Pharmacology-1 PHL 313 Parasympathetic Nervous System

Second Lecture

By

Abdelkader Ashour, Ph.D. Phone: 4677212 Email: aeashour@ksu.edu.sa

Cholinergic Transmission

ACh is synthesized in the cytoplasm from acetyl-CoA and choline through the catalytic action of the enzyme choline acetyltransferase (ChAT)

The terminals of cholinergic neurons contain large numbers of small membrane-bound vesicles (containing ACh) concentrated near the synaptic portion of the cell membrane

Acetyl-CoA is synthesized in mitochondria, which are present in large numbers in the nerve ending

Choline is transported from the extracellular fluid into the neuron terminal by a sodium-dependent membrane carrier (carrier A). This carrier can be blocked by a group of drugs called hemicholiniums

The action of the choline transporter is the rate-limiting step in ACh synthesis

Cholinergic Transmission

Once synthesized, ACh is transported from the cytoplasm into the vesicles by an antiporter that removes protons (carrier B). This transporter can be blocked by vesamicol (cholinergic physiological antagonist)

Release of ACh is dependent on extracellular Ca2+ and occurs when an action potential reaches the terminal and triggers sufficient influx of Ca2+ ions

The increased Ca2+ concentration "destabilizes" the storage vesicles by interacting with special proteins associated with the vesicular membrane (VAMPs)

Fusion of the vesicular membranes with the terminal membrane results in exocytotic expulsion of ACh into the synaptic cleft

The ACh vesicle release process is blocked by botulinum toxin (neurotoxic protein) through the enzymatic removal of two amino acids from one or more of the fusion proteins

Cholinergic Transmission

After release from the presynaptic terminal, ACh molecules may bind to and activate an ACh receptor (cholinoceptor)

Eventually (and usually very rapidly), all of the ACh released will diffuse within range of an acetylcholinesterase (AChE) molecule

AChE very efficiently splits ACh into choline and acetate, neither of which has significant transmitter effect, and thereby terminates the action of the transmitter

Most cholinergic synapses are richly supplied with AChE; the half-life of ACh in the synapse is therefore very short. AChE is also found in other tissues, e.g., red blood cells

Another cholinesterase with a lower specificity for ACh, butyrylcholinesterase [pseudocholinesterase], is found in blood plasma, liver, glia, and many other tissues

Demonstration of Muscarinic and Nicotinic Actions of ACh, Dale’s Experiment

Two kinds of effects produced by ACh.

A. Ach causes a fall in BP due to arteriolar vasodilatation and slowing of the heart

B. A larger dose of ACh also produces bradycardia, further reducing BP

C. Atropine blocks the effect of ACh in lowering BP

D. Still under the influence of atropine, a much larger dose of ACh produces nicotinic effects, causing a rise in BP and tachycardia due to stimulation of sympathetic ganglia ( vasoconstriction) and secretion of adrenaline

I. Nicotinic receptors, nAChRs (the nicotinic actions of ACh are those that can be reproduced by the injection of nicotine) ---- Nicotinic receptors are ligand-gated ion channels whose activation results in a rapid increase in cellular permeability to sodium and calcium.

Location: nAChRs are located …..1. At neuromuscular junctions of skeletal muscle (muscle type)

Postsynaptic Excitatory (increases Na+ permeability) Agonists: ACh, carbachol (CCh), suxamethonium Stimulate skeletal muscle (contraction) Antagonists: tubocurarine, hexamethonium

2. On postganglionic neurons in the autonomic ganglia (ganglion type) Postsynaptic Excitatory (increases Na+ permeability) Agonists: ACh, CCh, nicotine Stimulate all autonomic ganglia Antagonists: mecamylamine, trimetaphan

Parasympathetic Nervous System, Receptors for acetylcholine (cholinergic receptors)

Parasympathetic Nervous System, Cholinergic receptors

3. On some central nervous system neurons (CNS type)Pre- and postsynapticExcitatory (increases Na+ permeability)Agonists: nicotine, ACh Pre- and postsynaptic stimulation of many brain regionsAntagonists: mecamylamine, methylaconitine

4. On adrenal medullaACh stimulates secretion of adrenaline from adrenal medulla

Parasympathetic Nervous System, Cholinergic receptors

II. Muscarinic receptors, mAChRs (the muscarinic actions of ACh are those that can be reproduced by the injection of muscarine) ---- Muscarinic receptors are GPCRs: (odd-numbered members “M1, M3, M5) act through the inositol phosphate pathway, while the even-numbered receptors (M2, M4) act by inhibting adenylate cyclase, and thus reducing intracellular cAMP. mAChR also may activate or inhibit potassium channels and calcium channels.

Location: mAChRs are located … in tissues innervated by postganglionic parasympathetic neurons such as

On smooth muscle

On cardiac muscle

On gland cells

See next table for details

in postganglionic sympathetic neurons to sweat glands

In the central nervous system

P139 RDRM

Main locations

Cellular response

Functional response

Agonists

Antagonists

2. G-protein-Coupled Receptors, Targets

PIP2: phosphatidylinositol-4,5-bisphosphate

IP3: inositol-1,4,5-trisphosphate

DAG: 1,2-diacylglycerol

PIP2

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