Cholinergic neurotransmission

Question 1

Location of cholinergic neurons

Answer 1

• neuromuscular junction
• autonomic preganglionic fibers
• parasympathetic postganglionic fibers
• CNS

Question 2

High affinity Choline transporter (ChT)

Answer 2

• encoded by SLC5A7 gene
• plasmalemmal transporter
• carries choline into acetylcholine synthesizing neurons
• sodium and chloride dependent
• inhibited by: Hemicholiniums
• Km: >10nM in most tissues, 1-5mM in cholinergic neurons

Question 3

Choline actyltransferase

Answer 3

• combine choline and acetyl-CoA

Question 4

Vesicular acetylcholine transporter (VAChT)

Answer 4

• proton acetylcholine antiporter
• low pH (5,5) in vesicles- maintained by H+ATPase
• inhibited by: Vesamicol

Question 5

Vesicular exocytosis mechanism

Answer 5

1. Uptake of NT into vesicle (VAChT)
2. Formation of cluster of vesicles
3. Docking of vesicles in active zone
4. Priming- vesicles become competent for Ca2+ induced fusion
5. Ca2+ signal - fusion
6. Vesicle recycling
   * local recycling: vesicles refilled without undocking
   * fast recycling: undocks
   * classical endocytosis

Question 6

Vesicular exocytosis

Answer 6

1. Docking- no SNAREs
2. Priming - formation of SNARE complex and synaptotagmin
3. Ca2+ signal - synaptotagmin C2 domain into phospholipid membrane

Question 7

SNARE proteins

Answer 7

• vSNARE: incorporated into membrane during budding
• tSNARE: on membrane of target compartment
• 70 AAs homolog sequence
• receptors for NSF and SNAP
• on vesicle: VAMP + synaptobrevin
• on PM: syntaxin 1A/B + SNAP-25
• in cytosol SNARE motifs form alpha-helices that supercoil and parallel coil

Question 8

Proteins involved in exocytosis

Answer 8

• SNARE complex
• regulatory proteins:
• munc18-1
• complexins
• synaptophysin
• synapsin
• Ca2+ sensor:
• synaptotagmin

Question 9

Botulinum neurotoxins

Answer 9

• protein and neurotoxin produced by bacteria
• lethal dose: 1,3-2,1 ng intravenosuly, 13 ng/kg inhaled
• cause botulism
• used as botox
• 7 types (BoNT-A;B;C;D;E;F;G)
  
  • inhibitor of cholinergic neurotransmission
  • heavy chain 100 kDa, light chain 50 kDa
  • acceptor-mediated endocytosis
• in cytosol- catalytic domain inhibits exocytosis
• Zn2+ dependent endoproteases break down SNAREs

Question 10

M1 receptor

Answer 10

• Gq - phospholipase C - PIP2 hydrolysis - IP3 + Ca2+
• mediates slow ESPS in postganglionic nerve
• common in exocrine glands and CNS

Question 11

M2 receptor

Answer 11

• Gi - decrease cAMP - inhibit VGCC - increase K+ efflux
• location: heart
• function: slow HR to normal, slow speed of depol., reduce contractile force of atrial cardiac m. and ventricular m., reduce contraction velocity of AV node

Question 12

M3 receptor

Answer 12

• Gq pathway
• location: smooth m in vessels and lungs, glands
• contraction of smooth m -> bronchoconstriction
• NO release from endothelium - relaxation

Question 13

M4 receptor

Answer 13

• Gi pathway
• location: CNS
• possible bronchospasm if stimulated by agonist

Question 14

M5 receptor

Answer 14

• Gq pathway

- location not known

Question 15

Atropine

Answer 15

• competitive antagonist fo muscarinic receptors
• used as treatment for bradycardia - blocks vagus nerve and increases HR
• used as cycloplegic to paralyze accommodation reflex and as mydriatic to dilate pupils

Question 16

Nicotinic receptors

Answer 16

• 5 subunits, symmetrically around pore
• each subunit has 4 transmembrane domains with both N and C terminal extracellularly
• ligand gated ion chanels
• mediates fast signal transmission in synapses
• either neuronal or muscle-type

Question 17

Muscle-type Nicotinic AchR

Answer 17

• location: neuromuscular junction
• responsible for muscle tone
• 2alpha, beta, delta, gamma subunits in embryo
• 2alpha, beta, delta, epsilon in adult
• Ach binding site is at alpha and gamma/delta

Question 18

Neuronal type Nicotinic AchR

Answer 18

• location: between neurons in CNS
• involved in cognitive functions, learning, memory, arousal, reward, motor control and analgesia
• binding of Ach causes conformational change and opens ion pore - rapid increase in cellular permeability of Na+ and Ca2+ - depol. - contraction
• Ca2+ influx leads to release of NT
• subunits: 2alpha, 3 beta // 5 alpha

Question 19

D- tubocuranine

Answer 19

• long-duration antagonist of nAchR

- skeletal muscle relaxant

Question 20

alpha- bungarotoxin

Answer 20

• 74 AA neurotoxin, 5 disulfide bonds
• competitive antagonist of muscle- nAchR
• binds irreversibly
• inhibits ion flow
• leads to paralysis

Question 21

Malignant hyperthermia

Answer 21

• genetic defect in ryanodine receptors
• causes persistent increase in Ca2+ i.c. conc.
• cause uncontrolled increase in skeletal muscle oxidative metabolism which overwhelms body’s capacity to supply O2, remove CO2, regulate body temp.
• leads to death if not treated

Question 22

Myasthenia gravis

Answer 22

• autoimmune disease- body creates antibodies that bind to nAchR
• droopy eyelids, muscle weakness, early fatigue
• treatment: acetylcholinesterase inhibitors or immunosuppressants

Question 23

Acetylcholinesterase

Answer 23

• serine protease enzyme
• located in synapse od cholinergic neurons
• breakdown of Ach to choline + acetate
• pseudocholinesterase: in plasma and liver, its function is hydrolysis of other esters

Question 24

Suxamethonium

Answer 24

• nAchR agonist
• induce muscle relaxation and short-term paralysis
• causes opening of monovalent cation channel - disorganized depol. of motor end plates - Ca2+ release from SR
• not hydrolyzed by acetylcholinesterase
• no repol - relaxation

Question 25

Reversible Acetylcholinesterase inhibitors

Answer 25

Physostigmine
Neostigmine
- used for Myasthenia gravis

Question 26

Irreversible Acetylcholinesternase inhibitors

Answer 26

Diisopropyl-flurophosphate

• parasympathomimetic drug
• combines with serine159 residue in active site
• proton from OH serine released
• electron rearrangement and fluorine release
• DFP-enzyme complex