Action of bicyclic isoxazole GABA analogues on GABA transporters and its relation to anticonvulsant activity

The inhibitory action of bicyclic isoxazole gamma-aminobutyric acid (GABA) analogues and their 4,4-diphenyl-3-butenyl (DPB) substituted derivatives ha s been investigated in cortical neurones and astrocytes as well as in human embryonic kidney (HEK 293) cells transiently expressing either mouse GABA transporter-1 (GAT-1), GAT-2, -3 or -4. It was found that 4,5,6,7-tetrahydr oisoxazolo(4,5-c)pyridin-3-ol (THPO) and 5,6,7,8-tetrahydro-4H-isoxazolo[4, 5-c]azepin-3-ol (THAO) displayed some inhibitory activity on GAT-1 and GAT- 2, where the compounds exhibited a slightly lower potency on GAT-2 compared to GAT-1. DPB substituted THPO displayed higher inhibitory potency than th e parent compound regarding the ability to inhibit GABA uptake via GAT-1 an d GAT-2. Concerning the inhibitory mechanism, THPO, THAO and DPB-THPO were competitive inhibitors on GAT-1 transfected HEK 293 cells and the same mech anism was observed for THPO in GAT-3 transfected cells. Regarding GABA upta ke into neurones and astroglia cells THAO and DPB-THAO both displayed compe titive inhibitory action. The observations that THPO, THAO as well as their DPB derivatives act as competitive inhibitors together with earlier findin gs such as potent anticonvulsant activity, lack of proconvulsant activity a nd the ability of THPO to increase extracellular GABA concentration, indica te that these bicyclic isoxazole GABA analogues and their DPB derivatives m ay be useful lead structures in future search for new antiepileptic drugs. (C) 1999 Elsevier Science B.V. All rights reserved.