Ligands for the isolation of GABA(B) receptors W. Froestl would like to dedicate this work to the first GABA(B) chemist, Cr Heinrich Keberle, on the occasion of his 77th birthday.

Since the discovery that the most abundant inhibitory neurotransmitter in t he mammalian brain, GABA (gamma-aminobutyric acid), interacts not only with ionotropic GABA(A) receptors, but also with metabotropic GABA(B) receptors (Bowery et al., 1980) much work has been devoted to the elucidation of the structure of GABA(B) receptors by either affinity chromatography purificat ion or by expression cloning. In 1997 Kaupmann et al. succeeded in cloning two splice Variants designated GABA(B) R1a (960 amino acids) and GABA(B) R1 b (844 amino acids). Although the amino acid sequences are now known, preci se information on the three-dimensional environment of the GABA(B) R1 bindi ng site is still lacking. Recent experiments demonstrated that the amino ac ids of the seven transmembrane helices are not essential for ligand binding as a soluble GABA(B) receptor fragment is still able to bind antagonists ( Malitschek et al., 1999). For the isolation and purification of the soluble N-terninal extracellular domain (NTED) of GABA(B) receptors potent ligands for affinity chromatography were synthesised with the aim of obtaining a c rystalline receptor fragment-ligand complex for X-ray structure determinati on. The most promising ligand [(125)]CGP84963 (K-D = 2 nM) combines, in one molecule, a GABA(B) receptor binding part, an azidosalicylic acid as a pho toaffinity moiety separated by a spacer consisting of three GABA molecules from 2-iminobiotin, which binds to avidin in a reversible, pH-dependent fas hion. (C) 1999 Elsevier Science Ltd. All rights reserved.