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J. Wang, V. J. Kefalov; Functional Cone-Specific Pigment Regeneration Pathway in the Amphibian Retina. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5833.
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The function of cone photoreceptors under bright light is made possible by the fast decay and regeneration of their pigment. A need to recycle cone chromophore faster than the classical pigment epithelium pathway can manage, as well as recent biochemical studies from cone-dominant species have brought up the idea for a second, cone-specific, pigment regeneration pathway located within the retina and involving the Müller cells. To establish the existence of such a pathway physiologically, we investigated whether the intact salamander retina can promote the recovery of cone sensitivity after bleaching the visual pigment with a bright flash.
Transretinal electroretinogram a-waves were recorded from whole retina, in eyecup or isolated and cleaned from residual pigment epithelium. Background light was used to saturate the rods and obtain pure red cone responses. Synaptic transmission blockers and barium were used to block higher order signals in the retina and obtain pure cone photoresponses.
10 Minutes following a bleach of isolated retina, removed from the pigment epithelium, the sensitivity of salamander red cones recovered to within 1/2 of its dark-adapted value. The 2-fold residual desensitization corresponds to loss of 12% of cone pigment and was most likely due to the loss of bleached chromophore to the perfusion solution. In contrast, the same bleach permanently reduced the sensitivity of rods from isolated retina 200-fold, corresponding to loss of ~90% of rod pigment. Following incubation of isolated retina in the Müller cell-specific metabolic inhibitor alpha-aminoadipic acid (L-alpha-AAA ), cone sensitivity recovered from the bleach slower and only to a level 34-fold lower than dark-adapted sensitivity, corresponding to 77% cone pigment loss. L-alpha-AAA did not block the pigment epithelium-driven dark adaptation of cones from retina in eyecup. However, the rate of dark adaptation of cones from retina in eyecup was faster in Ringer than following Müller cell inhibition.
Our results provide physiological evidence for the existence of a functional cone-specific pigment regeneration pathway within the retina of tiger salamander. This pathway provides chromophore, presumably 11-cis retinol, specifically for cones and not for rods. The rapid chromophore recycling within the retina involves the Müller cells and is independent of the pigment epithelium.
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