[H-3]tiagabine binding to GABA uptake sites in human brain

The binding of [H-3]tiagabine ((RS-1-(4,4-(3-methyl-2-thienyl)-3-butenyl)-3 carboxylic acid) to homogenates of frozen post-mortem human brain has been characterized. Inhibition experiments with gamma-aminobutyric acid (GAB,4) , GABA uptake inhibitors, ligands active at postsynaptic GABA receptors and receptors for other neurotransmitters, suggest that [H-3]tiagabine binds w ith high affinity to GABA uptake sites. Inhibition and kinetic experiments suggests that 70%-80% of the binding is to a high affinity site. Saturation experiments showed that the binding was saturable. B-max was 3.4 pmol/mg p rotein and K-d 16 nM in frontal cortex. The dissociation constants (K-d) me asured in kinetic and equilibrium experiments were in the same range (16-56 nM). The regional distribution was studied in nine brain regions and the b inding was heterogenous, with the highest binding in frontal cortex and par ietal cortex and the lowest binding in nucleus caudatus and putamen. This i s, to our knowledge, the first study on [H-3]tiagabine binding in human tis sue. It is concluded that [H-3]tiagabine binding can be used as a specific marker for the GABA transporter GAT-1 in homogenates of human brain. (C) 19 99 Elsevier Science B.V. All rights reserved.