STRUCTURE AND PLASTICITY OF NEWLY FORMED ADULT SYNAPSES - A MORPHOMETRIC STUDY IN THE RAT HIPPOCAMPUS

Increasing evidence suggests that synaptic structure represents a plas tic feature of the neuron, although the plastic nature of newly formed and existing adult synapses has not yet been fully characterized. Fol lowing ipsilateral entorhinal cortical lesions, the rat dentate gyrus offers an excellent model for studying synaptogenesis and plasticity i n the adult central nervous system. Unilateral entorhinal lesions were performed in young adult male rats. Synaptic counts and structural fe atures were quantified at 3, 6, 10, 15, and 30 days post-lesion. The l esions resulted in an 88% synaptic loss in the denervated dentate midd le molecular layer, which was followed by a period of rapid synaptogen esis. Synaptic element size decreased during the period of maximal syn aptogenesis, which was associated with a peak in the presence of non-v esicular and perforated synapses. Following this period, synapses show ed a gradual increase in the size of their pre- and postsynaptic eleme nts. These data support the suggestion that newly formed adult synapse s have smaller synaptic components than existing adult synapses (resem bling synapses seen during development), and increase in size over tim e with usage. The results are discussed in terms of synaptic structura l development and plasticity in the adult central nervous system.