Which mediator decreases norepinephrine release from presynaptic sympathetic neurons?

Which mediator decreases norepinephrine release from presynaptic sympathetic neurons?

• A. Nitric oxide
• B. Norepinephrine
• C. Epinephrine
• D. Angiotensin II

Answer: (A)

Presynaptic inhibition of sympathetic transmission can be produced by nitric oxide, a gaseous signaling molecule. It diffuses to the presynaptic terminal and activates soluble guanylate cyclase, raising cGMP levels. This shift reduces calcium influx and vesicle fusion, so less norepinephrine is released into the synapse. Nitric oxide acts as a local brake on transmitter release, helping dampen sympathetic signaling. The other substances listed don’t serve as local presynaptic inhibitors in this context: norepinephrine is the transmitter itself, epinephrine mainly acts systemically rather than as a presynaptic brake in sympathetic terminals, and angiotensin II tends to enhance sympathetic activity rather than inhibit NE release.

Presynaptic inhibition of sympathetic transmission can be produced by nitric oxide, a gaseous signaling molecule. It diffuses to the presynaptic terminal and activates soluble guanylate cyclase, raising cGMP levels. This shift reduces calcium influx and vesicle fusion, so less norepinephrine is released into the synapse. Nitric oxide acts as a local brake on transmitter release, helping dampen sympathetic signaling.

The other substances listed don’t serve as local presynaptic inhibitors in this context: norepinephrine is the transmitter itself, epinephrine mainly acts systemically rather than as a presynaptic brake in sympathetic terminals, and angiotensin II tends to enhance sympathetic activity rather than inhibit NE release.