Morphology and physiology of the polyaxonal amacrine cells in the rabbit retina

We examined the morphology and physiological response properties of the axo n-bearing, long-range amacrine cells in the rabbit retina. These so-called polyaxonal amacrine cells all displayed two distinct systems of processes: (1) a dendritic field composed of highly branched and relatively thick proc esses and (2) a more extended, often sparsely branched axonal arbor derived from multiple thin axons emitted from the soma or dendritic branches. Howe ver, we distinguished six morphological types of polyaxonal cells based on differences in the fine details of their soma/dendritic/axonal architecture , level of stratification within the inner plexiform layer (IPL), and trace r coupling patterns. These morphological types also showed clear difference s in their light-evoked response activity. Three of the polyaxonal amacrine cell types showed on-off responses, whereas the remaining cells showed on- center responses; we did not encounter polyaxonal cells with off-center phy siology. Polyaxonal cells respected the on/off sublamination scheme in that on-off cells maintained dendritic/axonal processes in both sublamina a and b of the IPL, whereas processes of on-center cells were restricted to subl amina b. All polyaxonal amacrine cell types displayed large somatic action potentials, but we found no evidence for low-amplitude dendritic spikes tha t have been reported for other classes of amacrine cell. The center-recepti ve fields of the polyaxonal cells were comparable to the diameter of their respective dendritic arbors and, thus, were significantly smaller than thei r extensive axonal fields. This correspondence between receptive and dendri tic field size was seen even for cells showing extensive homotypic and/or h eterotypic tracer coupling to neighboring neurons. These data suggest that all polyaxonal amacrine cells are polarized functionally into receptive den dritic and transmitting axonal zones. (C) 2001 Wiley-Liss, Inc.