Comparative analyses of synaptic densities during reactive synaptogenesis in the rat dentate gyrus

Advancements in the field of synaptic plasticity have created the need for a reexamination of classic paradigms using new and more precise techniques. One prime candidate for such a reexamination is the process of reactive synaptogenesis (RS). Since the time course of RS was initially outlined in the 1970s and 1980s, advances in stereology have allowed for better characterization of synaptic ultrastructure. Thus, a reexamination was undertaken in the hippocampal dentate gyrus by assessing the densities and proportions of several synaptic subtypes in Long-Evans hooded rats at 3, 6, 10, 15 and 30 days following induction of unilateral lesions of the entorhinal cortex. Although initial synaptic loss in the denervated region was similar to previous reports, recovery during the first 30 days is not as dramatic as previously observed. Following lesioning, concave and perforated synapses retained pre-lesion density despite massive degeneration, underscoring their theoretical importance in plasticity and maintenance of neural function. Convex synapses showed opposite changes, having implications for excitation/inhibition imbalance following lesion induction. These complementary alterations in synaptic structures support ultrastructural changes as a means for compensation following synaptic loss. Nearby areas also seem to participate in this response, with a striking similarity to other models of plasticity, such as long-term potentiation.