5.4
Cholinergic Neurons: Neurotransmission
Cholinergic neurotransmission involves the synthesis and the release of acetylcholine (ACh) in order to transmit nerve impulses across the synapse. Th…
Cholinergic neurons synthesize acetylcholine for neurotransmission from choline and acetyl-CoA.
Acetyl-CoA is synthesized in the mitochondria by oxidation and decarboxylation reactions, while choline is transported within the presynaptic neuron via choline transporters.
Within the neuron, the enzyme ChAT catalyzes the acetylation of choline to yield acetylcholine. Next, the VAT transports acetylcholine from the cytoplasm into synaptic vesicles for storage.
Upon arrival of an electrical impulse, Ca2+ enters the neurons through voltage-gated calcium channels. This triggers the exocytosis of acetylcholine.
The released acetylcholine binds to the postsynaptic receptors, such as the nicotinic or muscarinic receptors. This initiates a postsynaptic response.
Finally, the enzyme AChE hydrolyzes acetylcholine into choline and acetate.
Notably, certain organophosphates can irreversibly block AChE and inhibit the breakdown of acetylcholine.
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Q1: How is acetylcholine synthesized in cholinergic neurons?
Acetylcholine synthesis requires two precursors: acetyl-CoA, produced in the mitochondria through oxidation and decarboxylation reactions, and choline, transported into the neuron via choline transporters. The enzyme choline acetyltransferase (ChAT) catalyzes the acetylation reaction, combining these precursors to form acetylcholine within the cytoplasm.
Q2: What role do synaptic vesicles play in acetylcholine release?
Synaptic vesicles store acetylcholine after it is actively transported from the cytoplasm by the vesicle-associated transporter (VAT). When an electrical impulse arrives, calcium influx triggers vesicle fusion with the terminal membrane, causing rapid exocytosis of acetylcholine into the synaptic cleft for neurotransmission.
Q3: How does calcium trigger acetylcholine release at the synapse?
Voltage-gated calcium channels open when electrical impulses reach the axonal terminal, allowing calcium influx into the neuron. Calcium binds to synaptotagmin, a calcium-binding protein associated with vesicle-associated membrane proteins (VAMP), triggering vesicle membrane fusion and acetylcholine exocytosis into the synaptic cleft.
Q4: What happens to acetylcholine after it binds to postsynaptic receptors?
After acetylcholine binds to muscarinic or nicotinic receptors on the postsynaptic membrane and initiates a response, the enzyme acetylcholinesterase (AChE) rapidly hydrolyzes it into choline and acetate. This termination is crucial for signal control. Choline is then recycled back into the presynaptic neuron via choline transporters for resynthesis.
Q5: How do organophosphates affect acetylcholine neurotransmission?
Organophosphates irreversibly block acetylcholinesterase (AChE), preventing the breakdown of acetylcholine. This causes acetylcholine to accumulate in the synaptic cleft, leading to prolonged receptor activation and disrupted neurotransmission. Understanding this mechanism is essential for studying indirect acting cholinergic agonists mechanism of action and anticholinesterase agents poisoning and treatment.
Q6: What are the two main types of cholinergic receptors activated by acetylcholine?
Acetylcholine activates two postsynaptic receptor types: nicotinic receptors, which mediate fast synaptic transmission, and muscarinic receptors, which mediate slower modulatory responses. Both receptor types are essential targets for cholinergic drugs and are distributed throughout the central and peripheral nervous systems.
Q7: How is choline recycled after acetylcholine breakdown?
Acetylcholinesterase hydrolyzes acetylcholine into choline and acetate in the synaptic cleft. Choline is recycled by being transported back into the presynaptic neuron through choline transporters, including high-affinity choline transporter CHT1 and lower-affinity transporters. This recycled choline then reacts with acetyl-CoA to regenerate acetylcholine.