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BIOCHEMISTRY 3 > Cholinergic neurotransmission > Flashcards

Cholinergic neurotransmission Flashcards

1
Q

Location of cholinergic neurons

A
  • neuromuscular junction
  • autonomic preganglionic fibers
  • parasympathetic postganglionic fibers
  • CNS
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2
Q

High affinity Choline transporter (ChT)

A
  • 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
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3
Q

Choline actyltransferase

A
  • combine choline and acetyl-CoA
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4
Q

Vesicular acetylcholine transporter (VAChT)

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

Vesicular exocytosis mechanism

A
  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
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6
Q

Vesicular exocytosis

A
  1. Docking- no SNAREs
  2. Priming - formation of SNARE complex and synaptotagmin
  3. Ca2+ signal - synaptotagmin C2 domain into phospholipid membrane
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7
Q

SNARE proteins

A
  • 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
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8
Q

Proteins involved in exocytosis

A
  • SNARE complex
  • regulatory proteins:
  • munc18-1
  • complexins
  • synaptophysin
  • synapsin
  • Ca2+ sensor:
  • synaptotagmin
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9
Q

Botulinum neurotoxins

A
  • 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
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10
Q

M1 receptor

A
  • Gq - phospholipase C - PIP2 hydrolysis - IP3 + Ca2+
  • mediates slow ESPS in postganglionic nerve
  • common in exocrine glands and CNS
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11
Q

M2 receptor

A
  • 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
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12
Q

M3 receptor

A
  • Gq pathway
  • location: smooth m in vessels and lungs, glands
  • contraction of smooth m -> bronchoconstriction
  • NO release from endothelium - relaxation
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13
Q

M4 receptor

A
  • Gi pathway
  • location: CNS
  • possible bronchospasm if stimulated by agonist
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14
Q

M5 receptor

A
  • Gq pathway

- location not known

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15
Q

Atropine

A
  • 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
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16
Q

Nicotinic receptors

A
  • 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
17
Q

Muscle-type Nicotinic AchR

A
  • 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
18
Q

Neuronal type Nicotinic AchR

A
  • 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
19
Q

D- tubocuranine

A
  • long-duration antagonist of nAchR

- skeletal muscle relaxant

20
Q

alpha- bungarotoxin

A
  • 74 AA neurotoxin, 5 disulfide bonds
  • competitive antagonist of muscle- nAchR
  • binds irreversibly
  • inhibits ion flow
  • leads to paralysis
21
Q

Malignant hyperthermia

A
  • 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
22
Q

Myasthenia gravis

A
  • autoimmune disease- body creates antibodies that bind to nAchR
  • droopy eyelids, muscle weakness, early fatigue
  • treatment: acetylcholinesterase inhibitors or immunosuppressants
23
Q

Acetylcholinesterase

A
  • 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
24
Q

Suxamethonium

A
  • 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
25
Q

Reversible Acetylcholinesterase inhibitors

A

Physostigmine
Neostigmine
- used for Myasthenia gravis

26
Q

Irreversible Acetylcholinesternase inhibitors

A

Diisopropyl-flurophosphate

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

BIOCHEMISTRY 3 (17 decks)

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