Inhibitory circuit motifs of cortical somatosensory layer 5 SST interneurons are uniform within layers but specific across layers.

Layer (L) 5 is a hub in the cortical column in which a multitude of feedforward and feedback pathways converge. These inputs are then transmitted to distant sites by resident pyramidal neurons (PN). L5 PN are under the strong influence of local somatostatin (SST)-expressing interneurons (IN). To better understand the inhibitory control of L5 SST cells, which leads to disinhibition of excitatory cells, we used paired whole-cell patch-clamp recordings in acute brain slices. We investigated whether they receive intra- and translaminar inputs by parvalbumin (PV) and vasoactive intestinal polypeptide (VIP) IN and what type of short-term synaptic plasticity these inputs display. In triple transgenic mice we found that intralaminarly both PV and VIP IN effectively target L5 SST IN. PV to SST connections were depressing at all tested frequencies, whereas VIP to SST connections were facilitating at high-frequency VIP IN stimulation. In addition translaminar inputs from L2/3 VIP to L5 SST IN showed similar connectivity rates and short-term plasticity compared to their L5 counterparts. However L2/3 PV IN, despite numerous descending axon collaterals, showed hardly any connection. In summary we are able to show that intralaminar circuit motifs of L5 SST IN resemble those previously studied in L2/3. Furthermore we demonstrate a selective translaminar targeting by L2/3 VIP IN that was missing from PV IN. These results shine new light on the circuit layout that enables intra- and translaminar dis/inhibitory processing in the cortical column. KEY POINTS: L5 somatostatin (SST)-expressing interneurons (IN) are widely targeted by other GABAergic IN in their home layer. The origin of this afferent inhibition is from parvalbumin (PV), as well as vasoactive intestinal peptide (VIP)-expressing GABAergic IN within L5, whereas L2/3 translaminar input originates from VIP but not from PV IN. Many of these connections are formed in a bidirectional manner. PV to SST and VIP to SST connections displayed cell type-specific differences in unitary synaptic properties and short-term plasticity. Our results help us to better understand intra- and translaminar dis/inhibitory processing, which might optimize tactile information processing in the cortical column.