The synaptic connections of the narrow-field, bistratified rod amacrine cell (AII) in the inner plexiform layer (IPL) of the rabbit retina were reconstructed from electron micrographs of continuous series of thin sections. The AII amacrine cell receives a large synaptic input from the axonal endings of rod bipolar cells in the most vitreal region of the IPL (sublamina b, S5) and a smaller input from axonal endings of cone bipolar cells in the scleral region of the IPL (sublamina a, S1-S2). Amacrine input, localized at multiple levels in the IPL, equals the total number of synapses received from bipolar cells. The axonal endings of cone bipolar cells represent the major target for the chemical output of the AII amacrine cell: these synapses are established by the lobular appendages in sublamina a (S1-S2). Ganglion cell dendrites represent only 4% of the output of the AII amacrine and most of them are also postsynaptic to the cone bipolars which receive AII input. The AII amacrine is not presynaptic to other amacrine cells. Finally, the AII amacrine makes gap junctions with the axonal arborizations of cone bipolars that stratify in sublamina b (S3-S4) as well as with other AII amacrine cells in S5. Therefore, in the rabbit retina 1) the rod pathway consists of five neurons arranged in series: rod-->rod bipolar-->AII amacrine-->cone bipolar-->ganglion cell; 2) it seems unlikely that a class of ganglion cells exists that is exclusively devoted to scotopic functions. In ventral, midperipheral retina, about nine rod bipolar cells converge onto a single AII amacrine, but one of them establishes a much higher proportion of synaptic contacts than the rest. Conversely, each rod bipolar cell diverges onto four AII amacrine cells, but one of them receives the largest fraction of synapses. Thus, within the pattern of convergence and divergence suggested by population studies, preferential synaptic pathways are established.

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We have reconstructed from electron micrographs of a continuous series of thin sections the synaptic connections of the axonal arborizations of all the rod bipolar cells contained in a small region of the retina of the rabbit. We observed that all rod bipolars share the same pattern of connectivity and are probably functionally equivalent. As a rule, they do not contact ganglion cells. Their prevalent synaptic output is on narrow-field, bistratified, and indoleamine-accumulating amacrine cells. Their dominant inputs are the reciprocal synapses from the indoleamine-accumulating amacrines, but they also receive a sizable number of synaptic contacts from other, non-reciprocal, amacrine cells. The lateral spread of scotopic signals at the synapse between rod bipolars and narrow-field, bistratified amacrines is small. Finally, in the rabbit, as in the cat, a narrow-field, bistratified amacrine is inserted in series along the rod pathway.

Amacrine cells of the rabbit retina were studied by ''photofilling,'' a photochemical method in which a fluorescent product is created within an individual cell by focal irradiation of the nucleus; and by Golgi impregnation. The photofilling method is quantitative, allowing an estimate of the frequency of the cells. The Golgi method shows their morphology in better detail. The photofilled sample consisted of 261 cells that were imaged digitally in throughfocus series from a previous study (MacNeil and Masland [1998] Neuron 20:971-982). The Golgi material consisted of 49 retinas that were stained as wholemounts. Eleven of these subsequently were cut in vertical section. Of the many hundreds of cells stained, digital through-focus series were recorded for 208 of the Golgi-impregnated cells. The two methods were found to confirm one another: Most cells revealed by photofilling were recognized easily by Golgi staining, and vice versa. The greater resolution of the Golgi method allowed a more precise description of the cells and several types of amacrine cell were redefined. Two new types were identified. The two methods, taken together, provide an essentially complete accounting of the populations of amacrine cells present in the rabbit retina. Many of them correspond to amacrine cells that have been described in other mammalian species, and these homologies are reviewed.