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About Serotonin
This section tries to address, in a manner that will be understandable to non scientists, what serotonin is, what we think it does, and why we study it.
Why Study Serotonin:
Neuroscientists believe that brain function is the combined result of millions upon millions of nerve cells communicating with each other in the vast network that we recognize as the brain. Brain cells, or neurons, communicate with each other by releasing molecules, known as neurotransmitters. Serotonin is a neurotransmitter.
Although there are many neurotransmitters, there is good reason to study serotonin in particular. For example Fluoxetine (Prozac) and other related compounds exert their therapeutic effect on depression and obsessive compulsive disorder (OCD) by acting on serotonin in the brain. Furthermore, hallucinogenic drugs such as LSD are also thought to target serotonin sensitive cells by selectively mimicking some of the effects of serotonin. Finally, many normal behaviors also appear to depend on intact serotonin function in the brain. For these reasons, many of the members of the "serotonin club" have spent their scientific lives trying to unravel the biology of serotonin.
But how does serotonin do all of these things and how do drugs, therapeutic or otherwise, affect serotonin? We know that neurotransmitters are released by one nerve cell when that cell becomes electrically activated. Once released, the neurotransmitter contacts and influences the functioning of other cells downstream. From this we believe most drugs that target serotonin affects the ability of serotonin to influence the function of brain cells. Not surprisingly, one large area of research is devoted to trying to understand how this happens.
The Biology of Serotonin
Serotonin is a small molecule related to the essential amino acid tryptophan. In fact serotonin is made by a relatively small number of cells in the brain starting from this amino acid. Cells that make serotonin contain an enzyme called tryptophan hydroxylase. This enzyme adds a hydroxyl chemical group (-OH) to tryptophan to make 5-hydroxytryptophan (5-HTP). Then a second enzyne called amino acid decarboxylase removes a carboxyl group (-COOH) from 5-hydroxytryptophan to make 5-hydroxytryptamine (aka serotonin).
While the enzyme amino acid decarboxylase is found in many brain cells or neurons, the enzyme tryptophan hydroxylase is present only in a small subset of neurons. These cells are located mostly in a series of cell groups in the brainstem collectively known as the Raphe nuclei. It is only these neurons that can make serotonin. Since these cells make serotonin, we call these cells serotonergic neurons.
Neurons connect with each through processes called axons that can extend over long distances. Each serotonergic neurons give rise to an axon that branches extensively to contact many hundreds to thousands to neurons. As a result, the few thousand cells that can make serotonin combined contact a sizeable proportion of the 40 million or so neurons that make up the human brain. In fact most areas of the brain receive at least some inputs from serotonergic neurons. In neuro-speak we say that most areas of the brain receive at least some serotonergic innervation. No wonder serotonin is involved in so many brain functions! |
