Synaptic clustering of GABAC receptor ρ‐subunits in the rat retina

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Abstract
Polyclonal antibodies which recognize the ρ-subunits of the GABAC 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 GABAC 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 GABAC receptor ρ-subunits coassemble with GABAA receptor subunits, double-labelling experiments were performed with subunit specific antisera. Punctate, putative synaptic clustering was observed with all antisera applied, however, GABAC receptor expressing puncta did not coincide with GABAA receptor containing puncta. This suggests that there are no synaptic GABA receptors in the retina in which GABAA and GABAC receptor subunits are coassembled. Similar double-labelling experiments were also performed to find out whether GABAC receptors and glycine receptors are colocalized. They were clustered at different synapses. This suggests that synaptic GABAC receptors consist of ρ-subunits and are not coassembled with GABAA- or glycine-receptor subunits.

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