One month of hyperglycemia alters spectral responses of the zebrafish photopic ERG

Abstract

Prolonged hyperglycemia can alter retinal function ultimately resulting in blindness. Zebrafish adults exposed to alternating 2% glucose conditions display a 3x increase in blood sugar levels. After 4 weeks of treatment, electroretinograms (ERGs) were recorded from isolated, perfused, in vitro eyecups. Control animals were exposed either to alternating 2% mannitol (osmotic control) or alternating water (handling control). Two types of ERGs were recorded: (1) native ERGs measured using white light stimuli and media without synaptic blockers and (2) spectral ERGs measured with an AMPA/KA antagonist, isolating photoreceptor-to-ON-bipolar cell synapses, and a spectral protocol that separated R, G, B, and UV cone signals. Retinas were evaluated for changes in layer thickness and for the inflammatory markers GFAP and Nf-κB. In native ERG, hyperglycemic b- and d-waves were lower in amplitude than mannitol controls. Alteration of waveshape became severe, with b-waves becoming more transient, and ERG responses showing more PIII-like characteristics. For spectral ERGs, waveshape appeared similar in all treatment groups. However, a1- and b2-wave implicit times were significantly longer, and amplitudes significantly reduced, by hyperglycemic treatment, due to the functional reduction in signals from R, G and B cones. Nf-κB increased significantly in hyperglycemic retinas, the increase in GFAP was not significant, and retinal layer thickness was unaffected. Thus, prolonged hyperglycemia triggers an inflammatory response and functional deficits localized to specific cone types, indicating rapid onset of neural complications in the zebrafish model of diabetic retinopathy.