Intrinsic Light Responses of Ganglion Cells in the Rodless-coneless Mouse Retina

Abstract

Abstract: : Purpose: In the absence of classical photoreception in the rodless/coneless (rd/rd cl) mouse, the circadian phenotype is largely preserved consistent with the presence of a novel retinal photoreceptor (Freedman et al., 1999, Science, 284, 502). Indeed, ganglion cells that project to the suprachiasmatic nucleus (SCN), the location of the central circadian pacemaker, are intrinsically light sensitive (Berson et al., 2002, Science, 295, 1070). However, it is unclear whether all intrinsically light sensitive ganglion cells project to the SCN and furthermore the properties of the associated intracellular signalling mechanism remain unknown. Methods: Using fluorescence imaging of the Ca²⁺ sensitive indicator FURA-2AM in the wholemount rd/rd cl retina we investigated the role of Ca²⁺ in the generation of light-induced activity. This approach allowed us to sample the responses of several hundred cells in the ganglion cell layer per preparation. Results: In response to 470 nm light stimulation an increase in intracellular calcium was detected in a subset of the neurons (2.5 %, n = 18 retinae), often occurring in discrete clusters. Three types of Ca²⁺ change were observed: a sustained increase in the intracellular Ca²⁺ levels followed by gradual recovery, transient Ca²⁺ spikes, or repetitive Ca²⁺ spikes with a lack of recovery. The response to light stimulation was dependent on the intensity of the light stimulus (threshold: ≈ 100 µW/cm²) and the duration of the light pulse (threshold: ≈ 30 seconds). Light activity persisted in the presence of a glutamatergic blockade confirming these responses were independent of any residual rod/cone activity. Conclusions: The results suggest a network of intrinsically light sensitive neurones are present in the ganglion cell layer of the rd/rd cl retina that employ calcium as an intracellular signalling mechanism.

Keywords: ganglion cells • retina • calcium