23 Principles of Pharmacology - Cholinergic Transmission - Physiology and General Pharmacology
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Course
Pharmacology
Institution
Pharmacology
The object of the course is to teach students an approach to the study of pharmacologic agents. It is not intended to be a review of the pharmacopoeia. The focus is on the basic principles of biophysics, biochemistry, and physiology as to the mechanisms of drug action, biodistribution and metabolis...
Harvard-MIT Division of Health Sciences and Technology
HST.151: Principles of Pharmocology
Instructor: Prof. Gary Strichartz
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:
,HST-151 2
Fig1a
Fig1b
, HST-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
Hemicholinium Block choline uptake and
Interference with synthesis Cholinergic
deplete ACh
of transmitter
Adrenergic α-Methyltyrosine Deplete NE
Displacement of NE by, α
Metabolism by same path
Adrenergic α-Methyldopa methylNE, a false
as transmitter
transmitter
Blockade of transport at Accumulation of NE at
Adrenergic Cocaine, imipramine
nerve. terminal membrane receptors
Blockade of transport into NE depletion from
Adrenergic Reserpine
storage granules adrenergic terminal
Latrotoxin (black widow Cholinomimetic followed
Displacement of Cholinergic
venom) by block
transmitter from terminal
Adrenergic Amphetamine, tyramine Sympathomimetic
Prevent release of Cholinergic Botulinus toxin Anticholinergic
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