Status epilepticus: mechanisms of generation and associated neuropathology

Abstract

Status epilepticus (SE) is a prolonged and dangerous epileptic condition that can cause brain damage and may be one of the predisposing factors for the subsequent development of temporal lobe epilepsy. In this thesis, I investigate causes of SE as well as the receptors maintaining seizures and the associated neuropathological changes (cell death, sprouting) that occur in SE brain. An unequivocal role for a defect in either excitatory and inhibitory systems in SE generation has not been established. An alternative approach to unravelling the causes of SE may be to investigate whether an impairment of endogenous seizure termination mechanisms, likely to be mediated by adenosine may lead to SE development. The findings that specific A1-adenosine receptor antagonists with a central site of action were able to transform brief electrically-induced seizures into SE, while co-administration of specific A1-adenosine receptor agonists blocked this effect supports this hypothesis. Pertussis toxin-treated animals also developed SE after elicitation of a seizure suggesting inactivation of Gi-protein linked receptors are involved in SE development. Although GABAB and 5HT1A receptors are also coupled to the same subset of anticonvulsant K+ channels as the A1-adenosine receptor, antagonists at these receptors could not transform brief seizures into SE following seizure elicitation, although a GABAB, agonist had anticonvulsant effects. While chemically-induced SE models suggest that glutamatergic and cholinergic mechanisms are involved in SE initiation and maintenance, whether similar receptor mechanisms operate to initiate and maintain electrically-induced SE are unclear. Using specific antagonists of muscarinic and glutamate receptors, I have shown that NMDA receptors and possibly AMPA/kainate receptors are involved in the initiation of electrically-induced SE. AMPA/kainate receptors may be predominantly involved in maintaining SE, although NMDA receptors may sustain seizures in neocortical regions. Muscarinic and metabotropic glutamate receptors do not appear to have a role in initiation and maintenance of electrically-induced SE. Distinct neuropathological features associated with SE including cell loss and morphological changes in the hippocampus were observed. As early as six days after SE, selective damage to hippocampal neurons in the hilus, CA1 and CA3 pyramidal regions and interneurons immunoreactive for parvalbumin and somatostatin were observed after SE in the absence and presence of the A1-adenosine receptor antagonist 8-cyclopentyl-l, 3-dimethylxanthine (8-CPT). Seizure severity generated in the presence of 8-CPT is likely to account for the increased neuronal damage found in this model. Other changes included increases in selected GABAA receptor subunit (α1, α2, β2/β3 and γ2) immunoreactivity in the dentate granule cell and molecular layer of the hippocampus suggesting the possible increased formation of GABAA receptors with the subunit configurations α1,β2,γ2 and α1,β3,γ2. Neuronal injury was also accompanied by reactive gliosis and microglial proliferation and astrocytic expression of the growth factors basic fibroblast growth factor (bFGF) and insulin-like growth factor 1 (IGF-1). Long-term sequelae of SE-induced hippocampal damage such as the appearance of spontaneous seizures in the animals and synaptic reorganisation in the supragranular layer of the dentate gyrus were found 1-2 months after SE. The sprouting response was associated with an increase in brain-derived neurotrophic factor (BDNF) immunoreactivity in dentate granule cells and mossy fibre axons 1 and 2 months after SE. A BDNF and clusterin-immunoreactive band was also present in the supragranular layer at 2 months, suggesting these molecules may be involved in mediating the sprouting response. BDNF immunoreactivity was also present in mossy fibre axons in normal and epileptic human hippocampi but was only present in the inner molecular layer in epileptic tissue. In conclusion, the results in this thesis provide some new insights into possible mechanisms of SE development and associated neuropathological changes, which may be applicable to that found in humans.