Reliable detection of low visual acuity in mice with pattern visually evoked potentials.

Measuring the optokinetic response (OKR) to rotating sinusoidal gratings is becoming an increasingly common method to determine visual function thresholds in mice. This is possible also through direct electrophysiological recording of the response of the neurons in the visual cortex to the presentation of reversing patterned stimuli, i.e. the pattern visually evoked potential (pVEP). Herein, we optimized the conditions for recording pVEPs in wild-type mice: we investigated the optimal depth (1, 2, or 3 mm) of the inserted electrode and the optimal stimulus pattern (vertical, horizontal, or oblique black and white stripes, or a checkerboard pattern). Visual acuity was higher when measured with the optimal pVEP recording conditions, i.e., with the electrode at 2 mm and a vertical-stripe stimulus (0.530 ± 0.021 cycle/degree), than with OKR (0.455 ± 0.006 cycle/degree). Moreover, in murine eyes with optic nerve crush-induced low vision, OKR could not measure any visual acuity, while pVEPs allowed the reliable quantification of residual vision (0.064 ± 0.004 cycle/degree). Our results show that pVEPs allow more sensitive measurement of visual function than the OKR-based method. This technique should be particularly useful in mouse models of ocular disease and low vision.

Retained Plasticity and Substantial Recovery of Rod-Mediated Visual Acuity at the Visual Cortex in Blind Adult Mice with Retinal Dystrophy.

In patients born blind with retinal dystrophies, understanding the critical periods of cortical plasticity is important for successful visual restoration. In this study, we sought to model childhood blindness and investigate the plasticity of visual pathways. To this end, we generated double-mutant (Pde6ccpfl1/cpfl1Gnat1IRD2/IRD2) mice with absent rod and cone photoreceptor function, and we evaluated their response for restoring rod (GNAT1) function through gene therapy. Despite the limited effectiveness of gene therapy in restoring visual acuity in patients with retinal dystrophy, visual acuity was, unexpectedly, successfully restored in the mice at the level of the primary visual cortex in this study. This success in visual restoration, defined by changes in the quantified optokinetic response and pattern visually evoked potential, was achieved regardless of the age at treatment (up to 16 months). In the contralateral visual cortex, cortical plasticity, tagged with light-triggered transcription of Arc, was also restored after the treatment in blind mice carrying an Arc promoter-driven reporter gene, dVenus. Our results demonstrate the remarkable plasticity of visual circuits for one of the two photoreceptor mechanisms in older as well as younger mice with congenital blindness due to retinal dystrophies.

Measurement of Electroretinograms and Visually Evoked Potentials in Awake Moving Mice.

The development of new treatments for intractable retinal diseases requires reliable functional assessment tools for animal models. In vivo measurements of neural activity within visual pathways, including electroretinogram (ERG) and visually evoked potential (VEP) recordings, are commonly used for such purposes. In mice, the ERG and VEPs are usually recorded under general anesthesia, a state that may alter sensory transduction and neurotransmission, but seldom in awake freely moving mice. Therefore, it remains unknown whether the electrophysiological assessment of anesthetized mice accurately reflects the physiological function of the visual pathway. Herein, we describe a novel method to record the ERG and VEPs simultaneously in freely moving mice by immobilizing the head using a custom-built restraining device and placing a rotatable cylinder underneath to allow free running or walking during recording. Injection of the commonly used anesthetic mixture xylazine plus ketamine increased and delayed ERG oscillatory potentials by an average of 67.5% and 36.3%, respectively, compared to unanesthetized mice, while having minimal effects on the a-wave and b-wave. Similarly, components of the VEP were enhanced and delayed by up to 300.2% and 39.3%, respectively, in anesthetized mice. Our method for electrophysiological recording in conscious mice is a sensitive and robust means to assess visual function. It uses a conventional electrophysiological recording system and a simple platform that can be built in any laboratory at low cost. Measurements using this method provide objective indices of mouse visual function with high precision and stability, unaffected by anesthetics.