Alternative splicing generates a novel isoform of the rat metabotropic GABA(B)R1 receptor

Here we present a novel isoform of the metabotropic G-protein-coupled recep tor for g-aminobutyric acid (GABA). The isoform, termed GABA(B)R1c (R1c), d iffers from the recently identified R1a and R1b receptors by an in-frame in sertion of 31 amino acids between the second extracellular loop and the fif th transmembrane region. Analysis of the rat GABA(B)R1 gene demonstrates th at the insertion is the result of an alternative splicing event within a 56 7-bp intron between exons 16 and 17. In situ hybridization in the rat brain shows a wide distribution of R1c transcripts and an overlap with the R1a a nd R1b transcripts. The highest mRNA levels are found in cerebellar Purkinj e cells, cerebral cortex, thalamus and hippocampal CA1 and CA3 regions. Wes tern blots and immunodetection of recombinant epitope-tagged receptors as w ell as [I-125]CGP71872 photoaffinity labelling of cell membranes demonstrat e that R1c is correctly expressed, although at a lower level than the previ ously identified isoforms. When coexpressed with the newly characterized GA BA(B)R2, R1c functionally couples to G-protein-activated Kir3.1/3.2 channel s in Xenopus oocytes and to PLC-activating chimeric G alpha qo subunits in HEK-293 cells with a similar EC50 for agonists. These data suggest that the R1c isoform represents a functional GABA(B)R in the rat brain.