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

The binding of [3H]tiagabine ((RS-1-(4,4-(3-methyl-2-thienyl)-3-butenyl)-3-piperidine carboxylic acid) to homogenates of frozen post-mortem human brain has been characterized. Inhibition experiments with gamma-aminobutyric acid (GABA), GABA uptake inhibitors, ligands active at postsynaptic GABA receptors and receptors for other neurotransmitters, suggest that [3H]tiagabine binds with 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. Bmax was 3.4 pmol/mg protein and Kd 16 nM in frontal cortex. The dissociation constants (Kd) measured in kinetic and equilibrium experiments were in the same range (16-56 nM). The regional distribution was studied in nine brain regions and the binding was heterogenous, with the highest binding in frontal cortex and parietal cortex and the lowest binding in nucleus caudatus and putamen. This is, to our knowledge, the first study on [3H]tiagabine binding in human tissue. It is concluded that [3H]tiagabine binding can be used as a specific marker for the GABA transporter GAT-1 in homogenates of human brain.