Abstracts

RATIONALE: Most epileptic disorders described so far are associated with a dysfunction in GABAergic neuronal inhibition. In particular, postsynaptic GABAA receptors (GABAARs) present on dentate gyrus granule cells (DGCs) undergo drastic changes in their biophysical and pharmacological properties during the epileptogenic process. This point has been illustrated in several studies and can be summarized as follows : GABAAR currents recorded from [dsquote]epileptic[dsquote] DGCs become sensitive to zinc but are less potentiated by zolpidem (type I benzodiazepine) or allopregnanolone (neurosteroid). These observations likely indicate implication of GABAARs composed of [alpha]4 and [delta] subunits that have been described in DGCs but are only located at extrasynaptic sites. Indeed, the expression of these two subunits are though to increase in [dsquote]epileptic[dsquote] DGCs. To further analyze this phenomenon, we chose to study the pharmacological alterations exhibited by GABAARs at DGC synapses during the chronic phase of epilepsy in rats rendered spontaneously epileptic after experiencing a sustained episode of status epilepticus induced by lithium and pilocarpine (Li-Pilo).
METHODS: Whole cell patch-clamp recordings of DGCs were performed at room temperature (20-23[degree]C) in horizontal hippocampal slices from 6 month-old control (Li-saline) and epileptic (Li-Pilo) rats that exhibited spontaneous seizures for 3-4 months. Spontaneous GABAA receptor-mediated inhibitory synaptic currents (GABAAR sIPSCs) were isolated by blocking ionotropic glutamate receptors with steady-state concentrations of kynurenic acid. The sIPSCs recorded in control conditions were compared to those obtained with perfusion of allosteric modulators of GABAAR function in both Li-Pilo and Li-saline hippocampal slices.
RESULTS: When compared to control conditions, GABAAR sIPSCs recorded in GCs from Li-Pilo rats exhibited several differences in their kinetic parameters and frequency of occurrence. They had increased mean peak amplitude (40%), time to peak (45%), deactivation time (20%) and frequency of occurrence (65%). While perfusion of 1[mu]M diazepam prolonged the mean time to peak (17%) and deactivation time (89%) of GABAAR sIPSCs recorded from Li-saline DGCs, GABAAR sIPSC parameters remained unchanged in Li-Pilo DGCs. Interestingly, perfusion of the benzodiazepine antagonist flumazenil (10[mu]M) significantly accelerated the time to peak and deactivation time of a fraction of GABAAR sIPSCs recorded in Li-Pilo DGCs (40-60%).
CONCLUSIONS: In summary we have shown significant differences in the biophysical and pharmacological properties of synaptic GABAARs present in 751002epileptic751002 DGCs. These changes that have been described initially early after status epilepticus seem to persist during the chronic phase in this model. Although sIPSCs were apparently not modulated by acute perfusion of diazepam in Li-Pilo rats, our results suggest that benzodiazepine-sensitive GABAARs (non-[alpha]4[beta]n[delta]) may still be present at DGC synapses. It is therefore tempting to speculate that these receptors may be composed of other subunits but had 751002masked751002 or uncoupled benzodiazepine binding sites.
[Supported by: INSERM U398
ULP/CNRS UMR7519]