Abstracts

RATIONALE: Vigabatrin causes an increase in brain [GABA] as a result of inhibition of GABA transaminase. This is presumed to cause an increase in neuronal inhibition, but exactly how it does this is unclear. Gabapentin induces an increase in "carrier-mediated GABA release," i.e. nonvesicular GABA release due to the GABA transporter working in reverse. This form of release can be induced by an increase in extracellular [K+] to 12 mM, a level that occurs during seizures in vivo. Here, we have examined the effect of vigabatrin on this form of inhibition. METHODS: Hippocampal neurons were harvested from P0 rats, and grown in tissue culture for at least 3 weeks. Patch clamp recordings of GABAA receptor mediated currents were used to measure carrier-mediated GABA release from neighboring neurons and/or glia. Carrier-mediated GABA release was induced by either raising extracellular [K+] from 3 mM to 12 mM in the absence of calcium, or by applying nipecotic acid (NPA; 10 mM). RESULTS:_ Vigabatrin treatment for 12-24 hours induced a dose-dependent increase in nonvesicular GABA release induced by NPA, to 191% of control at 100 ?M. The response to 12 mM [K+] was also increased to 193% of control. Gabapentin (100 ?M) for 12-24 hours also led to an increase in nonvesicular GABA release induced by NPA. The response to NPA in "pure neuronal" cultures was as large as the response in mixed neuronal/glial cultures, indicating that neurons are the source of this carrier-mediated GABA release. Incubation of neurons with vigabatrin (100 ?M) for 48-96 hours induced a tonic release of GABA in Ringer solution with 3 mM [K+] and a tonic current of >400 pA that was blocked by bicuculline. This current was decreased by lowering extracellular [K+] to 1 mM. CONCLUSIONS: Vigabatrin induces an increase in inhibition of neurons due to carrier-mediated GABA release. Gabapentin results in the same effect, suggesting that they share a final common pathway. This nonvesicular GABA release is an activity dependent form of inhibition that is likely to occur during seizures, and can be modulated by anticonvulsants that increase intracellular [GABA]. Supported by NIH NS-06208 and the EFA.