Abstracts

Epileptic tissue is characterized by a number of metabolic alterations. We have been investigating the effects of altered metabolism on GABAergic neurotransmission; here we have examined the physiological effects of blocking GABA synthesis using slices from both control and epileptic rats. We have used 3-mercaptoproprionic acid (3-MPA) a selective glutamic acid decarboxylase (GAD) inhibitor for these studies. Our primary hypothesis is that alterations in excitability will only be seen under activated conditions as GABA stores will need to be depleted., Adult male rats were used for these studies. We used the systemic kainic acid (KA) model of temporal lobe epilepsy. Hippocampal slices were prepared and maintained using standard methods. Field potential recordings were obtained from the CA1 region. Stimuli were delivered to stratum radiatum at an intensity twice that needed to evoke a 1 mV population spike. 3-MPA was washed on and recordings were made in different slices. The effect of repetitive stimulation was determined by giving 10 Hz, 10 sec trains and monitoring both evoked and spontaneous activity., In control animals, 3-MPA was not associated with significant hyperexcitability in the absence of afferent input even after several hours exposure. However, following a series of stimulus trains there was a significant enhancement of the duration of the evoked response and the development of short bursts of population spikes. We assayed the role of neuronal GABA transport by blocking GABA uptake with NO-711. There was a further prolongation of the evoked response in NO-711 and 3- MPA demonstrating that neuronal transport can contribute to synaptic inhibition. However, we were unable to elicit spontaneous activity under either recording condition. We found that raising excitability slightly by bath applying 5 mM K+ ACSF, was associated with the development of interical-like activity in the presence of 3-MPA in all slices studied, however. These events became prolonged seizure-like events following repetitive stimulation.
In KA-treated rats there was a significantly greater effect of 3-MPA on the duration of the evoked responses compared to controls. In addition, intracellular recordings demonstrated the development of spontaneous activity, unlike control animals., These data demonstrate that blocking GABA synthesis does not significantly alter neuronal circuit excitability in the absence of robust afferent input and/or a global elevation in tissue excitability. Thus, neurons appear to have plentiful reserves of GABA under baseline conditions in vitro. In epileptic animals, however, there appears to be a reduced inhibitory reserve under conditions when GABA synthesis is reduced. However, this may reflect the higher baseline levels of activity., (Supported by NIH NS045792 to AW and a Scientific and Technical Research Council of Turkey Fellowship to ND.)