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E. D. Eggers, J. S. Klein; Rod and Cone Pathways of Inhibition. Invest. Ophthalmol. Vis. Sci. 2010;51(13):4800.
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© ARVO (1962-2015); The Authors (2016-present)
Visual information is separated into parallel pathways of ON, OFF and rod signaling at the bipolar cell (BC) dendrites. Inhibition to BC axon terminals has also previously been shown to vary between these pathways. However, as these recordings were made in dark-adapted retinas where rod signals are most active, it is not clear what roles cone pathways vs. rod pathways play in this inhibition. Previous studies have also suggested that the timecourse of inhibition might match the timecourse of excitation to BCs. This would suggest that the timecourse of inhibition should change when BCs transition between receiving slow rod-mediated excitation and faster cone-mediated excitation. To address these questions we recorded light-evoked inhibitory postsynaptic currents (L-IPSCs) and spontaneous IPSCs (sIPSCs) determined how inhibition changes between dark and light adapted rod and cone BCs.
L-IPSC and spontaneous IPSC recordings of BCs were made in the whole-cell voltage-clamp configuration from dark-adapted mouse retinal slices. Light responses were stimulated by full-field illumination with a white LED, and light stimuli were applied over dark and rod-saturating light adapted backgrounds. Recordings were made at the reversal potential for cation-mediated currents to isolate IPSCs. Sulfarhodamine B was included in the pipettes to morphologically identify bipolar cell types. The charge transfer (Q) and decay to 37% (D37) of the peak of L-IPSCs were measured to compare the magnitude and timecourse of inhibition, respectively.
To determine how L-IPSCs change when rod signals are saturated, we compared the total inhibitory responses in dark and light-adapted conditions. We found that L-IPSCs in rod and OFF cone BCs respond differently to light adaptation. Rod BCs showed very little light-evoked inhibition after light adaptation. In contrast the L-IPSCs in OFF cone BCs have a similar Q before and after light adaptation. However, in all cone BCs recorded, the D37 of the L-IPSC decreased when light adaptation occurred. Additionally we saw changes in BC sIPSCs. The frequency of sIPSCs was significantly reduced due to light adaptation, showing the loss of a rod-mediated pathway of inhibition.
Our results suggest that rod BCs may receive inhibition from rod-specific circuits while cone BCs receive significant inhibition from both rod and cone circuits, which places limitations on the amacrine cells that contact these BCs. Additionally our finding that light-adapted L-IPSCs have a briefer timecourse holds with our hypothesis that cone-mediated inhibition is faster than rod-mediated inhibition, to match the timecourse of excitatory and inhibitory inputs.
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