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FA Dunn, DM Berson; Are Intrinsically Photosensitive Retinal Ganglion Cells Influenced by Rods or Cones . Invest. Ophthalmol. Vis. Sci. 2002;43(13):2982.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Rat retinal ganglion cells innervating the hypothalamic suprachiasmatic nucleus (SCN) are intrinsically sensitive to light. We sought to learn whether these photosensitive ganglion cells (pRGCs) are also influenced by rod/cone-driven retinal networks. Methods: Rat pRGCs, identified by retrograde transport of fluorescent beads from the SCN, and conventional RGCs were recorded by whole-cell current clamp in flattened eyecups and filled with dye. Results: Most pRGCs exhibited no obvious rod/cone influence. Light evoked a slow, tonic depolarization indistinguishable from that observed in isolated retinas without functioning rods or cones. This was true even though rod and cone networks were functionally intact: conventional RGCs had robust light responses (n = 17) that were eliminated by synaptic blockade (either 2 mM cobalt; 50 mM AP5 and 20 mM DNQX; or 100 mM APB; n = 8 ON cells). In pRGCs, modulation of the rod/cone ON channel by application of agonists or antagonists of the mGluR6 receptor (100 mM APB; or 200 mM CPPG; in eyecups or isolated retinas) had no obvious effect on membrane potential or current-voltage relations (n=5), whereas in conventional ON RGCs CPPG evoked the expected depolarization. A few pRGCs (3/10) did appear to receive rod/cone ON-channel input. Light evoked not only the slow, tonic depolarization signaling intrinsic phototransduction, but also a distinct, very transient ON response. This had a much shorter latency and lower threshold than the intrinsic response, consistent with a rod/cone origin. Synaptic blockade (2 mM cobalt or 100 mM APB) eliminated the short-latency ON response without attenuating the intrinsic depolarization. Morphology was similar in pRGCs with and without apparent rod/cone input. OFF responses were never observed. Conclusion: At least some pRGCs appear to receive rod and/or cone signals, most likely through synaptic contacts onto their proximal dendrites in the ON sublayer of the inner plexiform layer. Thus, this novel photosensitive system appears to be subject to at least modest modulation by conventional retinal circuits. This may be related to spectral and threshold evidence that SCN neurons receive rod and cone input (Aggelopoulos & Meissl, 2000).
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