Structure of GABAB receptors in rat retina

In the search for yet unknown subtypes of GABA(B) receptors, the subunit architecture of GABAB receptors in the retina was analyzed using selective antisera. Immunopurification of the splice variants GABA(B1a) and GABA(B1b) demonstrated that both were associated with GABA(B2). Quantitative immunoprecipitation experiments indicated that practical the entire GABA(B) receptor population in the retina consists of the receptor subtypes GABA(B1a)/GABA(B2) and GABA(B1b)/GABA(B2), although low levels of GABA(B1c)/GABA(B2) cannot be excluded. The data rule out the existence of GABA(B) receptors containing the splice variants GABA(B1d) and GABA(B1e). Moreover, no evidence for homomeric GABA(B1) receptors was found. Among the splice variants, GABA(B1a) is by far the predominant one in neonatal and adult retina, whereas GABA(B1b) is expressed only late in postnatal development and in the adult retina. Since GABA(B1a) is expressed at high levels before functional synapses are formed, this specific receptor subtype might be involved in the maturation of the retina. Finally, subcellular fractionation demonstrated that GABA(B1a), but not GABA(B1b), is present in postsynaptic densities, suggesting a differential pre- and postsynaptic localisation of both splice variants.