Transport Response is a filial-specific behavioral response to maternal carrying in C57BL/6 mice.

BACKGROUND: A mother carries her young in many altricial mammals, such as cats, lions, rats and mice. During maternal carrying, the transported young assume a compact posture. We have recently shown that, in both humans and mice, the carried infants immediately calmed down and showed reductions in heart rate, distress vocalizations, and voluntary movement. The loss of the calming response in mouse pups hindered maternal retrieval efficacy. These findings suggested that the infant calming response functioned to reduce the maternal burden of carrying and was therefore conserved in a variety of mammalian species. However, it remains unclear how and when each component of this calming response develops and whether it is a filial-specific behavior. RESULTS: We dissected various components of the carrying-induced responses in mouse pups, collectively called the "Transport Response" herein. We showed that during the second postnatal week, pups exhibited characteristic compact posture with limb ventroflexion. The body trunk remained paradoxically pliable, suggesting complex neural regulation throughout the body. Pups also showed an increased pain tolerance to a tail pinch during the Transport Response. Analyses of the developmental courses of distinct components of the Transport Response revealed the independent regulation of each component: in the first postnatal week, the cessation of ultrasonic vocalizations was exhibited prominently; in the second postnatal week, immobilization reached its peak; and toward the third postnatal week, the postural component became fully matured. At the end of the third postnatal week, when the pups are able to transport by themselves, the pups no longer exhibited the Transport Response. CONCLUSIONS: This study has revealed the mouse Transport Response as a complex set of behavioral and physiological components, each of which has a specific postnatal time window but is orchestrated in a well-matched manner with the maturation of ambulatory ability in the pups. These findings collectively indicate that the Transport Response is a filial-specific, innate behavioral reaction and is distinct from a simple reflex or defensive freezing response. The Transport Response could be a novel index of primitive filial attachment behaviors, acting to smooth mother-infant interaction.

Oxytocin Facilitates Allomaternal Behavior under Stress in Laboratory Mice.

Oxytocin (Oxt) controls reproductive physiology and various kinds of social behaviors, but the exact contribution of Oxt to different components of parental care still needs to be determined. Here, we illustrate the neuroanatomical relations of the parental nurturing-induced neuronal activation with magnocellular Oxt neurons and fibers in the medial preoptic area (MPOA), the brain region critical for parental and alloparental behaviors. We used genetically-targeted mouse lines for Oxt, Oxt receptor (Oxtr), vasopressin receptor 1a (Avpr1a), vasopressin receptor 1b (Avpr1b), and thyrotropin-releasing hormone (Trh) to systematically examine the role of Oxt-related signaling in pup-directed behaviors. The Oxtr-Avpr1a-Avpr1b triple knock-out (TKO), and Oxt-Trh-Avpr1a-Avpr1b quadruple KO (QKO) mice were grossly healthy and fertile, except for their complete deficiency in milk ejection and modest deficiency in parturition secondary to maternal loss of the Oxt or Oxtr gene. In our minimal stress conditions, pup-directed behaviors in TKO and QKO mothers and fathers, virgin females and males were essentially indistinguishable from those of their littermates with other genotypes. However, Oxtr KO virgin females did show decreased pup retrieval in the pup-exposure assay performed right after restraint stress. This stress vulnerability in the Oxtr KO was abolished by the additional Avpr1b KO. The general stress sensitivity, as measured by plasma cortisol elevation after restraint stress or by the behavioral performance in the open field (OF) and elevated plus maze (EPM), were not altered in the Oxtr KO but were reduced in the Avpr1b KO females, indicating that the balance of neurohypophysial hormones affects the outcome of pup-directed behaviors.

Using maternal rescue of pups in a cup to investigate mother-infant interactions in mice/rodents.

Efficient parental care is indispensable for survival of the mammalian offspring, and therefore both parents and offspring cooperate to achieve the best performance. For example, when parents transport altricial offspring, the offspring immediately respond by reducing its cry and movement in both human infants and rodent pups. This coordinated set of central, motor and cardiac responses is designated as the Transport Response (TR) and is shown to facilitate maternal carrying in rodents. The present study aims to investigate the core behavioural characteristics of mother-infant interaction, and to investigate the mechanisms underlying the mother-pup cooperation using pharmacological and genetic manipulations (i.e. Oprm1-/). Along with the clear developmental changes of the pups' immobility and posture during maternal carrying as previously reported, there were also adaptations in maternal strategies, particularly in positioning of foothold and oral grasp over the pup's body, with the pups' age and pup's behaviour. Tree-based models elucidated that both of these maternal variables as well as percentage of pups' struggle predict the time required for pup retrieval from a cup. When the sensory-motor control in pups was disturbed by pharmacological or genetic manipulations, these core behaviours were inefficiently performed and impede maternal retrieval. Mother-infant mutual fit is a complex construct where several intermingled mechanisms are involved. Thus mothers and infants, when interacting, should be considered together as one whole system in which any change in one side or the other, affects the output of the whole dyad. The outcome of the interaction relays on a specific dynamic pattern of infant and maternal behaviours, which mutually change and adapt to fit each other's needs. Key features to reach a successful outcome of the interaction were the maternal retrieving strategy and infants' Transport Response behaviour.

Corticotropin-Releasing Factor Receptor 1 in the Anterior Cingulate Cortex Mediates Maternal Absence-Induced Attenuation of Transport Response in Mouse Pups.

A human infant initially shows non-selective sociality, and gradually develops selective attachment toward its caregiver, manifested as "separation anxiety." It was unclear whether such sophistication of attachment system occurs in non-human mammals. To seek a mouse model of separation anxiety, we utilized a primitive attachment behavior, the Transport Response, in that both human and mouse newborns immediately stop crying and stay immobile to cooperate with maternal carrying. We examined the mouse Transport Response in three social contexts: 30-min isolation in a novel environment, 30-min maternal absence experienced with littermates in the home cage, and the control home-cage condition with the mother and littermates. The pups after postnatal day (PND) 13 attenuated their Transport Response not only in complete isolation but also by maternal absence, and activated several brain areas including the periventricular nucleus of the hypothalamus, suggesting that 30-min maternal absence was perceived as a social stress by mouse pups after PND13. This attenuation of Transport Response by maternal absence was independent with plasma corticosterone, but was diminished by prior administration of a corticotropin-releasing factor receptor 1 (CRFR1) antagonist. Among 18 brain areas examined, only neurons in the anterior cingulate cortex (ACC) co-express c-fos mRNA and CRFR1 after maternal absence. Consistently, excitotoxic ACC lesions inhibited the maternal absence-induced attenuation of Transport Response. These data indicate that the expression of mouse Transport Response is influenced not only by social isolation but also by maternal absence even in their home cage with littermates after PND13, at least partly via CRF-CRFR1 signaling in the ACC.

Infant calming responses during maternal carrying in humans and mice.

BACKGROUND: Mother-infant bonding is the earliest and most critical social relationship of mammalian infants. To promote this bond, infants have innate behaviors to seek maternal proximity and protest upon separation via communication with the mother vocally and through body movement. However, the physiological mechanisms regulating these infant behaviors remain largely undefined. RESULTS: Here we show a novel set of infant cooperative responses during maternal carrying. Infants under 6 months of age carried by a walking mother immediately stopped voluntary movement and crying and exhibited a rapid heart rate decrease, compared with holding by a sitting mother. Furthermore, we identified strikingly similar responses in mouse pups as 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 signaling are required for induction, and parasympathetic and cerebellar functions mediate cardiac and motor output, respectively. The loss of the calming response hindered maternal rescue of the pups, suggesting a functional significance for the identified calming response. CONCLUSIONS: Our study has demonstrated for the first time 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. Our findings provide evidence for and have the potential to impact current parenting theory and practice, since unsoothable crying is the major risk factor for child abuse.

The frequency of magnesium consumption directly influences its serum concentration and the amount of elutable bone magnesium in rats.

We investigated the influence of Mg feeding frequency on the variation in serum Mg concentration and tissue Mg levels in Mg-deficient rats. Sprague-Dawley rats, which had been fed a Mg-deficient diet for 14 d, were divided into 3 groups that were kept on 3 diets differing in their Mg content. The rats were fed 0.5-fold (Mg250 group), 1-fold (Mg500 group), or 1.5-fold (Mg750 group) the amounts of recommended Mg in their standard AIN-93G diet (Mg: 478 mg/kg diet) during the recovery period (12 d). The Mg500 and Mg750 groups were intermittently fed (Mg500, every 2 d; Mg750, every 3 d) so that their total intake of Mg during the recovery period could equal the Mg intake of the Mg250 group. The serum Mg concentrations increased in the 3 groups after feeding with a Mg-containing diet. However, serum Mg levels were only maintained within the normal range in the Mg250 group. After feeding on the Mg-deficient diet, in the intermittently fed groups, serum Mg concentrations decreased. Urinary Mg excretion was higher and Mg retention was lower in the Mg500 and Mg750 groups than in the Mg250 group. Moreover, bone Mg, especially elutable bone Mg, was lower in the Mg500 and Mg750 groups than in the Mg250 group. The elutable fraction of bone Mg correlated to the coefficient of variation of serum Mg concentration. In conclusion, for the maintenance of serum Mg concentration, it is important to increase the amount of elutable bone Mg by frequent Mg consumption.

Absence of negative feedback on intestinal magnesium absorption on excessive magnesium administration in rats.

This study aimed to clarify the regulatory mechanism of Mg homeostasis on administration of excessive Mg in rats. Six-week-old male Wistar rats (n=30) were fed a Mg-deficient diet (D) or a control diet (M) in addition to which they received subcutaneous injections of saline (S) or additional Mg (M) for 14 d. Feces and urine were collected from the rats for 4 d every week. Between the MS and MM rats and the DS and DM rats, the injection of additional Mg increased Mg retention, but intestinal Mg absorption did not differ. Urinary Mg excretion in the MM rats was significantly greater than that in the MS rats, but fecal Mg excretion did not increase. Mg retention in the DM rats was approximately 30% of that in the MS rats, and urinary Mg excretion did not differ between the 2 groups, although the serum Mg in DM rats was low. There was no significant difference in the femoral Mg between the MM and MS groups. The physiological Mg pool in the bone appears to be limited. Therefore, there is no physiological Mg pool for the storage of excessive Mg, and there appears to be no negative feedback mechanism on intestinal Mg absorption upon administration of excessive Mg in the rats. In conclusion, it appears that the kidney is the only organ that regulates Mg in the body; apart from this, regulatory mechanisms corresponding to the physiological Mg requirement do not exist or are weak.