Two novel
nipecotic acid derivatives, 1-(3-(9H-Carbazol-9-yl)-1-propyl)-4-(4-methoxyphenyl)-4-piperidino l (NNC 05-2045) and 1-(3-(9H-Carbazol-9-yl)-l-propyl)-4-(2-methoxyphenyl)-4-piperidino l (NNC 05-2090) have been tested for inhibition of gamma-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 microM, respectively. In synaptosomes from inferior colliculus in the presence of 1 microM 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 microM, respectively. A receptor profile showed that
NNC 05-2045 has binding affinities to sigma-, alpha 1- and D2-receptors of 113, 550 and 122 nM, respectively.
NNC 05-2090 displayed alpha 1- 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 05-2090 dose-dependently inhibited sound-induced tonic and clonic convulsions in DBA/2 mice with ED50 values of 6 and 19 mumol/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 mumol/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 mumol/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 mumol/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.