Contribution of rod, on-bipolar, and horizontal cell light responses to the ERG of dogfish retina.
Simultaneous extracellular ERG and intracellular recordings from horizontal and ON-bipolar cells were obtained from the dark-adapted retina of the dogfish. The light intensity-peak response relation (IR) and time course of on-bipolar cell responses closely resembled that of the ERG b-wave, but only at low light intensities [<10 rhodopsin molecules bleached per rod (Rh*)]. Block of on-bipolar cell responses with 50 microM 2-amino-4-phosphonobutyrate (APB) abolished the b-wave and unmasked a vitreal-negative wave. Subtraction from the control ERG resulted in the isolation of a vitreal-positive ERG with an IR which matched that of on-bipolar cells over the full range of light intensities. The D.C. component of the ERG arises as a result of sustained depolarization of on-bipolar cells in response to long (>0.5 s) dim light stimuli, or following bright light flashes. The IR of horizontal cells and the vitreal-negative wave unmasked by APB could be matched by scaling at low light intensities (<5 Rh*). However, horizontal cell responses saturated at about 30 Rh*, while the vitreal-negative wave continued to increase in amplitude. The time course of horizontal cell membrane current with dim flashes could be matched to the rising phase of the vitreal-negative wave, assuming that the delay in generating the voltage response in horizontal cells is due to their long (100 ms) membrane time constant. Blocking post-photoreceptor activity resulted in a much smaller vitreal-negative wave than that unmasked by APB alone. We conclude that the b-wave arises from on-bipolar cell depolarization, while the leading edge of the a-wave is a composite of the change in extracellular voltage drop across the rod layer and a component (proximal PIII) reflecting a decrease in extracellular K+ as horizontal cell synaptic channels close with light.
Shiells
R A
RA
Physiology Department, University College, London, UK.
Falk
G
G
D000818:Animals; D003623:Dark Adaptation; D004284:Dogfish; D004596:Electroretinography; D008027:Light; D012160:Retina / Q000166:cytology / Q000502:physiology* / Q000528:radiation effects; D017948:Retinal Rod Photoreceptor Cells / Q000502:physiology* / Q000528:radiation effects; D014822:Vitreous Body / Q000502:physiology / Q000528:radiation effects