Dendritic Co‐stratification of ON and ON‐OFF directionally selective ganglion cells with starburst amacrine cells in rabbit retina

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Abstract
The morphology, dendritic branching patterns, and dendritic stratification of retinal ganglion cells have been studied in Golgi‐impregnated, whole‐mount preparations of rabbit retina. Among a large number of morphological types identified, two have been found that correspond to the morphology of ON and ON‐OFF directionally selective (DS) ganglion cells identified in other studies. These cells have been characterized in the preceeding paper in terms of their cell body size, dendritic field size, and branching pattern. In this paper, the two kinds of DS ganglion cell are compared in terms of their levels of dendritic stratification. They are compared with each other and also with examples of class III.1 cells, defined in the preceding paper with reference to our previous studies.Studies employing computer‐aided, 3D reconstruction of dendritic trees, as well as analysis of a pair of ON DS and ON‐OFF DS ganglion cells with overlapping dendritic trees show that the two types of DS ganglion cell partly co‐stratify in the middle of sublamina b (stratum 4). The report that some ON DS ganglion cells extend a few dendrites into sublamina a is confirmed. The study of pairs of ON‐OFF DS ganglion cells and starburst amacrine cells with overlapping dendritic trees reveals a precise co‐stratification of these two cell types, and many points of close apposition of starburst boutons with ON‐OFF DS ganglion cell dendrites in both sublaminae of the inner plexiform layer (IPL). This is confirmed by high‐resolution light microscopy and by electron microscopy. It is possible to conclude, therefore, that ON DS are also partly co‐stratified with type b starburst (cholinergic) amacrine cells, and are apparently also partly co‐stratified with type a starburst amacrine cells, when occasional dendrites rise to that level. The co‐stratification of the two kinds of DS ganglion cell is consistent with the sharing of some inputs in common, including some cone bipolar cell inputs. The co‐stratification of both with starburst amacrine cells agrees with the physiological demonstration of the powerful pharmacological effects upon ON and ON‐OFF DS ganglion cells reported for cholinergic agonists.The major difference in the dendritic stratification of bistratified ON‐OFF DS ganglion cells and generally unistratified ON DS ganglion cells is consistent with the bisublaminar organization of ON and OFF pathways in the IPL. The problem of occasional branches of ON DS cells in sublamina a is discussed in terms of a threshold for OFF responses. The slight difference in stratification between ON and ON‐OFF DS ganglion cells in sublamina b and lack of dendritic fasciculation of the two in the zone of overlap is interpreted to mean that ON DS cells receive less input from starburst/cholinergic neurons that ON‐OFF DS cells. These differences in the course of dendrites are consistent with a difference in inputs, particularly in selective GABAergic pathways likely to account for their notable physiological differences in the two sets of preferred directions and in two ranges of velocity preference.