Respiratory response to an odor throughout development in rats

Odor-induced sniffing has proven to be a useful behavioral readout for assessing olfactory performance in adult rats. However, little is known about how the respiratory response changes throughout ontogeny. Thus, this study aimed at characterizing respiratory response to an odor in rats using paradigms suitable to infants, juveniles, and adults. We first analyzed the respiratory response to a neutral, novel odor. Then the value of the odor was changed either through its repeated presentation (odor habituation), or its association with a foot-shock (odor fear). In the habituation task, we found that the first presentation of the novel odor induced a clear sniffing response at all 3 ages, 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 and the younger the animal, the faster the fading of the response. 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, but not in juveniles. In another group for which the odor was explicitly unpaired with the foot-shock, the respiratory response to the odor did not last as long over the session than in the paired condition at all 3 ages. Finally, we observed that shock delivery induced a similar respiratory response at the 3 investigated ages in the paired and unpaired conditions. Collectively, these data show that the respiratory response constitutes a faithful index to assess rat's olfactory abilities throughout ontogeny.