Functional Characterization Of The Oscillatory Activity In Rd-1 Mouse Retina

Abstract

Purpose: : In the rd-1 mouse retina (Bowes et al., 1990), photoreceptors degenerate within the first 28 postnatal days. Despite the loss of their outer segments and, thus, a loss of light sensitivity, some photoreceptor somata survive in the outer nuclear layer (ONL). In addition to morphological also alterations functional changes have been observed in the remaining retinal network. For example, light-independent activity has been reported in rd-1 retinal ganglion cells (Ye et al., 2007; Stasheff, 2008; Margolis et al., 2008). Spontaneous activity in degenerating retina may interfere with artificial stimulation using electronic implants (Zrenner, 2010) and, hence, it is important to understand the mechanisms underlying spontaneous activity. This study aims at identifying the cellular source(s) of light-independent activity in the rd-1 mouse retina.

Methods: : Cells in retinal whole mounts from rd-1 mice (postnatal days 28-45) were loaded with the Ca²⁺ indicator dye Fura-2 and then the retina was placed with the ONL side up under a microscope. Ca²⁺ signals were measured using the 340/380 nm ratio with a CCD camera. To indentify measured cell types after the recordings, immunohistochemistry was performed with antibodies against recoverin (photoreceptors and cone bipolar cells), calbindin (horizontal cells), and protein kinase C (rod bipolar cells). Furthermore, DAPI and FM 4-64 were applied to label nuclei and membranes, respectively.

Results: : We found that light-independent oscillatory activity - as measured by Ca²⁺ imaging - was present in approx. 50% of the cells in the outer retina of the rd-1 mouse. Using immunocytochemistry, these cells could be identified as photoreceptors, horizontal cells and bipolar cells. The Ca²⁺ transients in these cell classes varied in shape, amplitude and frequency. In photoreceptors and bipolar cells, we measured slow and rhythmic transient Ca²⁺ signals up to 1.5 Hz as well as bursts of faster events. Horizontal cells showed more sustained signals with durations of several seconds. Furthermore, activity in local groups of cells tended to be highly correlated.

Conclusions: : Our preliminary data show a cell type-specific and partially correlated activity in the degenerated outer retina of rd-1 mouse. This activity may be responsible for oscillations in the remaining retinal network and for the spontaneous ganglion cell spiking observed in previous studies.

Keywords: retinal degenerations: cell biology