Coupling from AII amacrine cells to ON cone bipolar cells is bidirectional

The AII amacrine cell is a critical interneuron in the rod pathway of the mammalian retina. Rod signals pass into cone pathways by means of gap junctions between AII amacrine cells and ON cone bipolar cells. Filling AII amacrine cells with Neurobiotin produces labeling of cone bipolar cells by means of these gap junctions. However, tracer injections into bipolar cells do not produce labeling of the AII network (Vaney [1997] Invest Ophthalmol Vis Sci. 38:267–273), which suggests that the AII/bipolar gap junctions allow the passage of tracer in only one direction. This mechanism stands in contrast to physiological results, which indicate that light adapted signals can pass from ON cone bipolar cells into the AII network (Xin and Bloomfield [1999] Vis Neurosci. 16:653–665). Here, we report that a variety of ON and OFF bipolar cells are sometimes anomalously coupled to the A‐type horizontal cell network. These relatively rare examples do not result from dye injection errors, but seem to represent minor developmental errors. However, this provides a method to obtain Neurobiotin‐filled cone bipolar cells without the necessity of impaling them with a microelectrode. Under these conditions, Neurobiotin spreads from ON cone bipolar cells into neighboring AII amacrine cells. The dye‐coupled AII amacrine cells, positively identified by double labeling with an antibody against calretinin, were centered around anomalously coupled ON bipolar cells. These results indicate that AII/bipolar cell gap junctions allow tracer coupling in both directions, consistent with previous physiological results. The previous failure to detect the passage of neuronal tracer from injected bipolar cells to AII amacrine cells may reflect electrode damage or perhaps the asymmetrical voltage sensitivity of a heterotypic gap junction. J. Comp. Neurol. 437:408–422, 2001. © 2001 Wiley‐Liss, Inc.