Neurotransmission
description
Transcript of Neurotransmission
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Neurotransmission
B.Sc 2002
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Archetypal neurotransmitter is acetylcholine;
We know more about cholinergic transmission than any other. However, cholinergic transmission is the exception not the rule in the CNS
+CH3 C OCH2CH2 N
CH3
CH3
CH3
O
acetylcholine
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Acetylcholine can be very rapidly hydrolysed, that is broken down by reaction with water. It may be this characteristic that makes it suitable as a transmitter
+ +
+
CH3 C OCH2CH2 N
CH3
CH3
CH3
O
+CH3 C OCH2CH2 N
CH3
CH3
CH
O
HOH
choline
acetylcholine
acetic acid
H O2
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Why is ACh a transmitter? What makes it suitable?
Ach is a quaternary amine; it has a nitrogen atom that is positively charged at one end of the molecule
Most compounds that are agonists at cholinergic synapses also have a positively charged nitrogen
+CH3 C OCH2CH2 N
CH3
CH3
CH3
O
acetylcholine
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Some General Cholinergic agonists
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Two types of cholinergic agonists;Nicotinic and Muscarinic
Examples of Specific Nicotinic agonists: Nicotine, Lobeline
N
N
N
CH3
Nicotine
Lobeline
O O
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N
N
N
CH3N
N
CH3
Nicotine
Lobeline
O O
H
+
H
+
Free base at pH 7
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Muscarinic agonists do not need positively charged nitrogen;
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Nicotinic: ion channel openingMuscarinic: G-protein coupled
• M1; ‘neural’; slow epsps in ganglia; increase IP3,DAG
• M2; ‘cardiac’ decrease heart rate; decrease cAMP
• M3;’glandular’ increase secretion increase IP3
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Atropine & scopolamine are antagonists at all muscarinic synapses
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Transmitter release
Acetylcholine is found in synaptic vesicles (50 nm diameter), clustered around release zones in the presynaptic membrane.
Voltage-gated N- or P- type calcium channels are found in large numbers in the presynaptic membrane
Blockade of these channels (black widow spider venom, conotoxin) prevents transmitter release
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After stimulation of axon, muscles normally show an ‘end plate potential’ that triggers AP. In quiescent muscles, spontaneous ‘miniature’ end plate potentials are observed.
10-30 ms
-70 mv-65 mv
Time of arrival ofaction potential inafferent terminals
Motoneuronemembranepotential
EPSP
-90 mv
-40 mv
Time of arrival ofaction potential atneuromuscluar junction
Musclemembranepotential
EPP
Action potential threshold
-90 mv
-85 mv
Musclemembranepotential
spontaneous miniature end-plate potentials,always of fixed sizes
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Fact that mepps were quantized (2 mV, 4mV, 6 mV, not e.g. 3.5 mV, was evidence that transmitter was released in packets, or quanta. This led to idea that transmitter was released from vesicles. Even in absence of stimulation, some vesicles were releasing transmitter
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Methods to study transmitter release mechanisms
• Toxins (mostly proteases) eg botulinus toxin, tetanus toxin
• Capacitance measurements of secreting cells
• Intracellular ion-sensitive dyes
• Mutation of intracellular vesicular & membrane proteins
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Synaptic docking proteins
On vesicle:
synaptotagmin (sytg), synaptobrevin (syb)
On plasma membrane:
SNAP-25, syntaxin (sytx)
In cytoplasm:
NSF (contains -SNAP)
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Synaptic Vesicle
SYB
SYTX
SYTG
SNAP-25
Plasma membrane
NSF
Ca channel++
Synaptic Vesicle
SYB
SYTX
SYTG
SNAP-25
Plasma membrane
NSF
Ca channel++
Docking complex formedVesicle separate
Syb: synaptobrevin, Sytg: synaptotagmin,sytx: syntaxin
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Synaptic Vesicle
SYB
SYTX
SYTG
SNAP-25
Plasma membrane
NSF
Ca channel++
Docking complex formed Fusion complex formed
Synaptic Vesicle
SYB
SYTX
SYTG
SNAP-25
Plasma membrane
NSF
Ca channel++
Calcium enters; activates sytg
Synaptic Vesicle
SYB
SYTX
SYTG
SNAP-25
Plasma membrane
NSF
Ca++
sytg interacts with sytx;Transmitter release
SYB
SYTX
SYTG
SNAP-25
Plasma membrane
NSF
Ca++
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Transmitter can ‘leak’ from fusion complexes before proper calcium triggering
Spontaneous release can occur from fluctuations in intracellular calcium
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Clathrin
DockedExocytosis
Endocytosis
Migration
Refilling
Pre-fusion
Storage pool
Vesicle Life-Cycle
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Studies on ACh receptors have used the electric eel‘torpedo’ as it contains very large amount of receptor protein in the electroplax organ.
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Complete ACh channel
outside cell
inside cell
NH2
COOH
One ACh channel subunit
subunit
pore
subunit seen from above
five subunitsarrangedin a ring
long (210 residues) N-terminalhydrophilic extracellularregion pointing towardsACh release site
subunit
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pore
subunit
Nicotinic ACh Receptor
ACh Binding site
2M 2M
2M4M
4M4M
4M
4M
2M
2M
Lipophilic bonding
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Transmitter inactivation
• ACh uses enzymic breakdown as mechanism of inactivation
• All other transmitters use reuptake as inactivation mechanism
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• UNSOLVED PROBLEMS:
• How does ACh unbind from binding site?
• What is actual mechanism of allosteric distortion?
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Cholinegic antagonists
• Depolarisation blockers eg suxamethonium
• Do not unbind from receptor, leave muscle depolarised and unable to fire second AP
• Curare-like agents bind to receptor but do not open ion channel
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N
NO
O
OH
OH
CH O3
H
H
H
3CH3CH
3CH
Tubocurare
+
+
Distance apart of two ACh N binding sites?+
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QuickTime™ and aCinepak decompressor
are needed to see this picture.