In mammals, gap junctions between retinal bipolar
cells are generally small and tracer coupling has not been
previously demonstrated. In this study, Neurobiotin was
injected into the Ba3-type cone bipolar cell, a medium-field
cone bipolar cell that ramifies in sublamina a
of the rabbit retina. Tracer spread to many other Ba3 bipolar
cells, presumably through gap junctions. It also spread
to a smaller field bipolar cell called the Ba1 that ramifies
at the same depth of the inner plexiform layer. Injection
of Neurobiotin into Ba1 bipolar cells did not produce staining
beyond the injected cell. Tracer coupling from the Ba3
was therefore both heterologous, in that different cell
types were stained, and asymmetric. The unusual properties
of this bipolar cell suggest that its function may differ
from that of most cone bipolar cells, which are narrow-field,
do not overlap, and are poorly coupled to one another.