Correlated somatosensory input in parvalbumin/pyramidal cells in mouse motor cortex

ABSTRACT In mammalian cortex, feedforward excitatory connections recruit feedforward inhibition. This is often carried by parvalbumin (PV+) interneurons, which may densely connect to local pyramidal (Pyr) neurons. Whether this inhibition affects all local excitatory cells indiscriminately or is targeted to specific subnetworks is unknown. Here, we test how feedforward inhibition is recruited by using 2-channel circuit mapping to excite cortical and thalamic inputs to PV+ interneurons and Pyr neurons in female and male mouse motor cortex. Single Pyr and PV+ neurons receive input from both cortex and thalamus. Connected pairs of PV+ interneurons and excitatory Pyr neurons receive correlated cortical and thalamic inputs. While PV+ interneurons are more likely to form local connections to Pyr neurons, Pyr neurons are much more likely to form reciprocal connections with PV+ interneurons that inhibit them. This suggests that Pyr neurons are embedded in local subnetworks. Excitatory inputs to M1 can thus target inhibitory networks in a specific pattern which permits recruitment of feedforward inhibition to specific subnetworks within the cortical column. SIGNIFICANCE STATEMENT Incoming sensory information to motor cortex (M1) excites neurons to plan and control movements. This input also recruits feedforward inhibition. Whether inhibition indiscriminately suppresses cortical excitation or forms specific subnetworks is unclear. Specific differences in connectivity in circuits promoting different movements might assist in motor control. We show that input to connected pairs of pyramidal (Pyr) excitatory neurons and parvalbumin (PV+) inhibitory interneurons is more strongly correlated than non-connected pairs, suggesting the integration of interneurons into specific cortical subnetworks. Despite sparse connections between these cells, pyramidal neurons are vastly more likely (3x) to excite PV+ cells connected to them. Thus, inhibition integrates into specific circuits in motor cortex, suggesting that separate, specific circuits exist for recruitment of feedforward inhibition.