What are four distinct general classes of synapse in the ANS ACh acts as the major neurotransmitter on?
Post-ganglionic sympathetic innervation of sweat glands
Pre-ganglionic sympathetic synapses
Pre-ganglionic parasympathetic synapses
Post-ganglionic parasympathetic fibre-target tissue (neuroeffector junction)
What is ACh synthesised from? What enzyme is involved? Where does synthesis occur? What is the rate limiting factor?
- Synthesied from Choline and Acetyl CoA
- Enzyme: Choline Acetyltransferase
- Synthesis occurs in the cytoplasm of cholinergic neurones in the axon terminal
- Rate limiting factor: availability of the choline which depends on uptake of choline into the neurone.
How is ACh packaged for release?
- After synthesis, ACh is transported into synaptic vesicles for storage via ACh/H+ antiport channel. An ATPase that pumps protons into the vesicle (V-ATPase) provides the energy necessary for this process.
- Cholinergic vesicles also contain ATP and heparan sulphate, which serve as counter ions for positively charged ACh.
- The increase in intracellular Ca2+ facilitates the binding of Synaptotagmin to the SNARE-Complex proteins, which together mediate vesicle-membrane attachment and fusion.
What are the events that occur to cause release of ACh by pre-ganglionic cholinergic fibres synapses?
Action potential arrives at the axon.
Depolarisation opens the voltage-gated Ca2+ channels in the presynaptic membrane.
Ca2+ enters down its diffusion gradient binding to synaptotagmin.
This binding brings the vesicle close to the membrane.
The vesicel binds to the snare complex to make a fusion pore.
ACh is released and diffuses into the cleft.
ACh binds to post-synaaptic receptors - nACHr
Ach is removed from synaptic cleft by Acetylcholinesterase.
Give an example of an agent which interferes with release of Acetylcholine
Cleaves specifc part of SNARE complex.
Impaired targeting to synapse
Reduced ACh release
Results in paralysis
Clinically used to treat a variety of conditions such as muscle spasm.
Also known as botox.
What mechanisms does the parasympathetic synapse have that rapidly terminate the action of ACh?
- Acetylcholinesterase breaks down ACh into choline and acetate.
- Butyrylcholinesterase also responsible for degrading ACh.
- Acetylcholinesterase located on the post-synaptic membrane hydrolyses ACh to acetate and choline.
- Inactivation of NA+ channels on the post-synaptic membrane.
What happens to free choline and acetate present in synapses?
- Choline and acetate diffuse back into the presynaptic neurone.
- Acetate is converted into Acetyl CoA in the mitochondria.
- Acetyl CoA and Choline are recombined in a reaction catalysed by choline acetyltransferase.
Briefly list some of the important sites of parasympathetic innervation
- Salivary glands - watery,high enzyme
- Heart - decreased heart rate, decreased conduction and velocity
- Stomach - digestion
- Penis - erection (erectile tissue has no sympathetic innervation)
- Bladder- urination
- Descending colon - peristalsis
- Preganglionic axons are myelinated and Postganglionic axons are not.
What is the effect of increased parasympathetic discharge in the heart?
- Parasympathetic innervation to the heart is via the Vagus Nerve (10th Cranial Nerve)
- Parasympathetic action on the:
- SA Node: decreases HR
- AV node and Purkinje fibres: decreases contractile velocity
- Atria and ventricles: decreases contractility (Contractility = force of contraction-stroke volume)
ACh is employed as the neurotransmitter at all parasympathetic nerve synapses. Are there any other synapses in the ANS that use ACh as a neurotransmitter?
- Most of the PNS
- All preganglionic sympathetic neurones.
- Sympathetic innervation of sweat glands
- Neuromuscular junctions
What naturally-occuring substances, which each either mimic or prevent some of the actions of ACH, were first used to show that two distinct classes of ACh receptor exist?
- ACh agonists: Nicotine and Muscarine
- ACh antagonists: Scopolamine and Atropine (both are anti-muscarinic)
- E.g. in bradycardia, you give atropine which blocks parasympathetic innervation.
Briefly, explain how the receptor subtype found at the ganglionic junction causes a post-ganglionic response when activated by ACh?
- 2 ACh receptors bind to the alpha-subunit of nAChRs.
- This causes Na+ influx and K+ efflux.
- Vm reaches -10mV threshold causing opening of voltage-gated Na+ Channels
What is the class of ACh receptor found at the parasympathetic neuroeffector junction?
- Muscarinic - this is the slow part of transmission (seconds to hours).
- These are G protein coupled receptors.
- Act directly via second messengers or act indirectly by kinases.
- There are different types of muscarinic receptors (M1-M5)
Name the Predominant G-Protein and effectors involved in transducing signals from M1, M2 and M3
Briefly describe the series of events that couple activation of M2 muscarinic receptors to the activation of the effectors.
For the tissues: SA node, Bronchi, Bladder, Glands and Parasympathetic nerve terminals at neuro-effector junctions i.e. 'pre-synaptic receptors', list the major physiological actions of ACh
What might be the advantage of synthesising cholinoceptor agonists which only interact with a particular receptor subtype? And are such agents available?
Certain cholinceptors such as M2 are primarly located within the heart therefore drugs targeted to this receptor can be specific without affecting other cholinoceptor pathways.
Direct agonists produce a pharmacological effect by receptor activation. Indirect agonists inhibit Acetylcholinesterase and therefore increase the levels of ACh causing an increased cholinergic response. Indirect agonists can be reversible or irreversible.
The need for specific drugs is that the more specific the drug, the less unwanted side effects occur in other areas of the body since the receptors are found in many areas of the body.
Yes, such agents are available
Are cholinoceptor agonists used clinically?
They are used clinically however as a group they show little specificity in their actions which limits their clinical usefulness.
One clinical use of muscarinic cholinoceptor antagonists is to treat gastrointestinal disorders. Also these agents are frequently given as a premedication for general anaesthesia. Why are muscarinic cholinoceptor antagonists used under these circumstanes?
- GI disorders e.g. IBS, Gi hypermobility and peptic ulcers: antagonism of smooth muscle contraction and exocrine secretion means there is reduced activity of the GI tract. Acts on M1 and M3 receptors. e.g. Scopolamine is a smooth muscle relacant.
- Premedication for Anaesthesia; e.g. atropine and glycopytrolate are used to suppress respiratory secretion prior to surgery, reduce anxiety and pain, causes tachycardia because drugs block the vagus nerve (anaesthetics causes bradycardia and hypotension from vasodilation so tachycardia is useful). Act on M1 - M5 receptors.
What unwanted side effects limit the usefulness of some muscarinic cholinoceptor antagonists?
- Atropine causes urinary retention, blurred vision, dry mouth, sandy eyes, constipation, confusion, restlessness, hallucinations, delirium (CNS toxicity)and this can progress to collapse of the circulatory and respiratory systems leading to death. Children are particularly sensitive.
- Scopolamine: dry mouth, tachycardia, dyshidrosis (type of eczema)
Draw a simple diagram to illustrate the anatomy of the human eye relevant to sites of sympathetic and parasympathetic innervation.
- Ciliary muscle innervated by both
- Lacrimal gland is parasympathetic
- Iris sphincter muscle is parasympathetic
- Pupil dilation is sympathetic
- Pupil constriction is parasympathetic
An abnormally raised intraocular pressure is termed glaucoma. Untreated this can lead to irreversible damage of the eye and blindness. What are the most likely causes of this condition?
- Ethnicity (greater risk in Africans, East Asians and Inuit)
- Sex (higher in women)
- Steroid induced
- Restricted blood flow to eye (diabetic retinopathy, central retinal vein occlusion)
- Ocular trauma
What are the consequences of increasing parasympathetic tone in the eye?
Increasing the parasympathetic tone in the eye increases the cillary muscle contraction which drains the fluid of Schlemm's canal, decreasing intraocular pressure and helps stop the progression of glaucoma
What receptor ligands are used clinically to treat glaucoma?
- Pilocarpine is muscarinic receptor agonist (M3) - activates mAChR
- Alpha1 agonist stimulates Phoslipase C
- Alpha agonist inhibits adenylyl cyclase
- Beta adrenergic antagonists decrease production of aqueous fluid by ciliary body and increase drainage to relieve pressure in the eye
If the desired effect is to increase the stimulation of muscarinic receptors, what alternative non-receptor strategy can be adopted? Which agents are used clinically?
Carbonic anydrase inhibitors e.g. acetazolamide and dorzolamide decrease secretion of aqueous humour.
What are the effects of increasing sympathetic tone in the eye?
Causes dilation of the pupil which increases aqueous humour production --> increased intraocular (in the eye) pressure
Which agents, active at adrenoceptors, are used clinically in the treatment of Glaucoma?
- Beta-adrenergic receptor antagonists decrease aqueous humour production by ciliary body.
- Alpha2-Adrenergic agonists decrease aqueous humour production and increase drainage.
- Alpha-adrenergic agonists decrease aqueous humour production through ciliary body blood vessels.
Explain the following statemnt: Noradrenaline is the major neurotransmitter at the neuroeffector junction of sympathetic post-ganglionic fibres.
In the sympathetic pathway:
Preganglionic neurones release ACh.
Postganglionic neurones release NA which binds to the alpha and beta adrenergic receptors on the effector organ.
Are there any post-ganglionic synapses in the sympathetic nervous system at which NA is not the transmitter?
- Innervation of merocrine sweat gland; neurotransmitter is ACh, receptor is M3
- Renal cortex: neurotransmitter is Dopamine, receptor is D1
The "biogenic amines" (dopamine, noradrenaline and adrenaline) are all synthesised from the amino acid tyrosine: outline the synthetic pathway