Light Responses in Ground Squirrel b2 and b3 Off Bipolar Cells

Abstract

Purpose:: We previously examined the responses of voltage-clamped ground squirrel b2 and b3 bipolar cells during pairs of brief cone depolarizations separated by a variable interval (DeVries, 2000). The epsc peak recovered with a time constant (τ) of 105 ± 35 ms in b2 cells and 706 ± 182 ms in b3 cells. Recovery at the b2 synapse was not limited by AMPA receptor recovery (τ =18 ms), but rather by turnover at the presynaptic ribbons. Recovery at the b3 cell synapse was attributed to slow recovery of kainate receptor sensitivity (τ =1400 ms). Based on these results, we predicted that brief light steps (<500 ms) would preferentially produce transient responses at light-off in b2 cells and sustained responses in b3 cells, however actual light responses have not been measured.

Methods:: Retinal slices were made under dark-adapted conditions. Bipolar cell membrane currents were recorded in the whole cell configuration with picrotoxin and strychnine in the bath. A bright light stimulus (~0.5-1x10⁷ photons/µm² s^-1, duration = 4-5000 ms) was used to completely suppress glutamate release from cones and permit vesicle redocking. Suppression was judged by a paucity of mepscs during the stimulus, and results that did not meet this criterion were excluded. Cells were filled with Alexa 568 and identified according to Li and DeVries, 2006. For each step duration we measured the peak amplitude of the transient inward current at light offset relative to the baseline current in the dark. We then fitted an exponential curve to a plot of amplitude vs. stimulus duration to obtain the recovery τ.

Results:: Bright light suppressed steady currents of 20 ± 18 pA in b2 (n=6) and 17 ± 11 pA in b3 (n=5) cells. Transient responses at light-Off (peak amplitude of 30-200 pA for b2 and 25-140 pA for b3 cells) recovered more rapidly in b2 (τ=0.25 ± 0.17 s, n=3) than in b3 (τ=1.09 ± 0.37 s, n=3) cells. In three additional cells (2 b2 and 1 b3), mepscs were absent during the light pulse, however transient responses did not begin to recover until the light step duration was >500 ms and recovered much more slowly (τ >2.3 s) thereafter. These slower recoveries could be due to pathologically slow turnover at the cone ribbon.

Conclusions:: The transient light-off responses of AMPA receptor-containing b2 cells appear to recover more rapidly than those of kainate receptor-containing b3 bipolar cells. These results parallel those obtained with dual cell voltage clamp (DeVries, 2000), and are consistent with the idea that b2 cells are part of a transient signaling pathway while b3 cells are part of a more sustained pathway.

Keywords: retina: distal (photoreceptors, horizontal cells, bipolar cells) • retinal connections, networks, circuitry • synapse