Synaptic rearrangement in the dentate gyrus: histochemical evidence of adjustments after lesions in immature and adult rats.

In immature animals, ablation of the entorhinal cortex elicited a rapid intensification of acetylcholinesterase (EC 3.1.1.7) staining in the outer one-quarter of the molecular layer of the dentate gyrus. Subsequent lesions of the septum eliminated this acetylcholinesterase intensification. Electron-microscopic histochemical analysis demonstrated a 30-fold increase in the number of acetylcholinesterase-positive synaptic endings in the intensification zone. The acetylcholinesterase augmentation thus appears attributable, in part at least, to an increase in the number of acetylcholinesterase-rich synaptic endings established by septo-hippocampal fibers. Observations in a comparative study of immature and adult rats point to the animal's developmental state as a major determinant of differences in these lesion-induced neuronal adjustments.