Norepinephrine acting on presynaptic alpha-2 receptors leads to what presynaptic effect?

Norepinephrine acting on presynaptic alpha-2 receptors leads to what presynaptic effect?

• A. Increases NE release
• B. Vasoconstriction
• C. Inhibits NE release
• D. Bronchodilation

Answer: (C)

Activation of presynaptic alpha-2 receptors on norepinephrine terminals serves as a negative feedback mechanism. When engaged, these Gi-coupled receptors inhibit adenylyl cyclase, reduce cAMP, and decrease calcium influx, which lowers vesicle fusion and exocytosis. The net presynaptic effect is inhibition of norepinephrine release. The other possibilities describe downstream postsynaptic actions or effects not related to the presynaptic release control (for example, vasoconstriction comes from postsynaptic alpha-1 effects, and bronchodilation relates to beta-2 activity, not this presynaptic mechanism).

Activation of presynaptic alpha-2 receptors on norepinephrine terminals serves as a negative feedback mechanism. When engaged, these Gi-coupled receptors inhibit adenylyl cyclase, reduce cAMP, and decrease calcium influx, which lowers vesicle fusion and exocytosis. The net presynaptic effect is inhibition of norepinephrine release. The other possibilities describe downstream postsynaptic actions or effects not related to the presynaptic release control (for example, vasoconstriction comes from postsynaptic alpha-1 effects, and bronchodilation relates to beta-2 activity, not this presynaptic mechanism).