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Comment on "On the Origin of Interictal Activity in Human Temporal Lobe Epilepsy in Vitro"

Science  25 Jul 2003:
Vol. 301, Issue 5632, pp. 463
DOI: 10.1126/science.1084237

For more than a decade, there has been extensive discussion about whether hippocampal sclerosis causes enhanced neuronal excitability as a prerequisite for seizure generation. The disorder known as Ammon's horn sclerosis (AHS) is characterized by pronounced cell loss and gliosis in various regions of the hippocampal formation, leaving the subiculum generally intact (1, 2). Given the hypothesis that epileptic activity is generated within the hippocampal formation when the CA3 and CA1 regions are damaged or even absent, it is feasible that the adjacent subiculum is uniquely responsible for the generation of limbic seizures. Using multielectrode recordings in hippocampal brain slices of patients with temporal lobe epilepsy (TLE) and hippocampal sclerosis, Cohen et al. (3) detected spontaneous, rhythmic spikes in the subiculum— but rarely in the CA3 or CA1 regions. This activity closely resembled the discharges seen on the electroencephalograms (EEGs) of these patients. Cohen et al. (3) therefore concluded that in patients with AHS, deafferentation of the subiculum initiates an epileptogenic plasticity that includes changes in glutamatergic or γ-aminobutyric acid (GABA)-ergic signaling.

We find that even in nonsclerotic hippocampal tissue, as graded by Wyler (4), the subiculum shows cellular and synaptic changes which suffice to generate an epileptic focus. To elucidate this issue further, we investigated the contribution of subicular cells to interictal activity recorded in EEGs of TLE patients with (AHS, Wyler score W3, W4; n = 7) and without hippocampal sclerosis (non-AHS, W0-W2; n = 6). In AHS tissue, the majority (75%) of subicular cells were regular firing cells [n = 18 (5)], whereas only six cells were bursting cells (Fig. 1A). More than half of the recorded cells (both cell types) displayed spontaneous, rhythmic activity of 1.45 ± 0.31 Hz that correlated with the occurrence and frequency (0.75 to 2 Hz) of interictal discharges recorded in the EEGs of the corresponding patients (Fig. 1B; n = 5 patients). Intracellular recordings showed spontaneous excitatory postsynaptic potentials that were suppressed by 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f) quinoxaline (NBQX), but not by D,L-2-amino-5-phosphovaleric acid (APV), and inhibitory postsynaptic potentials that were suppressed by bicuculline; together, these results indicated an AMPA/kainate- and GABAA receptor–mediated synaptic circuit (Fig. 1C, a and b). We never found APV-/NBQX-insensitive excitatory events that would suggest spontaneous depolarizing GABAergic responses. In contrast to (3), two patients with severe sclerosis in area CA1 (W4) showed no rhythmic activity in the EEGs or in the subicular slice preparations (n = 7 cells).

Fig. 1.

Relation of hippocampal sclerosis, interictal activity, and membrane properties in human TLE. (A) Membrane properties and discharge patterns of a human subicular bursting cell (left trace) and regular firing cell (right trace) upon negative and positive current steps [range: ± 0.9 nA; duration:300 ms; representative traces recorded in AHS tissue (5, 7)]. In AHS and non-AHS tissue, the majority of subicular cells were regular firing cells (75% and 79%, respectively). Arrow indicates spontaneous activity recorded in AHS tissue. (B) 56% (AHS tissue) and 28% (non-AHS tissue) of subicular principle cells displayed spontaneous, rhythmic activity (in vitro), which was always correlated with interictal spike and wave events recorded with sphenoidal electrodes (in vivo) in the same patient. (C) The synchronized spontaneous activity consisted of excitatory and inhibitory postsynaptic potentials that were suppressed by the AMPA/kainate receptor-antagonist NBQX (but not by the NMDA receptor-antagonist APV) (a) and the GABAA receptor-antagonist bicuculline (b). (D) fAHP and sAHP following a train of depolarization-induced action potentials (1 nA, 1 s) were significantly decreased in cells showing spontaneous rhythmic activity compared to cells without activity (9). Arrow indicates spontaneous activity in the course of the recording.

As in AHS tissue, the majority (79%) of cells in non-AHS tissue were regular firing cells (n = 15) and only four cells were bursting cells (6). Notably, nearly one-third of the cells showed spontaneous activity reminiscent of the activity recorded in sclerotic tissue (0.65 ± 0.15 Hz). As with AHS patients, this activity corresponded to the interictal spike and wave events of the EEG records (Fig. 1B; n = 3 patients).

In addition to synaptic alterations, persistent changes in neuronal membrane properties contribute to epileptogenesis. We found no differences in resting membrane potential, input resistance, and discharge properties of subicular neurons between AHS and non-AHS tissue (5, 6). Surprisingly, the ratio between bursting cells and regular firing cells in both tissues was virtually inverse compared to the ratio observed in the rat subiculum (7). These results challenge theories on seizure generation that support a crucial role for bursting cells in seizure activity (8). However, the fast afterhyperpolarization (fAHP) and slow afterhyperpolarization (sAHP) recorded following a train of action potentials were significantly decreased in cells showing spontaneous activity compared with “silent” cells [Fig. 1D; (9)].

Our data demonstrate that histologically identified sclerosis of the CA1 region does not necessarily promote spontaneous rhythmic activity in the adjacent subiculum in vivo or in vitro. Furthermore, spontaneous activity driven by excitatory and inhibitory circuits also occurs in the absence of AHS. Thus, the presence of hippocampal sclerosis is not mandatory for the development of an epileptic focus in the major output pathway of the hippocampus, as proposed by Cohen et al. (3). However, we do concede that according to the Wyler grading system, a moderate neuronal dropout may occur in a non-AHS specimen (4). Therefore, even in nonsclerotic tissue, a loss of afferents from CA1 to the subiculum is conceivable. Even though AHS in the resected hippocampus has important prognostic implications for freedom from seizures postoperatively, our data show that functional alterations on both the synaptic and the cellular level enhance seizure susceptibility in cases where deafferentation of the subiculum is absent or not as severe as the one observed in classical AHS (10).

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