Visual System Hyperexcitability and Compromised V1 Receptive Field Properties in Early-Stage Retinitis Pigmentosa in Mice

Abstract

Inherited retinal degenerative diseases are a prominent cause of blindness. Although mutations causing death of photoreceptors are mostly known, the pathophysiology downstream in the inner retina and along the visual pathway is incompletely characterized in the earliest disease stages. Here, we investigated retinal, midbrain and cortical visual function using electroretinography (ERG), the optomotor response (OMR), visual evoked potentials (VEPs), respectively, and single unit electrophysiology at the primary visual cortex (V1) in light-adapted juvenile (approximately one-month-old) and young adult (three-month-old) Rho^P23H/WT mice, representative of early-stage retinitis pigmentosa (RP). Photopic ERG revealed up to ∼30% hypersensitivity to light in Rho^P23H/WT mice, as measured by the light intensity required to generate half-maximal b-wave (I₅₀ parameter). Rho^P23H/WT mice also showed increased OMRs toward low spatial frequency (SF) drifting gratings, indicative of visual overexcitation at the midbrain level. At the V1 level, VEPs and single-cell recordings revealed prominent hyperexcitability in the juvenile Rho^P23H/WT mice. Mean VEP amplitudes for light ON stimuli were nearly doubled in one-month-old Rho^P23H/WT mice compared with controls, and more than doubled for light OFF. Single-cell recordings showed a significantly increased spontaneous V1 neuron firing in the Rho^P23H/WT mice, and persistent contrast and temporal sensitivities. In contrast, direction selectivity was severely compromised. Our data suggest that during early RP, the visual pathway becomes hyperexcited. This could have both compensatory and deleterious consequences for visual behavior. Further studies on the mechanisms of hyperexcitability are warranted as this could lead to therapeutic interventions for RP.

Significance Statement

Lost retinal function in many blinding retinal degenerative disorders could soon be alleviated by advanced therapies that restore photoreception. However, it is unknown whether a visual system rewired downstream of the photoreceptors can process signals adequately. We studied the functional consequences of early rod death along the visual pathway in young retinitis pigmentosa (RP) mice. Photopic inner retina responses were moderately hypersensitized in the electroretinograms (ERG) of RP mice. Reflex-based visual behavior and visual cortex electrophysiology showed hyperexcitability. Some aspects of complex visual processing were remarkably resistant to degeneration, whereas others were severely impacted. We conclude that the visual system adapts to lost photoreception by increasing sensitivity, but simultaneously becomes detrimentally hyperexcited. Mechanistic understanding could lead to therapeutic preservation and restoration of vision.