Functional pharmacology of cloned heterodimeric GABAB receptors expressed in mammalian cells

1 In this study we report a new assay of heterodimeric gamma-amino-butanoic acid subtype B (GABA(B)) receptors where either GABA(B)R1a or GABA(B)R1b are co-expressed with GABA(B)R2 and the chimeric G-protein G alpha q-z5 in tsA cells. In this manner we obtained a robust response to GABA(B) agonists measured as increase in phosphoinositide hydrolysis. 2 We used this assay to characterize a number of commonly used GABA(B) receptor ligands. Both splice variants displayed the same rank order of agonist potency; 3-aminopropyl(methyl)phosphinic acid (SKF-97541)> GABA > (R)-4-amino-3-(4-chlorophenyl)butanoic acid ((R)-baclofen)> (RS)-4-amino-3-(5-chloro-2-thienyl)butanoic acid (BCTG)>3-aminopropylphosphonic acid (3-APPA) and furthermore, the absolute agonist potency values were very close to each other. 3 3-APPA was a partial agonist displaying maximal responses of 41 and 61% compared to GABA at GABA(B)R1a and GABA(B)R1b, respectively. The antagonist (RS)-3-amino-2-(4-chlorophenyl)-2-hydroxypropylsulphonic acid (2-OH-saclofen) displayed K-B values of 15 and 7.8 mu M at GABA(B)R1a and GABA(B)R1b, respectively. 4 The rank order of agonist potency as well as the absolute ligand potencies correspond very well with those previously reported in different tissues, and this study thus provides a functional assay of cloned GABA(B) receptors which should be a valuable tool for further characterization of GABA(B) ligands. Finally, we can conclude that the functional pharmacological profiles of the two GABA(B)R1 splice variants are very similar.