Cholinergic Systems and Multiple Cholinergic Receptors in Ocular Tissues

Abstract

Acetylcholine (ACh), choline acetyltransferases and cholinesterases occur in cornea, iris-ciliary body complex and retina of several vertebrates. In cornea, ACh may serve as a sensory transmitter as well as a local hormone, the function of which is not delineated. The function of ACh as the parasympathetic neurotransmitter at the iris and ciliary body is well established. The muscarinic receptors on the iris smooth muscle are similar to the muscarinic receptors (M2 type in two way classification) at other smooth muscles towards their interaction with agonists and antagonists. Binding studies using radiolabeled antagonists and their displacement by agonists indicate that muscarinic receptors in membranes of iris-ciliary body complex are heterogeneous indicating more than one subtype of muscrinic receptors. A subtype other than M2 receptors may occur at the presynaptic sites of parasympathetic nerves, which have yet to be investigated using specific agonists and antagonists. Cholinergic markers, choline acetyltransferase and acetylcholinesterase, differ quantitatively and qualitatively in retinas of different species. However, amacrine cells are cholinergic in all vertebrate species. Although they make up 1% of retinal neurons, they influence the activity of a majority of ganglion cells. Cholinergic effects in ganglia are mediated through nicotinic and muscarinic receptors. Both of these types of cholinergic receptors are heterogeneous. They have yet to be investigated for their subtypes using specific agonists and antagonists. Although the role of cholinergic retinal neurons in the processing of visual information is not known, their input to ganglion cells generally increases the rate of spontaneous activity or the number of action potentials in light-evoked responses. Thus, the cholinergic input seems to modify the overall neuronal input to the ganglion cells from the receptive fields. Endothelial cells of blood vessels contain muscarinic receptors, which are activated by ACh to cause relaxation. Although retinal blood vessels provide recognizable characteristic signs in diabetes mellitus and hypertensive disease, no information is available on the muscarinic receptors of these vessels.

Acetylcholinesterase Inhibition Promotes Retinal Vasoprotection and Increases Ocular Blood Flow in Experimental Glaucoma

“…Retinal and brain endothelial cells abundantly express M1 and M3 mAChR subtypes, 66,67 which have been implicated in cholinergic-mediated vasomodulation in the brain, 68 and ACh-dependent vascular relaxation is lost in M3 mAchR knockout mice. 69 Intriguingly, adult RGCs do not express mAChR and muscarine does not elicit membrane currents measured in whole-RGC patch-clamp preparations.…”

Receptor Test (Pupillary Dilatation after Application of 0.01% Tropicamide Solution) and Determination of Central Nervous Activation (Fourier Analysis of Pupillary Oscillations) in Patients with Alzheimer’s Disease

“…Cholinergic innervation is prominent in the eye, and the function of acetylcholine as the parasympathetic neurotransmitter at the iris and ciliary body is well established. The muscarinic acetylcholine receptors on the smooth muscle of the iris interact with cholinergic agonists in a way comparable to extraocular tissues [16]. In animal models, diminished cholinergic input to the anterior segment of the eye results in supersensitivity to miotic effects of the cholinergic agonist pilocarpine [17].…”

The effects of nicotinic and muscarinic receptor activation on patch-clamped cells in the optic tectum of rana pipiens

“…We conclude that both of these receptor types could substantially modulate visual activity by changing either the input to tectal neurons or the level of their response to that input. Keywords frog; retinotectal; acetylcholine; visual system; cholinergic; superior colliculus Acetylcholine (ACh) is present in many regions throughout the visual system appearing in the retina, superior colliculus, lateral geniculate nucleus, suprachiasmatic nuclei and the visual cortex (Groos et al, 1983;Sastry, 1985;Hohmann and Berger-Sweeney, 1998;Bickford et al, 2000). Contributed by both intrinsic and extrinsic sources (Sherman and Koch, 1986;Nobili and Sannita, 1997;Binns, 1999), cholinergic activity influences system function, plasticity and development (Greuel et al, 1988;Hohmann and Berger-Sweeney, 1998;Lauder and Schambra, 1999).…”