Serotonin's Role in Alcohol's Effects on the Brain

Article excerpt

Serotonin is an important brain chemical that acts as a neurotransmitter to communicate information among nerve cells. Serotonin's actions have been linked to alcohol's effects on the brain and to alcohol abuse. Alcoholics and experimental animals that consume large quantities of alcohol show evidence of differences in brain serotonin levels compared with nonalcoholics. Both short- and long-term alcohol exposure also affect the serotonin receptors that convert the chemical signal produced by serotonin into functional changes in the signal-receiving cell. Drugs that act on these receptors alter alcohol consumption in both humans and animals. Serotonin, along with other neurotransmitters, also may contribute to alcohol's intoxicating and rewarding effects, and abnormalities in the brain's serotonin system appear to play an important role in the brain processes underlying alcohol abuse. KEY WORDS: acute AODE (alcohol and other drug effects); chronic AODE; serotonin receptors; brain; AODU (alcohol and other drug use) development; neurotransmission; synapse; neurotransmitters; serotonin uptake inhibitors; GABA; dopamine; receptor proteins; AOD abuse; AOD intoxication; AOD dependence; reinforcement; animal model; literature review

Neurotransmitters are chemicals that allow signal transmission, and thus communication, among nerve cells (i.e., neurons). One neurotransmitter used by many neurons throughout the brain is serotonin, also known as 5-hydroxytryptamine (5-HT). Serotonin released by the signal-emitting neuron subtly alters the function of the signal-receiving neurons in a process calle neuromodulation. For example, in some neurons serotonin alters the rate at which the cells produce the electrical signals (i.e., action potentials) used for relaying information within the cells, whereas in other neurons it modulate the release of other neurotransmitters. (For more informa tion on the mechanisms underlying signal transmission within and among neurons, see the article "The Principle of Nerve Cell Communication," pp. 107-108.) Although serotonin's effect on individual neurons can be rather mo est, its overall effect on the neurons in a given brain area can substantially influence brain functions such as learning and memory, perception of the environment, mood states, and responses to alcohol and other drugs of abuse.

This article reviews serotonin's functions in the brain and the consequences of acute and chronic alcohol consumption on serotonin-mediated (i.e., serotonergic) signal transmission. In addition, the article summarizes recent findings indicating that serotonin may play a pivotal role in the development of alcohol abuse.1

SEROTONIN'S FUNCTIONS IN THE BRAIN

Serotonin is produced in and released from neurons that originate within discrete regions, or nuclei, in the brain (Cooper et al. 1991). Many serotonergic neurons are located at the base of the brain in an area known as the raphe nucleus, which influences brain functions related to attention, emotion, and motivation. The axons of the neurons in the raphe nucleus extend, or project, throughout the brain to numerous regions with diverse functions. These brain regions include the amygdala, an area that plays an important role in the control of emotions, and the nucleus accumbens, a brain area involved in controlling the motivation to perform certain behaviors, including the abuse of alcohol and other drugs. In these brain regions, the axon endings of the serotonergic neurons secrete serotonin when activated. The neurotransmitter then traverses the small space separating the neurons from each other (i.e., the synaptic cleft) and binds to specialized docking molecules (i.e., receptors) on the recipient cell.

The binding of serotonin to its receptors initiates a series of biochemical events that converts the extracellular, chemical signal into an intracellular signal in the recipient cell. For example, the interaction of serotonin with one type of receptor stimulates the formation of small molecules (i. …