HST-151 1 CHOLINERGIC TRANSMISSION: PHYSIOLOGY AND GENERAL PHARMACOLOGY Objectives: The purpose of this lecture is to describe the mechanisms and pharmacology of nicotinic and muscarinic cholinergic transmission. Cholinergic transmission is defined by the physiological processes that utilize acetylcholine to communicate between cells. We will address the following questions: 1. Where does cholinergic transmission occur? 2. What biochemical events underly cholinergic transmission and how do drugs alter these events? 3. What are the physiological consequences of cholinergic transmission, and of its absence? I. Distributions and varieties of cholinergic transmission: Neurotransmission using acetylcholine (ACh) occurs in the peripheral (PNS) and central nervous systems (CNS). Direct control of skeletal muscle tension is mediated by ACh released at the neuromuscular junction (nmj), and modulation of timing (chronotropy) and tension (inotropy) in cardiac and smooth muscle is effected through ACh released by postganglionic parasympathetic neurons. The excitatory aspect of neurotransmission at autonomic ganglia requires ACh, as does a variety of still cryptic mechanisms in the CNS. Cholinergic receptors are broadly classified as nicotinic (nAChR) or muscarinic (mAChR), although these are further subdivided by their selective pharmacologies (more on this below). Common to all these neurotransmissions are basic processes for the synthesis, storage, release, and breakdown of acetylcholine by synaptic endings of neurons, and for the binding of transmitters by postsynaptic receptors and their subsequent activation. Specific examples of these processes and of agents that selectively interfere with them during neuromuscular transmission are shown in the following Figures: CholinergicPharm.doc Harvard-MIT Division of Health Sciences and TechnologyHST.151: Principles of PharmacologyHST-151 2 Fig1a Fig1b CholinergicPharm.docHST-151 3 Other examples are listed in Table 1 below, which also includes adrenergic transmission, the other aspect of autonomic synaptic activity whose actions, subserving sympathetic n.s. activation, often antagonize the effects of parasympathetic (cholinergic) innervation of end organs (e.g. heart, gut, etc., see also figure 2A). Details of adrenergic pharmacology will be presented later. Table 1 Mechanism System Agents Effect Interference with synthesis of transmitter Cholinergic Hemicholinium Block choline uptake and deplete ACh Adrenergic α-Methyltyrosine Deplete NE Metabolism by same path as transmitter Adrenergic α-Methyldopa Displacement of NE by, α-methylNE, a false transmitter Blockade of transport at nerve. terminal membrane Adrenergic Cocaine, imipramine Accumulation of NE at receptors Blockade of transport into storage granules Adrenergic Reserpine NE depletion from adrenergic terminal Displacement of transmitter from terminal Cholinergic Latrotoxin (black widow venom) Cholinomimetic followed by block Adrenergic Amphetamine, tyramine Sympathomimetic Prevent release of transmitter Cholinergic Botulinus toxin Anticholinergic Adrenergic Bretylium, guanethidine Antiadrenergic Agonist at postsynaptic receptors Cholinergic (Nicotinic) Nicotine, succinylcholine Cholinomimetic Cholinergic (Muscarinic) Muscarine, methacholine Cholinomimetic Adrenergic (α1) Phenylephrine Sympathomimetic CholinergicPharm.docHST-151 4 Agonist at postsynaptic receptors Adrenergic (α2) Clonidine Sympathomimetic (periphery), sympatholytic (CNS) Adrenergic (β1) Dobutamine Cardiac M. stimulation Adrenergic (β2) Albuterol Sm. M. relaxation Antagonist at postsynaptic receptors Cholinergic (nicotinic) d-Tubocurarine Anticholinergic Cholinergic (muscarinic) Atropine Anticholinergic Adrenergic (α1) Prazosin Vasodilation Adrenergic (β1) Metoprolol Cardiac blockade Inhibition of transmitter breakdown Cholinergic Physostigmine, DFP Cholinomimetic Adrenergic MAO inhibitors (pargyline), COMT inhibitors (entacapone) Potentiate indirect acting sympathomimetics At this point you will probably benefit by an anatomical review of the autonomic nervous system (see Katzung and Fig. 2 this handout). Accompanying the general anatomy of the sympathetic and parasympathetic n.s. are the specific effects of acetylcholine (and norepinephrine) on particular end organs. CholinergicPharm.docHST-151 5 Fig 2a Fig 2b CholinergicPharm.doc--HST-151 6 A. Nicotinic Cholinergic Transmission. Familiar to you from earlier lectures on neuromuscular transmission, nicotinic cholinergic transmission results directly from the binding of ACh (2 molecules) to the nAChR, yielding an example of a directly ligand-gated conductance. The nAChR (nR in scheme below), when bound by 2 agonist ligands (L), undergoes a conformational change to form a monovalent cation-selective pore through the postsynaptic membrane. (Closed) (Closed) (Closed) (Open) L + nR L · nR L2 · nR L2 · nR* 2k+ k --2k Lk+ β α Slow Fast Single open channels of the activated nAChR are about equally permeable to Na+ and K+ ions; their activation in a resting cell thus produces a net inward ionic current (Erev ~ -10mV) that depolarizes the postsynaptic membrane. Depolarization has a variety of consequences: e.g. depolarizations are additive, summing temporally and spatially ("integration") to bring a postsynaptic excitable cell's membrane to impulse threshold, thereby activating voltage-gated Ca2+ channels to increase intracellular Ca2+ or directly modulating other voltage-gated channels, as well as producing other, long term changes. The fast nicotinic depolarization is very brief (<10 ms), as the ACh is rapidly hydrolyzed by acetylcholine-esterase (AChEase) in the synaptic cleft and the receptor-CholinergicPharm.docHST-151 7 bound ACh dissociates quickly from the "closed" receptor. The time course of the various stages of neuromuscular transmission is outlined in figure 3: CholinergicPharm.docHST-151 8 Agents that prevent the binding of ACh to the receptor yet have no activating capacity of their own (e.g. d-tubocurare) are non-depolarizing antagonists of nicotinic transmission. Those that mimic the effects (e.g. carbachol) are agonists, and those that activate it, but less effaciously than ACh (e.g. methacholine), are mixed agonists-antagonists (sometimes called partial agonists; see Figure 4a). Cholinergic agonists that are resistant or insensitive to AChEase (e.g. succinylcholine) can have an apparent in