Functional Properties of Cortical Feedback Projections to the Olfactory Bulb

SummarySensory perception is not a simple feed-forward process, and higher brain areas can actively modulate information processing in “lower” areas. We used optogenetic methods to examine how cortical feedback projections affect circuits in the first olfactory processing stage, the olfactory bulb. Selective activation of back projections from the anterior olfactory nucleus/cortex (AON) revealed functional glutamatergic synaptic connections on several types of bulbar interneurons. Unexpectedly, AON axons also directly depolarized mitral cells (MCs), enough to elicit spikes reliably in a time window of a few milliseconds. MCs received strong disynaptic inhibition, a third of which arises in the glomerular layer. Activating feedback axons in vivo suppressed spontaneous as well as odor-evoked activity of MCs, sometimes preceded by a temporally precise increase in firing probability. Our study indicates that cortical feedback can shape the activity of bulbar output neurons by enabling precisely timed spikes and enforcing broad inhibition to suppress background activity.

► AON axons make monosynaptic excitatory connections on many classes of OB interneurons
► AON activity evokes disynaptic inhibition in MCs through multiple inhibitory circuits
► Unexpectedly, AON axons directly excite MCs and can trigger precisely timed spikes
► AON activity can modulate OB responses to odors