Abstract
Abstract: :
Purpose: To determine the contribution of action potentials of retinal neurons to the generation of oscillatory potentials (OPs) of the dark–adapted flash electroretinogram (ERG) of the rat. Methods: Dark–adapted ERGs (DC–300 Hz) were recorded differentially between DTL fiber electrodes from anesthetized (ketamine, 60 mg/kg/hr, xylazine 6 mg/kg/hr) female Sprague–Dawley rats before and after intravitreal injection of 2 micro liters of saline (control expts), tetrodotoxin (TTX, 4–40 micro molar conc to suppress action potentials of retinal neurons) or TTX and N–methyl–D–Aspartate (NMDA, 4 millimolar conc to suppress all inner–retinal activity). The pupils were dilated with phenylephrine (2.5%) and accommodation was blocked with atropine sulfate (1%). An Espion system (Diagnosys, MA) was used for stimulus presentation and data acquisition. The stimuli consisted of brief (<4ms) full field blue LED flashes of intensities ranging between –5 and 2 log scot cd.s/ sq m. OPs were isolated from the overall ERG waveform using a high pass Butterworth filter with cutoff at 75Hz. Results: At lower flash intensities (∼ –4.2 log scot cd.s/m sq) control ERGs showed negative and positive Scotopic Threshold Responses (STR) and typical dark–adapted b– and a–waves were obtained at higher intensities. OPs could be clearly isolated from noise from ∼ –3.7 log scot cd.s/m sq, the OPs increased in amplitude and decreased in latency as flash intensity was increased. Following injection of TTX, as the STR showed a dose dependent reduction in size the OPs also showed a small but significant decrease in size. The combination of TTX and NMDA had similar effects and did not eliminate the OPs. Conclusions: While action potentials of retinal neurons make significant contributions to the rodent OP amplitudes, retinal neurons distal to the retinal ganglion and amacrine cells are likely to make large contribution to the OPs.
Keywords: electroretinography: non–clinical • ganglion cells • amacrine cells