Cholinergic neurons in the rat hippocampus do not compensate for the loss of septohippocampal cholinergic fibers.

In recent studies a small number of choline acetyltransferase (ChAT)-immunoreactive, supposedly cholinergic, neurons intrinsic to the rat hippocampus have been described. Here we report that these neurons are not capable of sprouting in response to removal of the cholinergic input to the hippocampus from the medial septum/diagonal band complex. One month after unilateral transection of the fimbria-fornix an almost complete lack of cholinergic fibers persists in all layers of the dorsal hippocampus and fascia dentata ipsilateral to the lesion when compared to the contralateral hippocampus or to unlesioned control rats. These results indicate that the well-known phenomenon of collateral sprouting in response to partial deafferentation is a specific process that spares a distinct group of cholinergic cells in the rat hippocampus.