Tropicamide

Muscarinic acetylcholine receptors are involved in numerous ocular functions. The M3 subtype is predominantly expressed by smooth muscle cells of the sphincter pupillae, which is a circular muscle of the iris, and ciliary muscles. In response to light or binding of acetylcholine, M3 receptor signalling leads to contraction of the sphincter pupillae and pupil constriction. Contraction of the ciliary muscle via M3 receptor signalling also leads to accommodation, adjusting the lens for near vision. The eye is also innervated by parasympathetic nerves: ciliary ganglion neurons project to the ciliary body and the sphincter pupillae muscle of the iris to control ocular accommodation and pupil constriction.

Tropicamide is a non-selective muscarinic antagonist that binds to all subtypes of muscarinic receptors. By binding to muscarinic receptors, tropicamide relaxes the pupillary sphincter muscle and causes pupil dilation. By blocking the muscarinic receptors of the ciliary body, tropicamide also prevents accommodation. Like other muscarinic antagonists, tropicamide inhibits the parasympathetic drive, allowing the sympathetic nervous system responses to dominate. Tropicamide is thought to ameliorate sialorrhea by blocking M4 receptors expressed on salivary glands and reducing hypersalivation.