COMPLETE SYNAPTIC COVERAGE OF DENDRITES OF CALCIUM-BINDING PROTEIN EXPRESSING INTERNEURONS
Quantitative data on single GABAergic inhibitory interneurons (INs) as to the types and distribution of synaptic input on their dendrites are quite rudimentary. Our aim is to generate a quantitative electron microscopic (EM) database of the complete synaptic coverage of major subtypes of INs in the mouse primary sensory cortices. Here we report our newly-developed immunohistochemistry-correlated EM method (“mirror” technique) that proved successful in allowing of tracing long dendrite segments from their soma origin, and generates utmost ultrastructural quality which is free of deficits caused by the immunohistochemical procedure. Coronal vibratome sections were collected from tissue blocks containing the V1. Calbindin-D28K (CB), calretinin (CR) and parvalbumin (PV) immuno-positive INs were chosen from different cortical layers and identified by the “mirror” technique. 50 nm ultrathin serial sections covering the whole thickness of the vibratome sections were collected and processed for transmission EM analysis. Dendrites, belonging to CB, CR or PV containing INs, were traced (photographed) and reconstructed in 3D. The following synaptic parameters were determined along the dendrites: distribution of synapses; surface area and volume of presynaptic boutons; morphometric parameters of synaptic vesicles; area of the active zones. Significant differences were detected between CB+ and CR+ INs, including spatial distribution and morphometric parameters of the presynaptic boutons whereas PV+ INs were less different from the other INs as to the above metrics. The findings will be used in realistic neuron modelling of IN populations. Supported by the Human Brain Project (785907-SGA2) and the TKI (Nr.11008).