Brain Chemistry (Neurochemistry)

The brain communicates with itself by sending out chemical information from one neuron, or nerve cell to another. Brain chemistry is the sum of all the chemical messaging that takes place in the brain, which allows it to carry out its daily functions, such as generating movement, speaking, thinking, listening, regulating the systems of the body, and countless others.

Out of Balance: Hector's Story

Now a tenth-grader, Hector had been experiencing severe depression since he was in seventh grade. Everyone feels down or depressed every now and then, but Hector felt this way most of the time. He had a hard time making friends, he was not interested in his schoolwork, and he spent most of his time hanging out in his room alone. He had even thought about suicide. At first, his parents believed that this was just a phase he was going through, but then they became really concerned. What was happening to their son, who had been generally upbeat and friendly until a few years earlier?

At the insistence of his parents and teachers, Hector started seeing a psychiatrist ^* , who tried to help him talk about what he was feeling. Based on her meetings with Hector, she decided to prescribe a type of medication known as an antidepressant. This medication increases the amount of a brain neurotransmitter ^* called serotonin (ser-ah-TO-nin), which is associated with feelings of well-being and control. The medicine, a selective serotonin reuptake inhibitor (SSRI), works by preventing (inhibiting) neurons from reabsorbing (reuptaking) the chemical messenger serotonin once it is released into the brain. As a result, there is more serotonin available, and this sometimes helps alleviate the symptoms of depression. If Hector's depression were being caused by too little serotonin, the medication likely would help him.

Sure enough, it did. Hector continued to see his psychiatrist while taking the medication. After about 6 months, his doctor decided to try taking Hector off the SSRI. Hector was afraid that his terrible feelings would return, but they did not. He found that talking through any problems with his doctor was enough to keep him on track.

A Cascade of Chemicals

Every day, researchers are learning more about the chemicals that the neurons (NUR-ons) in the human brain use to communicate with each other. They now know that all the feelings and emotions that people experience are produced through chemical changes in the brain. The "rush" of happiness that a person feels at getting a good grade on a test, winning the lottery, or reuniting with a loved one occur through complex chemical processes. So are emotions, such as sadness, grief, and stress. When the brain tells the body to do something, such as to sit down or run, this also sets a chemical process in motion. These "chemical communicators," or neurotransmitters, are the "words" that make up the language of the brain and the entire nervous system.

The billions of tiny neurons in the brain communicate with each other across small spaces called synapses (SIN-ap-siz). When one neuron is charged into action, it releases its chemical messenger, which then moves across the synapse to the next neuron, where it is accepted by a special receiving area, called a receptor, on the surface of the neuron. The chemical will be accepted only by receptors that recognize it, in a kind of "lock and key" system, that is, certain keys work only in certain locks. Once attached to a receptor site on another neuron, different neurotransmitters either trigger "go" signals that prompt the neuron to pass certain messages on to other cells or produce "stop" signals that prevent certain messages from being forwarded. (For an illustration of how neurotransmitters work, see "Medications.")

Any one neuron may be receiving many chemical messages, both positive and negative (stop and go), from the other neurons surrounding it. These neurotransmitters may be "competing" to get the neuron to respond in different ways, or they may work together to produce a certain effect. Since all of this happens within a split second, the neurotransmitter must be cleared away quickly so that the same receptors can be activated again and again. This clearing away can happen in one of three ways. The chemical may be pumped back into the nerve ending it comes from, a process known as "reuptake," it may be destroyed by enzymes ^* near the receptor sites, or it may simply spread out into the surrounding area of the brain and be destroyed there.

Modifying Neurotransmission with Drugs

Many neurological (nur-a-LA-je-kal) conditions, ranging from emotional disorders, such as depression, to movement disorders, such as Parkinson disease, are associated with imbalances of certain neurotransmitters in the brain. Researchers have been able to develop many medications that work to correct these imbalances, improving people's symptoms and helping them lead more fulfilling lives. At the same time, many legal and illegal drugs, such as the nicotine in cigarettes and the street drugs heroin and cocaine, work by changing neurotransmitter levels. People report feeling "good" or "up" when they start taking these drugs, but soon the neurons in their brains become so accustomed to the change in chemical balance that it takes more of a drug to get that same feeling, and the brain starts to crave the substance. The result is chemical dependency or addiction. What happens when brain chemicals are modified is described in the three examples of neurotransmitters (serotonin, dopamine, and gamma-aminobutyric acid) below.

Serotonin

Many studies have linked low levels of the neurotransmitter serotonin to depression, impulsive and aggressive forms of behavior, violence, and even suicide. The class of medications called SSRIs, like the one that was given to Hector, prevents the neurons that release this chemical from taking it back in once it is in the synapse. As a result, the person has more serotonin available to attach to receptors in the brain, which can ease the symptoms of depression, as it did for Hector.

An illegal drug known as "Ecstasy," or MDMA, also changes the level of serotonin in the brain, but much more radically. It causes the serotonin-releasing neurons to dump their contents all at once, which floods the brain with the chemical and produces feelings of extreme happiness and hyperactivity (excessive activity). This feeling comes with a price. Since Ecstasy uses up the brain's supply of serotonin, the person is likely to feel depressed when the immediate "rush" of the drug's effects ends after a few hours. This "down" period lasts until the brain can build its supply of serotonin back up to normal levels. Repeated use of Ecstasy may lead to depression or other problems over time, since the neurons can "bounce back" only so often.

Dopamine

Neurons in the core of the brain release dopamine (DO-pa-meen), a neurotransmitter that affects processes that control movement, emotional response, and the ability to experience pleasure and pain. In people who have Parkinson disease, dopamine-transmitting neurons in this area of the brain die, which causes progressive loss of movement control. A medication called L-DOPA, which the brain can convert into dopamine, often helps control these symptoms. Some researchers have theorized that people with the mental disorder known as schizophrenia ^* are, in fact, overly sensitive to the dopamine in their brains. Some of these people seem to have been helped by medications that block dopamine receptors in the brain, thereby limiting the neurotransmitter's effect.

Another class of drugs known as amphetamines (am-FET-a-meenz) work by increasing the level of dopamine that neurons release and then preventing them from taking it back in through the reuptake process. These drugs have medical uses, such as the treatment of attention deficit hyperactivity disorder, but some people misuse amphetamines to help themselves stay awake or perform a task better.

Gamma-aminobutyric acid

Gamma-aminobutyric acid, or GABA, is the main neurotransmitter that works to inhibit the brain's neurons from acting. Research suggests that certain types of epilepsy, which is characterized by recurring seizures that affect a person's awareness and movements, may be the result of having too little GABA in the brain. The neuronal messaging system goes into overdrive, with tens of thousands of neurons sending messages intensely and simultaneously, which produces a seizure. Researchers believe that enzymes may be responsible for breaking down too much GABA, and they have developed medications that appear to help combat this process.

Hormones: Another Piece of the Brain
Chemistry Puzzle

Norepinephrine (nor-e-pi-NE-frin) is a neurotransmitter that is involved in various arousal systems in the brain (systems that bring about alertness and attention) and in the sympathetic nervous system ^* . In the sympathetic nervous system, it is norepinephrine that causes the blood vessels to narrow, raising blood pressure, and speeds the breathing and heart rates. Norepi-nephrine also functions as a hormone when it is released by the adrenal glands located just above the kidneys, with similar results. Norepinephrine, epinephrine, and other hormones produced by the adrenal gland are involved in the "fight or flight" response of the body to stress.

Hormones are chemical substances that are sent into the bloodstream by the endocrine ^* (EN-do-krin) glands found throughout the body. They carry messages that produce certain effects in the body, much as the nerv-ous system neurotransmitters do. In fact, there are many substances like norepinephrine that function both as neurotransmitters and hormones. This reflects the close relationship between these two body regulation systems. The brain plays an important role in regulating the release of hormones, and if hormone levels get out of balance (as neurotransmitter levels sometimes do), it can have an impact on how the brain functions and therefore on how a person feels.