| Abstract No.: | C-B3033 |
| Country: | USA |
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| Title: | SK CHANNELS REGULATE SPONTANEOUS ACTIVITY IN DEVELOPING MOUSE RETINAL GANGLION CELLS |
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| Authors/Affiliations: | 1 Juan Qu*; 1 Karen Myhr;
1 Department of Biological Sciences, Wayne State University, Detroit, MI, USA
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| Content: | Objectives: The apamin-sensitive small-conductance calcium-activated potassium channels (SK channels) modulate synaptic integration and underlie the afterhyperpolarization (AHP), which is important for regulating neuronal firing patterns and limiting firing frequency. Spontaneous activity spreads across the immature retina in waves appearing as bursts in single retinal ganglion cells (RGCs) and is critical for the development of RGCs. In this study, we investigated the effects of SK channels on mouse RGC activity during a critical developmental period, which is after bipolar cells innervate ganglion cells (postnatal day 8, P8) and before form vision begins at eye opening (P14). In this period, the spontaneous activity starts to be driven by bipolar cells instead of amacrine cells, and previously excitatory transmission from the amacrine cells switches to become inhibitory. However, the regular waves of activity are maintained through this rearrangement. We tested the hypothesis that SK channels help regulate the overall activity via effects on the network that generates the waves and via the intrinsic excitability of RGCs.
Materials and Methods: The effect of SK channels on RGC spontaneous activity was investigated with cell-attached recording, whole-cell patch-clamp recording and calcium imaging. The experiments were performed in acutely dissected whole mount retinas at 33 degrees Celsius with and without the SK channel blocker apamin. The expression pattern of the SK2 subunit, which is the SK subunit known to be expressed in rat retina, was determined by immunohistochemistry of both flat-mounted retinas and cross-sectioned retinas using standard fluorescent immunohistochemistry protocols.
Results: Cell-attached recordings revealed different effects of apamin on burst frequency depending on the baseline activity level of the RGCs. The burst frequency in relatively more active RGCs increased with bath application of apamin, while the burst frequency in less active RGCs decreased. Calcium imaging showed more detectable bursts at a higher frequency in many RGCs with apamin than without. It is possible that smaller amplitude calcium fluxes might fall below the resolution of the assay. SK2 immunoreactivity was present in the inner nuclear layer, outer plexiform layer and inner plexiform layer, where the cells that generate the waves are located. SK2 was also expressed diffusely in the ganglion cell layer. Different from the effects on burst frequency, within a burst the instantaneous action potential frequency was always enhanced by blockade of SK channels. Preliminary results of whole-cell patch-clamp recordings showed the presence of the apamin-sensitive AHP in a ganglion cell.
Conclusion: We conclude that SK channels help maintain the homeostasis of RGC activity during a dynamic developmental period. Through the retinal synaptic network, SK channels maintain the frequency of spontaneous waves at a moderate level. Meanwhile, the action potential frequency within each burst was reduced by the SK channels. The intraburst firing frequency could be limited intrinsically by SK channels expressed in the RGCs and/or be regulated globally by SK channels in the retina decreasing the amplitude of the waves.
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