Electroretinographic Abnormalities and Sex Differences Detected with Mesopic Adaptation in a Mouse Model of Schizophrenia: A and B Wave Analysis

Retinal electrophysiology in central nervous system disorders. A review of human and mouse studies

The retina and brain share similar neurochemistry and neurodevelopmental origins, with the retina, often viewed as a "window to the brain." With retinal measures of structure and function becoming easier to obtain in clinical populations there is a growing interest in using retinal findings as potential biomarkers for disorders affecting the central nervous system. Functional retinal biomarkers, such as the electroretinogram, show promise in neurological disorders, despite having limitations imposed by the existence of overlapping genetic markers, clinical traits or the effects of medications that may reduce their specificity in some conditions. This narrative review summarizes the principal functional retinal findings in central nervous system disorders and related mouse models and provides a background to the main excitatory and inhibitory retinal neurotransmitters that have been implicated to explain the visual electrophysiological findings. These changes in retinal neurochemistry may contribute to our understanding of these conditions based on the findings of retinal electrophysiological tests such as the flash, pattern, multifocal electroretinograms, and electro-oculogram. It is likely that future applications of signal analysis and machine learning algorithms will offer new insights into the pathophysiology, classification, and progression of these clinical disorders including autism, attention deficit/hyperactivity disorder, bipolar disorder, schizophrenia, depression, Parkinson's, and Alzheimer's disease. New clinical applications of visual electrophysiology to this field may lead to earlier, more accurate diagnoses and better targeted therapeutic interventions benefiting individual patients and clinicians managing these individuals and their families.

Sex-Specific Retinal Anomalies Induced by Chronic Social Defeat Stress in Mice

Major depressive disorder (MDD) is one of the most common consequences of chronic stress. Still, there is currently no reliable biomarker to detect individuals at risk to develop the disease. Recently, the retina emerged as an effective way to investigate psychiatric disorders using the electroretinogram (ERG). In this study, cone and rod ERGs were performed in male and female C57BL/6 mice before and after chronic social defeat stress (CSDS). Mice were then divided as susceptible or resilient to stress. Our results suggest that CSDS reduces the amplitude of both oscillatory potentials and a-waves in the rods of resilient but not susceptible males. Similar effects were revealed following the analysis of the cone b-waves, which were faster after CSDS in resilient mice specifically. In females, rod ERGs revealed age-related changes with no change in cone ERGs. Finally, our analysis suggests that baseline ERG can predict with an efficacy up to 71% the expression of susceptibility and resilience before stress exposition in males and females. Overall, our findings suggest that retinal activity is a valid biomarker of stress response that could potentially serve as a tool to predict whether males and females will become susceptible or resilient when facing CSDS.

Effects of D-serine treatment on outer retinal function

The role of the N-Methyl-D-Aspartate Receptor (NMDAR) in the outer retina is unclear despite expression of the NMDAR-complex and its subunits in the outer retina. The flash-electroretinogram (fERG) offers a non-invasive measurement of the retinal field potentials of the outer retina that can serve to clarify NMDAR contribution to early retinal processing. The role of the NMDAR in retinal function was assessed using a genetic mouse model for NMDAR hypofunction (SR^-/-), where the absence of the enzyme serine racemase (SR) results in an 85% reduction of retinal D-serine. NMDAR hypo- and hyperfunction in the retina results in alterations in the components of the fERG. The fERG was examined after application of exogenous D-serine to the eye in order to determine whether pre- and post-topical delivery of D-serine would alter the fERG in SR^-/- mice and their littermate WT controls. Amplitude and implicit time of the low-frequency components, the a- and b-wave, were conducted. Reduced NMDAR function resulted in a statistically significantly delayed a-wave and reduced b-wave in SR^-/- animals. The effect of NMDAR deprivation was more prominent in male SR^-/- mice. A hyperfunction of the NMDAR, through exogenous topical delivery of 5 mM D-serine, in WT mice caused a significantly delayed a-wave implicit time and reduced b-wave amplitude. These changes were not observed in female WT mice. There were temporal delays in the a-wave and amplitude and a decrease in the b-wave amplitude and implicit time in both hypo- and NMDAR hyperfunctional male mice. These results suggest that NMDAR and D-serine are involved in the retinal field potentials of the outer retina that interact based on the animal's sex. This implicates the involvement of gonadal hormones and D-serine in retinal functional integrity.

Electoretinographic evidence of retinal ganglion cell-dependent function in schizophrenia

Schizophrenia is a complex disorder that is diagnosed mainly with clinical observation and evaluation. Recent studies suggest that many people with schizophrenia have abnormalities in the function of the N-methyl-d-aspartate receptor (NMDAR). The retina is part of the central nervous system and expresses the NMDAR, raising the possibility of the early detection of NMDAR-related schizophrenia by detecting differences in retinal function. As a first-step, we used two non-invasive outpatient tests of retinal function, the photopic negative response (PhNR) of the light-adapted flash-electroretinogram (PhNR-fERG) and the pattern ERG (PERG), to test individuals with schizophrenia and controls to determine if there were measurable differences between the two populations. The PhNR-fERG showed that males with schizophrenia had a significant increase in the variability of the overall response, which was not seen in the females with schizophrenia. Additionally at the brightest flash strength, there were significant increases in the PhNR amplitude in people with schizophrenia that were maximal in controls. Our results show measurable dysfunction of retinal ganglion cells (RGCs) in schizophrenia using the PhNR-fERG, with a good deal of variability in the retinal responses of people with schizophrenia. The PhNR-fERG holds promise as a method to identify individuals more at risk for developing schizophrenia, and may help understand heterogeneity in etiology and response to treatment.