Intro to Cholinergic Pharmacology Flashcards
What are the 2 methods of ACh metabolism?
- De novo synthesis (minor)
2. Alternative synthesis (major) - most choline is recycled from ACh
Describe the steps of de novo ACh synthesis
- ACh synthesis is a 2-step conversion of serine to choline.
- Step 1 requires enzyme serine decarboxylase.
- Step 2 requires choline N-methyl transferase and S-adenosyl methionine. - The enzyme choline acetyltransferase (ChAT) is responsible for the acetylation of choline. ChAT resides in the cytoplasm so ACh must be in the cytoplasm. Acetyl-CoA comes from the inner layer of the mitochondria and comes out of the mitochondria as citrate.
What modifications must ACh undergo to have activity in the brain and nervous system?
ACh is the major NT in the brain, but choline is charged so there is a BBB to overcome. Once inside the brain, choline remains charged and has to be transported into the neurons into the brain. ChAT converts choline into ACh by adding an acetyl group which allows it to be an active NT in neurons.
How is ACh stored and released?
ACh is stored in synaptic vesicles. Cholinergic vesicles contain 2 important transporters which are required to concentrate ACh:
1. proton ATPase
2. ACh-H+ antiport
This has reverse vs. depot pools. ACh-H+ antiport concentrates ACh. The vesicle contains ATP and heparin proteoglycans. Every time ACh is released, heparin sulfate proteoglycans are also released. There are 2 types of pools for release:
1. readily release pool
2. reverse pool - this pool waits for an action potential to be released, then it moves into position for supply.
Release of ACh can be done either by spontaneous or stimulated release.
- spontaneous release from terminal has constitutive activity of the NT at the synapse and produces a synaptic potential
- stimulated release relies on stimulation by an action potential.
Describe the function and mechanism of SNARE proteins
SNARE proteins anchor and release vesicles that contain NTs. There are SNARE proteins on the vesicles (v-SNAREs): synaptotagmin and synaptobrevin as well as SNARE proteins on the terminal membrane: SNAP-25 and Syntaxin.
Synaptotagmin is a Ca2+ receptor and Synaptobrevin binds to syntaxin and SNAP-25 on the membrane to form the ternary complex.
There are 3 steps to vesicle release of NT: 1. docking, 2. priming, and 3. fusion.
n-sec-1 binds syntaxin in the initial state, but it disassociates in order for synaptobrevin to interact with syntaxin and SNAP-25.
This complex tightens and leads to fusion and exocytosis when Ca2+ enters the neuronal terminal and binds to synaptotagmin. This leads to release of NT into the synaptic cleft. Then ATP is required to disassemble the ternary complex by releasing synaptobrevin from syntaxin and SNAP-25 to disassociate from synaptobrevin.
How does the botulinum toxin drug act upon SNARE proteins?
Botulinum toxin degrades the SNAP-25 SNARE protein on the terminal membrane and thus prevents vesicle docking on the membrane. This prevents vesicle fusion and exocytosis of ACh, so that it is unable to be released from the vesicle into the synaptic cleft.
What are 3 ways that ACh is degraded and recycled and describe their mechanism of action.
Metabolism of ACh:
- ACh is made from choline and acetyl CoA
- ACh is broken down by AChase in synpatic cleft
- Choline is transported back to the axon terminal
- Acetylcholinesterase
- this is found in the vicinity of cholinergic synapses and in RBCs
- it has a high affinity for ACh and degrades the NT in the synaptic cleft - Pseudocholinesterases (plasma or butyrlcholinesterase_
- found in plasma
- T1/2 range from 8-16 hours
- concentrations may directly affect succinylcholine therapy - Fate of metabolites
- choline is taken up by special choline transporters into pre-synaptic neurons
- coenzyme A is released from the mitochondria
How is ACh activity implicated in pts with Alzheimer’s?
Acetylcholinesterase will break down ACh and stop it from having effects.
Pseudocholinesterase is used in Tx of Alzheimer’s disease.
What are properties of muscarinic cholinergic receptors?
- G-protein coupled receptors (Gq/11 or Gi/o)
- 5 distinct subtypes exist in humans: M1 - M5
- M1/3/5 are stimulatory and coupled to Gq and PLC - increase in Ca/IP3/DAG
- M2/4 are inhibitory and are coupled to adenylyl cyclase and K+ channels, enhancing K+ conductivity (hyperpolarity)
What are properties of nicotinic cholinergic receptors?
- nAChR exhibit direct ligand-gated conductance
- enhance K+ and Na+ conductance equally
- there are 2 subtypes of nAChR:
1. Nm - skeletal muscles and neuromuscular junction
2. Nn - autonomic ganglia, adrenal medulla, and CNS
For the M1 receptor, describe:
1) typical locations
2) responses
3) mechanisms
4) prototype agonist
5) prototype antagonist
1) autonomic ganglia and CNS
2) late excitatory postsynaptic potential (EPSP), complex: at least arousal, attention, analgesia
3) Gq/11 –> PLC –> increase IP3 + DAG –> increase Ca2+ and PKC
4) agonist: oxotremorine
5) antagonist: pirenzepine
For the M2 receptor, describe:
1) typical locations
2) responses
3) mechanisms
4) prototype agonist
5) prototype antagonist
1) heart SA node, AV node, atrium and ventricle
2) slowed spontaneous depolarization; hyperpolarization, decreased conduction velocity, decreased refractory period, decreased contractility force, slight decrease in contractility
3) Beta-gamma portions of G protein inhibit AC and increase K+ channel opening
4) agonist: -
5) antagonist: AF-DX 117
For the M3 receptor, describe:
1) typical locations
2) responses
3) mechanisms
4) prototype agonist
5) prototype antagonist
1) location: smooth muscle
2) responses: contraction
3) mechanism: same as M1 (Gq/11 –> PLC)
4) agonist: -
5) antagonist: hexahydrosiladifenidol
For the M4 receptor, describe:
1) typical locations
2) responses
3) mechanisms
4) prototype agonist
5) prototype antagonist
1) location: CNS
2) responses: -
3) mechanism: same as M2 (Gi)
4) agonist: -
5) antagonist: Himbacine
For the M5 receptor, describe:
1) typical locations
2) responses
3) mechanisms
4) prototype agonist
5) prototype antagonist
1) location: CNS
2) responses: -
3) mechanism: same as M1 (Gq/11)
4) agonist: -
5) antagonist: -
For the Nm receptor, describe:
1) typical locations
2) responses
3) mechanisms
4) prototype agonist
5) prototype antagonist
1) location: skeletal muscle at NMJ
2) response: end-plate depolarization; skeletal muscle contraction
3) mechanism: opening of Na+/K+ channels
4) agonist: phenyltrimethylammonium
5) antagonist: tubocurare
For Nn receptor, describe:
1) typical locations
2) responses
3) mechanisms
4) prototype agonist
5) prototype antagonist
1) locations: autonomic ganglia, adrenal medulla, CNS
2) response: depolarization and firing of postganglionic neuron, secretion of catecholamines, complex: at least arousl, attention, analgesia
3) opening of Na+/K+ channels
4) agonist: dimethylphenylpiperazinium
5) antagonist: trimethylaphan
Where do muscarinic receptors dominate in the CNS?
brainstem and spinal cord
Where do nicotinic receptors dominate in the CNS?
substantia nigra, locus coeruleus, and septum
What are 6 locations where both muscarinic and nicotinic receptors are found in the CNS?
- corpus striatum
- cerebral cortex
- hippocampus
- thalamus
- hypothalamus
- cerebellum
What are 5 major cholinergic projection centers?
- septum
- nucleus basalis
- diagonal band
- laterodorsal tegmentum
- pendiculopontine tegmentum
Because of their location in this region of the brain, what neurologic disease can cholinergic agents be used as Tx for relief?
Cholinergic agents can be used for relief of Parkinson’s Sx because the substantia nigra is packed with nicotinic receptors
What are 3 locations of nicotinic receptors in the peripheral nervous system?
- autonomic ganglia
- adrenal medulla
- neuromuscular junction
How do the subunits of Nn receptors differ from Nm receptors?
Nn receptors are composed only of alpha and beta subunits, whereas Nm is comprised of alpha2-beta-epsilon-gamma (major) or alpha2-beta-gamma-delta
What are 3 major locations of muscarinic receptors in the peripheral nervous system?
- autonomic ganglia
- end organs (parasympathetic, sweat glands)
- CNS
Describe the mechianism of M1, M3, and M5 receptors.
M1 (autonomic ganglia, CNS), M3 (smooth muscles), and M5 (CNS) are coupled to PLC activation and IP3, DAG, Ca2+ and PKC increases
Describe the mechanism of action of M2 and M4 receptors.
M2 (SA node, AV node, atrium, and ventricles) and M4 (CNS) are coupled to adenylyl cyclase inhibition and increase K+ conductance
Describe the cholinergic transmission dysfunction associated with dementia.
Dementia is characterized by cognitive decline that leads to reduced or complete loss of ability to carry out daily functions.
Alzheimer’s disease is by far the most frequent and economically important form of dementia. Dysfunction in cholinergic transmission within the CNS is the most consistent neurotransmitter deficit in Alzheimer’s disease.
Trouble begins with the accumulation of neurofibrillary tangles in the entorhinal cortex and hippocampus - 2 of the major destination of cholnergic projections in the CNS.
Where are the 2 locations in the brain that neurofibrillary tangles will be found in someone with dementia and why?
neurofibrillary tangles will be found in
1. entorhinal cortex
2. hippocampus
because these are the 2 of the major destinations of cholinergic projections in the CNS
What are 3 drugs that are used in Tx of Alzheimer’s disease and how do they work?
- Donepezil
- Rivastigmine
- Galantamine
These drugs work by inhibiting acetylcholinesterase which prolongs life of ACh in the synapse
What is the specific group of neurons in the substantia innominata of the basal forebrain which has a rich supply of ACh that undergoes degeneration in Alzheimer’s?
Nucleus basalis
What cholinergic dysfunction is characterized by Parkinson’s and what are the resulting symptoms?
Parkinson’s is characterized by selective irreversible loss of dopaminergic neurons in the substantia neigra pars compacta which leads to motor degeneration. These symptoms include:
- bradykinesia
- rigidity
- impaired postural balance
- rest tremors
What NT imbalance leads to the characteristic movement dysfunctions in Parkinson’s?
An imbalance between dopaminergic input and cholinergic transmission in the putamen
What are effective treatments for relieving Parkinson’s Sx and how do they work?
Anti-muscarinic agents such as trihexyphenidyl and benztropine treat Parkinson’s by reducing cholinergic tone in the CNS
How can cholinergic transmission be manipulated for a dilated fundus examination in the eyes?
anti-muscarinic agents can be used to induce MYDRIASIS (dilation of the pupil) to observe the inner structures of the eye. Muscles are loaded with M1 receptors - increase and dilate the pupil so the physician can see right through to the pupil
What are 3 drugs that can produce mydriasis?
anti-muscarinic agents:
- atropine
- scopolamine
- hyoscyamine
What dysfunction leads to glaucoma?
acute angle glaucoma is associated with partial or absolute blockade of the canal of Schlemm. Gluacoma is caused by an increase of aquemous humor which leads to increase pressure in the eyeball. This is due to either slow drainage or overproduction of the aqueous humor.
Normally, the chiliary body makes aqueous humar in the canal of Schlemm. In acute angle glaucoma, the canal of Schlemm is blocked in mydriasis
What are 2 drugs can help relieve Sx of glaucoma?
muscarinic agonists:
- pilocarpine
- phsostigmine
What are 3 disorders of the respiratory tract that are affected by disorders of cholinergic transmission?
- chronic bronchitis
- COPD
- Asthma
What leads to asthma?
M3 stimulation within airway bronchioles which mediate bronchochonstriction
What are effective Tx for asthma?
ipratropium and tiotropium - effective alternatives to B2 adrenergic agonists
What is an effective drug for COPD?
Tiotropium with a long plasma half life