Abstract
Purpose: :
Three known pathways allow rod photoresponses to traverse the retinal circuitry en route to the ganglion cell layer. Of these the Rod Bipolar pathway is believed to be the most sensitive by ~ 5-10-fold, relaying information near absolute threshold. At higher light levels the Rod-Cone and Rod-OFF pathways may participate in rod vision, however their contributions are obscured by convergence in the inner retina. To determine the threshold for activation, we used a combination of knockout mice and pharmacology to separate signaling in each pathway.
Methods: :
Rod contributions to the Rod-Cone and Rod-OFF pathways were studied in retinal slices from mice lacking cone transducin (Gnat2-/-). Perforated-patch and whole-cell voltage clamp (Vm=-60mV) recordings were made from rod and OFF cone bipolar cells, and AII amacrine cells in dark-adapted retinal slices superfused with heated Ames’ media. Cellular morphology was determined with fluorescent dyes in the intracellular solution. Strychnine (100 microM) and APB (8 microM) were used to eliminate contributions from the Rod Bipolar Pathway. Families of flash responses were recorded from which the threshold, or flash strength where the signal-to-noise ratio is 1, was calculated.
Results: :
Based on the properties of the signal and noise, threshold in rod bipolar cells was estimated to be ~ 0.2 R*/rod with threshold in AII amacrine cells ~ 0.02 Rh*/rod, consistent with their expected 10-fold increase in sensitivity. In the absence of strychnine and APB, threshold in OFF cone bipolar cells was ~ 0.4 Rh*/rod. Thus, the Rod Bipolar Pathway contributes to OFF bipolar responses at low light levels. When the Rod Bipolar Pathway is blocked with strychnine alone threshold was ~ 0.6 Rh*/rod, and increased to ~ 1 Rh*/rod with added APB.
Conclusions: :
The Rod-Cone and Rod-OFF Pathways appear to have an ~ 5-fold elevated threshold for rod signals compared to the Rod Bipolar Pathway, but the Rod Bipolar Pathway can provide input to OFF bipolar cells at light levels where the alternative pathways are inactive. The greater increase in threshold for OFF bipolar cells using APB compared to strychnine suggests an alternative route, other than glycinergic transmission from AIIs, for Rod Bipolar Pathway signals to reach OFF bipolar cells.
Keywords: retinal connections, networks, circuitry • bipolar cells • retina: distal (photoreceptors, horizontal cells, bipolar cells)