Preferential inputs from cholecystokinin-positive neurons to the somatic compartment of parvalbumin-expressing neurons in the mouse primary somatosensory cortex.

Parvalbumin-positive (PV+) neurons in the cerebral cortex, mostly corresponding to fast-spiking basket cells, have been implicated in higher-order brain functions and psychiatric disorders. We previously demonstrated that the somatic compartment of PV+ neurons received inhibitory inputs mainly from vasoactive intestinal polypeptide (VIP)+ neurons, whereas inhibitory inputs to the dendritic compartment were derived mostly from PV+ and somatostatin (SOM)+ neurons. However, a substantial number of the axosomatic inputs have remained unidentified. Here we show preferential innervation of the somatic compartment of PV+ neurons by cholecystokinin (CCK)+ neurons in the mouse primary somatosensory cortex. CCK+ neurons, a minor population of GABAergic neurons (3.2%), displayed no colocalization with PV or SOM immunoreactivity but partial overlap with VIP immunoreactivity (27.7%). Confocal laser scanning microscopy observation of CCK+ synaptic inputs to PV+ neurons revealed that CCK+ neurons preferred the somatic compartment to the dendritic compartment of PV+ neurons and provided approximately 33% of the axosomatic inhibitory inputs to PV+ neurons. Additionally, 20.9% and 12.1% of the axosomatic inputs were derived from CCK+/VIP+ and CCK+/VIP-negative (-) neurons, presumably double bouquet and large basket cells, respectively. Furthermore, the densities of the axosomatic inputs from CCK+ and/or VIP+ neurons to PV+ neurons were not significantly different among the cortical layers. The present findings suggest that, by preferentially innervating the cell bodies of PV+ neurons, both CCK+/VIP- basket and CCK+/VIP+ double bouquet cells might efficiently interfere with action potential generation of PV+ neurons, and that the two types of CCK+ neurons might have a large impact on cortical activity through PV+ neuron inhibition.