Activation of muscarinic acetylcholine receptors elicits pigment granule dispersion in retinal pigment epithelium isolated from bluegill

BACKGROUND: In fish, melanin pigment granules in the retinal pigment epithelium disperse into apical projections as part of the suite of responses the eye makes to bright light conditions. This pigment granule dispersion serves to reduce photobleaching and occurs in response to neurochemicals secreted by the retina. Previous work has shown that acetylcholine may be involved in inducing light-adaptive pigment dispersion. Acetylcholine receptors are of two main types, nicotinic and muscarinic. Muscarinic receptors are in the G-protein coupled receptor superfamily, and five different muscarinic receptors have been molecularly cloned in human. These receptors are coupled to adenylyl cyclase, calcium mobilization and ion channel activation. To determine the receptor pathway involved in eliciting pigment granule migration, we isolated retinal pigment epithelium from bluegill and subjected it to a battery of cholinergic agents. RESULTS: The general cholinergic agonist carbachol induces pigment granule dispersion in isolated retinal pigment epithelium. Carbachol-induced pigment granule dispersion is blocked by the muscarinic antagonist atropine, by the M(1 )antagonist pirenzepine, and by the M(3 )antagonist 4-DAMP. Pigment granule dispersion was also induced by the M(1 )agonist 4-[N-(4-chlorophenyl) carbamoyloxy]-4-pent-2-ammonium iodide. In contrast the M(2 )antagonist AF-DX 116 and the M(4 )antagonist tropicamide failed to block carbachol-induced dispersion, and the M(2 )agonist arecaidine but-2-ynyl ester tosylate failed to elicit dispersion. CONCLUSIONS: Our results suggest that carbachol-mediated pigment granule dispersion occurs through the activation of M(odd )muscarinic receptors, which in other systems couple to phosphoinositide hydrolysis and elevation of intracellular calcium. This conclusion must be corroborated by molecular studies, but suggests Ca(2+)-dependent pathways may be involved in light-adaptive pigment dispersion.