MAPPING OF INHIBITORY SYNAPSE SUBTYPES ONTO L2/3 PYRAMIDAL NEURONS IN MOUSE VISUAL CORTEX

It is increasingly evident that inhibitory circuits play a key role in neurological deficits as well as experience-dependent plasticity. Yet, we know little regarding the differential contribution of inhibitory synaptic subtypes. Inhibitory signaling in the cortex originates from GABAergic interneurons, which are broadly divided into three classes, based on their distinct expression of Somatostatin (SST), Parvalbumin (PV) and certain Serotonin-receptors. While there is a body of research showing that synaptic transmission differs functionally between these classes, studies examining their cellular targeting onto pyramidal neurons are often limited in resolution and/or coverage of the dendritic arbor.
Here we used Cre-dependent Synaptophysin-TdTomato expression to comprehensively label inhibitory subtype-specific presynaptic boutons synapsing onto sparsely labelled, YFP-cell-filled pyramidal neurons in Layer 2/3 of binocular mouse visual cortex. Synaptic contacts were then validated and proteomically characterized with epitope-preserving Magnified Analysis of Proteome, a combination of tissue clearing and expansion microscopy.
We were thus able to map the sub-cellular distribution of inhibitory synapses across the whole dendritic arbor of pyramidal neurons in relation to their inhibitory afferent identity. Contrary to popular belief, we found that SST synapses account for only 68% of dendritic inhibitory synapses, and that PV-expressing interneurons, in addition to innervating somata, account for another 25% of dendritic inhibitory synapses. Further, we see that subtype-specific synapses are heterogeneously distributed across individual dendrites with subtype-specific differences in synaptic clustering. This work is an important step in defining subtype-specific distribution of inhibitory synapses, towards understanding their distinct roles in dendritic computation and circuit function.