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).