The mammary pheromone of the rabbit: from where does it come?

Marsupial and monotreme milk-a review of its nutrient and immune properties

All mammals are characterized by the ability of females to produce milk. Marsupial (metatherian) and monotreme (prototherian) young are born in a highly altricial state and rely on their mother’s milk for the first part of their life. Here we review the role and importance of milk in marsupial and monotreme development. Milk is the primary source of sustenance for young marsupials and monotremes and its composition varies at different stages of development. We applied nutritional geometry techniques to a limited number of species with values available to analyze changes in macronutrient composition of milk at different stages. Macronutrient energy composition of marsupial milk varies between species and changes concentration during the course of lactation. As well as nourishment, marsupial and monotreme milk supplies growth and immune factors. Neonates are unable to mount a specific immune response shortly after birth and therefore rely on immunoglobulins, immunological cells and other immunologically important molecules transferred through milk. Milk is also essential to the development of the maternal-young bond and is achieved through feedback systems and odor preferences in eutherian mammals. However, we have much to learn about the role of milk in marsupial and monotreme mother-young bonding. Further research is warranted in gaining a better understanding of the role of milk as a source of nutrition, developmental factors and immunity, in a broader range of marsupial species, and monotremes.

Maternal areola pH: A chemical basis for mother-infant recognition

BACKGROUND

The human female's areolar region is the point of arrival of a natural progression from birth to breastfeeding continuum, supported by numerous functional features.

AIMS

The aim of this study was to look more closely into the areola' skin chemical signals that are uniquely important for mother-infant chemoemission, chemoreception, and breastfeeding continuum.

STUDY DESIGN

A dermatological study of the areolae and corresponding breast quadrants was undertaken on 71 healthy, at-term women. Areolar and corresponding breast quadrant pH were assessed pre-partum and 1 and 2 days post-partum using a corneometer (Soft Plus 5.5; Callegari S.P.A., Parma, Italy).

RESULTS

Pre-partum, the pH of the areola was significantly higher than the pH of the breast quadrant (4.25 ± 0.26 vs. 4.06 ± 0.44, p < 0.015). Post-partum, the pH of the areola was significantly higher that the pH of the breast quadrant both on day one (4.28 ± 0.31 vs 4.01 ± 0.25, p < 0.001) and on day two (4.39 ± 0.19 vs 4.01 ± 0.16, p < 0.001), respectively. In addition, pre-labor pH of the areola increased on day one post-partum and significantly on day two postpartum (4.25 ± 0.26 vs 4.39 ± 0.19, p 0.004). Moreover, the pH of the areola increased significantly from day one to day two post-partum (4.28 ± 0.31 v s 4.39 ± 0.19, p < 0.041).

CONCLUSIONS

Our findings show, for the first time, that the areola has a higher pH than the surrounding breast skin, and this increases from day one to day two postpartum. We believe that the pH changes of the areolar region may act as a unique chemical signal to guide the infant directly to the nipple.

High frequency of Taenia pisiformis metacestodes and high sex-associated susceptibility to cysticercosis in naturally infected wild rabbits

Sexual dimorphism is a well-documented phenomenon observed at all levels of the animal kingdom, with the inclusion of both sexes in clinical trials and basic research becoming mandatory. Regarding parasitosis, in several animal species, the signs and virulence of the disease may change depending on the sex of the affected animal. In the cestodiasis caused by Taenia solium and Taenia crassiceps, females are more susceptible to experimental infection than males. Cysticercosis by Taenia pisiformis in rabbits has acquired relevance due to its economic impact, namely affecting welfare and production. In America, specifically in Mexico, there are no formal reports on the infection with T. pisiformis metacestodes in populations of wild rabbits, despite being the country with more endemic species (about 15 species), among them, the volcanoes rabbits or the endangered teporingo (Romerolagus diazi). In this study, 31 wild rabbits were obtained by hunters of some regions of Morelos state during several hunting seasons, and sex, physiological stage, and number of metacestodes were recorded. A high frequency of infection by T. pisiformis metacestodes (67.7%) was found. Also, a higher susceptibility to this infection was observed in does (80% infected) compared to bucks (40%), finding 84.2% of metacestodes (235 metacestodes) in does and 15.8% of metacestodes (44 metacestodes) in bucks. The percentage of infection was higher in lactating compared with pregnant and non-pregnant does, with metacestodes lodging mainly in the uterus. Increasing our knowledge regarding parasitic infections can help us better understand transmission circles as well as the parasite-host interaction of these increasingly at risk rabbit species.

Newborns prefer the odor of milk and nipples from females matched in lactation age: Comparison of two mouse strains

Newborn mice are attracted to mammary odor cues carried in murine milk and nipple secretions. However, murine milk odor is not equally attractive along lactation. The present study focuses on the differential response of 2day-old mouse pups of C57Bl/6 (C) and Balb/C (B) strains to the odor of milk (Experiment 1) and nipples (Experiment 2) that are matched/unmatched in terms of pup's age or strain. In Experiment 1, C and B pups were tested in a series of tests simultaneously opposing either murine milk and a blank (water), or two milks collected in early and late lactation (lactation days 2 and 15, respectively) from females belonging to their own or the other strain. Results showed that C and B pups were attracted to the odor of the different milks regardless of the lactation age and the strain of the donor female. Nevertheless, C and B pups preferred the odor conveyed by early- than late-lactation milk of either strain. Moreover, early-lactation milk from C females was more attractive than early-lactation milk from B females for pups of either strain. In Experiment 2, differential nipple grasping response of C and B pups was measured when they were exposed to nipples of females in early or late lactation. The proportion of C pups that grasped a nipple was greater when they were exposed to a nipple in early lactation regardless of the strain of the donor females, whereas the proportion of B pups that grasped a nipple was greater when they were exposed to a nipple in early lactation, but only from own strain. Thus, newborn mice prefer the odor of milk and nipples from females that are matched in lactation age. This result is discussed in terms of reciprocally adaptive mechanisms between lactating females and their newborn offspring.

When the nose must remain responsive: glutathione conjugation of the mammary pheromone in the newborn rabbit

In insects, xenobiotic-metabolizing enzymes were demonstrated to regulate pheromones inactivation, clearing them from the olfactory periphery and keeping receptors ready for stimulation renewal. Here, we investigate whether similar processes could occur in mammals, focusing on the pheromonal communication between female rabbits and their newborns. Lactating rabbits emit in their milk a volatile aldehyde, 2-methylbut-2-enal, that elicits searching-grasping in neonates; called the mammary pheromone (MP), it is critical for pups which are constrained to find nipples within the 5 min of daily nursing. For newborns, it is thus essential to remain sensitive to this odorant during the whole nursing period to display several actions of sucking. Here, we show that the MP is enzymatically conjugated to glutathione in newborn olfactory epithelium (OE), in accordance with the high mRNA expression of glutathione transferases evidenced by quantitative reverse transcription-PCR. This activity in the nose is higher than in the liver and in OE of newborns compared with weanlings (no more responsive to the pheromone). Therefore, the results pinpoint the existence of a high level of MP-glutathione conjugation activity in the OE of young rabbits, especially in the developmental window where the perceptual sensitivity toward the MP is crucial for survival.

How does a newly born mouse get to the nipple? Odor substrates eliciting first nipple grasping and sucking responses

It is a mammalian female strategy to emit odor cues and signals that direct their inexperienced newborns to the nipple, and optimize their initial sucking success and, hence, viability. Here, natural odorous substrates that contribute to nipple grasping were investigated in mice, a species that has not been much scrutinized on this topic. The response of pups toward the nipples of lactating females (LF) versus nonlactating females (NLF) were first assessed right after watched birth, before and after the first suckling experience, and at 1 day old, after more extended suckling experience. It appeared that only nipples of LF induced grasping at these early ages, leading to take NLF as the baseline setting to present various odor substrates sampled from LF, viz. amniotic fluid, murine milk, LF saliva, pup saliva, LF urine, and an odorless control stimulus (water). Results indicate that: (1) only amniotic fluid and fresh milk induced nipple grasping before the first suckling experience; (2) LF saliva started inducing grasping after the first suckling experience; (3) pup saliva released grasping after 24-36 hr of suckling experience; finally (4) neither LF urine, nor water induced any nipple grasping. In conclusion, the activity of amniotic fluid and murine milk on neonatal pup behavior before any postnatal suckling experience suggests that either prenatal learning and/or predisposed olfactory mechanisms do operate, while the behavioral activation due to maternal and infantile salivas clearly depends on postnatal exposure.

Communication by olfactory signals in rabbits: its role in reproduction

Rabbits use a variety of olfactory signals to transmit information related with reproduction. Such cues are produced in skin glands (submandibular, anal, Harder's, lachrymal, preputial) and the mammary gland-nipple complex. Some signals are transmitted by active behaviors, for example, chin-marking, urination, and defecation, while others are transmitted passively (e.g., mammary pheromone (MP) and inguinal gland secretions). We show that sex steroids regulate: chinning frequency and the chin gland's size, weight and secretory activity in bucks and does by acting on specific brain regions or on the chin gland, respectively. The "mammary pheromone," identified in milk as 2-methyl-but-2-enal, is essential for guiding the pups to the nipples, but its origin (mammary gland, ventral skin, nipple) remains to be determined. Estradiol, progesterone, and prolactin regulate the emission of an olfactory cue that also triggers nipple-search behavior in the pups, but its chemical identity and relation with the MP are unclear.

Persistence of hormonal and metabolic rhythms during fasting in 7- to 9-day-old rabbits entrained by nursing during the night

Rabbit does nurse their litter once every 24h during the night. We hypothesized that corticosterone, ghrelin, leptin, and metabolites such as glucose, liver glycogen, and free fatty acids could be affected in the pups by the time at which does nurse them. Therefore, we measured these parameters in pups nursed at 02:00 h (nighttime for the doe) to compare them with results from a previous study where does nursed at 10:00 h, during daytime. From postnatal day 7, pups were sacrificed either just before their scheduled time of nursing or at 4, 8, 12, 16, or 20 h after nursing (n=6 at each time point); additional pups were sacrificed at 4h intervals between 48 and 72 h after nursing to study the persistence of oscillations during fasting. All pups developed locomotor anticipatory activity to nursing. Corticosterone, ghrelin, and free fatty acids exhibited a rhythm that persisted in fasted pups. Glucose concentrations were lower in fasted than in nursed pups, and glycogen was only detected in nursed subjects. Leptin values were stable and low in nursed subjects but increased significantly in fasted subjects up to 72 h after the expected nursing time. The rhythm of ghrelin persisted during fasting, contrary to our previous findings in pups nursed during daytime (i.e., outside the natural time of nursing for this species). Therefore, in 7-day-old rabbit pups, night nursing is a strong zeitgeber for corticosterone, ghrelin, free fatty acids, and energy metabolites but not for leptin.

A pheromone to behave, a pheromone to learn: the rabbit mammary pheromone

Birth is part of a continuum and is a major developmental change. Newborns need to adapt rapidly to the environment in terms of physiology and behaviour, and ability to locate the maternal source of milk is vital. Mechanisms have evolved resulting in the emission of olfactory cues by the mother and the processing of these cues by the young. Here, we focus on some sensory, cognitive and behavioural strategies developed by the European rabbit (Oryctolagus cuniculus) that optimize the early development of offspring. In this species, chemosensory communication between the mother and young plays a critical role in eliciting adaptive neonatal responses. In particular, lactating females release a molecule, the mammary pheromone, which has several functional impacts. It triggers orocephalic responses involved in the quick localization of nipples and sucking. Moreover, this unconditioned signal promotes rapid appetitive learning of novel odorants, acting as a potent organizer of neonatal cognition. The mammary-pheromone-induced odour memory requires consolidation/reconsolidation processes to be maintained in the long term. Finally, as this mode of conditioning also promotes learning of mixtures of odorants, it supports investigations related to the capacity of neonatal olfaction to extract biological value from the complex environment.