Molecularly defined olfactory bulb output neurons exhibit selective yet overlapping projections

Abstract

Early sensory processing decomposes rich signals into elemental features that can be conveyed through parallel neural pathways defined by cell types, with unique local circuit organization and long-range projection targets. Outputs from the olfactory bulb are typically categorized into two broad pathways corresponding to mitral and tufted cells. Previous high-throughput molecular profiling identified finer classes, but their morphology, projection patterns, and downstream convergence remain unclear. Here, we use an intersectional genetic targeting method with two Cre driver lines, retrograde projection mapping from six cortical targets, and RiboTag-based sequencing to study the subtypes of olfactory bulb output neurons. We found that retrograde labeling from the same cortical area recruited morphologically different OB neurons depending on the Cre driver used, suggesting a cortical area can be targeted by several subtypes. Comparing the marker expression, morphology, and downstream targets, we found that M1 neurons are densely labelled from the cortical amygdala, whereas M2 neurons project widely throughout the olfactory cortex but can be selectively recruited from the posterior piriform cortex. T1 cells differ from external tufted subtypes in their projections to the cap of the olfactory tubercle, while ET2 cells preferentially target the anterior olfactory nucleus pars externa. Electrophysiology of labelled neurons with different morphology and projection patterns differ in their intrinsic and functional properties. Further, our data suggest that extracellular recording methods may underrepresent mitral cells. Together, these findings reveal that molecularly defined olfactory bulb output neuron subtypes form anatomically and physiologically distinct channels that route information to overlapping yet selective downstream cortical targets.