Delayed sclerosis, neuroprotection, and limbic epileptogenesis after status epilepticus in the rat

Abstract

Purpose: Hippocampal sclerosis and massive neurodegeneration in other parts of the limbic system are considered hallmarks of temporal lobe epilepsy. Using the rat model of kainate-induced status epilepticus, we sought to determine if limbic sclerosis after an excitotoxic insult follows a delayed type of neurodegeneration and is thus accessible to neuroprotective intervention after the insult. Effective pharmacologic neuroprotection after status epilepticus also addresses the old question of whether degenerative morphologic changes after an epilepsy-inducing event like status epilepticus are the primary cause of epileptogenesis (i.e., the development of recurrent spontaneous seizures) during the following weeks. Methods: Female Wistar rats after 90 min of generalized status epilepticus were used. Molecular biologic and histologic techniques were used to demonstrate markers of delayed cell death (apoptosis) 48 h after the status. The neuroprotective effects of i.c.v. injections of caspase inhibitors and systemic injections of the anticonvulsant drugs (AEDs) dizocilpine and retigabine after the status epilepticus were studied. The effect of neuroprotective intervention on the development of recurrent spontaneous seizures was investigated by behavioral observation of the rats. Results: After generalized status epilepticus in Wistar rats, massive sclerosis of the hippocampus and the piriform cortex occurred. TUNEL labeling and electron microscopy revealed that apoptosis is involved in the degenerative processes. Immunohistochemical analysis of the time course of the expression of the proapoptotic protein Bax suggested a maximal induction of apoptosis 24-48 h after the status. Application of caspase inhibitors before or after the status did not reduce lesions, although Bax labeling was reduced. Injection of dizocilpine and to a lower extent also of retigabine after the status prevented limbic neurodegeneration and expression of markers of apoptosis. However, the neuroprotection by dizocilpine did not prevent the development of recurrent spontaneous seizures. Conclusions: Prolonged seizure activity can induce delayed sclerosis in the hippocampus and other parts of the limbic system. This delayed cell loss can be prevented by neuroprotective drugs after a status epilepticus. However, the damage in limbic brain regions is not the main reason for limbic epileptogenesis and the occurrence of recurrent spontaneous seizures.