Detection of novelty by perinatal rats

This study investigated the development of fetal/neonatal rats' ability to distinguish between a novel and familiar taste. Here, we report that neonatal rats alter their orofacial movements (e.g., mouth movements and licks) upon tasting saccharin (SAC) if it was experienced previously. We also sought to determine the origins and duration of this response. Fetuses of embryonic ages E17, E18, or E19 received an oral injection of 10 microL 0.3% SAC while in utero. These animals were then reexposed to SAC on postnatal day 3, (P3) and observations of orofacial motor responses were recorded. Only neonates that first experienced SAC on E19 exhibited a SAC-induced stimulation of mouthing and licking on P3. These data suggested that a taste-recognition memory (TRM) is maintained for up to 5 days (i.e., E19 to P3). However, in this paradigm, the youngest fetuses also have the longest retention interval. Could these data also reflect the limitations of the E17 and E18 fetuses in retaining the TRM? In a second study, we shortened the taste exposure-reexposure interval to 2 days in an attempt to detect the TRM in younger fetuses. As expected, E19 rats exhibited a TRM when tested on E21. However, neither the E17 nor E18 fetuses showed SAC-induced increases in mouthing and licking when tested 2 days after their initial exposure (E19 or E20). Finally, in order to determine whether a TRM could be detected in fetuses as well as neonates (see above), we conducted an additional study wherein E21 fetuses were tested before parturition. Like E21 neonates, E21 rat fetuses that had received SAC on E19 showed a differential response to SAC depending on whether it was novel or familiar. Thus, although E21 fetal orofacial movements were less frequent than those of the E21 neonate, the fetal-testing procedures were not sufficient to obscure the detection of a TRM. In summary, the data indicate that E19 rat fetuses can acquire a TRM and retain it for at least 2-5 days, whereas E17 and E18 fetuses cannot.