Drugs and the Nervous System

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Neuropharmacology is a branch of medicine that deals with the effects of drugs on the nervous system, especially drugs that mimic, enhance, or inhibit the action of neurotransmitters. A few examples will illustrate the clinical relevance of understanding neurotransmitter and receptor functions.

A number of drugs work by stimulating adrenergic and cholinergic neurons or receptors. Sympathomimetics11 are drugs that enhance sympathetic action by stimulating adrenergic receptors or promoting norepinephrine release. For example phenylephrine, found in such cold medicines as Chlor-Trimeton and Dimetapp, aids breathing by stimulating a1 receptors and dilating the bronchioles and by constricting nasal blood vessels, thus reducing swelling in the nasal mucosa. Sym-patholyticsu are drugs that suppress sympathetic action by inhibiting norepinephrine release or by binding to adrenergic receptors without stimulating them. Propranolol, for example, is a beta-blocker. It reduces hypertension (high blood pressure) partly by blocking p-adrenergic receptors and interfering with the effects of epinephrine and norepinephrine on the heart and blood vessels. (It also reduces the production of angiotensin !!, a hormone that stimulates vasoconstriction and raises blood pressure.)

Parasympathomimetics enhance parasympathetic effects. Pilo-carpine, for example, relieves glaucoma (excessive pressure within the eyeball) by dilating a vessel that drains fluid from the eye. Parasympatholytics inhibit ACh release or block its receptors. Atropine, for example, blocks muscarinic receptors and is sometimes used to dilate the pupils for eye examinations and to dry the mucous membranes of the respiratory tract before inhalation anesthesia. It is an extract of the deadly nightshade plant, Atropa belladonna. Women of the Middle Ages used nightshade to dilate their pupils, which was regarded as a beauty enhancement.13

The drugs we have mentioned so far act on the peripheral nervous system and its effectors. Many others act on the central nervous system. Strychnine, for example, blocks the inhibitory action of glycine on spinal motor neurons. The motor neurons then overstim-ulate the muscles, causing spastic paralysis and sometimes death by suffocation.

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