Serotonin
Authors: Azmitia Efrain C 
Published: April 2013

Serotonin was discovered in 1949 and has been detected in all living aerobic organisms and in every tissue of the human body. In animals, serotonin functions both as a neurotransmitter and a trophic factor. As a neurotransmitter, serotonin can modify a variety of biological and behavioural functions, including sex, aggression, appetite, locomotor activity, learning and memory, sleep and hormonal secretion. As a trophic factor, serotonin is involved in the neuronal neurogenesis and neural maturation and has been implicated in the release of the cytoskeletal stability factor, S100b. The trophic actions of serotonin in human foetus begin soon after conception and are dependent on supplies provided by the mother's synthesis of serotonin in the gut by enterochromaffin cells and subsequent transfer in blood platelets. The baby not only gets most of its serotonin from the mother while in the uterus, but also makes serotonin very early in gestation when serotonin neurons appear in the midbrain, and serotonergic fibres soon spread throughout the brain. Serotonin at these early times is a differentiating factor and enhances cell mitosis, migration and maturation in subcortical, cortical and peripheral tissues. Serotonin neurons are sensitive to a large number of trophic, neurotransmitter, hormonal and sensory inputs and it has been proposed that this single chemical system serves as a brain homoeostatic regulatory. It is not surprising that serotonin is implicated in a variety of human illnesses, such as depression, Alzheimer's disease, attention deficit disorder, anorexia nervosa, bulimia, autism and schizophrenia. Therefore, when discussing the function of serotonin, it should be remembered that this molecule is ancient and predates the formation of the nervous system in both phylogeny and ontogeny.

Key Concepts:

Serotonin is made from oxygen, tryptophan and reducing cofactors by two enzymes: tryptophan hydroxylase and L‐aromatic amino acid decarboxylase.

Tryptophan is able to absorb photons from sunlight (blue wave) and convert to biological energy and is essential for photosynthesis; producing oxygen in aerobic cells.

Serotonin made in large quantities in plants, where it appears to serve as regulatory mechanism to prevent excess oxygen from damaging cells.

Serotonin neurons in the human brain can activate at least 14 separate receptors and activation of the 5‐HT2A receptor by psilocybin, mescaline and lysergic acid diethylamide produces hallucinations.

Serotonin in humans is made in a restricted midline area of brainstem called the raphe nuclei and has axonal connections to nearly every region of the brain.

Serotonin neurons are born early in gestation by the action of several genes including
PET‐1
, a transcription factor specific for this neuronal system.

Serotonin acts as a trophic factor regulating cell proliferation, maturation and apoptosis by direct receptor actions as well as release of the glial protein, S100b.

A decrease in serotonin neurons is associated with depression and suicide, and neurons show evidence of neurodegeneration in autism, Alzheimer's disease, Parkinson's disease, frontal lobe dementia and Lewy‐body dementia.

Serotonin is made within enterochromaffin cells in the gut and collected in blood platelets for transfer to all cells in the body and serves as a vehicle by which the mother can influence the development of her baby.

Melatonin is made from serotonin.