Cocaine can be used to help diagnose Horner's syndrome by inhibiting the reuptake of which neurotransmitter?

• A. Dopamine
• B. Serotonin
• C. Norepinephrine
• D. Acetylcholine

Answer: (C)

Cocaine testing in Horner's syndrome works because it blocks reuptake of norepinephrine, the sympathetic transmitter that drives the iris dilator muscle. In a normal eye, blocking norepinephrine reuptake increases its concentration in the synapse, stimulating alpha-1 receptors on the dilator pupillae and causing pupil dilation. If Horner's is present, the postganglionic sympathetic pathway is disrupted, so little norepinephrine is released and cocaine cannot produce much dilation. That lack of dilation helps confirm the diagnosis. Other transmitters like dopamine, serotonin, or acetylcholine don’t play the key role in this iris reflex, and acetylcholine is part of the parasympathetic system that constricts the pupil, not dilates it.