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S. Sekaran, R. G. Foster, M. W. Hankins; Phototransduction in Photosensitive Retinal Ganglion Cells. Invest. Ophthalmol. Vis. Sci. 2009;50(13):75.
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The phototransduction cascade in melanopsin-expressing photosensitive retinal ganglion cells (pRGCs) is a subject of intense investigation. Recent evidence in dissociated cultured pRGCs pointed towards an invertebrate-like cascade (Graham et al., 2008). Using patch clamp recordings, light exposure was found to activate a G protein which in turn stimulated the enzyme phospholipase C (PLC). Opening of the light-gated ion channels was suggested to involve a membrane associated but diaclyglycerol (DAG)-independent mechanism. The current study attempted to confirm these results in an intact retinal preparation using fluorescent Ca2+ imaging.
Retinae were isolated from C3H mice at post natal day 4-5 (before the emergence of functional rod and cone photoreceptors). Retinae were loaded RGC side up with the Ca2+ sensitive indicator FURA-2AM. Fluorescent images were acquired every 2 seconds. Light responses were evoked by application of a 470 nm stimulus for 1 minute. Light-evoked responses were obtained before and during bath application of pharmacological agents.
The general G protein inhibitor suramin (100 µM) completely abolished the light evoked response. U73122 (10 µM), a PLC inhibitor, suppressed light responses by 23% on average. OAG, a DAG analogue, had no effect on light responsiveness at 25 µM. However, at 50 µM OAG significantly suppressed the light response in 5 out of 17 cells. The remainder of the cells continued to respond to light stimulation suggesting only a sub-population of the cells is OAG sensitive.
The data indicate that the light-evoked signaling pathway of pRGCs in an intact retinal preparation includes components of the invertebrate-like phototransduction cascade.
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