Abstract
Abstract: :
Purpose: To determine the mechanism generating the light response in retinal ‘On’-bipolar cells. Methods: Whole-cell voltage clamp recordings were made from dark-adapted salamander On-bipolar cells in retinal slices. Maximal channel opening was assessed as the peak inward current generated by bright light flashes. The strategy in these experiments was to block the activity of the G-protein with internally dialyzed GDP-ß-S and determine which components are required in the electrode solution to open transduction channels while the G-protein is bypassed. This approach may broadly identify possible mechanisms for channel gating. The current at -60 mV was used to assess channel opening versus time after whole-cell break in. Results: Light responses recorded from ‘On’-bipolar cells maintained > 50% of their amplitude for 20 or more minutes during whole-cell recordings, provided the electrode solution contained 1 mM ATP and 0.2 mM GTP. With the inclusion of 500 µM GDP-ß-S, the On-light response decayed to < 20% of its maximal value within 5-12 minutes with little change in current (n = 16). Including 1 mM cGMP in the recording electrode with GDP-ß-S also resulted in a decay of the light response without an increase in current (n = 10). Conclusions: During the block of G-protein activity with GDP-ß-S we find that transduction channels do not open, suggesting that the channels are intrinsically closed. The inclusion of 1 mM cGMP does not open transduction channels, arguing that the channels are not gated directly by cGMP. The lack of channel opening during G-protein block also suggests that the mechanism of channel opening requires G-protein activity. While a majority of evidence to date suggests that Go is involved in channel closure, our results suggest that Go may also be involved in channel opening, or perhaps a second glutamate-independent G-protein controls channel opening.
Keywords: retina: distal(photoreceptors, horizontal cell • bipolar cells • neurotransmitters/neurotransmitter systems