Interhemispheric asymmetry of c-Fos expression in glomeruli and the olfactory tubercle following repeated odor stimulation

Odor adaptation allows the olfactory system to regulate sensitivity to different stimulus intensities, which is essential for preventing saturation of the cell-transducing machinery and maintaining high sensitivity to persistent and repetitive odor stimuli. Although many studies have investigated the structure and mechanisms of the mammalian olfactory system that responds to chemical sensation, few studies have considered differences in neuronal activation that depend on the manner in which the olfactory system is exposed to odorants, or examined activity patterns of olfactory-related regions in the brain under different odor exposure conditions. To address these questions, we designed three different odor exposure conditions that mimicked diverse odor environments and analyzed c-Fos-expressing cells (c-Fos+ cells) in the odor columns of the olfactory bulb (OB). We then measured differences in the proportions of c-Fos-expressing cell types depending on the odor exposure condition. Surprisingly, under the specific odor condition in which the olfactory system was repeatedly exposed to the odorant for 1 min at 5-min intervals, one of the lateral odor columns and the ipsilateral hemisphere of the olfactory tubercle had more c-Fos+ cells than the other three odor columns and the contralateral hemisphere of the olfactory tubercle. However, this interhemispheric asymmetry of c-Fos expression was not observed in the anterior piriform cortex. To confirm whether the anterior olfactory nucleus pars externa (AONpE), which connects the left and right OB, contributes to this asymmetry, AONpE-lesioned mice were analyzed under the specific odor exposure condition. Asymmetric c-Fos expression was not observed in the OB or the olfactory tubercle. These data indicate that the c-Fos expression patterns of the olfactory-related regions in the brain are influenced by the odor exposure condition and that asymmetric c-Fos expression in these regions was observed under a specific odor exposure condition due to synaptic linkage via the AONpE.