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Johan Pahlberg, Rikard Frederiksen, Kiyoharu Miyagishima, Soile Nymark, Alapakkam Sampath, M Cornwall; Bleach-Induced Desensitization and Response Compression of the Photocurrent and Photovoltage in Mouse Rods. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4081.
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Most of our visual experience occurs during the day when a substantial fraction of the visual pigment in our photoreceptor cells, the rods and cones, is bleached. Under these conditions it is widely believed that rods are saturated and do not contribute substantially to downstream signaling. Here we characterize the properties of rod photoreceptors following substantial pigment bleaching in transgenic mice lacking cone photoresponses.
To study how the sensitivity of rod photoresponses is modulated following pigment bleaching, we made electrophysiological recordings of the outer segment photocurrent with suction electrodes, the rod inner segment photovoltage with patch electrodes, and the average photovoltage with electroretinogram (ERG) measurements from whole retina. Photopigment bleaching in rods was quantified using microspectrophotometry.
Following substantial pigment bleaching, we found that rod outer segment current experienced a persistent reduction in sensitivity that is explained by the combined loss of quantum catch and cellular desensitization of the phototransduction proportional to the concentration of the bleached pigment. Surprisingly, the rod photovoltage measured from single cells or ERGs was not desensitized to the same extent as the outer segment photocurrent. This photovoltage sensitivity was reduced if whole-cell measurements were made with an internal solution or pharmacological agents that blocked rod K+ conductances.
As shown previously, bleached pigment activates phototransduction and desensitizes rod photoresponses more than predicted from purely the loss of visual pigment. The desensitization of rod photocurrent can be explained by adaptation of phototransduction by bleached pigment and response compression. Surprisingly, the photovoltage, measured in single cells or by ERG, is not subject to the same extent of desensitization. We further find that inner segment K+ conducances (like Ih) enhance the magnitude of the photovoltage to increase its sensitivity.
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