The previous section described dopamine as the major player in drug addiction. Yet several other neurotransmitters are important and deserve attention. They include serotonin, norepinephrine, acetylcholine, glutamate, and gamma-amino butyric acid (GABA).
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, 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.
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.
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 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 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.
Addiction Science Research and Education Center
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Director: Carlton Erickson, Director