Nasal Airflow Rate Affects the Sensitivity and Pattern of Glomerular Odorant Responses in the Mouse Olfactory Bulb

Sniffing is a characteristic odor sampling behavior in various mammalian species, which is associated with an increase in both nasal airflow rate and breathing frequency. Although the importance of sniffing in olfaction is well recognized, it has been challenging to separate the effect of airflow rate and sniffing frequency in vivo. In this study, we examined the individual effects of airflow rate and frequency on odorant responses of glomeruli in the mouse olfactory bulb (OB) using calcium imaging techniques and an artificial sniffing system. We found that nasal airflow rate, but not sniffing frequency, affected the apparent glomerular responses. When measured using OB imaging, apparent sensitivity for some of the odorants was significantly greater at the high nasal flow rates, while other odorants exhibited the opposite effect. In a single defined glomerulus, the sensitivity shift caused by changes in flow rate varied between odorants, suggesting that the flow rate effect is dependent on the chemical properties of an odorant rather than on the specific characteristics of the expressed olfactory receptor. Using natural flavors containing a variety of odorants, different glomerular activation patterns were observed between breathing and sniffing condition, likely due to odorant-dependent flow rate effects. Our results provide important information on in vivo odorant recognition and suggest that odor representation in the OB is not fixed but rather varies significantly depending on the respiratory state.