Hyperpolarizing, small-field, amacrine cells in cone pathways of cat retina

Intracellular recording and horseradish peroxidase (HRP) staining of amacrine cells in the isolated arterially perfused cat retina have revealed examples of small-field cells that hyperpolarize to light. Two were examined in detailed electron microscopic reconstructions to determine patterns of synaptic relationships within the inner plexiform layer (IPL). The cells were morphologically similar to A8 and A13 types as described in Golgi-impregnated material (Kolb et al. [1981] Vision Res. 21:1081-1114). Both types received ribbon synaptic input from rod and cone bipolar cells. The latter input was numerically predominant, occurred in both a and b sublaminae of the IPL, and arose from at least three cone bipolar types. Reciprocal synapses were evident between A13 cells and cone bipolar cells. Amacrine input occurred throughout the dendritic tree of both A8 and A13 types, and numerically exceeded bipolar cell input for A13. Gap junctions between stained, and similar-appearing unstained dendritic profiles were observed for both amacrine types. In addition, A8 engaged in gap junctions with cone bipolar profiles in sublamina b which also provided ribbon input. Synaptic output for both amacrine types occurred primarily upon amacrine and ganglion cells in sublamina a. Both cells were presynaptic upon single OFF-center beta ganglion cells running through the middle of their dendritic trees. Mixtures of rod and cone signals were found in the centrally evoked hyperpolarizations of each type. Center mechanism space constants of such types ranged from 100 to 400 microns, with antagonistic surround in 1 of 5 cases. Dopamine (250 microM) reduced receptive field space constants by one-third in one case. The synaptic organization and potential circuitry implications of these cone system-dominated amacrine types are compared and contrasted to the better-known AII and A17 types previously described for the rod system.