IMMUNOCYTOCHEMICAL LOCALIZATION OF THE SYNAPSE-ASSOCIATED PROTEIN SAP102 IN THE RAT RETINA

An elaborate network of transmitter receptors, synapse associated prot eins (SAPs), and cytoskeletal elements, generally known as the postsyn aptic density, is involved with efficient synaptic signaling. The loca lization of the synapse associated protein SAP102 was studied in the r at retina by using immunocytochemical methods. Immunofluorescence for SAP102 was most prominent in the inner plexiform layer (IPL). It had a punctate appearance, suggesting a synaptic clustering of SAP102 in th e IPL. Electron microscopy by use of pre-embedding immunocytochemistry showed that SAP102 is concentrated in the IPL in processes which are postsynaptic at bipolar cell ribbon synapses (dyads).As a rule, only o ne of the two postsynaptic members of the dyad was labeled for SAP102. Double-labeling experiments were performed in order to find out wheth er SAP102 is involved with the clustering the N-methyl-D-aspartate (NM DA) receptor 2A subunit (NR2A). Only a fraction (approximately 23%) of the SAP102 clusters expressed NR2A, suggesting SAP102 is also associa ted with other subunits or receptors. Distinct SAP102 labeling was als o present in horizontal cell processes in the outer plexiform layer (O PL), which are inserted as lateral elements into photoreceptor ribbon synapses (triads). The optic nerve fibre layer was also diffusely immu noreactive for SAP102. The postsynaptic aggregation of SAP102 at bipol ar cell dyads and at photoreceptor triads suggests SAP102 is associate d with the clustering of transmitter receptors. However, the labeling of the optic nerve fibre layer indicates additional functions of SAP10 2 in the retina. J. Comp. Neurol. 397:326-336, 1998. (C) 1998 Wiley-Li ss, Inc.