotonin Deficiency

A serotonin deficiency, either on its own or in combination with a deficiency in melatonin or dopamine, can cause depression, anxiety, difficulty concentrating, insomnia, migraines, fibromyalgia (nonspecific connective tissue pain), sensory disturbances (MCS), or severe fatigue. You can learn more about this in Part 2 of my discussion.

Summary

We have seen that serotonin performs a wide variety of functions in both the body and the brain. To synthesize serotonin, we need tryptophan and vitamin B6. Too much or too little serotonin can affect not only important physical functions such as body temperature, digestion, or blood clotting. Mental functions such as appetite regulation, mood, and sexual behavior are also influenced by serotonin.

In Part 2 of my discussion, I’d like to address possible clinical presentations, particularly depression associated with serotonin deficiency. I’ll also describe ways to positively influence serotonin levels through exercise and diet.

Until then, I send you my best wishes and hope you stay in good spirits!

Yours,

Holger Gugg

www.body-coaches.de