Olfactory sensory cues are processed in the rodent central nervous system via at least two complementary pathways, the main and accessory olfactory pathway. This dichotomy is reflected structurally, by both the peripheral sensory organs and the first central processing stages. While sensory neurons in the main olfactory epithelium detect various chemosensory stimuli and project to the main olfactory bulb, sensory neurons in the vomeronasal organ are specialized to transduce semiochemicals as well as other socially relevant chemostimuli and project to the accessory olfactory bulb. Despite their proximity, main and accessory olfactory bulbs are structurally and functionally independent and are thought to target largely segregated downstream nuclei.
We set out to delineate mitral/tufted cell axonal projections from either bulb using state-of-the-art tracing techniques in largely intact brain samples. Our implementation of stereotactic microinjection of recombinant adeno-associated viruses and Cre-dependent viral genome expression enabled selective investigation of either main or accessory olfactory bulb principal neuron projections. Moreover, viral genome expression was selectively restricted to mitral/tufted cells using the transgenic t-box protein 21 (tbx21) Cre driver mouse line. Axonal projections were subsequently assessed utilizing whole-brain slice preparations as well as cleared tissue samples, enabling the identification of three-dimensional projection pathways and secondary olfactory integration areas.
Altogether, we describe axonal projections of main and accessory olfactory bulb principal neurons, which innervate both unique and common central nuclei. Our findings provide the structural basis for future physiological investigations aimed at unraveling neuronal integration of information that is relayed along both olfactory pathways.