IMMUNOCYTOCHEMICAL LOCALIZATION OF THE POSTSYNAPTIC DENSITY PROTEIN PSD-95 IN THE MAMMALIAN RETINA

Synapse-associated proteins are the scaffold for the selective aggrega tion of ion channels at synapses; they provide the link to cytoskeleta l elements and possibly are involved with the regulation of synaptic e fficacy by electrical activity. The localization of the postsynaptic d ensity protein PSD-95 was studied in different mammalian retinae (rat, monkey, and tree shrew) by using immunocytochemical methods. Immunofl uorescence for PSD-95 was most prominent in the outer plexiform layer (OPL). The axon terminals of rods and cones, the rod spherules and con e pedicles, were strongly labeled. Electron microscopy, using preembed ding immunocytochemistry, showed PSD-95 localized presynaptically with in the photoreceptor terminals. Distinct PSD-95 labeling was also pres ent in the inner plexiform layer (IPL). It had a punctate appearance s uggesting the synaptic clustering of PSD-95 in the IPL. Electron micro scopy showed that PSD-95 was concentrated in processes that were posts ynaptic at bipolar cell ribbon synapses (dyads). As a rule, only one o f the two postsynaptic members of the dyad was labeled for PSD-95. Dou ble-labeling experiments were performed for PSD-95 and for SAP 102 or PSD-93, respectively, two other members of the family of synapse-assoc iated proteins. All three were found to be colocalized in the synaptic hot spots in the IPL. In the OPL, however, PSD-95 and PSD-93 were fou nd presynaptically, whereas SAP 102 was located postsynaptically at ph otoreceptor synapses. Double-labeling experiments also were performed for PSD-95 and for the NR1 subunit of the NMDA receptor. They were fou nd to be colocalized in synaptic hot spots in the IPL.