PERSISTING AXONAL DEGENERATION IN THE HIPPOCAMPUS AFTER TRANSECTION OF THE FIMBRIA-FORNIX

Degeneration within the hippocampus was examined at the light microsco pic level using the Gallyas silver stain two, four or nine months afte r bilateral transection of the fimbria-fornix and commissural connecti ons. At two or four months after the lesion the strata oriens and radi atum of the subicular end of the CA1 subfields were strongly argyrophi lic as was the inner third of the molecular layer of the dentate gyrus . At nine months post-lesion argyrophilia diminished but clearly persi sted in the same layers. Electron microscopic examination revealed a l arge number of electron-dense axon terminals in the argyrophilic areas , most of them making asymmetric synaptic contacts with dendritic spin es. These findings suggest that at least a portion of the Schaffer col laterals of the CA3 pyramidal cells and associational collaterals of h ilar neurons were in a process of acute degeneration at all time point s after the initial surgical trauma. This persistent synaptic reorgani zation of intrahippocampal circuits may be related to abnormal electri cal activity observed several months after fimbria-fornix transection.