Antiepileptic activity of zonisamide on hippocampal CA3 neurons does not depend on carbonic anhydrase inhibition.

Zonisamide (ZNS) is an anticonvulsant drug known to affect various neuronal channels and transmitter systems. ZNS has also been reported to inhibit carbonic anhydrase activity and may thus influence neuronal activity via changes of pH. Therefore, we analyzed effects of ZNS in vitro using epileptic model systems which are sensitive to carbonic anhydrase inhibition and pH changes. Intracellular recordings from CA3 neurons (hippocampal slice, adult guinea pigs) were carried out under bicarbonate-buffered conditions. Epileptiform activity was induced by either 4-aminopyridine or theophylline. In parallel experiments, intracellular pH (pHi) was determined in the CA1 and CA3 subfields of 2',7-bis(2-carboxyethyl)-5(6)-carboxyfluorescin-acetoxymethyl ether (BCECF-AM) loaded slices. The ammonium prepulse method was used to test for effects of ZNS on pHi regulation. ZNS (50 microM) reversibly reduced the frequency of 4-AP induced epileptiform bursting and the number of action potentials per bursts but had no effect on input resistance and membrane potential. Theophylline-induced epileptiform bursting, although sensitive to hypercapnic acidosis, was not affected by ZNS. There was also no effect on steady-state pHi and pHi regulation of BCECF-AM loaded hippocampal tissue. Clinically relevant concentrations of ZNS strongly inhibit 4-AP induced epileptiform activity of hippocampal CA3 neurons in vitro, but this effect was unlikely based on carbonic anhydrase inhibition or changes of neuronal pHi.