PRESYNAPTIC AND POSTSYNAPTIC LOCALIZATION OF GABA(B) RECEPTORS IN NEURONS OF THE RAT RETINA

The recently cloned GABAB receptors were localized in rat retina using specific antisera. Immunolabelling was detected in the inner and oute r plexiform layers (IPL, OPL), and in a number of cells in the inner n uclear layer and the ganglion cell layer. Double-labelling experiments for GABA (gamma-aminobutyric acid) and GABA(B) receptors, respectivel y, demonstrated a co-localization in horizontal cells and amacrine cel ls. Electron microscopy showed that GABA(B) receptors of the OPL were localized presynaptically in horizontal cell processes invaginating in to photoreceptor terminals. In the IPL, GABA(B) receprors were present presynaptically in amacrine cells, as well as postsynaptically in ama crine and ganglion cells. The postnatal development of GABA(B) recepto rs was also studied, and immunoreactivity was observed well before mor phological and synaptic differentiation of retinal neurons. The presen t results suggest a presynaptic (autoreceptor) as well as postsynaptic role for GABA(B) receptors. In addition, the extrasynaptic localizati on of GABA(B) receptors could indicate a paracrine function of GABA in the retina.