Reduced cholinergic status in hippocampus produces spatial memory deficits when combined with kainic acid induced seizures.

ABSTRACT

Alzheimer's disease is the most common form of dementia in North America today. Though many risk factors have been suggested to increase the likelihood of developing this disease, an accurate etiology has yet to be described. One of these risk factors commonly associated with Alzheimer's disease is the loss of cholinergic neurons of the medial septum that project to the hippocampus, leading to depletion in cholinergic activity. A second risk factor is the presence of seizures, which can increase the risk of excitotoxic cell death. To examine the interaction between these two common risk factors, we gave rats a focal cholinergic lesion of the medial septum using the specific immunotoxin 192-IgG Saporin, followed 2 weeks later by a non-convulsive dose of kainic acid. We then assessed the rats for seizure severity, hippocampal damage and performance on a spatial memory task. The combination of the two factors resulted in a trend towards increased seizure severity in the cholinergic depleted rats, but more importantly, the lesioned rats that had non-convulsive seizures were significantly impaired on a spatial version of the Morris water maze when compared with either the rats with a cholinergic depletion or non-convulsive seizure alone. This result could not be explained by seizure severity or the extent of hippocampal damage, suggesting a more subtle interaction between these two risk factors in the development of a hippocampal based memory impairment.