Elsevier

Epilepsy Research

Volume 36, Issues 2–3, September 1999, Pages 233-241
Epilepsy Research

Effect of long-term spontaneous recurrent seizures or reinduction of status epilepticus on the development of supragranular mossy fiber sprouting

https://doi.org/10.1016/S0920-1211(99)00054-6Get rights and content

Abstract

In a recent report we have shown that a protein synthesis inhibitor, cycloheximide (CHX), is able to block the mossy fiber sprouting (MFS) that would otherwise be triggered by pilocarpine (Pilo)-induced status epilepticus (SE), and also gives relative protection against hippocampal neuronal death. Under this condition animals still showed spontaneous recurrent seizures (SRS) which led us to question the role played by sprouted mossy fibers in generating those seizures. In both patients and animal models of epilepsy the relative contribution of SE (when present) and/or SRS for the development of MFS is not known. In the present study we investigated the relationship between MFS, SE and SRS, and evaluated whether the CHX-induced blockade of MFS was transient or permanent in nature. We performed a chronic study which included animals subject to Pilo-induced SE in the presence of CHX and sacrificed between 8 and 10 months later, and animals that were subject to Pilo-induced SE in the presence of CHX and underwent a reinduction of SE with Pilo, 45 days after the first induction, but this time in the absence of CHX. Re-induction of SE or a long period of chronic seizures, were able to trigger supragranular MFS even in animals where the first (or only) SE event was triggered in the presence of CHX. MFS did not show any association with the frequency of SRS, and thus seemed to depend more critically on time. Our current findings allow us to suggest that MFS are neither the cause nor the consequence of SRS in the pilocarpine model.

Introduction

A proposed classification of temporal lobe epilepsy based on tissue analysis has indicated three major groups: medial temporal lobe epilepsy (MTLE), mass associated medial temporal lobe epilepsy (MaMTLE) and paradoxical medial temporal lobe epilepsy (PaTLE) (Spencer and Spencer, 1994). In patients with MTLE retrospective analysis of clinical histories often indicate that a previous cerebral injury has taken place prior to the onset of their habitual complex partial seizures (Falconer et al., 1964) in addition to major neuronal loss and synaptic reorganization (Babb et al., 1984, Babb et al., 1991). Both the kainate (in its various versions) and the pilocarpine models have been shown to mimic many features of MTLE (Cavalheiro et al., 1982, Tauck and Nadler, 1985, Cavalheiro et al., 1991, Mathern et al., 1995, Mathern et al., 1997, Wuarin and Dudek, 1996) MaMTLE, is most often associated with a glioma and is associated with much more subtle neuronal loss and synaptic reorganization (Mathern et al., 1995, Mathern et al., 1997). We have recently suggested that induction of status epilepticus with pilocarpine (or intrahippocampal kainate) in the presence of a protein synthesis inhibitor (cycloheximide) might mimic certain features of MaMTLE (Longo and Mello, 1998). Finally, PaTLE patients have a variable history and a neuropathological profile similar to MaMTLE (Spencer and Spencer, 1994). So far no model has been proposed to specifically study this later condition.

A long-standing debate in TLE is centered on the relationship between seizures and neuropathology. Do repeated seizures cause neuronal loss? Do repeated seizures cause synaptic reorganization? Or rather than cause, are they both the consequences of seizures? Or are they both cause and consequence? The answers to these questions might be different for each of the different above mentioned conditions MTLE, MaMTLE and PaTLE.

From its original description in the kainate model by Tauck and Nadler (1985), supragranular mossy fiber sprouting (MFS) rose to become the leading theory to explain, at least in part, the hyperexcitability of epileptic tissue. The description of intense MFS in human material was fundamental in this process (Sutula et al., 1989, Babb et al., 1991). In 1988, it was shown that kindling also could induced MFS (Sutula et al., 1988). Similarly to all kindled rats, many patients with MFS do not have a history of preceding status epilepticus (SE). Whereas in human cases there is rarely (if ever) 100% confidence on the patients clinical history this is not the case with rats. Therefore, such data supported the notion that SE was not necessary for MFS to develop. As for neuronal loss in kindling the data tends to support the notion that neuronal degeneration might follow kindled seizures (Cavazos and Sutula, 1990) despite evidence on the contrary (Bertram et al., 1990).

Data from SE models (mostly kainate and pilocarpine) tend to support the notion that SE itself is a crucial event for the onset of neuronal loss, MFS and spontaneous seizures (Ben-Ari, 1985, Tauck and Nadler, 1985, Cronin and Dudek, 1988, Cavalheiro et al., 1991, Mello et al., 1993, Lemos and Cavalheiro, 1995). The influence of single spontaneous seizures on neuronal damage in the pilocarpine model has been shown to concentrate on specific cell populations and not to contribute significantly to the overall cell loss (Covolan and Mello, 1996). The influence of single spontaneous seizures on MFS, however, has been a more elusive subject. Here we used the induction of SE with pilocarpine in the presence of cycloheximide, a condition that blocks SE-induced MFS but does not affect epileptogenesis and the onset of spontaneous seizures (Longo and Mello, 1997) to address this and other issues.

Section snippets

Methods

Adult male Wistar EPM-1 rats (200–250 g) were kept on a standard light/dark cycle (12/12 h) and fed ad libitum. The protocols were approved by the Animal Care and Use Ethic Committee at the University.

Results

For the two experiments a total of 85 animals were injected with Pilo (n=32) or CHX+Pilo (n=53). Twenty eight animals (33% of the total) did not develop SE, 20 (37%) were CHX+Pilo and 8 (25%) were Pilo (χ2 P=0.23). Of these, five animals (one Pilo-injected and four CHX+Pilo-injected) were kept together with the experimental groups and sacrificed after 9 months of the attempted (unsuccessful) SE induction. Of the 33 animals injected with CHX+Pilo that developed SE, 16 (48%) died prior to

Discussion

Our findings indicate that, whatever the mechanisms through which CHX is able to block Pilo-induced supragranular MFS, this synaptic reorganization can still be induced in these same animals after either a long time interval or SE-reinduction. As previously shown (Longo and Mello, 1997, Longo and Mello, 1998), a single SE event triggered by Pilo or intrahippocampal kainate in the presence of CHX is not able to induce supragranular MFS (as assessed 60 days after SE induction). Animals undergoing

Acknowledgements

We thank Ivone de Paulo for technical assistance. This work was supported by: FAPESP, CNPq and PRONEX; B. Longo is a PhD fellow of FAPESP (96/6623-2).

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