Synaptic contacts between identified neurons visualized in the confocal laserscanning microscope. Neuroanatomical tracing combined with immunofluorescence detection of post-synaptic density proteins and target neuron-markers

The axons of neurons in the CNS with their delicate ramification patterns and terminal boutons can be visualized with conventional neuroanatomical techniques with a high degree of accuracy. Whether identified terminal boutons form synaptic contacts with target neurons identified by a second and different marker needs resolution beyond that offered by conventional light microscopy. The morphological elements associated with synaptic connectivity consist of specialized pre- and post-synaptic junctional complexes known as the pre- and post-synaptic densities. Electron microscopy of these junctional complexes consumes much time and resources. In an attempt to increase the speed with which we can analyze networks of neurons we developed a high-resolution triple-fluorescence approach including neuroanatomical tracing, immunofluorescence, confocal laserscanning and 3D-computer reconstruction to pinpoint at the light microscopic level the three elements involved in synaptic connectivity: afferent fibers and their terminal boutons, close apposition with neurons identified by the presence of a fluorescent marker, and sandwiched in between a post-synaptic density marker. We used morphological criteria for the detection of axon terminals (swellings on fibers). Antibodies against ProSAP2/Shank3, a post-synaptic density-associated scaffolding protein, were used to pinpoint the location of the synaptic junctions. The results show the existence of sandwich-like configurations: pre-synaptic fiber, ProSAP2/Shank3, post-synaptic neuron. Thus we feel that we can minimize (and perhaps completely eliminate) the need for electron microscopy and hence dramatically increase the overall efficiency of neuroanatomical tracing and network analysis.