Responses of Human Neonates to Highly Diluted Odorants from Sweat.

Conjugated forms of odorants contributing to sweat odor occur not only in human sweat but also in amniotic fluid, colostrum, and milk. However, it is unclear whether the released odorants are detected and hedonically discriminated by human newborns. To investigate this issue, we administered highly diluted solutions of (R)/(S)-3-methyl-3-sulfanylhexan-1-ol (MSH), (R)/(S)-3-sulfanylhexan-1-ol (SH), (E)/(Z)-3-methylhex-2-enoic acid (3M2H), and (R)/(S)-3-hydroxy-3-methylhexanoic acid (HMHA) to 3-d-old infants while their respiratory rate and oro-facial movements were recorded. Adult sensitivity to these odorants was assessed via triangle tests. Whereas no neonatal stimulus-specific response was found for respiratory rate, oro-facial reactivity indicated orthonasal detection of MSH and SH by male neonates, and of HMHA by the whole group of neonates. Dependent on the dilution of odorants, newborns evinced neutral responses or longer negative oro-facial expressions compared with the reference stimuli. Finally, newborns appeared to be more sensitive to the target odorants than did adults.

Responsiveness of human neonates to the odor of 5α-androst-16-en-3-one: a behavioral paradox?

The odorous steroid 5α-androst-16-en-3-one (AND) occurs in numerous biological fluids in mammals, including man, where it is believed to play a chemocommunicative role. As AND was recently detected in milk and amniotic fluid, sensitivity and hedonic responses to this substance were assessed in human neonates. To this aim, respiration and facial expressions were recorded in 3-day-old newborns in response to aqueous solutions of AND, ranging from 500ng/mL to 0.5 fg/mL. Although analyses of respiratory rate did not lead to clear-cut results, the newborns changed their facial expressions at concentrations not detected by adults in a triangle test. Newborns displayed negative facial actions of longer duration to AND relative to an odorless control. Thus, AND may be considered to be offensive to newborns, which is a counterintuitive outcome as they are exposed to this compound in the womb (and it should therefore be familiar), in milk, and on the mother's skin surface (and it should therefore be conditioned as positive). Multiple reasons for this perceptual-behavioral paradox are discussed.

An overlooked aspect of the human breast: areolar glands in relation with breastfeeding pattern, neonatal weight gain, and the dynamics of lactation.

The early nursing-sucking relationship is not to be taken for granted in humans. A number of factors can either facilitate or mitigate its optimal establishment on the mother's or newborn's sides. Among these factors, a morphological feature of human mothers' breasts--the areolar glands (AG)--has been identified as potentially important. Three day-old infants display attraction during the presentation of the native secretions of the AG, suggesting that they could influence the newborn's behaviour during breastfeeding. The present study assessed this topic in a sample of 121 Caucasian mother-infant dyads. The areolae of these women were screened during the first 3 postnatal days in parallel with the infant's sucking performance, body weight fluctuations and time to lactation onset. On average, 97% of the women bore AG, 80.2% having 1-20 units per areola and 33% showing AG excreting a visible fluid. The endowment in AG appeared positively linked with neonatal growth after birth and with the speed of lactation onset: infants of primiparous women with lower AG numbers had a lower weight gain than those of mothers with higher AG numbers. Further, it took longer to primiparae with lower AG counts to set on lactation. This study confirms and extends the fact that AG, in interaction with maternal experience, might influence the initiation of the breastfeeding relationship.

The secretion of areolar (Montgomery's) glands from lactating women elicits selective, unconditional responses in neonates.

BACKGROUND: The communicative meaning of human areolae for newborn infants was examined here in directly exposing 3-day old neonates to the secretion from the areolar glands of Montgomery donated by non related, non familiar lactating women. METHODOLOGY/PRINCIPAL FINDINGS: The effect of the areolar stimulus on the infants' behavior and autonomic nervous system was compared to that of seven reference stimuli originating either from human or non human mammalian sources, or from an arbitrarily-chosen artificial odorant. The odor of the native areolar secretion intensified more than all other stimuli the infants' inspiratory activity and appetitive oral responses. These responses appeared to develop independently from direct experience with the breast or milk. CONCLUSION/SIGNIFICANCE: Areolar secretions from lactating women are especially salient to human newborns. Volatile compounds carried in these substrates are thus in a position to play a key role in establishing behavioral and physiological processes pertaining to milk transfer and production, and, hence, to survival and to the early engagement of attachment and bonding.

Mammary olfactory signalisation in females and odor processing in neonates: ways evolved by rabbits and humans.

Mammalian females have long been known to release olfactory attraction in their offspring. Mammary odor cues control infant state, attention and directional responses, delay distress responses, stimulate breathing and positive oral actions, and finally can boost learning. Here, we survey female-offspring odor communication in two mammalian species - European rabbits and humans - taken as representatives of evolutionary extremes in terms of structure and dynamics of mother-infant relations, and level of neonatal autonomy. Despite these early psychobiological differences, females in both species have evolved mammary structures combining multiple sources of endogenous and exogenous odorants, and of greasy fixatives, conferring on them a chemocommunicative function. To process these mammary chemosignals, neonates have co-evolved multiple perceptual mechanisms. Their behaviour appears to be driven by plastic mechanism(s) calibrated by circumstantial odor experience in preceding and current environments (fetal and postnatal induction of sensory processes and learning), and by predisposed mechanisms supported by pathways that may be hard-wired to detect species-specific signals. In rabbit neonates, predisposed and plastic mechanisms are working inclusively. In human neonates, only plastic mechanisms could be demonstrated so far. These mammary signals and cues confer success in offspring's approach and exploration of maternal body surface, and ensuing effective initial feeds and rapid learning of maternal identity. Although the duration of the impact of these mammary signals is variable in newborns of species exposed to contrasting life-history patterns, their functional role in setting on infant-mother interaction in the context of milk transfer can be crucial.

The "smellscape" of mother's breast: effects of odor masking and selective unmasking on neonatal arousal, oral, and visual responses.

Lactating women emit odor cues that release activity in newborns. Such cues may be carried in various substrates, including milk or areolar secretions. The present study aimed to examine the responses of infants facing their mother's breast and to sort out the source(s) of active volatile compounds emitted by the lactating breast. Infants (aged 3-4 days) were presented their mother's breast in two consecutive trials of 90 s each: a scentless condition (breast entirely covered with a transparent film) paired with one of four odorous conditions (fully exposed breast: n = 15; nipple only exposed: 15; areola only exposed: 13; and milk exposed: 12). The infants were more orally activated when facing any of the odorous breast conditions than when facing the scentless breast. They cried earlier and longer, and opened their eyes less, when facing the scentless breast. Nipple, Areola, and Milk odors appeared to be equivalent to the whole breast odor in stimulating oral activity and in delaying crying onset. This study shows that volatile compounds originating in areolar secretions or milk release mouthing, stimulate eye opening, and delay and reduce crying in newborns.

Human breast areolae as scent organs: morphological data and possible involvement in maternal-neonatal coadaptation.

In humans, areolar skin glands (AG) enlarge during pregnancy and lactation. Their role in mother-infant interactions may pertain to protective, mechanical, and communicative functions. It was questioned here whether more profuse AG could be related to more optimal adaptation to breastfeeding. A morphological study of the areolae was undertaken between birth and day 3 to assess the number, secretory status, and spatial distribution of AG. These data were related to infants' weight variation, mothers' perception of their infant's behavior at breast, and time between delivery and onset of lactation. AG were seen in virtually all women but with great interindividual variations; their areolar distribution was nonrandom, and about 1/5 of the women had AG giving off a secretion. The AG number was positively related with neonatal weight gain between birth and day 3, and with the mother's perception of infant's latching speed and sucking activity. AG numbers were also positively related with the onset of lactation in first-time mothers. In conclusion, the maternal endowment in AG may contribute to the infants' breastfeeding performance, early growth, and the mother's lactation onset.