Diverse types of interneurons generate thalamus-evoked feedforward inhibition in the mouse barrel cortex

Sensory information, relayed through the thalamus, arrives in the neocortex as excitatory input, but rapidly induces strong disynaptic inhibition that constrains the cortical flow of excitation both spatially and temporally. This feedforward inhibition is generated by intracortical interneurons whos e precise identity and properties were not known. To characterize interneur ons generating feedforward inhibition, neurons in layers IV and V of mouse somatosensory ("barrel") cortex in vitro were tested in the cell-attached c onfiguration for thalamocortically induced firing and in the whole-cell mod e for synaptic responses. Identification as inhibitory or excitatory neuron s was based on intrinsic firing patterns and on morphology revealed by intr acellular staining. Thalamocortical stimulation evoked action potentials in similar to 60% of inhibitory interneurons but in <5% of excitatory neurons . The inhibitory interneurons that fired received fivefold larger thalamoco rtical inputs compared with nonfiring inhibitory or excitatory neurons. Tha lamocortically evoked spikes in inhibitory interneurons followed at short l atency the onset of excitatory monosynaptic responses in the same cells and slightly preceded the onset of inhibitory responses in nearby neurons, ind icating their involvement in disynaptic inhibition. Both nonadapting (fast- spiking) and adapting (regular-spiking) inhibitory interneurons fired on th alamocortical stimulation, as did interneurons expressing parvalbumin, calb indin, or neither calcium-binding protein. Morphological analysis revealed that some interneurons might generate feedforward inhibition within their o wn layer IV barrel, whereas others may convey inhibition to upper layers, w ithin their own or in adjacent columns. We conclude that feedforward inhibi tion is generated by diverse classes of interneurons, possibly serving diff erent roles in the processing of incoming sensory information.