Cholinergic systems and multiple cholinergic receptors in ocular tissues.

Area of Science:

• Ophthalmology
• Neuroscience
• Pharmacology

Background:

• Acetylcholine (ACh), choline acetyltransferases, and cholinesterases are present in the vertebrate eye, including the cornea, iris-ciliary body, and retina.
• The precise function of ACh in the cornea is not fully understood, though it may act as a sensory transmitter or local hormone.
• ACh is a well-established parasympathetic neurotransmitter for the iris and ciliary body.

Purpose of the Study:

• To review the established and potential roles of acetylcholine (ACh) and its receptors in the vertebrate eye.
• To highlight areas requiring further investigation, particularly regarding muscarinic and nicotinic receptor subtypes in the iris-ciliary body and retina.
• To discuss the influence of cholinergic neurons on retinal ganglion cell activity and vascular endothelial cells.

Main Methods:

• Review of existing literature on cholinergic systems in the vertebrate eye.
• Analysis of binding studies using radiolabeled antagonists to characterize muscarinic receptors.
• Discussion of quantitative and qualitative differences in cholinergic markers across species.

Main Results:

• Muscarinic receptors in the iris-ciliary body complex are heterogeneous, suggesting the presence of subtypes beyond M2.
• Cholinergic amacrine cells are found in all vertebrate retinas, influencing a majority of ganglion cells via nicotinic and muscarinic receptors.
• ACh activates muscarinic receptors on retinal blood vessel endothelial cells, causing relaxation.

Conclusions:

• The iris-ciliary body likely possesses heterogeneous muscarinic receptors, with potential presynaptic subtypes needing investigation.
• Cholinergic input to retinal ganglion cells modulates neuronal activity, but its precise role in visual processing remains unknown.
• Further research is warranted to investigate specific receptor subtypes and their functions in the eye, particularly in retinal blood vessels.