Psychoactive drugs affect communication within the Central Nervous System (CNS, Brain and Spinal Cord) and between the CNS and the body. There is two-way communication between the CNS and the motor system (musculature), the endocrine system (the collection of glands within the body which release hormones into the blood supply) the sensory systems, and all of the internal organs.
As you can see, this inter-communication within the brain and between the brain and the rest of the body makes specific psycho-pharmacologic interventions nearly impossible. To prescribe a drug to alter communication in one area, can alter communication throughout the body; hence, the potential for side-effects is immense, and response to medication can vary widely across individuals.
Within the central nervous system, communication within nerve cells (neurons) is mediated electrically, while communication between neurons is mediated chemically. The purpose of the electrical communication within neurons is to regulate the release of chemicals which the neuron uses to communicate with other neurons. The purpose of the chemical communication between neurons is to influence the electrical excitability of the receiving cell. As you can see, this relationship is circular.
The chemicals which the neurons use to communicate with each other are referred to, collectively, as neurotransmitters. Many of these neurotransmitters you may have heard of. Serotonin is the neurotransmitter which is enhanced by Prozac, Paxil, Zoloft, and several other antidepressants. Dopamine is the neurotransmitter which is deficient in Parkinson's disease, due to the selective death of dopamine releasing neurons. The L-DOPA medication which Parkinson's patients take elevates dopamine levels in the brain. On the other hand, if the brain is excessively sensitive to dopamine, schizophrenia will result. Antischizophrenic drugs, such as thorazine, reduce dopamine communication in the brain. It should not be surprising that if antischizophrenic treatment is overly aggressive (as to dose) the psychiatric patient can become Parkinsonian. Conversely, if a Parkinson's Disease patient is over-medicated, he or she can suffer the symptoms of schizophrenia, such as hallucinations. The neurotransmitter, acetylcholine is released by neurons in the base of the brain which degenerate in Alzheimer's disease. Care must be taken when Alzheimer's patients are treated with drugs which boost acetycholine levels, because this neurotransmitter is also used by the body to cause contraction of skeletal muscles and to inhibit the contraction of smooth muscle. The heart is a smooth muscle, and elderly people have heart disease. Since Alzheimer's disease is a disease of the elderly, treating these patients with drugs to enhance acetylcholine levels can have disastrous consequences for their cardiovascular health.
MODULATION OF BIOLOGICAL COMMUNICATION BY DRUGS
What psychoactive (neuroactive) drugs do is increase or decrease the strength of these neurotransmitter signals. For a neurotransmitter to communicate with a target cell (another neuron, a muscle, or a gland), the target cell must have on its surface specialized receptor molecules specific to that neurotransmitter. Hence, a cell with dopamine receptors, acetylcholine receptors, and serotonin receptors can receive communication from cells which use one of these neurotransmitters to communicate with their targets. When a neurotransmitter chemically binds with a receptor (e.g., dopamine binding to a dopamine receptor) the electrical excitability of the receiving cell is either enhanced or diminished.
Direct Drug Interactions With the Receptor. Some synthetic drugs and plant
extracts are so similar to the natural neurotransmitter in composition and three-dimensional
shape that they can bind to the receptor and stimulate it in a manner identical
to the neurotransmitter. If the neurotransmitter would have produced electrical
excitation, so will the drug. Similarly, if the drug would have inhibited excitation
in the receiving cell, the drug will do the same. Essentially, the receptor
or the receiving cell thinks it has received communication via the neurotransmitter.
Thus, these compounds are termed as "mimetics"; they mimic the effect
of whichever neurotransmitter they resemble. Nicotine mimics acetylcholine,
and heroine and morphine mimic the neurotransmitters called enkephalins and
Other compounds have somewhat less chemical similarity to various neurotransmitters. This allows these compounds to bind to receptors, but not stimulate them. Hence, there is no effect on the electrical activity of the receiving cell. On the other hand, if these receptors are already bound by one of these compounds, then the naturally occurring neurotransmitter cannot bind to the receptor, and communication is blocked. As a group, these compounds are referred to as receptor blocking agents. Cobra venom blocks acetylcholine receptors.
Since mimetics and receptor blocking agents go directly to the receptor and either stimulate the receptor (mimetics) or block it, these agents are referred to as Directly Acting agents.
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