Input architecture of cortical calretinin interneurons: dendritic synaptome

We have developed an immunohistochemistry-correlated electron microscopy (EM) method - the "mirror" technique - by which long series of EM ultrathin sections can be obtained of neurochemically identified interneurons (INs) with the preservation of high-fidelity ultrastructure. This approach allows quantitative assessment of morphometric parameters of synaptic inputs along the whole length of dendrites originating from the parent soma. Adjoining sections always have cut cell bodies which appear on the facing surface of both sections. Using the mirror-technique, in one of the sections the histochemical marker of the GABAergic subtype, calretinin (CR) was revealed. After identifying immuno-positive neurons which were cut on the surface the remaining part of the very same cell bodies were identified on the surface of the adjoining section. Here we provide quantitative data on the synaptic coverage of two dendrite segments belonging to two CR immuno-positive INs. The spatial distribution of asymmetric and symmetric synapses, surface area and volume of the presynaptic boutons, area extent of the active zones and morphometric parameters of synaptic vesicles were determined.