Expression of synaptophysin in sprouting neurons after entorhinal lesion in the rat

Expression of the synaptic vesicle protein synaptophysin was studied in lesion-induced sprouting neurons of the contralateral entorhinal cortex and in the contralateral dentate gyrus using immunocytochemistry at the light- and electron-microscopic level. Perikaryal immunoreactivity for synaptophysin was found between 8 and 10 days postlesion. Light microscopy revealed that synaptophysin immunostaining was present in almost all neurons of layers II and III of the contralateral medial entorhinal cortex. These neurons give rise to the sprouting, crossed temporodentate pathway. In addition, some hilar neurons of the contralateral dentate gyrus, which are the parent cells of sprouting commissural fibers, were immunostained for synaptophysin. Transient immunostaining for synaptophysin was observed within cell bodies and dendrites. Additionally, the cell bodies were outlined by immunoreactive puncta, identified by electron microscopy as nerve terminals. Our results revealed that sprouting neurons express the major synaptic vesicle protein synaptophysin during reactive synaptogenesis in a pattern that reflects biosynthesis and sorting of this protein as seen in developing neurons during synapse formation.