Studies on the 3D ultrastructure of synaptic inputs to distinct GABAergic neurons in the primary visual cortex

Abstract

The dendritic spine is a representative post-synaptic structure found primarily in pyramidal neurons, functions as an amplifying postsynaptic potential. Recently, these structures have been revealed sparsely in GABAergic interneurons, but it is not known how these structures are distributed differently in subtypes divided by gene expression patterns. To address this question, the intensive ultrastructural approach can be implemented about the input structure identification nearby spine structures. In this paper, I used serial block-face scanning electron microscopy (SBEM) to reconstruct and compare perisomatic synaptic input structures between somatostatin- (SST$^+$) and parvalbumin- (PV$^+$) neurons in the primary visual cortex (V1) of mice. The results demonstrated that a subset of SST$^+$ neurons has highly dense spines on their perisomatic structure, while PV$^+$ neurons have a low density of spines. The spines from spiny SST$^+$ neurons showed a larger number of synapses and volumes than a classical pyramidal neuron. Further evaluation of the synaptic inputs onto each cell-type showed that only spiny SST$^+$ neurons can have an equivalent density of excitatory inputs through synapses on the spine, while other aspiny SST$^+$ neurons receive less excitatory inputs on their shaft structure compared to PV$^+$ neurons. Moreover, further analysis of the synaptic structure pattern of inputs illustrated that excitatory and inhibitory axons predominantly form multi-synaptic boutons (MSBs) and multi-bouton inputs (MBIs), respectively. Hence, these results imply the different roles of spiny and aspiny GABAergic neurons in the cortical network and unique mechanism of excitatory and inhibitory inputs in cortical circuitry.