Drugs affecting cholinergic function

• The parasympathetic (cholinergic) nervous system stimulates the GI system and decreases metabolism and cardiovascular and respiratory function to preserve energy. It is considered the "rest and digest" system. • In the parasympathetic system, the transmitter is acetylcholine, and the receptors are considered cholinergic. Cholinergic may be further divided into muscarinic or nicotinic. • Parasympathetic or cholinergic drugs can be stimulating or blocking in their action. The cholinergic stimulating drugs are known as cholinergic agonists, and the cholinergic blocking agents are known as anticholinergics. There are additional terms for these drugs (see Table 14.1). • Acetylcholine is inactivated by acetylcholinesterase, which is sometimes referred to as cholinesterase. • Drugs that interfere with acetylcholinesterase's breakdown of acetylcholine (i.e., they inhibit the action of acetylcholinesterase) are known as anticholinesterase agents. • Therapeutic uses of cholinergic drugs are varied and related to providing extra cholinergic stimulation or blockage to normal autonomic nervous system functioning. • Excessive stimulation of cholinergic receptors is known as cholinergic poisoning or cholinergic crisis. The symptoms include muscle twitching and paralysis of all muscles, including respiratory muscles. This can lead to respiratory failure. • Overstimulation of anticholinergic receptors can be remembered by the saying "red as a beet, dry as a bone, blind as a bat, and mad as a hatter." • A prototype of a direct-acting muscarinic agonist is pilocarpine, which is used for simple and acute glaucoma, preoperative and postoperative intraocular tension, and mydriasis. • A prototype direct-acting nicotinic agonist is nicotine, which is used as an adjunct to smoking cessation programs. • A prototype indirect-acting cholinergic receptor stimulant is neostigmine, which is used to control the symptoms of myasthenia gravis. It inhibits the action of acetylcholinesterase. As with any of the cholinergic drugs, adverse effects involve many of the major organ systems. • Atropine is the prototype of the antimuscarinic anticholinergic drugs. It is most commonly used to raise heart rate in cardiac emergencies and preoperatively to dry oral secretions. • The most common adverse effects of anticholinergic drugs are blurred vision, dry mouth, constipation, and urinary retention.