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

Abstract

Prolonged hyperglycemia can alter retinal function, ultimately resulting in blindness. Adult zebrafish adults exposed to alternating conditions of 2% glucose/0% glucose display a 3× increase in blood sugar levels. After 4 weeks of treatment, electroretinograms (ERGs) were recorded from isolated, perfused, in vitro eyecups. Control animals were exposed to alternating 2% mannitol/0% mannitol (osmotic control) or to alternating water (0% glucose/0% glucose; handling control). Two types of ERGs were recorded: (1) native ERGs measured using white-light stimuli and medium without synaptic blockers; and (2) spectral ERGs measured with an AMPA/kainate receptor antagonist, isolating photoreceptor-to-ON-bipolar-cell synapses, and a spectral protocol that separated red (R), green (G), blue (B) and UV cone signals. Retinas were evaluated for changes in layer thickness and for the inflammatory markers GFAP and Nf-κB (RelA or p65). In native ERGs, hyperglycemic b- and d-waves were lower in amplitude than the b- and d-waves of mannitol controls. Alteration of waveshape became severe, with b-waves becoming more transient and ERG responses showing more PIII-like (a-wave) characteristics. For spectral ERGs, waveshape appeared similar in all treatment groups. However, a1- and b2-wave implicit times were significantly longer, and amplitudes were significantly reduced, in response to hyperglycemic treatment, owing to the functional reduction in signals from R, G and B cones. Nf-κB increased significantly in hyperglycemic retinas, but 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 the rapid onset of neural complications in the zebrafish model of diabetic retinopathy.

Diabetic photoreceptors: Mechanisms underlying changes in structure and function

“…Lacking a blood-retinal barrier to hinder glucose delivery directly to neurons, in vitro neuroretinal cultures, ex vivo explants, and isolated eyecups have been used to test the effects of high glucose in the absence of vascular influence, with or without experimental supplementation of relevant cellular and metabolic stressors. In general, such studies indicate that short-term hyperglycemia is beneficial to photoreceptor integrity and electrophysiology, whereas chronic or long-term hyperglycemia increases susceptibility to a range of cellular stressors present in culture or established within the systemic background prior to isolation of the retina (Han et al, 2013;Layton, 2015;Matteucci et al, 2015;Tanvir et al, 2018;Calbiague et al, 2019). Although research efforts to dissect effects of hyperglycemia in photoreceptors are gaining momentum, existing studies collectively suggest an extreme adaptability of these outer retinal neurons, and a need for precise control of hyperglycemic duration as well as the extracellular milieu to understand their role in early DR.…”

Photoreceptor Degeneration Accompanies Vascular Changes in a Zebrafish Model of Diabetic Retinopathy

“…14,15 Hyperglycemic zebrafish, caused by high glucose incubation or toxin-induced loss of beta cells, showed retinal thinning and vasculature changes, as well as disruption of cone cells. [16][17][18] However, there was no evidence for neovascularization, perhaps due to limited duration of the treatments. Thus far there have been no zebrafish models for PDR, or for earlier stages of DR based on genetically induced diabetes.…”

Neurodegeneration, Neuroprotection and Regeneration in the Zebrafish Retina

“…The amplitude of the native photopic (white light) ERG was found to be significantly lower in hyperglycemic fish compared to mannitol-treated fish, which act as a control for osmotic changes. B-wave parameters were also reduced in response to spectral ERG stimuli, indicating cone-pathway deficits [ 145 ]. Hyperglycemia also significantly reduced the amplitude of the d-wave, which is thought to be derived from the OFF-bipolar cells.…”