Centrifugal Innervation of the Olfactory Bulb: A Reappraisal.

The inter-regional connectivity of sensory structures in the brain allows for the modulation of sensory processing in manners important for perception. In the olfactory system, odor representations in the olfactory bulb (OB) are modulated by feedback centrifugal innervation from several olfactory cortices, including the piriform cortex (PCX) and anterior olfactory nucleus (AON). Previous studies reported that an additional olfactory cortex, the olfactory tubercle (OT), also centrifugally innervates the OB and may even shape the activity of OB output neurons. In an attempt to identify the cell types of this centrifugal innervation, we performed retrograde tracing experiments in mice utilizing three unique strategies, including retrobeads, retrograde adeno-associated virus (AAV) driving a fluorescent reporter, and retrograde AAV driving Cre-expression in the Ai9-floxed transgenic reporter line. Our results replicated the standing literature and uncovered robustly labeled neurons in the ipsilateral PCX, AON, and numerous other structures known to innervate the OB. Surprisingly, consistent throughout all of our approaches, no labeled soma were observed in the OT. These findings indicate that the OT is unique among other olfactory cortices in that it does not innervate the OB, which refines our understanding of the centrifugal modulation of the OB.