In the rabbit retina, there are two types of horizontal cell (HC). The A-type HC is axonless and extensively coupled. The B-type HC is axon bearing; the somatic dendrites are radially symmetric and form a second coupled network, while the axon branches expansively to form a complex terminal structure. The B-type axon terminals (ATs) are independently coupled to form a third network in the outer plexiform layer. We have modified our dye-injection methods to obtain detailed fills of the three different horizontal cell networks for analysis via confocal microscopy. We have confirmed that A-type HCs and the somatic dendrites of B-type HCs receive input exclusively from cones, whereas the B-type ATs receive input only from rods. A single B-type AT may receive input from as many as 1,000 rods, but, surprisingly, our data reveal only one end terminal per rod spherule. The somatic dendrites of A- and B-type HCs form clusters at each cone pedicle coincident with GluR2/3 and GluR4 glutamate receptor subunits. The B-type ATs have GluR2/3- or GluR4-labeled glutamate receptors in two locations: small puncta on the end terminals within the rod spherule invagination and large clusters on the terminal stalks, approximately 1.5 microm from the rod synaptic ribbon. We conclude that AMPA receptors of the same or similar composition mediate photoreceptor input to all types of HCs.
2006 Wiley-Liss, Inc.