K1vii: Topic: Major neurotransmitters & their role
Definition
- Neurotransmitter = a chemical agent released by a neuron which transfers an impulse to neighbouring neurons/cells
- NT Criteria
- Synthesised by neuron
- Stored in nerve terminal
- Released in sufficient [ ] to affect postsynaptic cell
- Mechanism for removal
Synaptic Transmission
- Synthesis: of NT molecule from precursors
- Storage: of NT molecules into vesicles
- Release: AP causes Ca2+ influx → vesicles fuse with membrane → NT molecules released
- Binding: NT activates post synaptic receptor → INHIBITORY/EXCITATORY RESPONSE
- INHIBITORY POST SYNAPTIC POTENTIAL (IPSP) makes it more difficult for post synaptic neuron to generate AP
- EXCITATORY POST SYNAPTIC POTENTIAL (EPSP) makes it easier for post synaptic neuron to generate AP
- Removal: via reuptake/metabolism
Classification of Neurotransmitters
- Amino acids e. glutamine, GABA, glycine
- Biogenic amides
- From decarboxylation of amino acids
- e. Adrenaline, NA, dopamine, histamine
↓ further subdivides
- EXCITATORY: NMDA, nACh → membrane depolarisers
- INHIBITORY: GABA → membrane hyperpolarisation
Classificaiton of Neurotransmitter Receptors
Ionotropic (direct)
- A membrane protein
- Made of subunits
- Allows passage of an ion
Metabrotropic (indirect)
- Transmitter binds to a GPCR
- Activates 2nd messenger pathways
FOR EXAMPLE – Cholinergic
nAChR → Excitatory Ionotropic
- 5 subunit receptor
- Central ion pore
- ACh binds 2 α subunits
- Opens pore
- Na+ influx/small Ca2+ influx/K+ outflux
- Depolarisation
- Propagation of AP across NMJ
mAChR → Inhibitory & Excitatory Metabrotropic
- 7 transmembrane domains
- GPCR
- M1 (parietal cells)
- Phospholipase C activation
- ↑DAG/IP3
- ↓K+ conductance
- Membrane depol
- M2 (PM cells)
- ↓AC
- ↑K+ conductance
- Membrane hyperpolarisation
- ↑time to depol
- ↓HR
FOR EXAMPLE – Adrenergic = ALL metabrotopic
α Receptors
- α1
- ↑Phospholipase C
- ↑IP3/DAG
- ↑Ca2+
- Depol
- VC (excitatory)
- α2
- ↓AC
- ↓cAMP
- ↓Ca2+
- ↓NA release presynaptically (inhibitory)
- β receptors:
- ↑AC
- ↑cAMP
- ↑Ca2+ (excitatory)
β1 = ↑FoC, ↑HR
β2 = VD, bronchodilation
β3 = brown fat lipolysis
NT
Glutamate
(amino acid)
Action
1° excitatory NT
Synthesis
From ketoglutamate (Krebs)
Secretion
CNS – brain & SC
Receptors
Ionotropic
3 types
- NMDA
- AMPA
- Kainate
Uptake/Metabolism
Active transport back into neurons
NT
GABA
(amino acid)
Action
1° inhibitory NT
Synthesis
From glutamate
Secretion
CNS – SC, brain
Receptors
GABAA ionotropic
Uptake/Metabolism
GABA transaminase & reuptake
NT
Glycine
(amino acid)
Action
Excitatory & inhibitory
Synthesis
–
Secretion
CNS
Receptors
Ionotropic
Uptake/Metabolism
–
NT
Dopamine
(biogenic amine)
Action
Excitatory & inhibitory (i.e. inhibits NA release = VD)
Synthesis
From tyrosine
Secretion
CNS
PNS
Receptors
GPCR metabrotropic
Uptake/Metabolism
Metabolism by COMT/MAO
NT
NA
(biogenic amine)
Action
Excitatory & inhibitory
Synthesis
From tyrosine
Secretion
CNS
PNS
Receptors
GPCR metabrotropic
Uptake/Metabolism
Metabolism by COMT & MAO
NT
Adrenaline
(biogenic amine)
Action
Excitatory & inhibitory
Synthesis
From tyrosine
Secretion
CNS
PNS
Receptors
GPCR metabrotropic
Uptake/Metabolism
Metabolism by COMT/MAO
NT
Serotonin
(biogenic amine)
Action
Excitatory & inhibitory
Synthesis
From tryptophan
Secretion
CNS
Receptors
Ionotropic & metabrotropic
Uptake/Metabolism
Reuptake & MAO
NT
Histamine
(biogenic amine)
Action
Multiple actions
Synthesis
From histidine
Secretion
CNS
Receptors
–
Uptake/Metabolism
–
NT
ACh
(no category)
Action
Main NT of PNS
Synthesis
From choline + Acetyl CoA
Secretion
PNS
Receptors
Nicotinic – ionotropic
Muscarinic metabrotropic
Uptake/Metabolism
ACh-ase