Pharmacology of the Autonomic Nervous System
From the foregoing discussion, it is obvious that intra-venous injection of norepinephrine causes essentially the same effects throughout the body as sympathetic stimulation. Therefore, norepinephrine is called a sympathomimetic or adrenergic drug. Epinephrine and methoxamine are also sympathomimetic drugs, andthere are many others. They differ from one another in the degree to which they stimulate different sympa-thetic effector organs and in their duration of action. Norepinephrine and epinephrine have actions as short as 1 to 2 minutes, whereas the actions of some other commonly used sympathomimetic drugs last for 30 minutes to 2 hours.
Important drugs that stimulate specific adrenergic receptors but not others are phenylephrine (alpha receptors), isoproterenol (beta receptors), and albuterol (only beta2 receptors).
Drugs That Cause Release of Norepinephrine from Nerve Endings.
Certain drugs have an indirect sympathomimetic action instead of directly exciting adrenergic effector organs. These drugs includeephedrine, tyramine, and ampheta-mine. Their effect is to cause release of norepinephrinefrom its storage vesicles in the sympathetic nerve endings. The released norepinephrine in turn causes the sympathetic effects.
Drugs That Block Adrenergic Activity. Adrenergic activitycan be blocked at several points in the stimulatory process, as follows:
1. The synthesis and storage of norepinephrine in the sympathetic nerve endings can be prevented. The best known drug that causes this effect is reserpine.
2. Release of norepinephrine from the sympathetic endings can be blocked. This is caused by guanethidine.
3. The sympathetic alpha receptors can be blocked. Two drugs that cause this effect are phenoxybenzamine and phentolamine.
4. The sympathetic beta receptors can be blocked. A drug that blocks beta1 and beta2 receptors is propranolol. One that blocks mainly beta1receptors is metoprolol.
5. Sympathetic activity can be blocked by drugs that block transmission of nerve impulses through the autonomic ganglia. They are discussed in a later section, but an important drug for blockade of both sympathetic and parasympathetic transmission through the ganglia is hexamethonium.
Parasympathomimetic Drugs (Cholinergic Drugs). Acetyl-choline injected intravenously usually does not cause exactly the same effects throughout the body as parasympathetic stimulation because most of the acetyl-choline is destroyed by cholinesterase in the blood and body fluids before it can reach all the effector organs. Yet a number of other drugs that are not so rapidly destroyed can produce typical widespread parasympa-thetic effects, and they are called parasympathomimeticdrugs.
Two commonly used parasympathomimetic drugs are pilocarpine and methacholine. They act directly on themuscarinic type of cholinergic receptors.
Drugs That Have a Parasympathetic Potentiating Effect—Anti- cholinesterase Drugs. Some drugs do not have a directeffect on parasympathetic effector organs but do poten-tiate the effects of the naturally secreted acetylcholine at the parasympathetic endings. That potentiate the effect of acetylcholine at the neuromuscular junction. They include neostigmine, pyridostigmine, and ambenonium.
These drugs inhibit acetylcholinesterase, thus prevent-ing rapid destruction of the acetylcholine liberated atparasympathetic nerve endings. As a consequence, the quantity of acetylcholine increases with successive stimuli, and the degree of action also increases.
Drugs That Block Cholinergic Activity at Effector Organs— Antimuscarinic Drugs. Atropineand similar drugs, such ashomatropine and scopolamine, block the action of acetylcholine on the muscarinic type of cholinergic effec-tor organs. These drugs do not affect the nicotinic actionof acetylcholine on the postganglionic neurons or on skeletal muscle.
Drugs That Stimulate Autonomic Postganglionic Neurons. Thepreganglionic neurons of both the parasympathetic and the sympathetic nervous systems secrete acetylcholine at their endings, and this acetylcholine in turn stimulates the postganglionic neurons. Furthermore, injected acetylcholine can also stimulate the postganglionic neurons of both systems, thereby causing at the same time both sympathetic and parasympathetic effects throughout the body.
Nicotine is another drug that can stimulate postgan-glionic neurons in the same manner as acetylcholine because the membranes of these neurons all contain the nicotinic type of acetylcholine receptor. Therefore, drugsthat cause autonomic effects by stimulating postgan-glionic neurons are called nicotinic drugs. Some other drugs, such as methacholine, have both nicotinic and muscarinic actions, whereas pilocarpine has only mus-carinic actions.
Nicotine excites both the sympathetic and parasym-pathetic postganglionic neurons at the same time, resulting in strong sympathetic vasoconstriction in the abdominal organs and limbs but at the same time result-ing in parasympathetic effects such as increased gas-trointestinal activity and, sometimes, slowing of the heart.
Ganglionic Blocking Drugs. Many important drugs blockimpulse transmission from the autonomic preganglionic neurons to the postganglionic neurons, including tetraethyl ammonium ion, hexamethonium ion, and pen-tolinium. These drugs block acetylcholine stimulation ofthe postganglionic neurons in both the sympathetic and the parasympathetic systems simultaneously. They are often used for blocking sympathetic activity but seldom for blocking parasympathetic activity because their effects of sympathetic blockade usually far overshadow the effects of parasympathetic blockade. The ganglionic blocking drugs especially can reduce the arterial pres-sure in many patients with hypertension, but these drugs are not very useful clinically because their effects are difficult to control.
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