Contribution of voltage‐gated sodium channels to the b‐wave of the mammalian flash electroretinogram

Abstract

Voltage-gated sodium channels (Na v channels) in retinal neurons are known to contribute to the mammalian flash electroretinogram (ERG) via activity of third-order retinal neurons, i.e. amacrine and ganglion cells. This study investigated the effects of tetrodotoxin (TTX) blockade of Na v channels on the b-wave, an ERG wave that originates mainly from activity of second-order retinal neurons. ERGs were recorded from anaesthetized Brown Norway rats in response to brief full-field flashes presented over a range of stimulus energies, under dark-adapted conditions and in the presence of steady mesopic and photopic backgrounds. Recordings were made before and after intravitreal injection of TTX (∼3 μM) alone, 3-6 weeks after optic nerve transection (ONTx) to induce ganglion cell degeneration, or in combination with an ionotropic glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 200 μM) to block light-evoked activity of inner retinal, horizontal and OFF bipolar cells, or with the glutamate agonist N -methyl-D-aspartate (NMDA, 100-200 μM) to reduce light-evoked inner retinal activity. TTX reduced ERG amplitudes measured at fixed times corresponding to b-wave time to peak. Effects of TTX were seen under all background conditions, but were greatest for mesopic backgrounds. In dark-adapted retina, b-wave amplitudes were reduced only when very low stimulus energies affecting the inner retina, or very high stimulus energies were used. Neurophysiological studies of blockade of Na v channels using tetrodotoxin (TTX) (Narahashi et al. 1964;Kao, 1966) in isolated mammalian retinal slice preparations or dissociated neurons have shown that these channels are prominent and functional in retinal ganglion cells (e.g. Lipton & Tauck, 1987;Skaliora et al. 1993) and in some This paper has online supplemental material. Loss of ganglion cells following ONTx did not affect b-wave amplitudes, and injection of TTX in eyes withtypes of amacrine cells (e.g. AII, A17, A18 and starburst amacrine cells; Boos et al. 1993;Feigenspan et al. 1998;Hartveit, 1999;Cohen, 2001). More recently, functional Na v channels in isolated bipolar cells and bipolar cells in retinal slice preparations have been observed in several species: rat (Pan & Hu, 2000), goldfish (Zenisek et al. 2001), salamander Ichinose & Lukasiewicz, 2007), ground squirrel (Saszik & DeVries, 2005) human (Miyachi et al. 2006;Ohkuma et al. 2007). In the rat retina, functional Na v channels were found in cone but not rod bipolar cells (Pan & Hu, 2000). Their absence from rod bipolar cells, suggests an additional postreceptoral divergence in physiological properties of rod versus cone circuits (Pan & Hu, 2000;Ma et al. 2005). The activity of Na v channels in bipolar cells has been shown recently to be modulated by ambient background illumination in the tiger salamander retinal slice (Ichinose & Lukasiewicz, 2007) where the Na v channels contributed more to the bipolar cell response under low light conditions than high light conditions. The authors suggested that the ...

Sip1 regulates the generation of the inner nuclear layer retinal cell lineages in mammals

“…S3). The b-wave is generated primarily by BP cells (Mojumder et al, 2008). Therefore, these results were consistent with the alterations in Sip1 mutant retinae, including the disruption in the OPL, which outputs to the BPs, and the reduction in BP numbers.…”

Testing retinal toxicity of drugs in animal models using electrophysiological and morphological techniques

“…The photopic negative response (PhNR) also reflects ganglion cells' activity in humans and nonhuman primates [31,32]; however, its use in mammals (rats) is still questionable since its neural source is still under debate. One study attributed the PhNR in rats to ganglion cells' activity [33] and another to amacrine cells [34]. The recently developed multifocal ERG recording can be applied to laboratory animals when electrophysiological mapping of the retina is needed in order to identify physiologic changes in localized retinal regions [65,66].…”

“…The photopic negative response (PhNR) has been shown recently to reflect ganglion cells' activity in human [31] and non-human primates [32]. However, experiments in a rat model yielded conflicting results; one study attributed the PhNR to ganglion cells' activity [33], while in another, the amacrine cells were suggested as the source of this response [34]. Recordings of the different types of ERG responses can help separating drug's effects on the different retinal layers, and sometimes also on specific retinal cells.…”

Degeneration and Dysfunction of Retinal Neurons in Acute Ocular Hypertensive Rats: Involvement of Calpains

“…5 and 6), and this agrees with a previous report. 19 Thus, hypertensioninduced apoptosis and loss of electrophysiological function occurs before histologic degeneration of the inner retinal layers.…”

“…The tissues were then incubated overnight with primary monoclonal antibody for Brn-3a (MAB1585, 1:100; Chemicon/ Millipore), a transcription factor specifically expressed in the nuclei of ganglion cells. 19 After washing 3 times with blocking solution for 90 min, the tissues were incubated overnight with secondary antibody conjugated to Alexa Fluor 488 (1:200). After washing 3 times for 90 min in PBS with 5% DMSO, whole retinas were isolated.…”