Photoreceptor and Post-Photoreceptoral Contributions to Photopic ERG a-Wave in Rhodopsin P347L Transgenic Rabbits

Abstract

PURPOSE.The a-wave of the photopic electroretinogram (ERG) of macaque monkeys is made up of the electrical activities of cone photoreceptors and post-photoreceptoral neurons. However, it is not known whether the contributions of these two components change in retinas with inherited photoreceptor degeneration. The purpose of this study was to determine the contributions of cones and post-photoreceptoral neurons to the a-wave of the photopic ERGs in rhodopsin Pro347Leu transgenic (Tg) rabbits. METHODS. Ten Tg and 10 wild-type (WT) New Zealand White rabbits were studied at 4 and 12 months of age. The a-waves of the photopic ERGs were elicited by xenon flashes of different stimulus strengths before and after the activities of post-photoreceptoral neurons were blocked by intravitreal injections of a combination of 0.2 to 0.4 mM of 6-cyano-7-nitrouinoxaline-2,3(1H,4H)-dione, disodium (CNQX) and 2 to 4 mM of (Ϯ)-2-amino-4-phosphonobutyric acid. RESULTS. The percentage contribution of the cone photoreceptors to the photopic ERG a-waves increased with increasing stimulus strength, and the percentage ranged from 54% to 75% in 4-month-old WT rabbits. In contrast, the percentage contribution of the cone photoreceptors in 4-month-old Tg rabbits ranged from 32% to 51% (P Ͻ 0.05). The mean percentage contribution of cone photoreceptors became still smaller at 11% to 48% in 12-month-old Tg rabbits. CONCLUSIONS. These results suggest that the relative contribution of cone photoreceptors to the photopic ERG a-wave is smaller in retinas with inherited photoreceptor degeneration. This indicates that the a-waves of the photopic ERGs in patients with retinitis pigmentosa must consider this lower contribution from the cone photoreceptors. (Invest Ophthalmol Vis Sci. 2012;53:1467-1472 The origins of the photopic or light-adapted a-wave of the ERG in macaque monkeys was studied by Sieving et al.3-5 They injected glutamate agonists and antagonists intravitreally to dissect the retinal circuits. They found that the a-wave of the photopic ERG received contributions not only from the cone photoreceptors but also from post-photoreceptoral neurons (e.g., OFF-bipolar cells and horizontal cells) 3,4 because cis-2,3-piperidine dicarboxylic acid (PDA) or kynurenic acid reduced the a-wave amplitude. A later study by Robson et al. 6 showed that the PDA-sensitive post-photoreceptoral a-wave component started at much earlier times of approximately 5 ms in macaques. Frieberg et al.7 also estimated the time course of the cone photoreceptor response in normal human ERGs using the paired-flash technique, in which an intense "probe" flash was delivered at different times after a "test" flash. Their results showed that the photopic ERG a-wave of the human ERG contains an appreciable postphotoreceptoral component, similar to that reported in monkeys. [3][4][5][6] These studies, which were designed to determine the origins of the photopic ERG a-wave, have been performed primarily on normal macaque monkeys and human eyes.3-7 It is not known whether the contr...

Long-Term Protection of Genetically Ablated Rabbit Retinal Degeneration by Sustained Transscleral Unoprostone Delivery

“…26 However, rabbits show large photopic a-and b-waves. 27 The relatively large eye balls also make it possible to examine the variable effects of treatments, such as intravitreal injections and drug delivery systems. 28 To examine the effects of these novel approaches, Tg rabbits imitating human RP are therefore useful.…”

“…The amplitudes of the postphotoreceptor components in Tg rabbits are reported to be significantly larger than those of the Wt rabbits at lower stimulus strengths of 0.2 and 0.7 log (cd*seconds/m 2 ). 27 This may be why significant differences in the amplitudes of photopic a-and b-waves were observed at higher light stimulus intensities. The amplitudes of rod a-and bwaves in the URD-treated eyes were also higher than those of the placebo-URD-treated and nontreated eyes (Fig.…”

Contributions of Second- and Third-Order Retinal Neurons to Cone Electroretinograms After Loss of Rod Function in Rhodopsin P347L Transgenic Rabbits

“…The intravitreal concentrations were 4 mM for APB, 0.4 mM for CNQX, 4 mM for TTX, and 5 mM for NMDA assuming that the vitreous volume of the NZW rabbit is 1.5 mL. 18 The drugs were dissolved in phosphate-buffered solution, and 0.05 mL was injected into the midvitreous. Four right eyes of four WT and Tg rabbits were analyzed after the injections of APB followed by CNQX.…”

“…Interestingly, several ERG studies using pharmacologic dissection of the retinal circuits in this model revealed an Copyright 2017 The Authors iovs.arvojournals.org j ISSN: 1552-5783 enhancement of the function of the second-or third-order retinal neurons. [16][17][18] This rabbit model of RP has at least two benefits in ERG analyses compared with the rodent models of eyes with RP. The first is that rabbits have large eyes, which allowed us to perform multiple intravitreal injections of different agents.…”

Cytoprotective Effects of a Novel Nrf2 Activator, RS9, in Rhodopsin Pro347Leu Rabbits

“…1 Transgenic rabbits with this mutation show decreased electroretinogram responses and progressive retinal degeneration. 3 Although several approaches are suggested such as ribozyme, neurotrophic factors and antioxidant therapies, there are currently no proved effective treatments for RP. However, antioxidant therapy should be given further consideration as a therapeutic modality, as reduced aqueous humor glutathione levels have been reported in RP patients.…”