Abstract
Mother-infant relationship is critical for well being and development of mammalian infants. To promote this bond, infants have innate behaviours to seek maternal proximity and protest upon separation. However, the physiological mechanisms regulating these infant behaviours require further investigations. Here we present a novel infant cooperative response during maternal carrying. Infants under six months of age carried by a walking mother immediately stop voluntary movement and distress vocalization, compared with holding by a sitting mother, and infant heart rate rapidly decreased after the mother started walking. Furthermore, a strikingly similar response has been found in mouse preweaning pups, defined by immobility and diminished ultrasonic vocalizations and heart rate. Using pharmacologic and genetic interventions in mouse pups, we identified the upstream and downstream neural systems regulating the calming response. Somatosensory and proprioceptive input signalling are required for induction, and parasympathetic and cerebellar functions mediate cardiac and motor output, respectively. The loss of calming response hindered the maternal rescue of distressed pups, suggesting a functional significance for the identified calming response. These results demonstrate that the infant calming response to maternal carrying is a coordinated set of central, motor and cardiac regulations, and is a conserved component of mammalian mother-infant interactions. The possible clinical applications will be discussed, including (i) using the carrying test to assess the infants' autonomic nerve responsivity, which may contribute to early diagnosis and prognosis of perinatal brain damage; (ii) screening for abnormal sensory integration as an early biomarker of autism spectrum disorders. These findings may also impact current parenting theory and practice, since unsoothable crying is the major risk factor for child abuse.
