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Alapakkam P. Sampath, K. J. Miyagishima, A. C. Arman, Soile Nymark, Johan Pahlberg, Carter Cornwall; Bleaching and Background Adaptation Shift Rod-Driven Signals from Primary to Secondary Rod Pathways in the Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4551.
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Rod vision ranges over 8 orders of magnitude of light intensity. To allow this range of responsiveness, rod photoresponses are processed in several parallel pathways that vary in their degree of convergence and their ability to adapt. The rod bipolar (primary) pathway relays signals to the retinal output under conditions where photon flux is low. As light level increases secondary rod pathways progressively increase their signaling efficacy. While the functional equivalence of bleaching and background adaptation in rods is well accepted, the mechanisms whereby this encoding takes place in primary and secondary rod pathways is not well understood.
Voltage clamp recordings (Vm = -60 mV) were made from retinal slices in mice lacking cone transducin (Gnat2-/-; Chang et al., 2006, IOVS 47: 5017-5021) to determine the sensitivity of the excitatory input to rod and ON and OFF cone bipolar cells to flashes of light. The relative sensitivity of rod and cone bipolar cells was assessed after 70% bleaching of the visual pigment. Background light exposures were adjusted to produce similar desensitization of rod phototransduction.
In dark-adapted retina, rod bipolar cells were ~1.5 fold more sensitive than neighboring ON or OFF cone bipolar cells, consistent with previous studies. After 70% bleaching of the visual pigment this was reversed such that both ON and OFF cone bipolar cells were ~3-fold more sensitive than rod bipolar cells. A similar shift in sensitivity was also observed during adaptation to background light producing 300 Rh*rod-1s-1. These shifts to secondary rod pathways occurred due to the more profound desensitization of rod bipolar cells to bleaching and background light (~16-fold) compared to the ON and OFF cone bipolar cells (~4-fold).
We find that during adaptation evoked by bleaching or background light, rod bipolar cells are more profoundly desensitized than neighboring ON or OFF cone bipolar cells. These differences manifest in the cone bipolar cells carrying more sensitive rod-driven signals than rod bipolar cells. The switch in sensitivity of retinal pathways is consistent with models whereby rod signals are more effectively pooled by cone pedicles as light levels increase. We aim to characterize the bleaching and background light levels where pathway switching occurs, and to determine whether these stimuli produce functionally equivalent modulation of sensitivity downstream in the retina.
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