Regional specificity of raphe graft-induced recovery of behavioral functions impaired by combined serotonergic/cholinergic lesions.

We compared the effects of embryonic raphe grafted into either the hippocampus or the entorhinal cortex on the ability of rats to perform a spatial memory water-maze task. Serotonin depletion or partial cholinergic lesion of the hippocampus (by injection of colchicine into the septum) did not affect the ability of rats to perform the task, but the combined treatment did. Double-lesioned rats, with raphe grafts in the hippocampus, but not in the entorhinal cortex, performed similar to control or single-lesioned rats. The results suggest that the functioning of the serotonergic innervation of the hippocampus, and not of its afferents, is crucial for the ability of rats to perform spatial memory tasks, especially when the septohippocampal cholinergic connection is disrupted.