Oxytocin, a neuropeptide hormone, is indispensable for milk ejection during nursing and is important for uterine contractions during parturition. The exact functions of oxytocin in postpartum maternal behaviors and motivations require further investigation. To this end, we characterized the role of oxytocin in components of maternal motivations during the mid‐postpartum period, which has not been previously studied. To maintain suckling stimuli, postpartum oxytocin knockout (Oxt−/−) and heterozygous (Oxt+/−) littermates were co‐housed with a wild‐type lactating mother and its litter, and were examined for their ability to retrieve pups under standard or high‐risk conditions, nursing behavior, maternal aggression towards an unfamiliar intruder, and motivation to regain contact with separated pups. One‐third of Oxt−/− mothers exhibited prolonged parturition but were otherwise grossly healthy. Despite their inability to eject milk, Oxt−/− mothers displayed nursing behaviors for similar durations to Oxt+/− mothers during the second postpartum week. In addition, Oxt−/− mothers were essentially intact for pup retrieval under standard conditions and were motivated to stay close to pups, although they showed a mild decrease in maternal care under high‐risk conditions and increased anxiety‐like behaviors in pup‐related contexts. The present findings indicate that oxytocin is dispensable for nursing behavior and maternal motivations, yet suggest that oxytocin may be relevant for stress resilience in the postpartum period.
The mother–infant relation is key to infant physical, cognitive and social development. Mutual regulation and cooperation are required to maintain the dyadic system, but the biological foundation of these responses remains to be clarified. In this study, we report the maternal calming responses to infant suckling during breastfeeding. Using behavioral measures and a Holter electrocardiogram as a readout of the maternal autonomic nervous system, the maternal activities during resting, sitting with her infant on her lap, and breastfeeding were assessed. We found that during breastfeeding, mothers talked less and maternal heart rate was lower than during sitting with the infant without breastfeeding. Congruently, maternal heart rate variability measurements indicated a higher parasympathetic activity during breastfeeding. Time-locked analyses suggested that this maternal calming response was initiated by the tactile stimulation at the breast by the infant face or mouth latch, which preceded the perceived milk ejection. These findings suggest that somatosensory stimuli of breastfeeding activate parasympathetic activity in mothers. Just as how the infant Transport Response facilitates the carrying of infants, the maternal calming responses during breastfeeding may promote efficient milk intake by inhibiting spontaneous maternal activities.
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