Structure of GABAB receptors in rat retina

In the search for yet unknown subtypes of GABAB receptors, the subunit architecture of GABAB receptors in the retina was analyzed using selective antisera. Immunopurification of the splice variants GABAB1a and GABAB1b demonstrated that both were associated with GABAB2. Quantitative immunoprecipitation experiments indicated that practical the entire GABAB receptor population in the retina consists of the receptor subtypes GABAB1a/GABAB2 and GABAB1b/GABAB2, although low levels of GABAB1c/GABAB2 cannot be excluded. The data rule out the existence of GABAB receptors containing the splice variants GABAB1d and GABAB1e. Moreover, no evidence for homomeric GABAB1 receptors was found. Among the splice variants, GABAB1a is by far the predominant one in neonatal and adult retina, whereas GABAB1b is expressed only late in postnatal development and in the adult retina. Since GABAB1a 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 GABAB1a, but not GABAB1b, is present in postsynaptic densities, suggesting a differential pre- and postsynaptic localisation of both splice variants.