Which neurotransmitter is released by parasympathetic nerves that causes pupil constriction?

The neurotransmitter responsible for pupil constriction in response to parasympathetic nerve activation is acetylcholine. This neurotransmitter is released by the postganglionic neurons of the parasympathetic nervous system, specifically at the iris sphincter muscle, where it binds to muscarinic receptors, leading to muscle contraction and subsequent constriction of the pupil.

The action of acetylcholine contrasts with other neurotransmitters listed. Dopamine is primarily associated with pathways related to reward and motor control and does not play a direct role in the autonomic control of the pupils. Norepinephrine, on the other hand, is mainly involved in the sympathetic nervous system, which generally causes pupil dilation through different receptor mechanisms. Serotonin is more widely recognized for its role in mood regulation and does not have a direct influence on pupil size. Thus, acetylcholine is the key player in mediating pupil constriction through parasympathetic signaling.