Abstract
The broadly-distributed, non-topographic projections to and from the olfactory cortex may suggest a flat, non-hierarchical organization in odor information processing. Layer 2 principal neurons in the anterior piriform cortex (APC) can be divided into 2 subtypes: semilunar (SL) and superficial pyramidal (SP) cells. Although it is known that SL and SP cells receive differential inputs from the olfactory bulb (OB), little is known about their projections to other olfactory regions. Here, we examined axonal projections of SL and SP cells using a combination of mouse genetics and retrograde labeling. Retrograde tracing from the OB or posterior piriform cortex (PPC) showed that the APC projects to these brain regions mainly through layer 2b cells, and dual-labeling revealed many cells extending collaterals to both target regions. Furthermore, a transgenic mouse line specifically labeling SL cells showed that they send profuse axonal projections to olfactory cortical areas, but not to the OB. These findings support a model in which information flow from SL to SP cells and back to the OB is mediated by a hierarchical feedback circuit, whereas both SL and SP cells broadcast information to higher olfactory areas in a parallel manner.
Highlights
Sensory perception emerges from the confluence of bottom-up and top-down inputs
Bead injections in the olfactory bulb (OB) led to labeling profiles comparable to what has previously been described in the literature, notably with ipsilateral labeling of somata in the anterior olfactory nucleus (AON) pars principalis, but not in the AON pars externa; and contralateral labeling in AON pars principalis and pars externa, and labeling of the ipsilateral horizontal limb of the diagonal band of Broca[27,28,29,30,31] (Supplementary Fig. S1)
We characterized the laminar distribution of projecting populations and identified a substantial fraction of neurons dually projecting to the OB and the posterior piriform cortex (PPC)
Summary
Sensory perception emerges from the confluence of bottom-up and top-down inputs. In olfaction, feedback projections innervate the first brain relay for information processing: the olfactory bulb (OB). Odor information encoded by assemblies of APC cells is transmitted to a variety of olfactory regions such as the anterior olfactory nucleus (AON), posterior piriform cortex (PPC), cortical amygdala (CoA), and lateral entorhinal cortex (LEnt) These olfactory cortical areas project to higher, non-sensory brain regions such as the orbitofrontal cortex (OFC). Reconstruction studies of individual neurons suggest that APC principal cells project axons to the OB, AON, and to downstream olfactory regions such as the PPC, LEnt, and CoA20, 21. It is unclear how prevalent cells projecting both in feedforward and feedback directions are. We found that genetically labeled SL cells projects widely to olfactory areas, but not back to the OB
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