ELECTRON-MICROSCOPIC STUDY OF LESION-INDUCED SYNAPTOGENESIS IN DENTATE GYRUS OF ADULT RAT .1. MAGNITUDE AND TIME COURSE OF DEGENERATION

Synapses in the rat dentate gyrus are rapidly lost after removal of the primary input from the entorhinal cortex. The extent and time course of degeneration was described. The synapses of entorhinal afferents were remarkably concentrated in their zone of termination. Unilateral removal of the rat entorhinal cortex resulted in the loss of about 86% of all synapses in the outer 3/4 of the molecular layer of the ipsilateral dentate gyrus. Entorhinal synapses were all asymmetric (Gray type I) and terminate on dendritic spines. Analysis of the degeneration reaction provided a means to examine the characteristics of the loss of a relatively homogeneous afferent on a single cell type. The morphological characteristics of the degenerating terminals showed some heterogeneity; both the electron lucent and electron dense types of degenerating terminals were identified. The electron lucent type was observed only at short survival times. The time course of the loss of degenerating terminals was resolvable into 2 components, each of which followed 1st order decay kinetics. Thus degenerating entorhinal terminals behaved as a population which disappeared randomly at a rate dependent on the fraction of terminals present at any time. The loss of degenerating terminals was accompanied by the loss of postsynaptic sites. At short survival times the majority of postsynaptic sites (defined by the presence of a postsynaptic density) had disappeared. There was also a loss of complex spines and some shrinkage of the molecular layer.