Abstract
Purpose: :
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.
Methods: :
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.
Results: :
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.
Conclusions: :
The data indicate that the light-evoked signaling pathway of pRGCs in an intact retinal preparation includes components of the invertebrate-like phototransduction cascade.
Keywords: ganglion cells • photoreceptors • receptors: pharmacology/physiology