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D. Tu, M.L. Batten, K. Palczewski, R.N. Van Gelder; Characterization of Non-visual Photoreception in Isolated Chicken Iris Sphincter Muscle . Invest. Ophthalmol. Vis. Sci. 2003;44(13):424.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Iris sphincter muscle isolated from embryonic chicken (Gallus gallus) is able to constrict in response to photic stimuli, independent of neural input from the retina or CNS. While isolated embryonic chicken pupillary sphincter muscle has previously been recognized as photoresponsive, the underlying mechanism remains unknown. We characterize the photopigment(s) and phototransduction pathway driving this non-visual light response. Methods: The iris sphincter muscle (and overlying cornea) of embryonic chickens (incubation day 15-18) was dissected away from the posterior eyecup and mounted corneal surface down while maintained in Tyrode’s physiologic solution. Further dissection removed: remnants of retina, lens, vitreous body, tissue anchoring the iris radial dilator muscles, and most of the iris dilator muscle itself. The pupillary light response (PLR) of these preparations was utilized as the endpoint in the following experiments: (1) generation of an action spectrum from 400nm to 600nm, (2) response to bright light, (3) response to retinal depletion via UV light exposure followed by retinal quantification via HPLC and UV spectrophotometry. Results: The action spectrum of the PLR has a peak plateau from 400 to 450nm and loses most sensitivity by 530nm. The PLR is sensitized by prior exposure to bright light. There are no detectable retinoids in the chicken iris by HPLC (limit of detection ≤1 pmole/iris). Prolonged UV exposure that would deplete >95% of any remaining retinoids does not significantly reduce the sensitivity of the PLR. Conclusions: The photoresponsive constriction of embryonic chicken iris sphincter muscle is preferentially sensitive to blue light. In contrast to classical opsin-based phototransduction (in which bright light desensitizes the phototransduction and bleaches pigment), in the chicken iris bright light exposure sensitizes the phototransduction process. The isolated chicken iris PLR appears to be retinal/retinol-independent, suggesting that the active photopigment is not an opsin. The sensitization phenomenon is more consistent with a flavin-based pigment such as cryptochrome. In summary, these data demonstrate the existence of an ocular, non-visual, non-opsin photopigment within the pupillary sphincter muscle of embryonic chicken irises.
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