To test the sensitivity of novel imaging biomarkers to phototransduction, the use of Light vs. dark-adapted C57BL/6J mice, and mice null for transducin (i.e., Gα_t; Gnat1^−/−) that can not undergo phototransduction, were studied. We compared the extent of dark-induced thinning of the external limiting membrane-retinal pigment epithelium (ELM-RPE) thickness, a known proxy for phototransduction / cGMP / mitochondrial-driven / pH-triggered / RPE-water removal from the subretinal space to the magnitude of a hyporeflective band (HB) at the rod tips, a biomarker that is correlated with ELM-RPE thickness in wildtype mice, but shows a pH-independent response. An additional comparison was perform against a new parameter, the shape of the light-scattering, mitochondria-rich IS measured in an unbiased manner with a quantitative ellipse descriptor to generate the minor: major aspect ratio (AR) from its AC profile. 

Dark-adapted C57BL/6 mice showed the expected thinning of the ELM-RPE, as well as reduction in HB magnitude compared to that in light-adapted conditions. In addition, we also found that the IS AC AR was significantly greater in the dark than in the light. In contrast, Gnat1^−/− mice did not show a light-dark differences in ELM-RPE, HB magnitude, or IS AC AR.

Our first-in-kind data unequivocally demonstrate that the HB and shape of the IS AC profile are functionally regulated by phototransduction, a major regulator of rod mitochondria activity, as has been shown earlier for the ELM-RPE thickness. This suite of OCT-based biomarkers is expected to address a so-far unmet gap in the diagnosis and treatment of rod energy-landscape abnormalities in patients with neurodegenerative diseases.