Elsevier

Appetite

Volume 161, 1 June 2021, 105147
Appetite

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

https://doi.org/10.1016/j.appet.2021.105147Get rights and content

Abstract

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.

Introduction

Rodents are able to recognize their environment through different cues. Newborns show a preference for their maternal odor. This is determined by previous experiences, that is, it can be either their biological mother or a foster one (Sanchez-Andrade & Kendrick, 2009). Moreover, infant rats display an enhanced capacity for odor-preference learning, accompanied by a decreased capacity for avoidance learning, during postnatal days (PD) 1–9 (Moriceau, Roth, & Sullivan, 2010). During this period, pups even develop a preference for odors paired with mild aversive stimuli (e.g., a 0.5 mA electric shock; Camp & Rudy, 1988). These particular learning capabilities appear to be beneficial for them for two reasons. Firstly, since the mother's odor can be modified by diet (Leon, 1992), this mechanism allows the pups to readily learn approaching behaviors toward her. In accordance to this, newborn babies are able to learn from the flavor compounds they receive through their mother's milk. Studies show that the human milk compound is continuously changing; showing that the babies receive a variation of chemosensory experiences (Hausner, Bredie, Mølgaard, Petersen, & Møller, 2008). These variations will impact their diet choices later in life (Cooke & Fildes, 2011).

It has been shown that, in humans, flavors experienced via the mother's milk can modify the infants' preferences for those flavors. For example, Mennella, Daniels, and Reiter (2017) found that human infants whose mothers drank vegetable juice for one month (beginning at two weeks postpartum) showed an increased intake of carrot-flavored cereal. Similar results were found when the mothers consumed garlic capsules or carrot juice in the nursing context; the babies spent more time breastfeeding when they were re-exposed to this flavor (Cooke & Fildes, 2011; Mennella & Beauchamp, 1993).

Interestingly, this can also be observed in non-human animals. When rat dams were administered onions, the pups showed a preference toward this flavor, which may indicate that there is a transmission of food's taste that the mother consumes through maternal milk to the pup, that determines later food preferences (Wuensch, 1978). Similarly, adult mice whose mothers were exposed to an artificial sweetener during lactation exhibited a greater preference for sweet foods, when compared to the offspring of non-exposed dams (Zhang et al., 2011). Several studies, in which the factors that influence ethanol consumption were evaluated, showed that prenatal pre-exposure to the drug through i.g. administration to the mother can affect postnatal preferences towards it (Arias & Chotro, 2005; Pueta, Abate, Haymal, Spear, & Molina, 2008; Spear & Molina, 2005). This phenomenon was also observed when the flavor of alcohol was detected by the pup through maternal milk, which can influence the acceptance of this solution later in life (Hunt, Kraebel, Rabine, Spear, & Spear, 1993). This begs the question of whether exposing infant rats to a novel odor, through the mother's milk, can modify their reactions to flavors.

Although taste reaction responses comprise an innate component, they are plastic and depend on previous experiences and learning. Studies with numerous species and at various stages of development showed that there is a tendency to accept foods with sweet flavors, and to reject bitter, acidic and salty ones (Berridge, 2000). However, under certain environmental manipulations, reactions to the sweet taste may resemble those that occur when a bitter taste is presented and vice versa (Suárez, Ifran, Pautassi, & Kamenetzky, 2017; Suárez, Pautassi, & Kamenetezky, 2016). Other studies from our lab showed that prenatal exposure to an odor through an i.g. administration can induce a preference for typically-rejected flavors after birth. Kamenetzky, Suárez, Ifran, Nizhnikov, and Pautassi (2018) exposed rats to a lemon scent in-utero in order to assess whether this manipulation would alter their first sucking responses towards a bitter solution -which is usually rejected by the pups-delivered through a surrogate nipple. Three hours after being born, the subjects were tested while being stimulated by the same scent. The authors found an increased intake of quinine in pups that were pre-exposed to the scent in the womb, when compared to a control group that was not previously exposed. Similar effects were found when exposure to the odor was carried out immediately after birth. Kamenetzky, Suárez, Pautassi, Mustaca, and Nizhnikov (2015) exposed newborn rats to a lemon scent during 1 h immediately after birth. Three hours later, they stimulated the pups with a surrogate nipple containing a quinine solution, odorized with this scent. There was an increased intake of this solution when compared with a control group that was not previously exposed to the scent. The increase in consumption did not occur when the artificial nipple contained a sucrose solution, or a higher concentration of quinine. These results suggest that the presence of a familiar odor facilitates the acceptance of usually rejected flavors, as long as it is a moderate concentration.

This novel phenomenon is apparently robust, since it was replicated under various conditions. Animals that were evaluated in the presence of maternal odor increased the ingestion responses of a quinine solution and decreased the latency until the first intake response, which suggests that the presence of the mother's odor facilitates the acceptance of a naturally rejected flavor (Ifrán, Suárez, Pautassi, & Kamenetzky, 2018). Previous results suggest that this phenomenon occurs within a sensitive period, given that at Postnatal day (PD) 15 the phenomenon is no longer observed (Ifran, Suárez, Pautassi, & Kamenetzky, 2020).

In order to assess whether this phenomenon could be replicated using other aversive solutions, we conducted an experiment with newborn rats, in which the first sucking experience toward an artificial nipple was evaluated. Half of the rats were pre-exposed to a lemon scent immediately after birth. This was achieved by placing the pups near an embedded cotton swab for an hour. Three-hours later all the animals were evaluated in the presence of the lemon odor, and independent groups received different solutions through the surrogate nipple: quinine (bitter), citric acid (sour), saline solution (salty) or distilled water. The pre-exposed animals showed an increase in the grasp frequency toward the artificial nipple, containing the quinine and citric acid solutions, in comparison to the animals that were not pre-exposed or received the saline solution (unpublished data).

The present study assessed whether 3 day-old rat pups would increase their ingestive behavior towards commonly rejected solutions if presented with a lemon scent, previously exposed through the dam's milk. The objectives of this study were: 1. To determine whether the effect of increased intake of a bitter solution in the presence of a pre-exposed odor generalizes to other solutions with non-palatable flavors (i.e., sour and salty); 2. To evaluate whether breast milk functions as a means of propagation of odors, and whether this results in a modification of the ingestion responses of the offspring. The hypothesis was that subjects presented with a familiar odor, previously perceived during nursing, would increase their intake of quinine, citric acid, and saline solution, when compared to subjects exposed to distilled water or who had not been previously fed breast with milk contaminated with the scent.

Section snippets

Subjects

A total of 95 Wistar, 3-day-old rats (48 males and 47 females), derived from 17 mothers, served as subjects. One male and one female were used per experimental condition to avoid litter overrepresentation. The animals were born and reared at the vivarium of the Instituto de Investigaciones Médicas Dr. Alfredo Lanari (IDIM-CONICET, Argentina) under a 12/12hs light/dark cycle, with lights on at 7 a.m., and controlled temperature (22-24 °C) and humidity. PD 0 was defined as the day of parturition.

Results

Fig. 1 shows the percentage of body weight gained, number of grasps, total time of grasp, mean grasp duration, and latency to grasp across odor conditions. The figure suggests that the animals that had been pre-exposed to the lemon essence through breast milk showed an increased response to the artificial nipple, when it contained the quinine or citric acid solution. Variance analyzes revealed that for percentage of body weight gained, a significant interaction among Pre-exposure solution and

Discussion

The pups pre-exposed to the scent showed an increase in seeking and ingesting behavior toward an artificial nipple containing quinine or citric acid, when compared to pups receiving saline or control (non-pre-exposed) pups. Strikingly, we observed high variability in the citric acid and quinine groups in the pre-exposed groups’ response that could be explained by the in-between-litter variations; while the grasp response is usually homogeneous within the control groups. Although we found high

Author contribution

All authors participated in the design, interpretation of the studies, and analysis of the data and review of the manuscript. MI, AS, and GK conducted the experiments and analyzed the data. MI, AS, MA, and GK wrote the manuscript.

Funding

This work was supported by the PIP-CONICET-2012 [grant number 11220110100228], BID-PICT 2014[grant number 3212] and the Universidad Abierta Interamericana, awarded to Giselle Kamenetzky.

Ethical statement

This research was conducted with full compliance of research ethics norms, and more specifically the codes and practices established by the Guide for the Care and Use of Laboratory Animals (National Research Council, 1996). The protocol was approved by the Animal Care and Use Committee of IDIM-CONICET at Instituto de Investigaciones Médicas Alfredo Lanari (Approval number 039-16).

References (38)

  • M. Pueta et al.

    Ethanol exposure during late gestation and nursing in the rat: Effects upon maternal care, ethanol metabolism and infantile milk intake

    Pharmacology, Biochemistry and Behavior

    (2008)
  • T.L. Roth et al.

    Memory of early maltreatment: Neonatal behavioral and neural correlates of maternal maltreatment within the context of classical conditioning

    Biological Psychiatry

    (2005)
  • G. Sanchez-Andrade et al.

    The main olfactory system and social learning in mammals

    Behavioural Brain Research

    (2009)
  • B. Schaal

    Mammary odor cues and pheromones: Mammalian infant-directed communication about maternal state, mammae, and milk

    Vitamins & Hormones

    (2010)
  • A.B. Suárez et al.

    Changes in sucrose and quinine taste reactivity patterns in infant rat pups after exposure to the other tastant

    Appetite

    (2017)
  • R.M. Sullivan et al.

    Transitions in sensitive period attachment learning in infancy: The role of corticosterone

    Neuroscience & Biobehavioral Reviews

    (2010)
  • G.K. Beauchamp et al.

    Early flavor learning and its impact on later feeding behavior

    Journal of Pediatric Gastroenterology and Nutrition

    (2009)
  • G.K. Beauchamp et al.

    Flavor perception in human infants: Development and functional significance

    Digestion

    (2011)
  • L.L. Camp et al.

    Changes in the categorization of appetitive and aversive events during postnatal development of the rat

    Developmental Psychobiology: The Journal of the International Society for Developmental Psychobiology

    (1988)
  • Cited by (0)

    View full text