Mental Illness, Serotonin, and Genetics

The Serotonin System

Serotonin is a neurotransmitter, one of the chemicals that nerve cells use to communicate. The bodies of most of the nerve cells that use serotonin are located in a region of the brain stem called the raphe nuclei. These nerve cells send their axons to many other regions of the brain, relaying messages to areas that are involved in emotion, planning, sleep-wake cycles, eating, drinking, and many other important processes (Lucki, 1998).

In the search for biological causes of mental illness, researchers have paid a great deal of attention to the serotonin neurotransmitter system. Several lines of evidence implicate serotonergic dysfunction in a number of mental illnesses. For example, important evidence that serotonin is involved in depression is that drugs that reduce depression symptoms have direct effects on the serotonin system. Each of the three major classes of antidepressant drugs—the tricyclics, the monoamine oxidase inhibitors, and the selective serotonin reuptake inhibitors (SSRIs)—affects serotonin system function. In addition to depression, drugs that act on the serotonin system have been shown to be somewhat effective in treating other mental illnesses, such as obsessive compulsive disorder, anxiety disorders, schizophrenia, and alcoholism (Blier & de Montigny, 1999).

Serotonin System Components

The serotonin system, like other neurotransmitter systems, is made up of several components that enable nerve cells to communicate with other nerve cells (Baumgarten & Gothert, 1999). Each component has a role in communication, and while it appears that some components are not vital for survival, because they can be genetically deleted (i.e., knocked out) in mice and the mice do not perish, it is likely that a healthy serotonin system requires all components [Page 713]to function properly. In humans, there are common genetic differences that affect how well some of these components function. Although it is clear that genetic factors underlie serotonin system dysfunction, it is not well understood how combinations of genetic differences in serotonin system components contribute to serotonin system dysfunction.

One component synthesizes serotonin (tryptophan hydroxylase), while another breaks it down (monoamine oxidase A). Serotonin receptors are important components in nerve cell communication; they are located on the outer surface of nerve cells and receive serotonin much like a baseball glove catches a baseball. At least 14 different types of serotonin receptors have been identified. Such variation is likely to enable serotonin to convey more than one message.

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