The presence of chemosensory cues transmitted through the mother's milk increases the search and ingestion responses toward bitter and sour solutions.

Olfaction is of major importance during early stages of life in altricial species. This sense allows newborns to develop different behaviors that will allow them to survive. Odors tend to be associated to contextual stimuli (such as warmth); this, in turn, enables the pups to recognize when to withdraw or approach. At the same time, olfaction modulates the acceptance of aversive flavors. The increase of approach responses toward a bitter substance during early life is enhanced by stimulation with familiar, pre-exposed odors. Newborn rats exhibit heightened grasp responses toward an artificial nipple dispensing quinine, and drink more of this bitter solution, in the presence of a pre-exposed odor (lemon or the mother's odor). The present research assessed the replicability of previous results by pre-exposing the subjects to the scent through maternal milk and using solutions with different aversive tastes. Half of the subjects (3 day-old Wistar rats) were pre-exposed to lemon odor through the maternal milk (the mother had previously ingested the lemon essence via an intragastric injection); 4 h later, all the rats were evaluated in the presence of the lemon odor with an artificial nipple containing quinine, citric acid, saline solution, or water. The results showed enhanced seeking and intake of the bitter (quinine) and sour solution (citric acid). However, this did not occur when the nipple contained water or saline solution. The evidence suggests that: During the early stages of development, familiar odors regulate the acceptance of non-palatable, otherwise rejected, flavors; and that the route of transmission of the pre-exposed odor can be through air, or through food (amniotic fluid in previous studies and, in this case, breast milk), that is, via the retronasal and orthonasal routes.

Sensitive period for the acceptance of unpalatable flavors in the presence of a preexposed odor in infant rats.

It has been shown that exposure to familiar odors facilitate the acceptance of bitter flavors in preweanling rats, yet it unknown how long this phenomenon persists. This study assessed, in 9- or 15-day-old Wistar rats, the influence of a familiar scent (i.e., lemon) on the intake of and behavioral responsiveness (i.e., mouthing, paw lick, chin rub, head shake, among other taste reactivity responses) elicited by a 0.1% quinine solution. The results showed heightened quinine intake in 9-day-old rats that had been preexposed to the odor, when compared to non-preexposed controls. This result was replicated in Experiment 2, which also documented no alterations in behavioral responsiveness toward quinine in the 9-day-old rats, as a function of the pre-exposure. More importantly, 15-day-old rats exhibited no alterations in intake or behavioral responsiveness toward quinine as a function of odor pre-exposure. These results suggest that the effects of odor pre-exposure upon acceptance of bitter tastes may occur within a sensitive period for the acceptance of bitter food.

Exposure to maternal odor enhances intake of a taste that mimicks the sensory attributes of ethanol.

Early exposure to ethanol increases subsequent acceptance of this drug. Little attention, however, has been devoted to the interaction of the taste of the drug with other, familiar or non-familiar, odors contingent with ethanol access, particularly early in ontogeny. This study assessed the influence of exposure to maternal odor on intake and grasp responses to an artificial nipple providing a solution (a sucrose-quinine mix) that emulates the taste of alcohol, in 4-day-old rat pups. The results showed that the mother's odor enhanced intake from and seeking responses to an artificial nipple that provided the solution that mimicked the taste of alcohol (Experiment 1). This pattern of results was not evoked by the odor of an unrelated dam (Experiment 2), nor was it observed when the nipple delivered water. The main new finding of the present study is that 4-day-old rats tested in the presence of the mother (and hence exposed to her odor cues) exhibited enhanced seeking and intake of a solution that mimics the chemosensory properties of ethanol.

Maternal Odor Exposure Modulates Acceptance of a Bitter Taste in Newborn and Infant Rats.

The acceptance of bitter, aversive, substances during early life is enhanced by stimulation with familiar, pre-exposed odors. Newborn rats exhibited heightened grasp responses toward an artificial nipple dispensing quinine, and drank more of this bitter solution, if concurrently stimulated with a lemon odor they had been exposed to shortly after birth. It yet unknown, however, if odors made familiar via normative developmental milestones also acquire modulatory influence upon seeking and intake of basic tastants. The current study assessed the influence of exposure to maternal odor on intake and grasp responses toward a surrogate nipple providing quinine, in 3-day (Experiment 1) or 12-day (Experiment 2) old, Wistar rat pups. The results revealed enhanced seeking and intake of the bitter solution, but not of water, in animals tested in the presence of the mother (and hence exposed to its odor cues), at both ages, compared to counterparts given either no explicit odor stimulation or stimulation to the odor of an unrelated dam. These results, obtained with a biologically relevant odor, are consistent with those previously found with a neutral, arbitrary odor. It seems that during the early stages of development, familiar odors regulate the acceptance of non-palatable, otherwise rejected, flavors.