LOCALIZATION OF GABA-A RECEPTOR SUBTYPES IN THE TIGER SALAMANDER RETINA

Dry autoradiography was used to determine the distribution of GABA(A) binding sites in tiger salamander retina. High-affinity binding of [H-3]-flunitrazepam ([H-3]-FNZ) was used to localize benzodiazepine receptors (BZR) and [H-3]-muscimol was used to localize the GABA(A) recognition site. Specific [H-3]-FNZ binding was present only in the inner retina, primarily in the inner plexiform layer (IPL). Co-incubation with GABA enhanced [H-3]-FNZ binding by 20-50%. [H-3]-muscimol binding was found throughout the IPL and in the outer plexiform layer (OPL). MOuse monoclonal antibodies 62-3G1 and BD-17, that recognize the GABA(A)beta(2),beta(3) polypeptides, and BD-24, that recognizes the GABA(A)alpha(1) polypeptide, did not label either the OPL or IPL, despite numerous variations in the fixation and immunoprocessing methods. GABA(A) receptor location, as revealed by [H-3]-muscimol binding, matches the distribution of presumed GABAergic terminals in the OPL and IPL. We suggest that there are at least two subtypes of GABA(A) receptor in the tiger salamander retina: one type is present only in the inner retina, primarily in the IPL and is functionally coupled to BZRs; the other type is located in the OPL and is not coupled to the BZRs. Furthermore, GABA(A) receptors in the tiger salamander retina appear to be of a different epitope than GABA(A) receptors in numerous other preparations that are recognized by mAbs 62-3G1, BD-17, and BD-24.