Abstract
Purpose :
The global neural retinal response to light is compromised in early diabetes, but the underlying mechanisms are not well understood. We previously found rod bipolar cell (BC) output to AII amacrine cells is increased due to decreased GABAergic inhibition to rod BCs in early diabetes. The purpose of this study was to determine if reduced rod BC GABAergic inhibition is mediated by altered amacrine cell GABA release and if reduced reciprocal A17 amacrine cell inputs contribute to this decrease.
Methods :
Diabetes was induced in C57BL/6J mice by 3 i.p. injections of streptozotocin (STZ, 75 mg/kg), and confirmed by blood glucose levels >200 mg/dL. Six weeks post injections whole-cell voltage clamp recordings of light-evoked (L) and feedback (f) inhibitory postsynaptic currents (IPSCs) were made from rod BCs in dark-adapted retinal slices. Cells were held at the reversal potential for cations to isolate IPSCs. A 30ms full field LED stimulus was used to elicit light responses. Receptor (R) inputs were pharmacologically isolated. Transmitter release was estimated by deconvolution analysis. The peak amplitude, charge transfer and decay to 37% of the peak (D37) were measured for fIPSCs and transmitter release.
Results :
Deconvolution analysis to estimate transmitter release that underlies L-IPSCs showed a significant decrease in the peak, charge transfer and D37 of release onto rod BC GABAAR (p<0.001) and GABACR (p<0.01) at multiple light intensities in diabetic mice. The D37 of combined GABAergic fIPSCs was faster in diabetic mice (p<0.05) compared to control mice. Isolated GABACR fIPSCs from diabetic mice had a faster D37 compared to control mice. There was no significant difference in the GABAAR fIPSC D37. The charge transfer of total, GABAAR and GABACR-mediated fIPSCs was on average lower in diabetic mice but this was not significant.
Conclusions :
Analysis of quantal release suggests that reduced light-evoked GABAergic inhibition to rod BCs in early diabetes is due to decreased amacrine cell GABA release. We also found that the timing of rod BC fIPSCs was shortened which may decrease A17 GABAergic inhibition. These results show that diabetes affects A17 inputs as well as release from GABAergic amacrine cells that provide reciprocal and non-reciprocal inhibition to rod BCs, and may contribute to the deficits in retinal electrical signaling seen in diabetes.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.