Forebrain cholinergic plasticity in rats with chronic epilepsy induced by status epilepticus

Abstract

The brain cholinergic system may undergo structural and functional alterations both in human epilepsy and in respective animal models, but the causal relationships between these alterations and epilepsy remain to be established. In this study, we attempted to examine how the inhibition of epilepsy-related cholinergic plasticity may be reflected in seizure susceptibility and/or in the development of chronic epilepsy and its neurological consequences. For this purpose, adult Wistar rats received intrahippocampal injections of low doses of 192 IgG-saporin (SAP) to produce a moderate, but significant loss of septohippocampal cholinergic cells and to suppress their plasticity. Then, animals were treated with kainic acid to induce status epilepticus, which leads to the development of chronic epilepsy later in life. It was found that SAP-pretreatment was associated with longer latency to the onset of status epilepticus and with reduced mortality rate, suggesting that increased activity of septal cholinergic cells may potentiate seizures. Interestingly, months later, a greater percentage of rats with intact septohippocampal cholinergic connections showed spontaneous seizures, when compared to SAP-pretreated rats. Treatment with kainic acid produced death of 40-50% of hippocampal neurons and this effect was not ameliorated by prior cholinergic depletion. Moreover, the kainate induced cognitive deficits were detected in both SAP-pretreated and sham-pretreated groups. These data suggest that seizure-induced plasticity of cholinergic cells may indeed enhance seizure susceptibility and contribute to epileptogenic processes. They do not support the hypothesis that epilepsy-related hypertrophy of cholinergic neurons may potentiate hippocampal cell loss and respective behavioral impairments. In a subsequent part of our project, we studied the effects of epilepsy on GABAergic neurons of the medial septum. Immunohistochemistry techniques were used to identify cells stained for parvalbumin known to be co-expressed by septal GABAergic neurons. We found a 35% reduction in this neuronal population, which can justify some of the clinical findings in epileptic individuals, like learning and memory impairments. Also, we established a functional interaction between the GABAergic and cholinergic systems by showing that cholinergic fiber varicosities are co-localized with perikarya and neurites immunoreactive for parvalbumin in both KA-treated and control animals. Notably, the density of cholinergic fiber varicosities in the medial septum was increased by 45% in epileptic animals, suggesting enhanced cholinergic drive upon septohippocampal GABAergic cells, which can be explained as a compensatory mechanism for the decrease of GABAergic neurons, or as a contributing factor for septohipocampal pathway disruption and, ultimately, epileptogenesis.