Sniffing response to an odor through development in rats: modulatory role of repetition and association with an aversive event

Sniffing has proven to be a useful behavioral readout for assessing olfactory performance in adult rats. However, little is known about how sniffing response changes through ontogeny. This study thus aimed at characterizing respiratory response to an odor through development in rats using paradigms applicable to both young pups and adults. We first analyzed the sniffing response to the arrival of a novel neutral odor. Then the value of the odor was changed through either its repeated presentation (odor habituation), or its association with a foot-shock resulting in odor fear. In the habituation task, we found that at the three ages, the first presentation of the novel odor induced a clear sniffing response but the peak respiratory frequency was higher in adults than in juveniles and infants. When the odor was presented repeatedly, the sniffing response gradually faded. This habituation of the response took more trials as the animals age increases. In the fear conditioning task, the odor induced an increase in respiratory rate that persisted until the end of the session in adults and infants, while it faded rapidly in juveniles. When the odor was explicitly unpaired with the foot-shock, at the three ages the respiratory response to the odor lasted less over the session than in the paired condition. Finally, we observed that shock delivery induced a similar respiratory response at the three ages in paired and unpaired condition. Collectively these data show that sniffing response constitutes a faithful index to assess rats olfactory abilities through ontogeny.