Ultrastructure of axons in stereotaxically placed ibotenic acid-induced lesions of the hippocampus in the adult rat. Evidence for demyelination and degeneration of dispersed axons of passage.

The fate of axons in lesions following stereotaxic injections of ibotenic acid into the rat hippocampus was studied on the ultrastructural level. Myelinated and unmyelinated axons were examined in the, presumably, axon sparing lesions induced by circumscribed in vivo injections of ibotenic acid. In lesions placed in the dentate gyrus or parts of the Ammon's horn a considerable amount of dispersed myelinated axons of passage were affected, showing a characteristic pattern of demyelination and suggesting the involvement of oligodendroglia processes. However, the structural integrity of unmyelinated axons of passage, studied with the example of the mossy fiber system, was completely unaffected after exposure to ibotenic acid. Thus, at least in the hippocampus, in situ injections of ibotenic acid cannot be looked upon as being completely axon sparing as frequently proposed.