Lecture 6: mAChR

Question 1

What effect does acetylcholine have on the heart?

Answer 1

Slows down the heart rate.

Question 2

Where is the M3 muscarinic receptor predominantly found, and what functions is it associated with?

Answer 2

• Glands (e.g., salivary glands) and smooth muscles in the gut.
• Associated with the secretion of various substances and can influence peristalsis.

Question 3

What are the functions of the M4 muscarinic receptor?

Answer 3

• Primarily located in the central nervous system (CNS)
• Modulates synaptic transmission but is too slow to mediate fast synaptic transmission like nicotinic receptors.

Question 4

How do muscarinic receptors differ in their distribution and effects?

Answer 4

• Polypeptide chain composition
• Binding pockets, distribution
• Effects they mediate
• Intracellular signaling cascades they couple to.

Question 5

What effect does activation of the M2 muscarinic receptor have on the heart?

Answer 5

Slowing of the heart rate and reduction of force in contraction.

Question 6

What type of receptors are muscarinic receptors, and what type of proteins are they associated with?

Answer 6

GPCRs associated with various types of G proteins

Question 7

What is the role of Gαq protein in muscarinic receptor signaling?

Answer 7

• Gαq protein, associated with M1, M3, and M5 muscarinic receptors, activates phospholipase C β (PLCβ) → breakdown of phosphatidylinositol 4,5-bisphosphate (PIP2).
  
  • This breakdown produces two products:
    ○ Diacylglycerol (DAG) -> activates protein kinase C (PKC)
    Inositol 1,4,5-trisphosphate (IP3) -> releases calcium ions from internal stores -> smooth muscle contraction and other cellular responses.

Question 8

What is the function of Gαi protein in muscarinic receptor signaling?

Answer 8

• Inhibits the actions of adenylate cyclase, which converts ATP to cyclic AMP (cAMP)
  
  • Inhibition reduces cAMP levels -> decreased activation of PKA
  • Reduced activity in cardiac tissue, including decreased force of contraction and reduced heart rate.

Question 9

How do muscarinic receptors and adrenergic receptors influence cellular activity?

Answer 9

• Through various effector molecules via G proteins.
• EXAMPLE: muscarinic receptors activate different types of G proteins, such as Gαq and Gαi -> diverse downstream signaling cascades and physiological effects.

Question 10

What are the effects of carbachol on various physiological processes?

Answer 10

• Constriction of muscles in the eye n bronchi
• Increases secretion (e.g., sweating, salivation, mucus production)
• Decreases intraocular eye pressure,
• Beneficial for conditions like glaucoma. However, it is not commonly used clinically.

Question 11

How does pilocarpine affect physiological functions?

Answer 11

• Increases secretion (e.g., lacrimation, salivation, sweating) and causes bronchoconstriction and increased mucus production.
• It is clinically used for treating glaucoma by decreasing intraocular pressure through muscle constriction, facilitating the drainage of aqueous humor from the anterior chamber.

Question 12

What are the therapeutic uses of cevimeline?

Answer 12

• Increases gut motility and relaxation of sphincters → defecation
• Clinically used to manage dry mouth (xerostomia) and dry eyes associated with Sjögren’s syndrome, stimulating the secretion of fluids in these conditions

Question 13

How does bethanechol affect bladder function?

Answer 13

• Bethanechol increases bladder constriction and sphincter relaxation → micturition (urination).
• Blood vessels to dilate → decreased blood pressure.
• Stimulate the activity of smooth muscle in the gastrointestinal and urinary tracts, particularly post-operatively.

Question 14

How can atropine be used to affect gut motility?

Answer 14

• Derived from belladonna
  
  • Parasympatholytic agent that inhibits the actions of the parasympathetic nervous system
  • USE: reduce gut motility.

Question 15

What are some common uses of parasympatholytic agents?

Answer 15

• Act by inhibiting the activation of muscarinic receptors.
  
  • Atropine: dilate pupils for eye exams
  • Co-phenetrope: reduces gut motility
  • Oxitropium: bronchodilator for asthma treatment
  • Benzhexol: Parkinson’s Disease
  • Hyoscine: motion sickness

Question 16

How is atropine utilized during medical procedures?

Answer 16

• Atropine can be used w anesthesia [vasodilation of the lungs but also reduces bronchial secretion so airways are clear during procedure]
  
  • Can be used during the reversal of neuromuscular blockade
    ○ Non-depolarizing neuromuscular blockers (nicotinic receptor antagonist)
    ○ Use AChE inhibitor to reverse their actions -> elevate ACh levels at the synapse at the NMJ -> elevate Ach everywhere around the heart
    § Atropine to reduce effects of ACh on the heart

Question 17

What was the myth regarding the M1 receptor and gastric acid secretion n how was it falsified?

Answer 17

• MYTH: M1 receptor is important for gastric acid secretion
  ○ EVIDENCE: treatment of stomach/gastric ulcers w pirenzepine (believed to be antagonist of M1 receptor)
  
  • FALSIFICATION
    ○ Applying molecular genetics to mice
    § Mutate/KO/remove proteins in mice
    ○ Measure gastric acid secretion in response to carbachol (agonist of muscarinic receptor)
    ○ No difference in gastric acid release whether there is an M1 receptor or not

Question 18

What conclusion was drawn from the falsification of the M1 myth?

Answer 18

• CONCLUSION: M3 is important is instead [associated w secretion]
  ○ When M3 is knocked out in mice, carbachol applied -> greatly reduces gastric acid being released
  ○ Some contribution w M5 receptors

Question 19

What further evidence supported the identification of the M3 receptor’s role in gastric acid secretion?

Answer 19

• Further evidence that pirenzepine that is thought to be active is the fact that
  ○ Pirenzepine is still effective in the M1 KO mice (i.e. blocks gastric acid secretion even in mice lacking M1 receptor
  ○ Residual gastric acid n histamine secretion in M3 KO blocked by pirenzepine
  • Instead of using musacrinic receptor antagonist in the treatment of stomach ulcers, we actually use compounds that target histamine H2 receptors

Question 20

What are the two types of acetylcholinesterase (AChE)?

Answer 20

• True AChE
  ○ Present at cholinergic synapses where ACh is released
  ○ Bound to the postsynaptic membrane in the synaptic cleft
  
  • Pseudo-cholinesterase
    ○ Widely distributed n found in plasma, circulate the blood stream
    ○ Important in inactivating the depolarizing neuromuscular blockers, suxamethonium
    § Suxamethonium has a short lasting effect in the majority of ppl bc they are broken down by circulating AChE
    Both types are inhibited equally by most clinically relevant anti-AChE

Question 21

Why is pseudo-cholinesterase relevant in the breakdown of suxamethonium?

Answer 21

Suxamethonium has a short-lasting effect in the majority of people because it is broken down by circulating pseudo-cholinesterase.

Question 22

Why does suxamethonium have a short-lasting effect in the majority of people?

Answer 22

Broken down by circulating pseudo-cholinesterase.

Question 23

How does acetylcholinesterase (AChE) function in terminating the action of acetylcholine (ACh)?

Answer 23

• Acetylcholine sits in the binding pocket of ACh enzyme
  
  • AChE has 2 sites: esteric site and anionic site
    ○ Attracts acetylcholine molecule to the catalytic site of the enzyme
  • Bond is cleaved, resulting in acetic acid (acteate) and choline
  • Splits the molecule apart thereby terminating the action of Ach
  • Making the choline available for re-uptake to the presynaptic terminal -> can be introduced into the synthetic pathway for Ach

Question 24

What happens to the choline produced by the cleavage of acetylcholine?

Answer 24

• Made available for re-uptake to the presynaptic terminal.
• It can then be introduced into the synthetic pathway for acetylcholine -> synthesis of new acetylcholine molecules

Question 25

What are the characteristics of reversible acetylcholinesterase inhibitors?

Answer 25

• Doesn’t hv particularly long lasting effects
  Useful for increasing ACh lvl temporarily to e.g. overcome NMJ block of non-depolarizing blockers

Question 26

How does neostigmine function as a reversible acetylcholinesterase inhibitor?

Answer 26

• Neostigmine sits in the catalytic pocket
  ○ Serine residue takes carbamyl group from neostigmine
  ○ Serine residue becomes carmabylated -> activates enzyme
  § If the carbamyl group is present, ACh can’t bind
  § So the enzyme is inactivated n can’t metabolize ACh for a period of time (allows ACh level to increase in the synapse where it’s been released)
  
  • Reversible reaction
    ○ Slowly carbamyl-serine bond becomes hydrolyzed
    ○ Essentially serine becomes free to participate in metabolism of ACh

Question 27

What is the mechanism of irreversible acetylcholinesterase inhibitors?

Answer 27

• Organophosphate ACh characterized by e.g. dyflos
  ○ Organophosphate phosphorylate serine residue -> covalent bond which is difficult to hydrolyze hence irreversible
  ○ V serious bc no ACh that can be broken down bc it’s been inactviated
  Clinical signs of organophosphate poisoning

Question 28

How do oximes function in the reactivation of acetylcholinesterase inhibited by organophosphates?

Answer 28

• Oxines bind to anioinic site, allowing transfer of phosphate to oxine, freezing serine
  
  • Phosphate has been transferred to oxine
    Allow the reactivation of AChE

Question 29

What is the significance of the aging process in the context of organophosphate poisoning?

Answer 29

• Aging of the bond b/w the phosphate group n serine
  
  • Longer you wait to deliver oxine, more difficult it becomes for AChE to be reactivated
    Has to be delivered quickly to those poisoned by organophosphate

Question 30

What are the general uses of acetylcholinesterase inhibitors like physostigmine?

Answer 30

• Increases the concentration of the naturally occurring agonist acetylcholine.
• Constriction of the muscles in the eyes -> accumulated fluid to be expelled.

Question 31

What is the observed pattern of loss of nicotinic receptors in Alzheimer’s Disease (AD)?

Answer 31

The loss of nicotinic receptors in AD follows the distribution of cholinergic fibers in the brain.

Question 32

What is the impact of amyloid proteins on cholinergic function in AD?

Answer 32

Inhibit the enzyme choline acetyltransferase (ChAT) -> cholinergic deficit.

Question 33

How does the reduction in vesicular acetylcholine (ACh) transporters relate to dementia in AD?

Answer 33

Varying degrees of dementia in AD are associated with a reduction in vesicular ACh transporters, crucial for packaging ACh into synaptic vesicles.

Question 34

How do acetylcholinesterase (AChE) inhibitors play a role in AD treatment?

Answer 34

AChE inhibitors are used to reduce the activity of AChE in the brain, allowing more acetylcholine to be released and activate receptors, attempting to overcome the cholinergic deficit.

Question 35

How does the cholinergic hypothesis explain AD?

Answer 35

The cholinergic hypothesis proposes that cholinergic inputs into various brain regions are affected in AD, involving factors like inhibition of ChAT by amyloid proteins, loss of muscarinic receptors, and reduction in nicotinic receptors.

Question 36

What cognitive improvements are observed with AChE inhibitors in AD?

Answer 36

AChE inhibitors have been shown to improve cognitive performance, as demonstrated in tests like the Stroop test, where increased ACh levels lead to better performance.

Question 37

How do antibodies against nicotinic receptors contribute to myasthenia gravis (MG)?

Answer 37

In myasthenia gravis, an autoimmune disease, antibodies are produced against nicotinic receptors at the NMJ

Question 38

What structural changes occur at the NMJ in myasthenia gravis?

Answer 38

• Normal conditions: highly invaginated
  
  • MG: fewer receptors
    ○ Receptors can be cross linked by antibodies -> provoking internalization -> less able to contribute to neuromuscular transition
  • Can block activation of receptors by antibodies binding to them
    All this reduces skeletal muscle contraction

Question 39

How can the effects of organophosphate nerve agent poisoning be countered?

Answer 39

• Atropine to block excessive stimulation of ACh receptor esp ones in the vasculature that might result in dilation of blood vessels -> decrease in BP
  
  • Oximes to reactivate enzymes but aging can occur
  • Valium
    ○ Treatmetn of seizures
  • Reversible antiAChEs as prophylactics
    ○ Dose soldiers ahead of time
    ○ Complement of AChE that was occupied by neurostigmine
    ▪ If you were to be exposed to organophosphate, you would just stop the neurostigmine
    Would still hv some active AChE [neurostigmine would come off q quickly]

Question 40

What are the two compounds present in a dual injector pack given to soldiers in battlefield situations where they might encounter organophosphates?

Answer 40

Atropine and an oxime
WHY?
- Atropine blocks ACh receptor stimulation -> decrese in BP
- OXines reactivate enzymes