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L. A. Ostrin, Y. Liu, J. Mok Yee, V. Choh, C. F. Wildsoet; Autonomic Innervation to the Intraocular Muscles of the Anterior Segment in Chicks. Invest. Ophthalmol. Vis. Sci. 2010;51(13):689.
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Studies show that the embryological chick iris has only muscarinic receptors, while the mature iris muscles have both nicotinic and muscarinic receptors. Mature chick ciliary muscle is reported to have nicotinic receptors; whether they also have muscarinic receptors during early development has not been fully resolved. The possibility of dual cholinergic receptors has implications for chick emmetropization studies. An in vitro pharmacological study was undertaken to characterize receptor subtypes on intraocular muscles of chick anterior segment at various ages.
Eyes from 42 normal White-Leghorn chickens, aged embryonic day 18 to postnatal 6 weeks, were used. Eyes were enucleated and anterior segments mounted and scanned in a lens scanning instrument (ScanTox). As indices of muscle contraction, lens back vertex distance (BVD), a measure of accommodation, and pupil size were determined. Individual preprations were tested with increasing concentrations of either pilocarpine (muscarinic agonist), carbachol (muscarinic/nicotinic agonist) or nicotine (nicotinic agonist). The response to a maximum effective concentration of nicotine was also measured from both eyes in 6 previously monocularly iridectomized chicks, aged 6 wks.
Nicotine had the greatest effect on accommodation and pupil size, with a 100 µM concentration producing the maximum decrease in BVD of 35.5±11%, and pupil size of 54.0±16%, averaged across all ages. With carbachol, a higher concentration (5000 µM) was required to achieve maximal decreases in BVD and pupil size, 29.6±8 and 15.4±16% respectively, which were less than that of nicotine. Up to 1000 µM pilocarpine had little effect on either BVD or pupil size, even for the youngest chicks (4.4±7%, 4.1±3% respectively, E18 to D7;). Iridectomized eyes showed minimal decrease in BVD to 100 µM nicotine (6.2±5% compared to 41.5±20% for fellow eyes).
Nicotinic receptors primarily mediate the response of the ciliary muscle and iris in chicks, from age E18 onwards. A possible role of muscarinic receptors in the youngest eyes is suggested by subtle decreases in BVD and pupil size with pilocarpine, and the iris was more sensitive to carbachol than the older eyes. The lack of response to nicotine in iridectomized eyes confirmed results of a previous study and implies that the iris is necessary for accommodation in vitro, although a parallel in vivo study of the same eyes showed that accommodation was unaffected.
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