Aberrant phosphorylation of neurofilaments accompanies transmitter-related changes in rat septal neurons following transection of the fimbria-fornix

Lesions of the fimbria-fornix (FF) have been reported to cause retrograde changes in neurons of the medial septal nucleus (MSN). To analyze the nature and time course of these events, we investigated changes in cytoskeletal elements (phosphorylated and nonphosphorylated neurofilament (NF) proteins) and transmitter-related enzymes (choline acetyltransferase (ChAT) in MSN neurons following FF transection. During the first week postlesion, ChAT immunoreactivity and size of many perikarya were reduced. Irregular, swollen cholinergic fibers appeared first at postlesion day 2 in caudal septum and soon spread rostrally, reaching rostral septum by day 7. A few perikarya developed abnormal accumulations of phosphorylated NFs. At postlesion days 7-10, many neurons did not stain for ChAT. Phosphorylated NFs were present in many perikarya. At this time, cell loss was apparent in Nissl-stained material. Cholinergic cell loss continued through postlesion week 6-8 but at a much slower rate than during the first week. Phosphorylated NF accumulations in MSN perikarya persisted until postlesion week 6, disappearing thereafter. Double-immunostaining procedures showed that MSN neurons expressed both ChAT and phosphorylated NF immunoreactivity at postlesion day 3; however, at days 7 and 14, cells that accumulated phosphorylated NFs did not stain for ChAT. The results of this study indicate that FF transection leads to perikaryal shrinkage with loss of ChAT immunoreactivity, perikaryal phosphorylation of NFs, cholinergic fiber abnormalities, and cell loss. Recent evidence suggests that reduction of transmitter markers and aberrant phosphorylation of NFs may be involved in the pathogenesis of several neurodegenerative disorders, including Alzheimer's disease. Therefore, FF transection provides a useful animal model for further investigations of complex disorders of the central nervous system that involve degeneration of transmitter-specific pathways.