Synaptic clustering of GABA C receptor ρ‐subunits in the rat retina

Polyclonal antibodies which recognize the ρ‐subunits of the GABA C receptor were applied to sections of the rat retina. Strong punctate immunoreactivity was found in the inner plexiform layer (IPL), which was shown by electron microscopy to represent a clustering of the GABA C receptors at synaptic sites. During postnatal development diffuse ρ‐immunoreactivity was first observed at postnatal day P3. Distinct labelling of bipolar cells appeared at P7 and punctate, synaptic labelling was observed at P10. In order to show that the ρ‐immunoreactive puncta coincide with the axons of bipolar cells, double immunostainings of retinal sections with an antiserum against syntaxin 3 and with the ρ‐antiserum were performed. The experiments showed that ρ‐immunoreactive puncta are preferentially located on the axon terminals of rod and cone bipolar cells. In order to determine whether GABA C receptor ρ‐subunits coassemble with GABA A receptor subunits, double‐labelling experiments were performed with subunit specific antisera. Punctate, putative synaptic clustering was observed with all antisera applied, however, GABA C receptor expressing puncta did not coincide with GABA A receptor containing puncta. This suggests that there are no synaptic GABA receptors in the retina in which GABA A and GABA C receptor subunits are coassembled. Similar double‐labelling experiments were also performed to find out whether GABA C receptors and glycine receptors are colocalized. They were clustered at different synapses. This suggests that synaptic GABA C receptors consist of ρ‐subunits and are not coassembled with GABA A ‐ or glycine‐receptor subunits.