QUANTITATIVE AUTORADIOGRAPHIC CHARACTERIZATION OF THE BINDING OF [H-3]-TIAGABINE (NNC-05-328) TO THE GABA UPTAKE CARRIER

The kinetic properties and regional distribution of [H-3]tiagabine 4,4 -bis(3-methyl-2-thienyl)but-3-en-1-yl]nipecotic acid) binding to the c entral GABA uptake carrier was examined in the rat brain using quantit ative receptor autoradiography. In slide mounted sections of frontal c ortex, the binding of [H-3]tiagabine was saturable, reversible and sod ium dependent. The kinetics of association and dissociation of [H-3]ti agabine were monophasic, and Scatchard transformation of saturation is otherms resulted in a linear plot with a K-d = 58 +/- 7 nM and a B-max = 58.9 +/- 0.9 pmol/mg protein. The autoradiogaphic distribution of [ H-3]tiagabine binding sites in rat brain was heterogenously distribute d. The highest density of [H-3]tiagabine binding sites was present in the cerebral cortex, mammillary body, globus pallidus, substantia nigr a pars reticulata, hippocampus, dorsal raphe, superior colliculus (out er layer), and cerebellum. The distribution of GABA uptake sites, as m easured by [H-3]tiagabine binding, in the rat brain is highly consista nt with the organization of GABAergic terminals and cell bodies. The p resent investigation characterized the use of [H-3]tiagabine as a nove l radioligand for the GABA uptake carrier using quantitative receptor autoradiography. [H-3]Tiagabine has several major advantages over the currently utilized radioligand for the GABA uptake carrier [H-3]nipeco tic acid, in that [H-3]tiagabine has an increased affinity, specificit y, and is not transported intracellularly via the GABA uptake carrier. These data suggest that [H-3]tiagabine represents a novel and highly useful ligand for studying the GABA uptake carrier using quantitative receptor autoradiography.