Pharmacological characterisation of a cell line expressing GABAB1b and GABAB2 receptor subunits

Abstract The γ-aminobutyric acid (GABA B ) receptor has been shown to be a heterodimer consisting of two receptor subunits, GABA B1 and GABA B2 . We have stably co-expressed these two subunits in a CHO cell line, characterised its pharmacology and compared it to the native receptor in rat brain membranes. Radioligand binding using [ 3 H ]CGP54626A demonstrated a similar rank order of potency between recombinant and native receptors: CGP62349>CGP54626A>SCH 50911>3- aminopropylphosphinicacid (3- APPA )> GABA > baclofen > saclofen > phaclofen . However, differences were observed in the affinity of agonists, which were higher at the native receptor, suggesting that in the recombinant system a large number of the receptors were in the low agonist affinity state. In contrast, [ 35 S ]GTPγS binding studies did not show any differences between recombinant and native receptors with the full agonists GABA and 3-APPA. Measurement of cAMP accumulation in the cells revealed a degree of endogenous coupling of the receptors to G-proteins. This is most likely to be due to the high expression levels of receptors ( B max =22.5±2.5 pmol/mg protein) in this experimental system. There was no evidence of GABA B2 receptors, when expressed alone, binding [ 3 H ]CGP54626A, [ 3 H ]GABA, [ 3 H ]3-APPA nor of GABA having any effect on basal [ 35 S ]GTPγS binding or cAMP levels.