SYNAPTIC CIRCUITRY OF NEUROPEPTIDE-CONTAINING AMACRINE CELLS IN THE RETINA OF THE CANE TOAD, BUFO-MARINUS

Synaptic connections of amacrine cells with substance P-like or neurop eptide Y-like immunoreactivity (SP-LI or NPY-LI) in the retina of the cane toad, Bufo marinus, were investigated using ultrastructural immun ocytochemistry. The perikarya of SP-LI or NPY-LI amacrine cells were l ocated in the innermost row of the inner nuclear layer. The synapses a ssociated with SP-LI amacrine cells were distributed mainly in sublami nae 3 and 4 with about 10% in sublamina 1 of the inner plexiform layer . The synapses formed by NPY-LI amacrine cells were found in sublamina e 1, 2, and 4 with approximately equal frequency. Of a total of 175 SP -LI profiles, 56% were in presynaptic positions and 44% in postsynapti c positions. The synaptic inputs to SP-LI profiles predominantly deriv ed from other unlabeled amacrine cell dendrites, and to a lesser exten t, from bipolar cell terminals. The majority of synaptic outputs from SP-LI amacrine cell dendrites were directed onto unlabeled amacrine ce ll processes. The SP-LI profiles also made synapses onto bipolar cell terminals and formed synapses onto presumed ganglion cell dendrites. O f a total of 200 NPY-LI profiles, 48% were in presynaptic positions an d 52% in postsynaptic positions. The profiles of NPY-LI amacrine cells mainly received their synaptic inputs from other unlabeled amacrine c ell processes, and to a lesser extent, from bipolar cell terminals. Th e majority of NPY-LI amacrine cell profiles gave their synaptic output s onto unlabeled amacrine cell dendrites, and others formed synapses o nto presumed ganglion cell processes. These results suggest that these two populations of neuropeptide-containing amacrine cells in the Bufo retina are involved in different synaptic circuits.