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Second‐order input to the medial amygdala from olfactory sensory neurons expressing the transduction channel TRPM5

Recent anatomical tracing experiments in rodents have established that a subset of mitral cells in the main olfactory bulb (MOB) projects directly to the medial amygdala (MeA), traditionally considered a target of the accessory olfactory bulb. Neurons that project from the MOB to the MeA also show activation in response to conspecific (opposite sex) volatile urine exposure, establishing a direct role of the MOB in semiochemical processing. In addition, olfactory sensory neurons (OSNs) that express the transient receptor potential M5 (TRPM5) channel innervate a subset of glomeruli that respond to putative semiochemical stimuli. In this study, we examined whether the subset of glomeruli targeted by TRPM5‐expressing OSNs is innervated by the population of mitral cells that projects to the MeA. We injected the retrograde tracer cholera toxin B (CTB) into the MeA of mice in which the TRPM5 promoter drives green fluorescent protein (GFP). We found overlapping clusters of CTB‐labeled mitral cell dendritic branches (CTB+) in TRPM5‐GFP+ glomeruli at significantly greater frequency than expected by chance. Despite the significant degree of colocalization, some amygdalopetal mitral cells extended dendrites to non‐TRPM5‐GFP glomeruli and vice versa, suggesting that, although significant overlapping glomerular innervation is observed between these two features, it is not absolute J. Comp. Neurol. 520:1819–1830, 2012. © 2011 Wiley Periodicals, Inc.