Abstracts

Glutamate glutamine cycling between neurons and glia is critical for normal synaptic transmission and has been shown to be impaired in patients with temporal lobe epilepsy. In the present study, we investigated the effects of blocking glutamine uptake with the structural analog, MeAIB. We hypothesized that while interrupting this cycle will affect both glutamatergic and GABAergic neurotransmission, that GABAergic function may be affected first as the bulk of GABA is synthesized from glutamate generated from glutamine. We used standard electrophysiological techniques to address this hypothesis using both field potential and intracellular recordings from the CA1 region in rat hippocampal slices. All drugs were bath applied and the ACSF osmolarity was adjusted as necessary. Synaptic stimuli were delivered to stratum radiatum using a twisted bipolar electrode. Field potential recordings in the cell body layer showed a significant decrease in the slope following bath application of MeAIB with no significant alteration in the amplitude or duration of the synaptic events. Little effect was seen with 10 mM MeAIB. However, there was a decrease in the stimulus threshold needed to evoke a population spike in 40% of slices studied. MeAIB was associated with a slight increase in excitability however, as current clamp recordings showed an enhancement of the evoked response measured both as an increase in the area and duration of the response. When we examined GABAergic events, there was no consistent effect of 20 mM MeAIB on the IPSP conductance in slices stimulated at low (1 Hz) frequencies. However, we noted that there was a significant and persistent use-dependent depression of the IPSP conductance of 67.9 [plusmn] 10.9% following 10 Hz stimulus trains in MeAIB that was not seen under control conditions. Similarly, when monosynaptic IPSPs (recorded in APV and CNQX) were examined in control ACSF there was no persistent decrease in IPSP conductance in response to 10 Hz stimulation. However, in MeAIB, we noted a 38.4% decrease in the monosynaptic IPSP conductance following a stimulus train. The effects of MeAIB on synaptic function were modest given previous work suggesting that interrupting glutamate glutamine cycling can disrupt synaptic transmission. However, we noted use-dependence of these effects, such that there was little effect of 20 mM MeAIB on GABAergic function in unstimulated slices, but that there was a profound decrease in synaptic inhibition following a stimulus train. That this was seen for both polysynaptic and monosynaptic events suggests that GABAergic transmission may be more vulnerable than glutamatergic transmission following blockade of glutamine uptake during periods of high synaptic levels, including seizures, possibly due to a depletion of GABA. (Supported by grant R01 NS45792 to AW and an award from the Scientific and Technical Research Council of Turkey to ND.)