Anticonvulsant properties of two GABA uptake inhibitors NNC 05-2045 and NNC 05-2090, not acting preferentially on GAT-1

Abstract 

Two novel nipecotic acid derivatives, 1-(3-(9H-Carbazol-9-yl)-1-propyl)-4-(4-methoxyphenyl)-4-piperidinol (NNC 05-2045) and 1-(3-(9H-Carbazol-9-yl)-1-propyl)-4-(2-methoxyphenyl)-4-piperidinol (NNC 05-2090) have been tested for inhibition of γ-amino butyric acid (GABA) transporters in synaptosomal preparations of rat cerebral cortex and inferior colliculus and found to differ markedly from gabitril® (tiagabine), a selective GAT-1 inhibitor. IC₅₀ values for inhibition of [³H]GABA uptake into synaptosomes from cerebral cortex for NNC 05-2045 and NNC 05-2090 were 12±2 and 4.4±0.8 μM, respectively. In synaptosomes from inferior colliculus in the presence of 1 μM 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid (NNC 05-0711), a highly potent and selective GAT-1 inhibitor, IC₅₀ values for inhibition of [³H]GABA uptake were 1.0±0.1 and 2.5±0.7 μM, respectively. A receptor profile showed that NNC 05-2045 has binding affinities to sigma-, α₁- and D₂-receptors of 113, 550 and 122 nM, respectively. NNC 05-2090 displayed α₁- and D₂-receptor affinity of 266 and 1632 nM, respectively. The anticonvulsant action of both compounds was tested in four rodent models after intra peritoneal (i.p.) injection. Both NNC 05-2045 and NNC 05-2090 dose-dependently inhibited sound-induced tonic and clonic convulsions in DBA/2 mice with ED₅₀ values of 6 and 19 μmol/kg, respectively. NNC 05-2045 also antagonized sound-induced seizures in genetic epilepsy prone rats (GEP rats) with ED₅₀ values against wild running, clonic and tonic convulsions of 33, 39 and 39 μmol/kg, respectively (NNC 05-2090 was not tested in GEP rats). Both NNC 05-2045 and NNC 05-2090 dose-dependently antagonized tonic hindlimb extension in the maximal electroshock (MES) test with ED₅₀ values of 29 and 73 μmol/kg, respectively. In amygdala kindled rats NNC 05-2045 and NNC 05-2090 significantly (P<0.05) reduced generalized seizure severity (seizure grade 3–5) at highest doses (72–242 μmol/kg) and NNC 05-2090 also significantly reduced afterdischarge duration at these doses (P<0.05). These data show that inhibition of GABA uptake through non-GAT-1 transporters has different anticonvulsant effects than selective GAT-1 inhibitors (e.g. tiagabine) in that enhanced efficacy against MES and reduced efficacy against kindled seizures is observed. Although a contribution of adrenergic agonistic effects cannot be entirely ruled out, it is proposed that inhibition of GAT-3 (mouse GAT4) is primarily responsible for the anticonvulsant action of these two nipecotic acid derivatives in MES, amygdala kindled rats and in sound-induced seizures in GEP-rats and DBA/2 mice.

Keywords:  GABA transporter, GAT-1, GAT-3, Epilepsy, Tiagabine