Figure 4.

Pharmacoresistance is observed with the epileptiform discharges in the in vitro model and implicates involvement of the astrocytic GABA-glutamate-glutamine cycle in the regulation of GABA-ergic inhibition in this model. The addition of a variety of conventional antiepileptic drugs at their reported minimal inhibitory concentration (A) has elicited minimal inhibitory response in the interictal discharges. At the suprathreshold concentration (B), minimal response is still observed with the majority of the tested antiepileptics except for pentobarbital, which was able to afford significant suppressive effect (n = 6 for baseline and phenytoin, n = 5 for all other treatment groups). The addition of GABA (C) was also able to afford significant suppression at the high concentration of 2.5 mM (n = 8). When co-applied with the benzodiazepine lorazepam, the lower concentration of GABA (0.5 mM) was able to produce a synergistic effect creating a maximal inhibitory effect towards the interictal discharges (D) (n = 4 for lorazepam and lorazepam + GABA group, n = 6 for GABA group). As GABA is synthesized via the intermediary of the astrocytic GABA-glutamate-glutamine cycle, substrates of this cycle were tested for its anticonvulsant properties in this model. Interestingly, addition of glutamate (E) was unable to afford any suppressive effect whereas glutamine (F) was able to exert a significant anticonvulsant property at the equimolar concentration as GABA (n = 6 for glutamate, n = 5 for glutamine). *P ≤ 0.05, **P ≤ 0.01, ***P ≤ 0.001.