Neurons, Brain Chemistry and 
   Neurotransmission

 2.3 Other Neurotransmitters

Other Neurotransmitters Compared to Dopamine 

The previous section described dopamine as the major player in drug addiction. Yet several other neurotransmitters are important and deserve attention. They include serotonin, norephinephrine, acetylcholine,  glutamate  and gamma-amino butyric acid (GABA).

   Serotonin

Serotonin plays a major role in emotional disorders such as depression, suicide, impulsive behavior, and aggression. Neurons using serotonin as a neurotransmitter are found in the brain, primarily in a cluster of cells called the pons.

Serotonin is normally involved in temperature regulation, sensory perception, and mood control. The hallucinogenic drug LSD acts on serotonin receptors; so do some antidepressant drugs.

As mentioned earlier, neurotransmitters usually bind and stimulate their receptors, then travel back to their sending neurons. These are the normal events in the reuptake system. Reuptake occurs in order to keep neurotransmitter levels steady and maintain homeostasis. In effect, the receiving neuron says "That's enough!" to the sending neuron that has been releasing neurotransmitters. The sending neuron quickly picks up the leftover neurotransmitters and quits releasing new ones. This is an example of negative feedback.

Prozac and some of the other drugs used to treat severe depression prevent the normal reuptake of serotonin. As a result, there is more serotonin floating around to grab on to receptors and trigger impulses in receiving neurons. This leads to increased stimulation of serotonin neurons in depressed people, who find that the drugs help to relieve their symptoms.

 Norepinephrine

Norepinephrine, also called noradrenaline, is a neurotransmitter that doubles part-time as a hormone. (Hormones are chemicals that regulate many body functions, including growth, digestion, and fluid balance.) As a neurotransmitter, norepinephrine helps to regulate arousal, dreaming, and moods. As a hormone, norepinephrine acts to increase blood pressure, constrict blood vessels, and increase heart rate - responses that occur when we feel stress.

 Acetylcholine

Another major neurotransmitter named acetylcholine excites neurons in the brain and many other parts of the body, including muscle tissues and glands. Acetylcholine is released where nerves meet muscles and is therefore responsible for muscle contraction.

After acetylcholine stimulates its receptors, it is quickly inactivated and destroyed by an enzyme. Drugs that keep this enzyme from working are used to treat myasthenia gravis, a disease of muscle weakness and fatigue. These drugs lead to an excess of acetylcholine in synapses and overstimulation of the muscles. The result in patients with extreme muscle weakness is normal muscle contraction.

Glutamate  and

gamma-amino butyric acid (GABA)

Certain amino acids also act as neurotransmitters, including glutamate and gamma-amino butyric acid (GABA). Glutamate strongly excites neurons, while GABA strongly inhibits neurons.

Glutamate and GABA are unique in several ways:

The number of synapses using glutamate and GABA is much greater than those using all other types of neurotransmitters combined.

Glutamate and GABA neurons are found in many brain regions. As a result, glutamate and GABA work all over the brain while other neurotransmitters do not.

Both glutamate and GABA have important functions in the body in addition to their role as neurotransmitters. For example, they are needed by our body's metabolism to break down food and make energy-rich molecules in cells.

The fact that GABA and glutamate are so widely present makes it likely that they will be altered during drug addiction. This fact also makes it difficult to treat addiction with drug therapy. Say that a drug affects GABA and glutamate in a  way that relieves craving. Because GABA and glutamate are so widely present, these drugs could produce a mess of side effects as well. If we had drugs that could selectively stimulate or block certain receptors, then we could treat addiction and avoid doing people more harm than good.

Drugs Interfere With Neurotransmitters

Drugs can interfere with just about every step in the work of neurotransmitters. To understand this point, consider an analogy. In your apartment you perform various tasks: working on a computer, watching television, listening to music on a stereo system, and more. When you leave your apartment, you make sure the door is locked. You'd hate for people to get a key so similar to yours that they could somehow jimmy your door open and break in. Once in your apartment, they could vandalize your property-take your computer and VCR, break your TV, bust out your lights, or drop your stereo. You could then no longer perform your daily tasks.

Something like this can happen in your brain. Remember that each receptor is designed to bind only a certain neurotransmitter. A drug of abuse that is structurally similar to a neurotransmitter could be a "key" that fits into a receptor's "lock." In this way, the drug could disrupt neuron activity in the same way that an intruder disrupts your apartment and damages your property.

More specifically, drugs can:

Stop the chemical reactions that create neurotransmitters.

Empty neurotransmitters from the vesicles where they're normally stored and protected from breakdown by enzymes.

Block neurotransmitters from entering or leaving vesicles.

Bind to receptors in place of neurotransmitters.

Prevent neurotransmitters from returning to their sending neuron (the reuptake system).

Interfere with second messengers, the chemical and electrical changes that take place in a receiving neuron.

One example of drug interference: the effect of cocaine.

In other words, drugs can damage your intellectual property by blocking nerve impulses, preventing neurotransmitters from getting where they're supposed to be, or producing too many or too little neurotransmitters. As a result, neurons may be overstimulated or not stimulated at all, crippling the nervous system's ability to carry out its functions. 

In each of these ways and more, drugs can damage and vandalize the complicated circuit of nerve pathways in your body. Treatment for drug addiction stops this cycle. A network of so intricately designed to reason, imagine, compute, remember, and dream is truly incredible-not something to be tampered with. Central to treatment is the idea that the vast network of neurons in our bodies can be treated with care and respect.

References:  (1) Understanding Addiction:  Other Neurotrans-mitters.
                      (2) Understanding Addiction:  Drugs    Interfere  With Neurotransmitters / Addiction Science and Research Education Center~ College of Pharmacy ~ University of Texas
 Austin, Texas 78712

Deborah Shrira,Editor 2008 August