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 μM 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.
Spontaneous Slow Potential Fluctuations in the Limulus Photoreceptor
