Immunocytochemical analysis of synaptic proteins provides new insights into diabetes-mediated plasticity in the rat hippocampus.

The hippocampus, an important integration center for learning and memory in the mammalian brain, undergoes neurological changes in response to a variety of stimuli that are suggestive of ongoing synaptic reorganization. Accordingly, the aim of this study was to identify markers of synaptic plasticity using rapid and reliable techniques such as radioimmunocytochemistry and confocal microscopy, thereby providing a "birds-eye view" of the whole hippocampus under hypercorticosteronemic conditions. The regulation of microtubule-associated protein 2, synaptophysin and postsynaptic density-95 was examined in two different animal models of hypercorticosteronemia: corticosterone administration and streptozotocin-induced diabetes using both a short-term (1 week) and long-term (5 weeks) treatment. Glucocorticoids and/or hyperglycemia increased synaptophysin expression in CA1, CA3 and the dentate gyrus, regions that exhibit synaptic plasticity in response to glucocorticoid exposure. In these models, postsynaptic density-95 expression increased in the CA3 region, particularly in the diabetic rats, while microtubule-associated protein 2 exhibited more selective changes. Fluoro-Jade histochemistry did not detect neuronal damage, suggesting that glucocorticoids and/or hyperglycemia induce plastic and not irreversible neuronal changes at these time points. Collectively, these results demonstrate that changes in the expression and distribution of synaptic proteins provide another measure of synaptic plasticity in the rat hippocampus in response to glucocorticoid exposure, changes that may accompany or contribute to neuroanatomical, neurochemical, and behavioral changes observed in experimental models of type 1 diabetes.