Synaptic distribution of ionotropic glutamate receptors in the inner plexiform layer of the primate retina.

Abstract

The distribution and synaptic clustering of glutamate receptors (GluRs) were studied in the inner plexiform layer (IPL) of the macaque monkey retina by using subunit specific antisera. A punctate immunofluorescence pattern was observed in the IPL for all subunits tested, and electron microscopy confirmed that the immunoreactive puncta represent clustering of receptors at sites postsynaptic to the bipolar cell ribbon synapses (dyads). Usually only one of the two postsynaptic processes at the dyads expressed a given subunit. Immunoreactive GluR2, GluR2/3, and GluR4 puncta were found at high density throughout the IPL and are probably expressed at every dyad. The GluR1 subunit was expressed at lower density. The N-methyl-D-aspartate (NMDA) receptor subunits NR2A and NR1C2' were restricted to synapses localized in two broad bands in the center of the IPL. They were often colocalized with GluR2/3 and GluR4 subunits. The orphan receptor subunits delta 1/2 predominated in three horizontal bands. The kainate receptor subunits GluR6/7 were clustered in large postsynaptic densities adjacent to bipolar cell axon terminals but lacking a synaptic ribbon on the presynaptic side. This might represent a conventional synapse made by a bipolar axon terminal. The results suggest that GluR2/3 and GluR4, together with NMDA receptors, are preferentially expressed on ganglion cell dendrites, whereas kainate receptors and the delta 1/2 subunits are mostly localized on amacrine cell processes.