Correct option is A
- Norepinephrine: Norepinephrine is synthesized from dopamine. The enzyme dopamine β-hydroxylase (DBH), which catalyzes the final conversion of dopamine to norepinephrine, is located inside the synaptic vesicles. Therefore, dopamine is synthesized in the cytoplasm, taken up into the vesicle, and then converted to norepinephrine within the vesicle.
Why other options are incorrect:
Epinephrine: Epinephrine is synthesized from norepinephrine in the cytoplasm. The enzyme that catalyzes this reaction, phenylethanolamine N-methyltransferase (PNMT), is in the cytoplasm. Epinephrine is then transported into vesicles.
Acetylcholine: Acetylcholine's synthesis is unique. The enzyme choline acetyltransferase, which synthesizes acetylcholine from choline and acetyl-CoA, is located inside the synaptic vesicles. The precursors are transported in, and the entire synthesis happens within the vesicle. It's not synthesized in the cytoplasm first.
Serotonin: Serotonin is synthesized from tryptophan in the cytoplasm. It is then transported into synaptic vesicles by a specific transporter.
Information Booster:
- Choline acetyltransferase (ChAT): The enzyme responsible for acetylcholine synthesis within synaptic vesicles.
- Vesicular transporters: Proteins that transport neurotransmitters into synaptic vesicles. For example, VMAT transports norepinephrine, serotonin, and dopamine into synaptic vesicles.
- Synaptic vesicle function: These vesicles store neurotransmitters and release them into the synaptic cleft in response to an action potential.
- Neurotransmitter synthesis location: The cytoplasm is where many neurotransmitters, like dopamine and serotonin, are synthesized before being loaded into vesicles. In contrast, acetylcholine is synthesized directly inside the vesicle.
- Neurotransmitter release: Once stored in vesicles, neurotransmitters are released into the synaptic cleft by the process of exocytosis, triggered by an influx of calcium ions.
