Ligands for expression cloning and isolation of GABA(B) receptors

Outlined is the rationale behind the syntheses of radioligands [125I]CGP64213 and [125I]CGP71872, which led to the identification of cloned GABA(B) receptors 1a and 1b 17 years after the first pharmacological characterisation of native GABA(B) receptors by Bowery et al. [Nature 283 (1980) 92-94]. More recently it was shown that the N-terminal extracellular domains of GABA(B) receptors 1a and 1b contain the binding sites for agonists and antagonists [B. Malitschek et al., Mol. Pharmacol. 56 (1999) 448-454]. In order to isolate the extracellular domain(s) of GABA(B) receptors 1a (or 1b) and to purify and crystallise these proteins a third ligand [125I]CGP84963 was designed, which combines, in one molecule, a GABA(B) receptor binding part, an azidosalicylic acid as photoaffinity moiety and 2-iminobiotin, which binds to avidin in a reversible, pH-dependent fashion [W. Froestl et al., Neuropharmacology 38 (1999) 1641-1646].