Selective lesion of medial septal cholinergic neurons followed by a mini-stroke impairs spatial learning in rats

Reduced levels of hippocampal acetylcholine are a common finding in patients diagnosed with Alzheimer's disease, but it remains unclear what role this depletion plays in the development of dementia. It is possible that the reduced levels of acetylcholine increases the vulnerability of hippocampal neurons to future insults which could lead to neuronal death and cognitive impairment. One insult that is commonly observed in the demented elderly and often co-exists with Alzheimer's disease is stroke. In the current experiment, we used the immunotoxin 192 IgG-Saporin to specifically lesion the cholinergic neurons of the medial septum that project to the hippocampus. We then explored the effects of small, localised strokes in the hippocampus on spatial learning and memory. The combination of cholinergic depletion and stroke resulted in significant impairment on the spatial water maze compared to the performance of rats receiving either factor alone. Quantification of hippocampal damage revealed no difference in the overall lesion size of stroke-only or combined (cholinergic depletion and stroke) rats, suggesting that a more subtle mechanism is responsible for the observed impairment. We propose that healthy hippocampal neurons may normally be able to withstand, and compensate for a small ischemic insult. However, in the absence of cholinergic projections from the medial septum, these compensatory processes in the hippocampus may be compromised resulting in the spatial learning impairment reported here. This suggests an association between the cholinergic depletion observed during aging and the potential for functional recovery following stroke.