Enhanced spontaneous glutamatergic activity in the hippocampal samples obtained from patients with MTLE-HS
For all the electrophysiological experiments, we have used tumour periphery non-seizure control tissues for comparing with hippocampal samples obtained from patients with MTLE-HS. Passive membrane properties of pyramidal neurons in slice preparation obtained from hippocampal samples of patients with MTLE-HS and that of tumour periphery non-seizure controls were comparable (Cell capacitance 163 ± 16pF in non-seizure control vs 171 ± 21pF in MTLE-HS; input resistance 148 ± 23MΩ in non-seizure control vs 157 ± 27MΩ in MTLE-HS). We recorded spontaneous excitatory postsynaptic currents (EPSCs) from pyramidal neurons at -70 mV holding potential under whole-cell voltage clamp configuration in 300 µM thick slice preparations obtained from the MTLE-HS and non-seizure control groups. These glutamatergic events were inward currents which were completely abolished following 10 minutes bath perfusion of the slices with ACSF containing NMDA receptor antagonist APV (50 µM) and AMPA receptor antagonist CNQX (10 µM), implying that these events were mediated by both NMDA and AMPA/kainate receptors (Figure 3a). These events were unaffected by bath perfusion with GABAA receptor antagonist bicuculline (10 μM), indicating that the resting state spontaneous glutamatergic activity was independent of GABAergic influence, in these slice preparations. The frequency and amplitude of the spontaneous EPSCs were significantly increased in the hippocampal samples (Table 1). The normalised cumulative distribution of inter-event intervals shifted significantly to left side, whereas that of peak amplitudes shifted significantly to right side in the hippocampal samples (Figure 3b and c). However, Rise time and τd was not affected (Table 1). These findings suggested that resting state glutamatergic activity was enhanced in the hippocampal samples of the MTLE-HS patients.
We recorded the spontaneous inhibitory postsynaptic currents (IPSCs) at 0 mV from the pyramidal neurons in the slice preparations of resected brain specimens obtained from patients with MTLE-HS and non-seizure controls. The spontaneous IPSCs were completely blocked following 15 minutes bath perfusion of those slices with ACSF containing bicuculline (10 μM; Figure 3d). However, we did not observe any significant alteration in the frequency as well as amplitude of IPSCs between the two groups (Table 1). The cumulative distribution of inter-event intervals as well as peak amplitudes remain unchanged (Figure 3e and f). Then we calculated the ratio of frequency and amplitude of glutamatergic-GABAergic activities in a cohort of neurons where EPSCs and IPSCs were recorded from the same neuron. We found that the ratio of frequency as well as amplitude were significantly increased in the hippocampal samples (control, n=5; MTLE-HS, n=5; frequency ratio 0.39 ± 0.015 in non-seizure control vs 0.62 ± 0.010 in MTLE-HS; amplitude ratio 0.53 ± 0.014 in non-seizure control vs 0.71 ± 0.21 in MTLE-HS; Figure 3g).