Synaptic circuitries of two types of ON–cone bipolar cells in the rabbit retina

Abstract

Abstract: : Purpose: To elucidate the synaptic circuitries of two types of ON–cone bipolar cells in the rabbit retina, as a part of our efforts to extend the knowledge about the function and role of the cone bipolar cell in visual processing. Methods: Using immunocytochemistry and electron microscopy with two specific ON–bipolar cell markers, antibodies against calbindin and neurokinin 1 (NK1) receptor, the synaptic connections of two types of ON–cone bipolar cells in the rabbit retina were examined. Results: Axons of both calbindin–immunoreactive (IR) and NK1 receptor–IR ON–cone bipolar cells ramified in sublamina b of the inner plexiform layer (IPL). Both bipolar axon terminals were located between rod bipolar terminals and ON–type starburst amacrine processes. The NK1 receptor–IR bipolar cell axon terminals stratified distally to those of calbindin–IR bipolar cell and stratified within and slightly more proximally to the ON–plexus of starburst cells. Axons of both bipolar cells received synaptic inputs from a heterogeneous group of amacrine cells through conventional synapses, and from AII amacrine cells via gap junctions. Gap junctions were found more frequently in the calbindin–IR bipolar cells than in the NK1 receptor–IR bipolar cells. The major output from both bipolar cell types was onto amacrine cell processes. The most frequent postsynaptic dyads comprised an amacrine cell process and a ganglion cell dendrite, and two amacrine cell processes, in calbindin–IR and NK1 receptor–IR bipolar cells, respectively. Conclusions: The calbindin–IR and the NK1 receptor–IR cone bipolar cells may play an important role in transferring the rod signal to the ON–ganglion cell of the cone pathway in the rabbit retina. In addition, NK1–IR bipolar cells may provide synaptic inputs onto the ON–starburst cells and/or the ON–plexus of the ON–OFF direction–selective ganglion cells.

Keywords: retina: proximal (bipolar, amacrine, and ganglion cells) • retinal connections, networks, circuitry • microscopy: electron microscopy