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Annika Balraj, Ralph F Nelson, Tara Suresh; Zebrafish retinal ON-bipolar cells compensate for background induced sensitivity loss. Invest. Ophthalmol. Vis. Sci. 2017;58(8):2222.
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© ARVO (1962-2015); The Authors (2016-present)
A previous study of isolated cone signals (ERG PIII) in zebrafish larvae showed a high-sensitivity UV-cone peak (370nm), and a lower sensitivity red-cone peak (530nm). The red-cone peak was suppressed by red backgrounds. We hypothesize that ERG signals of bipolar cells (b2) amplify cone PIII signals by a fixed amount, but otherwise mirror PIII in spectral shape.
Larvae (roy/roy) were studied at 5, 6, 7, and 12 dpf. Isolated eyes were perfused with oxygenated MEM containing either 20mM Na Aspartate to isolate cone PIII signals, or 50µM CNQX to isolate ON-bipolar-cell b2 signals. Using a trans-corneal patch electrode, ERG responses were recorded at 9 wavelengths (330-650nm). Backgrounds were either infrared (IR, RG770 filter) as a control, or red (627nm). Spectra were fit by a ‘sum of Hill functions’ model (Nelson & Singla, 2009). Spectra from cone PIII signals were compared to ON-bipolar b2-signals from the same eye.
Sensitivity of b2 signals tended to decrease throughout development. The UV-cone and red-cone peaks ranged from 370-410nm and 530-550nm, respectively. Red backgrounds increased the UV-cone peak sensitivity, but left the red-cone peak sensitivity unchanged for all larval stages except 12dpf, where sensitivities were higher for both peaks. We defined amplification as the ratio of b2 to PIII spectral sensitivities. For an IR background, the b2/PIII ratio was close to 1 for the red-cone peak (maximum of 1.8 times its PIII response), and less than 1 for the UV-cone peak. Red backgrounds increased amplification particularly for red cone signals. The b2 UV-cone peak ranged from 1.7-2.7x the PIII response, but the b2 red-cone peak was from 5.8-23.8x the PIII response.
For cone PIII the red-cone peak in larval spectral sensitivity is decreased as expected; however, for ON-bipolar-cell generated b2 responses, this is not the case. The hypothesis that ON bipolar cells perform fixed amplification of PIII responses is rejected. The results suggest that bipolar cells adjust synaptic gain for different lighting conditions and may do so selectively for different cone input types. As CNQX, an AMPA/kainate antagonist, blocks horizontal-cell synapses with cones, the gain adjustment mechanism is likely to be intrinsic to the cone-to-bipolar synapse, and not involve feedback circuitry.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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