Chemicals can influence neurotransmission in a wide variety of ways. Those that block or inhibit the normal effect of synaptic transmission are called antagonists, those that facilitate it are called agonists. (The Greek word agon means ”contest”). Every synapse between an axon and the cell membrane of a skeletal muscle employ the neurotransmitter acetylcholine (ACh). Thus a substance that interferes with the action of ACh (an antagonist) can prevent nerve impulses from crossing these synapses and thus can produce paralysis. If the paralysis involves the muscles used in breathing! the results can be fatal. The deadly effects of Botulinus toxin come about in precisely this way, by blocking the release of ACh. (Botulinus toxin is sometimes present in improperly canned foods; it can be destroyed if the food is boiled for 5 minutes before eating.)
Other poisons block the uptake of ACh by the postsynaptic membrane; these include curare and cobra venom. Still others, such as the venom of the black widow spider and remain nerve gases developed for chemical warfare (agonists) prevent the breakdown of ACh, prolonging its action and causing convulsions.
Norepinephrinw (NE) is a neurotransmitter released by neurons whose firing causes a general activation of various body spheres. The stimulant amphetamine is an agonist since it stimulates the release of NE and blocks its breakdown in the synaptic cleft as does the stimulant cocaine. Thus both drugs produce a highly activated wide awake state. In contrast, the drug reserpine is an antagonist since it causes the vesicles to ”leak” norepinephrine before the action potential allowing the NE to be broken down. It is often used as a relaxant to combat hypertensive (high blood pressure).
Dopamine is a neurotransmitter closely related to norepinephrine and is affected in a similar fashion by the drugs reserpine and amphetamine. Of special interest is the possibility that neurons releasing dopamine may be involved in the serious mental illness schizophrenia. The antipsychotic chlorpromazine seems to block dopamine receptors in the brain (it is a dopamine antagonist) and reduces symtoms of schizophrenia.
Serotonin is the neurotransmitter that seems to be affected by the ”mind-bending” (hallucinogenic) drug LSD (lysergic acid). LSD ants as an antagonist of serotonin by increasing the activity of neurons in the brain that are normally inhibited by that neurotransmitter.
This is only a very partial list of neurotransmitters and substances that affect their action. It is presented primarily to illustrate the complexity of the chemical processes involved in neural activity.
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