We studied the light-evoked responses of AII amacrine
cells in the rabbit retina under dark- and light-adapted
conditions. In contrast to the results of previous studies,
we found that AII cells display robust responses to light
over a 6–7 log unit intensity range, well beyond
the operating range of rod photoreceptors. Under dark adaptation,
AII cells showed an ON-center/OFF-surround receptive-field
organization. The intensity–response profile of the
center-mediated response component followed a dual-limbed
sigmoidal function indicating a transition from rod to
cone mediation as stimulus intensities were increased.
Following light adaptation, the receptive-field organization
of AII cells changed dramatically. Light-adapted AII cells
showed both ON- and OFF-responses to stimulation of the
center receptive field, but we found no evidence for an
antagonistic surround. Interestingly, the OFF-center response
appeared first following rapid light adaptation and was
then replaced gradually over a 1–4 min period by
the emerging ON-center response component. Application
of the metabotropic glutamate receptor agonist APB, the
ionotropic glutamate blocker CNQX, 8-bromo-cGMP, and the
nitric oxide donor SNAP all showed differential effects
on the various center-mediated responses displayed by dark-
and light-adapted AII cells. Taken together, these pharmacological
results indicated that different synaptic circuits are
responsible for the generation of the different AII cell
responses. Specifically, the rod-driven ON-center responses
are apparently derived from rod bipolar cell synaptic inputs,
whereas the cone-driven ON-center responses arise from
signals crossing the gap junctions between AII cells and
ON-center cone bipolar cells. Additionally, the OFF-center
response of light-adapted AII cells reflects direct synaptic
inputs from OFF-center cone bipolar cells to AII dendritic
processes in the distal inner plexiform layer.
Light Adaptation/Acclimation of Photosynthesis and the Regulation of Ribulose-1,5-Bisphosphate Carboxylase Activity in Sun and Shade Plants
