Myristic acid in amniotic fluid produces appetitive responses in human newborns
Introduction
Among the signaling systems, chemical cues that consist of pheromones [1] can cause notable behavioral responses, including anxiety [2], when perceived by other individuals in the group. The opposite is also true. Some pheromones can act as cues that indicate the existence of a safe environment [1] by informing other individuals of the same species about the absence of danger or presence of food through sensorial systems. Some chemical cues begin to achieve salience in the intrauterine milieu before birth under two conditions: a functional sensorial system and the presence of such cues in the intrauterine milieu (i.e., amniotic fluid). Such prenatal training may guide newborns to similar substances that are contained in colostrum and milk, acting as a guide to the maternal breast and nursing.
In humans as early as the 24th gestational week, the olfactory mucosa is well developed [3] and contains ciliated olfactory receptors that have a mature appearance [4]. Olfactory marker proteins (an indicator of neuroreceptor functionality) [5] and connectivity with mitral cells in the main olfactory bulb [6] are present in epithelia by the 28th gestational week and in the main olfactory bulb between the 32nd and 35th gestational weeks [7]. Similarly, at approximately the 12th gestational week, the human vomeronasal organ is well developed [8] and visible in the developing fetus [3], [9], [10] and newborns [11]. From an anatomical perspective, human fetuses have a reasonably well developed olfactory system before birth.
Some specific components of the interphase between mothers and newborns (e.g., amniotic fluid, colostrum, and milk) may serve as sensorial cues. Shortly after birth, human newborns and other mammals display movements of the head toward the maternal mammary gland [12], [13], its own amniotic fluid odor [14], [15], [16], and maternal axillary odor [17]. These observations suggest that prior exposure to sensorial cues, likely during intrauterine life, facilitates orientation toward the natural source of feeding (i.e., the mammary gland). Human amniotic fluid is a complex mixture of many substances. Eight fatty acids from amniotic fluid, colostrum, and maternal milk have consistently been identified, and an artificial fatty acid mixture that is based on the content and concentration of fatty acids in human amniotic fluid (i.e., lauric acid, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, elaidic acid, and linoleic acid) produces appetitive responses in human newborns [18]. Therefore, at least in humans, the initial steps toward infant-mother interactions appear to occur after birth when the newborn seeks substances that are similar to those that it experienced during intrauterine life, such as those that emanate from the maternal breast [15], [19], including fatty acids [19], [20].
However, unknown is whether the entire fatty acid mixture produces these seeking behaviors or whether only some of the fatty acids do. Therefore, the present study evaluated human newborns a few hours after birth. We recorded appetitive responses to each of these eight fatty acids and compared the results with scents from vanilla, the vehicle, and neutral saline.
Section snippets
Ethics
The present study strictly followed the Code of Ethics of the World Medical Association (Declaration of Helsinki) for experiments that involve humans. The local research ethics committee (Biomedical Research Institute of the National Autonomous University of Mexico and Hospital Civil de Perote, Veracruz, Mexico) approved the study. During the invitation sessions, all of the mothers received a detailed explanation of the purpose and risks of the study. Two physicians who did not participate in
Results
The mean age of the mothers was 22.8 ± 1.25 years (range, 14–32 years). All of the deliveries occurred by the vaginal route. No resuscitation or intensive care was needed in any of the cases. The Silverman score was zero. The newborns' average Capourro score was 39.5 ± 0.16. Twelve newborns were included in the behavioral study (five girls and seven boys). The newborns were tested approximately 22.0 ± 1.21 h after delivery (range, 18–24 h).
Discussion
The present study found that myristic acid may act as an olfactory cue that guides newborns to the maternal breast. The behaviors that are elicited by chemical cues must be elaborated in brain regions that modulate emotional processing. Such processes involve the hippocampus and other deep temporal lobe structures, such as the amygdaloid complex [24], the mesolimbic system [25], and interactions among these structures and the prefrontal cortex [26], [27], among other connections. A typical
Conclusion
The present findings are consistent with previous studies. Some components of amniotic fluid seemingly act as a primary intrauterine signal for the subsequent recognition of similar substances after birth. We found a significantly greater duration of appetitive responses when myristic acid was the stimulus, suggesting that it may be one of the main olfactory sensory cues that guide newborns to the maternal breast during the transition from intrauterine life. Our findings also suggest that there
Author contributions
A.G. Gutiérrez-García had primary responsibility for protocol execution, preliminary data analysis, and writing and revising the manuscript. C.M. Contreras conceived the study and experimental design and wrote and revised the manuscript. C. Díaz-Marte was responsible for patient screening.
Conflicts of interest
The authors declare that there are no conflicts of interest.
Acknowledgements
The authors thank Michael Arends for revising and editing the English of this paper.
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