Functional segregation of gap junctionally connected amacrine cells in the inner plexiform layer

Purpose. In amacrine cells (ACs), three major physiological classes: i.e transient ON-OFF sustained depolarizing and sustained hyperpplarizing, are known. ACs of the same class are interconnected by gap junctions with either tip-contacts or crossing-contacts. The receptive fiefd of ACs with tipcontacts differs from those with crossing-contacts. Thus each physiological class is classified into at least two separate functional types. It's possible that the homotypic connections between ACs of the same type perform the segregation of electrically coupled networks in the inner plexiform layer (IrL). To evaluate functional roles of gap junctiona) networks between ACs, 1) neu retransmitter contents were investigated with anti-GABA antiserum and 2) chemical synaptic contacts were analyzed by electron microscopy. Methods. Intracellular recordings were obtained from ACs in the isolated retina of Japanese dace Tribolodqn hakpnensis). Following ejectrophysiolpgical measures, cell;; were injected with Lucifer Yellow ana either bipcytin or Neurobiotin. Intracellular labeling was revealed histochemicaHy and retinas were then processed for electron microscopic observation. For GABA irnmunorea-tivity, retinas were incubated in avidinrhodamine to reveal biotin-labeling, frozen sectioned, incubated with rabbit anti-GABA and then treated with goat FITC-conjugated anti-rabbit. Sections were examined by laser scanning confocal microscopy using a HeNe or Argon laser. Results and Conclusions. The present study confirmed that each of physiological types constructed discrete gap junctional networks in individual sublayers of the IPL. Physiologically identified ACs with homotypic gap junctions were all GABA-immunopositive. ACs with tipcontacts made conventional synaptic outputs onto ganglion cells and other amacrine cells, while ACs with crossing-contacts made reciprocal synapses onto bipolar cell axons. These results suggest that gap junctional networks consisting of ACs of the samt; physiological type are functionally segregated in the IPL. Homotypic gap junctions between ACs may extend inhibitory synapses in functional sun-layers of the IPL. None.