Abstract
Abstract: :
Purpose:Background light reduces the sensitivity of mouse rod bipolar light responses and speeds their kinetics. Our goal is to assess the relative contributions of adaptation in both rods and rod bipolar cells to background adaptation measured in rod bipolar cells. Methods:Suction electrode recordings were made from mouse rod photoreceptors. Whole–cell or perforated–patch voltage clamp (Vm = –60 mV) recordings were made from rod bipolar cells in mouse retinal slices. Sensitivity changes in rod bipolar cells were measured by delivering a series of flashes of increasing strength and fitting these response families with a Hill equation. Both the Hill exponent and the half–maximal flash strength were used to estimate the cell's sensitivity as the background light level was increased. Sensitivity changes in rods were measured by determining the change in fractional suppression of the dark current per photon with increasing background light. The response kinetics in both cell types was determined using the time–to–peak of the responses. Results:The extent of nonlinear signal transfer between rods and rod bipolar cells was reduced in a graded manner with background illumination. Other than the relieving the nonlinearity, background light did not cause significant extra adaptation in rod bipolar cells compared with rod photoreceptors. The half–maximal flash strength did not change substantially for background lights dimmer than 100 photons/μm2/s, of similar magnitude to background lights that halved the fractional suppression of the dark current per photon in rods. The time–to–peak of the rod bipolar response was shortened by backgrounds as dim as 10 photons/μm2/s, similar to the intensity where the rod's time–to–peak was reduced. To isolate adaptation within rod bipolar cells, we perfused the retina with 2μM of the mGluR6 antagonist LY341495 (LY) in darkness, to mimic the effect of background light. Similar to background light, LY reduced the extent of nonlinearity, but did not produce a speeding of the time–to–peak. Conclusions:While background light reduces the extent of nonlinearity in rod bipolar cells, changes in sensitivity and kinetics of the light response appear to reflect adaptation within the rod photoreceptors themselves, with little extra contribution from the rod bipolar cell transduction machinery.
Keywords: retina: distal (photoreceptors, horizontal cells, bipolar cells) • bipolar cells • photoreceptors