Push–pull model of the primate photopic electroretinogram: A role for hyperpolarizing neurons in shaping the b-wave

Abstract

Existing models of the primate photopic electroretinogram (ERG) attribute the light-adapted b–wave to activity of depolarizing bipolar cells (DBCs), mediated through a release of potassium that is monitored by Müller cells. However, possible ERG contributions from OFF-bipolar cells (HBCs) and horizontal cells (HzCs) have not been explored. We examined the contribution of these hyperpolarizing second-order retinal cells to the photopic ERG of monkey by applying glutamate analogs to suppress photoreceptor transmission selectively to HBC/HzCs vs. DBCs.ERGs of Macaca monkeys were recorded at the cornea before and after intravitreal injection of drugs. Photopic responses were elicited by bright 200–220 ms flashes on a steady background of 3.3 log scotopic troland to suppress rod ERG components.2–amino-4–phosphonobutyric acid (APB), which blocks DBC light responses, abolished the photopic b–wave and indicated that DBC activity is requisite for photopic b–wave production.However, applying cis–2,3–piperidine dicarboxylic acid (PDA) and kynurenic acid (KYN), to suppress HBCs/HzCs and third-order neurons, revealed a novel ERG response that was entirely positive and was sustained for the duration of the flash. The normally phasic b–wave was subsumed into this new response. Applying n–methyl-dl-aspartate (NMA) did not replicate the PDA+KYN effect, indicating that third-order retinal cells are not involved. This suggests that HBC/HzC activity is critical for shaping the phasic b–wave.Components attributable to depolarizing vs. hyperpolarizing cells were separated by subtracting waveforms after each drug from responses immediately before. This analysis indicated that DBCs and HBC/HzCs each can produce large but opposing field potentials that nearly cancel and that normally leave only the residual phasic b–wave response in the photopic ERG.Latency of the DBC component was 5–9 ms slower than the HBC/HzC component. However, once activated, the DBC component had a steeper slope. This resembles properties known for the two types of cone synapses in lower species, in which the sign-preserving HBC/HzC synapse has faster kinetics but probably lower gain than the slower sign-inverting G-protein coupled DBC synapse.A human patient with “unilateral cone dystrophy” was found to have a positive and sustained ERG that mimicked the monkey ERG after PDA+KYN, indicating that these novel positive photopic responses can occur naturally even without drug application.These results demonstrate that hyperpolarizing second-order neurons are important for the primate photopic ERG. A “Push-Pull Model” is proposed in which DBC activity is requisite for b–wave production but in which HBC/HzC activity limits the amplitude and controls the shape of the primate photopic b–wave.

Postreceptoral contributions to the light-adapted ERG of mice lacking b-waves

“…However, the slow time to peak in the mouse distinguishes it from d-waves recorded in other species such as humans and monkeys where it rises to a peak rapidly (e.g. Sieving et al, 1994). This peak was not obvious in Control mice before APB injection (see inset to Fig.…”

“…Miyake et al, 1887;Pesch et al, 2000). Blockade of transmission between photoreceptors and DBCs by a glutamate analogue the binds to mGlur6 receptors, APB (L-2-amino-4-phosphonobutyric acid), similarly results in no ERG b-wave under photopic and scotopic conditions in the macaque retina, a retina very similar to that of humans (Khan et al, 2005;Knapp & Schiller, 1984;Sieving et al, 1994), and in the retina of normal mice (Kang Derwent et al, 2007;Sharma et al, 2005), In contrast, blockade of transmission from photoreceptors to HBCs using an iontotropic glutamate receptor antagonist in macaques and mice results in a reduced photopic ERG a-wave and in macaques, elimination of the d-wave (Maeda et al, 2003;Sharma et al, 2005;Sieving et al, 1994).…”

“…Sieving et al, 1994). Pharmacologic blockade studies using APB also have not revealed dwaves in rodents (Sharma et al, 2005;Xu et al, 2001).…”

Thenob2mouse, a null mutation inCacna1f: Anatomical and functional abnormalities in the outer retina and their consequences on ganglion cell visual responses

“…While diminished, both the b-wave and highfrequency oscillatory potentials are retained in nob2 mice and CSNB2 patients, indicating the retention of some inner retinal activity in the absence of α 1F . While there are important differences in the origins of the primate and mouse cone ERG (Sieving et al, 1994;Sharma et al, 2005), these responses also indicate the presence of inner retinal activity in the absence of α 1F . A recent publication has described a KO mouse for α 1F (Mansergh et al, 2005).…”

Dynamic, in vivo, real-time detection of retinal oxidative status in a model of elevated intraocular pressure using a novel, reversibly responsive, profluorescent nitroxide probe

“…The a-wave is the response generated by photoreceptors (Penn and Hagins, 1969) and the b-wave is predominantly generated by the depolarization of the ON bipolar cells (Bush and Sieving, 1996;Hood and Birch, 1996;Robson and Frishman, 1995;Robson and Frishman, 1998;Sieving et al, 1994). The oscillatory potentials (OPs) arise from activity in the inner retina, including a major contribution from amacrine cells (Wachtmeister and Dowling, 1978) Prior to employing and evaluating our ME-TRN probe as a real-time reporter of retinal oxidative status in vivo, we initially examined its 'short and long term' toxicity to retinal tissues and their function.…”