GABA-level increasing and anticonvulsant effects of three different GABA uptake inhibitors

The present study examines the effect of tiagabine (a selective inhibitor of GABA transporter 1, GAT-1), SNAP-5114 (a semi-selective inhibitor of rat GAT-3/mouse GAT4) and NNC 05-2045 (a non-selective GABA uptake inhibitor) in modulating GABA levels in the hippocampus and thalamus. Anticonvulsant effects of the same compounds were assessed (after intranigral administration) after maximal electroshock (MES) in juvenile rats. Anticonvulsant effects were also tested after intraperitoneal (i.p.) administration against audiogenic seizures in DBA/2 mice and against pentylentetrazole (PTZ)-induced tonic convulsions or MES in NMRI mice. Tiagabine (30 microM, perfused through the microdialysis probe in halothane anaesthetized rats) increased GABA levels to (% basal+/-SEM) 645+/-69 in the hippocampus and 409+/-61 in the thalamus. SNAP-5114 (100 microM) increased GABA levels in the thalamus (% basal+/-SEM) to 247+/-27 but had no effect on hippocampal GABA-levels. NNC 05-2045 (100 microM) increased GABA levels both in the hippocampus (% basal+/-SEM, 251+/-51) and in the thalamus (298+/-27). All compounds protected against tonic hindlimb extension (THE) in juvenile male rats after intranigral administration. Sound induced convulsions in DBA/2 mice were dose-dependently inhibited by all compounds (administered intraperitoneal, i.p.) with ED(50) values of 1, 6 and 110 micromol/kg, for tiagabine, NNC 05-2045 and SNAP-5114, respectively. Tiagabine and NNC 05-2045 but not SNAP-5114 protected against PTZ-induced tonic convulsions whereas only NNC 05-2045 protected against MES-induced tonic convulsions in NMRI mice. However, tiagabine and NNC 05-2045 exerted a synergistic effect in the MES model. These findings substantiate and extend previous findings of different effects of selective versus non-selective GABA uptake inhibitors in animal models of epilepsy.