Foveal RGCs develop abnormal calcium dynamics weeks after photoreceptor ablation

OBJECTIVE OR PURPOSE: Physiological changes in retinal ganglion cells (RGCs) have been reported in rodent models of photoreceptor (PR) loss but this has not been investigated in primates. By expressing both a calcium indicator (GCaMP6s) and an optogenetic actuator (ChrimsonR) in foveal RGCs of the macaque, we reactivated RGCs in vivo and assessed their response in the weeks and years following PR loss. DESIGN: We used an in vivo calcium imaging approach to record optogenetically evoked activity in deafferented RGCs in primate fovea. Cellular scale recordings were made longitudinally over a 10 week period following photoreceptor ablation and compared to RGC responses from RGCs that had lost photoreceptor input more than two years prior. PARTICIPANTS: Three eyes received photoreceptor ablation, OD of a male Macacus mulatta (M1), OS of a female Macacus fascicularis (M2) and OD of a male Macacus fascicularis (M3). Two animals were used for in vivo recording, one for histological assessment. METHODS: Cones were ablated with an ultrafast laser delivered through an adaptive optics scanning light ophthalmoscope (AOSLO). A 0.5 s pulse of 25Hz 660nm light was delivered to optogenetically stimulate the deafferented RGCs, and the resulting GCaMP fluorescence signal was recorded from RGCs with an AOSLO. These measurements were repeated over the 10 weeks immediately after photoreceptor ablation and at 2 years. MAIN OUTCOME MEASURES: The rise time, decay constant and response magnitude of the deafferented RGCs responding to optogenetic stimulation was derived from GCaMP fluorescence recordings from 221 RGCs (Animal M1) and 218 RGCs (Animal M2) in vivo. RESULTS: The mean time to the peak calcium response was stable in deafferented RGCs over the 10-week post ablation observation period, while the mean decay constant of the calcium response decreased by 1.5 fold 1.6±0.5 s to 0.6±0.3 s within 10 weeks in subject 1 and 2.1 fold (2.5±0.5 s to 1.2±0.2 s SD) within 8 weeks in subject 2. CONCLUSIONS: We observe abnormal calcium dynamics developing in primate foveal RGCs in the weeks after photoreceptor ablation. The mean decay constant of the optogenetic mediated calcium response decreased 1.5 – 2-fold. This is the first report of this phenomenon in primate retina and further work is required to understand the role these changes play in cell survival and activity. Nevertheless, the presence of optogenetic mediated responses 2 years after PR loss and the stable rise time remain promising for vision restoration therapies.