COLOCALIZATION OF GEPHYRIN AND GABA(A)-RECEPTOR SUBUNITS IN THE RAT RETINA

Gephyrin is a protein that copurifies with the glycine receptor (GlyR) and is required for the clustering of GlyRs at postsynaptic sites. Pr eviously, it was thought that antibody mAb 7a, directed against gephyr in, was a specific marker for GlyR. However, there is evidence that ge phyrin can also be found at nonglycinergic synapses. Here, immunocytoc hemistry was applied to show this directly for the rat retina. Both ge phyrin and different subunits of the gamma-aminobutyric acid (GABA)(A) receptor were localized to discrete puncta in the inner plexiform lay er, and these puncta were shown by electron microscopy to represent sy naptic sites. Double immunocytochemistry revealed that GABA(A) recepto rs and GlyRs are not colocalized. However, gephyrin and different subu nits of GABA(A) receptors were found to occur at the same synapses. Th e amount of colocalization varied with the GABA(A) receptor subunit co mposition and was most extensive for the alpha 2 subunit, less for the alpha 3 subunit, and minimal for the alpha 1 subunit. The gephyrin pr esent at GABAergic synapses of the retina might also be involved with clustering of receptors at the postsynaptic sites. Hence, localization of gephyrin can no longer be considered as a unique marker of glycine rgic synapses. (C) 1995 Wiley-Liss, Inc.