Selective synaptic distribution of AMPA and kainate receptor subunits in the outer plexiform layer of the carp retina

The subunit composition of ionotropic glutamate receptors (GluRs) is extrem ely diverse and responsible for the diversity of postsynaptic responses to the release of glutamate, which is the major excitatory neurotransmitter in the retina. To understand the functional consequences of this diversity, i t is necessary to reveal the synaptic localization and subunit composition of GluRs. We have used immune light and electron microscopy to localize AMP A and kainate (GluR1, GluR2/3, GluR4, GluR5-7) subunits in identified carp retinal neurons contributing to the outer plexiform layer. GluR1 could not be detected within the outer plexiform layer. Rod and cone horizontal cells all express only GluR2/3 at the tips of their invaginating dendrites. Thes e receptors are also inserted into the membrane of spinules, light-dependen t protrusions of the horizontal cell dendrites, flanking the synaptic ribbo n of the cone synapse. Bipolar cells express GluR2/3, GluR4, and GluR5-7 at their terminal dendrites invaginating cone pedicles and rod spherules. Col ocalization data suggest that each subunit is expressed by a distinct bipol ar cell type. The majority of bipolar cells expressing these receptors seem to be of the functional OFF-type; however, in a few instances, GluR2/3 cou ld also be detected on dendrites of bipolar cells that, based on their loca lization within the cone synaptic complex, appeared to be of the functional ON-type. The spatial arrangement, of the different subunits within the cav ity of the cone pedicle appeared not to be random: GluR2/3 was found predom inantly at the apex of the cavity, GluR4 at its base and GluR5-7 dispersed between the two. (C) 2001 Wiley-Liss, Inc.