Morphological and functional alteration of light perception circuits in AD patients
Disruption of sleep and circadian rhythms is one of the earliest symptoms of Alzheimers disease (AD). Circadian entrainment and modulation of alertness are non-visual responses to light driven by intrinsically photosensitive retinal ganglion cells (ipRGCs). To explore structural and functional changes of ipRGCs and ipRGC circuits in AD, we analyzed the retinas and brains of 13 elderly patients ranging from normal cognition to AD and performed ex vivo extracellular electrophysiological recordings on freshly harvested retinas. While rods and cones were moderately impaired, there was a severe loss of ipRGCs in AD donors. Importantly, the remaining ipRGCs exhibited morphological alterations, hyperexcitability, and were not able to sustain high levels of activation. These changes may be ipRGC subtype-specific and correlated with disease progression. Altered ipRGC circuits and function could contribute to the disruption of sleep and circadian rhythms reported in AD patients. Measuring ipRGC-dependent responses to light could be a promising way to predict or monitor pathological changes in the brain.