C6-O-04An excitatory and inhibitory synapse density on various nonpyramidal cells in the rat cerebral cortex

The neocortex has numerous classes of GABAergic nonpyramidal cells that are distinct in their firing, morphological, and chemical characteristics. Morphological studies show distinct axonal projection patterns from these different classes of nonpyramidal neuron, suggesting selective synapse formation onto postsynaptic targets. However, the dendritic inputs to individual GABA cell subtypes have been rarely studied. In order to reveal rules governing selective synapse formation among and between GABAergic neurons, we labeled the somata and dendrites of 5 chemically defined nonpyramidal neuron subtypes positive for somatostatin (SOM), substance P receptor (SPR), parv album in (PV), calretinin or CCK by pre-embedding immunohistochemistry. Excitatory and inhibitory synapses onto the stained dendrites were investigated by postembedding GABA immunohistochemistry in ultra thin sections using conventional serial electron microscopy. We subsequently reconstructed somata and dendrites with identified synaptic inputs three-dimensionally and quantified synapse morphologies and synapse distributions along identified dendritic segments. Whereas the density of inhibitory synapses onto the soma was not different among neuronal subtypes, the density of excitatory input was correlated with postsynaptic neuronal chemical identity. SOM-only expressing somata had a higher proportion of inhibitory somatic input than did PV-only expressing somata or SPR-positive somata which were also SOM-positive. To estimate total number of the excitatory and inhibitory synaptic inputs on the cell subtypes, we further investigated the synapse distribution on whole cell recorded FS basket cell, Martinotti cell, double bouquet cell and large basket cell using the EM reconstruction method. We found they received 1100-4400 excitatory and 500-1000 inhibitory synapse inputs.