LIGHT-INDUCED MODULATION OF COUPLING BETWEEN AII AMACRINE CELLS IN THE RABBIT RETINA

The rod-driven, AII amacrine cells in the mammalian retina maintain ho mologous gap junctions with one another as well as heterologous gap ju nctions with on-cone bipolar cells. We used background illumination to study whether changes in the adaptational state of the retina affecte d the permeabilities of these two sets of gap junctions. To access cha nges in permeability, we injected single AII amacrine cells with the b iotinylated tracer, Neurobiotin, and measured the extent of tracer cou pling to neighboring AII cells and neighboring cone bipolar cells. We also measured the center-receptive field size of AII cells to assess c oncomitant changes in electrical coupling. Our results indicate that i n well dark-adapted retinas, AII cells form relatively small networks averaging 20 amacrine cells and covering about 75 mu m. The size of th ese networks matched closely to the size of An cell on-center receptiv e fields. However, over most of their operating range, AII cells forme d dramatically larger networks, averaging 326 amacrine cells, which co rresponded to an increased receptive-field size. As the retina was lig ht adapted beyond the operating range of the AII cells, they uncoupled to form networks comparable in size to those seen in well dark-adapte d retinas. Our results, then, indicate that the adaptational state of the retina has a profound effect on the extent of electrical coupling between AII amacrine cells. Although we observed light-induced changes in the number of tracer-coupled cone bipolar cells, these appeared to be an epiphenomenon of changes in homologous coupling between An amac rine cells. Therefore, in contrast to the robust changes in AII-AII co upling produced by background illumination, our data provided no evide nce of a light-induced modulation of coupling between AII cells and on -cone bipolar cells.