VEGF as a Direct Functional Regulator of Photoreceptors and Contributing Factor to Diabetes-Induced Alteration of Photoreceptor Function

Vascular endothelial growth factor (VEGF) is a major therapeutic target for blood–retina barrier (BRB) breakdown in diabetic retinopathy (DR), age-related macular degeneration (AMD), and other hypoxic retinal vascular disorders. To determine whether VEGF is a direct regulator of retinal neuronal function and its potential role in altering vision during the progression of DR, we examined the immediate impact of recombinant VEGF (rVEGF) on photoreceptor function with electroretinography in C57BL6 background wild-type (WT) and Akita spontaneous diabetic mice. Shortly after intravitreal injections, rVEGF caused a significant reduction of scotopic ERG a-wave and b-wave amplitudes and photopic ERG b-wave amplitudes in a dose-dependent manner in dark-adapted 1.5-mo-old WT mice. Compared with WT controls, 5-mo-old Akita spontaneous diabetic mice demonstrated a significant reduction in scotopic ERG a-wave and b-wave amplitudes and photopic ERG b-wave amplitudes. However, the effect of rVEGF altered photoreceptor function in WT controls was diminished in 5-mo-old Akita spontaneous diabetic mice. In conclusion, our results suggest that VEGF is a direct functional regulator of photoreceptors and VEGF up-regulation in DR is a contributing factor to diabetes-induced alteration of photoreceptor function. This information is critical to the understanding of the therapeutic effect and to the care of anti-VEGF drug-treated patients for BRB breakdown in DR, AMD, and other hypoxic retinal vascular disorders.

Retinal A2A and A3 adenosine receptors modulate the components of the rat electroretinogram

Adenosine is a neuromodulator present in various areas of the central nervous system, including the retina. Adenosine may serve a neuroprotective role in the retina, based on electroretinogram (ERG) recordings from the rat retina. Our purpose was to assess the role of A2A and A3 adenosine receptors in the generation and modulation of the rat ERG. The flash ERG was recorded with corneal electrodes from Sprague Dawley rats. Agonists and antagonists for A2A and A3 receptors, and adenosine were injected (5 µl) into the vitreous. The effects on the components of the single flash scotopic and photopic ERGs were examined, and ERG flicker. Adenosine (0.5 mM) increased the mean amplitudes of the scotopic ERG a-waves (68 ± 8 to 97 ± 14 µV, P = 0.042), and b-waves (236 ± 38 µV to 305 ± 42 µV). A2A agonist CGS21680 (2 mM) reduced the mean amplitude of the ERG b-wave, from 298 ± 21 µV in response to the brightest stimulus to 212 ± 19 µV (P = 0.005), and mean scotopic oscillatory potentials (OPs) from 100 ± 9 µV to 47 ± 11 µV (P = 0.023). ZM241385 [4 mM], an A2A antagonist, decreased the scotopic b-wave of the ERG. A3 agonist 2-CI-IB-MECA (0.5 mM) increased the a-wave, while decreasing the scotopic and photopic ERG b-waves, and the scotopic OPs. A3 antagonist VUF5574 (1 mM) increased the mean amplitude of the scotopic a-wave (66 ± 8 to 140 ± 29 µV, P = 0.046) and b-wave (224 ± 20 to 312 ± 39 µV, P = 0.0037). No significant effects on ERG flicker were found. We conclude that retinal neurons containing A2A and/or A3 adenosine receptors contribute to the generation of the ERG a- and b-waves and OPs.

Assessing the Contribution of the Oscillatory Potentials to the Genesis of the Photopic ERG with the Discrete Wavelet Transform

The electroretinogram (ERG) is composed of slow (i.e., a-, b-waves) and fast (i.e., oscillatory potentials: OPs) components. OPs have been shown to be preferably affected in some diseases (such as diabetic retinopathy), while the a- and b-waves remain relatively intact. The purpose of this study was to determine the contribution of OPs to the building of the ERG and to examine whether a signal mostly composed of OPs could also exist. DWT analyses were performed on photopic ERGs (flash intensities: −2.23 to 2.64 log cd·s·m−2in 21 steps) obtained from normal subjects (n=40) and patients (n=21) affected with a retinopathy. In controls, the %OP value (i.e., OPs energy/ERG energy) is stimulus- and amplitude-independent (range: 56.6–61.6%; CV = 6.3%). In contrast, the %OPs measured from the ERGs of our patients varied significantly more (range: 35.4%–89.2%;p<0.05) depending on the pathology, some presenting with ERGs that are almost solely composed of OPs. In conclusion, patients may present with a wide range of %OP values. Findings herein also support the hypothesis that, in certain conditions, the photopic ERG can be mostly composed of high-frequency components.

Scotopic vision in the monkey is modulated by the G protein-coupled receptor 55

The endogenous cannabinoid system plays important roles in the retina of mice and monkeys via their classic CB1 and CB2 receptors. We have previously reported that the G protein-coupled receptor 55 (GPR55), a putative cannabinoid receptor, is exclusively expressed in rod photoreceptors in the monkey retina, suggesting its possible role in scotopic vision. To test this hypothesis, we recorded full-field electroretinograms (ERGs) after the intravitreal injection of the GPR55 agonist lysophosphatidylglucoside (LPG) or the selective GPR55 antagonist CID16020046 (CID), under light- and dark-adapted conditions. Thirteen vervet monkeys (Chlorocebus sabaeus) were used in this study: four controls (injected with the vehicle dimethyl sulfoxide, DMSO), four injected with LPG and five with CID. We analyzed amplitudes and latencies of the a-wave (photoreceptor responses) and the b-wave (rod and cone system responses) of the ERG. Our results showed that after injection of LPG, the amplitude of the scotopic b-wave was significantly higher, whereas after the injection of CID, it was significantly decreased, compared to the vehicle (DMSO). On the other hand, the a-wave amplitude, and the a-wave and b-wave latencies, of the scotopic ERG responses were not significantly affected by the injection of either compound. Furthermore, the photopic ERG waveforms were not affected by either drug. These results support the hypothesis that GPR55 plays an instrumental role in mediating scotopic vision.

The Intoxication Effects of Methanol and Formic Acid on Rat Retina Function

Objective. To explore the potential effects of methanol and its metabolite, formic acid, on rat retina function.Methods. Sprague-Dawley rats were divided into 3- and 7-day groups and a control. Experimental groups were given methanol and the control group were provided saline by gavage. Retinal function of each group was assessed by electroretinogram. Concentrations of methanol and formic acid were detected by GC/HS and HPLC, respectively.Results. The a and b amplitudes of methanol treated groups decreased and latent periods delayed in scotopic and photopic ERG recordings. The summed amplitudes of oscillatory potentials (OPs) of groups B and C decreased and the elapsed time delayed. The amplitudes of OS1, OS3, OS4, and OS5 of group B and OS3, OS4, and OS5 of group C decreased compared with the control group. The IPI1 of group B and IPI1-4 of group C were broader compared with the control group and the IPI1-4 and ET of group B were broader than group C. Conclusions. Both of scotopic and photopic retinal functions were impaired by methanol poisoning, and impairment was more serious in the 7-day than in the 3-day group. OPs, especially later OPs and IPI2, were more sensitive to methanol intoxication than other eletroretinogram subcomponents.