Fast and slow feedforward inhibitory circuits for cortical odor processing

Feedforward inhibitory circuits are key contributors to the complex interplay between excitation and inhibition in the brain. Little is known about the function of feedforward inhibition in the primary olfactory (piriform) cortex. Using in vivo two-photon-targeted patch clamping and calcium imaging in mice, we find that odors evoke strong excitation in two classes of interneurons - neurogliaform (NG) cells and horizontal (HZ) cells - that provide feedforward inhibition in layer 1 of the piriform cortex. NG cells fire much earlier than HZ cells following odor onset, a difference that can be attributed to the faster odor-driven excitatory synaptic drive that NG cells receive from the olfactory bulb. As a result, NG cells strongly but transiently inhibit odor-evoked excitation in layer 2 principal cells, whereas HZ cells provide more diffuse and prolonged feedforward inhibition. Our findings reveal unexpected complexity in the operation of inhibition in the piriform cortex.

Automated summary

Excitation triggered by odors in the primary olfactory cortex activates two types of interneurons - neurogliaform (NG) cells and horizontal (HZ) cells - which provide feedforward inhibition. NG cells fire earlier than HZ cells due to a faster excitatory synaptic drive, leading to strong but transient inhibition in layer 2 principal cells, while HZ cells offer more diffuse and lasting feedforward inhibition. These findings highlight the intricate operations of inhibition in the piriform cortex.