ÿþEditors: Finkel, Richard; Clark, Michelle A.; Cubeddu, Luigi X.
Title: Lippincott's Illustrated Reviews: Pharmacology, 4th Edition
Copyright Â©2009 Lippincott Williams & Wilkins
> Table of Contents > Unit II - Drugs Affecting theAutonomic Nervous System > Chapter 4 - Cholinergic Agonists
Drugs affecting the autonomic nervous system are divided into two groups according to the type of neuron involved in their mechanism of action. The cholinergic drugs, which are described in this and the following chapter, act on receptors that are activated by acetylcholine. The second groupâ¬ the adrenergic drugs (discussed in Chapters 6 and 7)â¬ act on receptors that are stimulated by norepinephrine or epinephrine. Cholinergic and adrenergic drugs both act by either stimulating or blocking receptors of the autonomic nervous system. Figure 4.1 summarizes the cholinergic agonists discussed in this chapter.
Figure 4.1 Summary of cholinergic agonists.
II. The Cholinergic Neuron
The preganglionic fibers terminating in the adrenal medulla, the autonomic ganglia (both parasympathetic and sympathetic), and the postganglionic fibers of the parasympathetic division use acetylcholine as a neurotransmitter (Figure 4.2). In addition, cholinergic neurons innervate the muscles of the somatic system and also play an important role in the central nervous system (CNS). [Note: Patients with Alzheimer's disease have a significant loss of cholinergic neurons in the temporal lobe and entorhinal cortex. Most of the drugs available to treat the disease are acetylcholinesterase inhibitors (see p. 102).]
A. Neurotransmission at cholinergic neurons
Neurotransmission in cholinergic neurons involves sequential six steps. The first fourâ¬ synthesis, storage, release, and binding of acetylcholine to a receptorâ¬ are followed by the fifth step, degradation of the neurotransmitter in the synaptic gap (that is, the space between the nerve endings and adjacent receptors located on nerves or effector organs), and the sixth step, the recycling of choline (Figure 4.3).
Synthesis of acetylcholine: Choline is transported from the extra-cellular fluid into the cytoplasm of the cholinergic neuron by an energy-dependent carrier system that cotransports sodium and that can be inhibited by the drug hemicholinium. [Note: Choline has a quaternary nitrogen and carries a permanent positive charge, and thus, cannot diffuse through the membrane.] The uptake of choline is the rate-limiting step in acetylcholine synthesis. Choline acetyltransferase catalyzes the reaction of choline with acetyl coenzyme A (CoA) to form acetylcholineâ¬ an esterâ¬ in the cytosol. Acetyl CoA is derived from the mitochondria and is produced by the Krebs cycle and fatty acid oxidation.
Storage of acetylcholine in vesicles: The acetylcholine is packaged into presynaptic vesicles by an active transport process coupled to the efflux of protons. The mature vesicle contains not only acetylcholine but also adenosine triphosphate (ATP) and proteoglycan. [Note: ATP has been suggested to be a cotransmitter acting at prejunctional purinergic receptors to inhibit the release of acetylcholine or norepinephrine.] Cotransmission from autonomic neurons is the rule rather than the exception. This means that most synaptic vesicles will contain the primary neurotransmitter, here acetylcholine, as well as a cotransmitter that will increase or decrease the effect of the primary neurotransmitter. The neurotransmitters in vesicles will appear as bead-like structures, known as varicosities, along the nerve terminal of the presynaptic neuron.