The superfused retinal slice preparation was used to examine
the morphology and glutamate-activated whole-cell currents of
rabbit bipolar cells. There were six morphologically distinct
types of cone bipolar cells and a rod bipolar cell that had
axon terminals stratifying in stratum 3 to 5 of
sublamina-b. All of these bipolar cell types exhibited
an outward current in response to the application of the
metabotropic glutamate receptor, mGluR6, agonist AP-4 (APB),
and had I/V curves indicative of membrane channel closure.
Conversely, there were no currents activated during the application
of kainate, the AMPA/kainate receptor agonist. These data
demonstrate they were on-bipolar cells. In addition, there were
six morphologically distinct cone bipolar cells that stratified
in sublamina-a. Every cell with axonal arborizations
in stratum 1 and 2 exhibited an inward current when the ionotropic
glutamate receptor agonist kainate was applied. This current
was blocked by application of the AMPA/kainate receptor antagonist
CNQX. These cells also decreased their membrane resistance in
response to kainate, a characteristic of the opening of channels
within the plasma membrane. Without exception, no cells stratifying
in sublamina-a responded to the mGluR6 agonist AP-4,
further identifying them as off-bipolar cells.
