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Widespread lesions of forebrain cholinergic or noradrenergic projections by intraventricular administration of 192 IgG-saporin or 6-hydroxydopamine, respectively, accelerate kindling epileptogenesis. Here we demonstrate both quantitative and qualitative differences between the two lesions in their effects on hippocampal kindling in rats. Epileptogenesis was significantly faster after noradrenergic as compared to cholinergic denervation, and when both lesions were combined, kindling development resembled that in animals with 6-hydroxydopamine lesion alone. Furthermore, whereas the 192 IgG-saporin lesion promoted the development only of the early stages of kindling, administration of 6-hydroxydopamine or both neurotoxins accelerated the late stages also. To investigate the contribution of different subparts of the basal forebrain cholinergic system to its seizure-suppressant action in hippocampal kindling, 192 IgG-saporin was injected into medial septum/vertical limb of the diagonal band of Broca or nucleus basalis magnocellularis, leading to selective hippocampal or cortical cholinergic deafferentation, respectively. The denervation of the hippocampus facilitated kindling similar to the extensive lesion caused by intraventricular 192 IgG-saporin, whereas the cortical lesion had no effect. These results indicate that although both noradrenergic and cholinergic projections to the forebrain exert powerful inhibitory effects on hippocampal kindling epileptogenesis, the action of the cholinergic system is less pronounced and occurs specifically prior to seizure generalization. In contrast, noradrenergic neurons inhibit the development of both focal and generalized seizures. The septo-hippocampal neurons are responsible for the antiepileptogenic effect of the cholinergic system in hippocampal kindling, whereas the cortical projection is not significantly involved. Conversely, we have previously shown [Ferencz I. et al. (2000) Eur. J. Neurosci., 12, 2107-2116] that seizure-suppression in amygdala kindling is exerted through the cortical and not the hippocampal cholinergic projection. This shows that, depending on the location of the primary epileptic focus, i.e. the site of stimulation, basal forebrain cholinergic neurons operate through different subsystems to counteract seizure development in kindling.

Intraventricular 192 IgG-saporin was used to induce a selective lesion of basal forebrain cholinergic neurons in rats. When subjected to 40 rapid hippocampal kindling stimulations with 5-min intervals, these animals exhibited increased number of generalized seizures and a higher mean seizure grade in response to the first five stimulations, and required fewer stimuli to develop focal behavioural seizures, as compared to non-lesioned rats. In contrast, both groups showed similarly enhanced responsiveness when test stimulated four weeks later. Using in situ hybridization, cholinergic denervation was found to cause a significant decrease of basal brain-derived neurotrophic factor messenger RNA levels in the hippocampal formation and piriform cortex, whereas gene expression for nerve growth factor, neurotrophin-3, and TrkB and TrkC was unchanged. Four weeks after rapid kindling stimulations, basal levels of brain-derived neurotrophic factor messenger RNA in the dentate granule cells were restored to normal in the lesioned rats, whereas neurotrophin-3 messenger RNA levels were decreased. No differences in the seizure-evoked levels of neurotrophin and Trk messenger RNAs were detected, except in the dentate granule cell layer, which had significantly higher brain-derived neurotrophic factor messenger RNA expression in the lesioned animals at 2 h. In conclusion, the basal forebrain cholinergic system (i) dampens the severity of recurring seizures induced by rapid hippocampal kindling stimulations, but has no effect on the subsequent delayed phase of epileptogenesis; and (ii) exerts a tonic stimulation of basal brain-derived neurotrophic factor messenger RNA levels in the hippocampal formation and piriform cortex. The findings also indicate that the cholinergic lesion does not affect neurotrophin and Trk gene expression after recurring seizures, and that the kindling process leads to long-term changes in basal brain-derived neurotrophic factor and neurotrophin-3 messenger RNA levels in the denervated animals.

The immunotoxin 192 IgG-saporin induces an efficient and specific lesion of low-affinity nerve growth factor receptor-bearing cholinergic neurons in the basal forebrain. Intraventricular injection of 192 IgG-saporin, which caused a complete loss of cholinergic afferents to the hippocampus and neocortex and a partial denervation of amygdala and piriform cortex, was found to markedly facilitate the initial stages of seizure development in hippocampal kindling. In contrast, the progression of kindling process from focal to generalized seizures was not affected. In situ hybridization demonstrated that basal levels of brain-derived neutrotrophic factor messenger RNA in the hippocampal formation and piriform cortex were significantly decreased by the lesion, which also attenuated the seizure-induced increase of brain-derived neurotrophic factor messenger RNA expression in the hippocampus and frontal cortex. In the dentate gyrus, the 192 IgG-saporin lesion selectively reduced the upregulation of messenger RNAs for brain-derived neurotrophic factor exons I and III after a generalized seizure, whereas the increase of exon II messenger RNA was unchanged. The lesion abolished the seizure-evoked increase of nerve growth factor and TrkC messenger RNA levels and decrease of neutrophin-3 messenger RNA expression in dentate granule cells, while TrkB messenger RNA levels were not affected. We conclude that the basal forebrain cholinergic system (1) suppresses kindling epileptogenesis in the hippocampus, and (2) enhances both basal and seizure-evoked brain-derived neurotrophic factor synthesis in the hippocampal formation and some cortical areas through a specific pattern of activation of promoters within the brain-derived neurotrophic factor gene.