Parental brain through time: The origin and development of the neural circuit of mammalian parenting.

This review consolidates current knowledge on mammalian parental care, focusing on its neural mechanisms, evolutionary origins, and derivatives. Neurobiological studies have identified specific neurons in the medial preoptic area as crucial for parental care. Unexpectedly, these neurons are characterized by the expression of molecules signaling satiety, such as calcitonin receptor and BRS3, and overlap with neurons involved in the reproductive behaviors of males but not females. A synthesis of comparative ecology and paleontology suggests an evolutionary scenario for mammalian parental care, possibly stemming from male-biased guarding of offspring in basal vertebrates. The terrestrial transition of tetrapods led to prolonged egg retention in females and the emergence of amniotes, skewing care toward females. The nocturnal adaptation of Mesozoic mammalian ancestors reinforced maternal care for lactation and thermal regulation via endothermy, potentially introducing metabolic gate control in parenting neurons. The established maternal care may have served as the precursor for paternal and cooperative care in mammals and also fostered the development of group living, which may have further contributed to the emergence of empathy and altruism. These evolution-informed working hypotheses require empirical validation, yet they offer promising avenues to investigate the neural underpinnings of mammalian social behaviors.

Effects of oxytocin ablation on pup rescue, nursing behaviors and response to pup separation in early-to-mid postpartum mice.

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.

Maternal physiological calming responses to infant suckling at the breast.

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.

A method to soothe and promote sleep in crying infants utilizing the transport response.

Approximately 20%-30% of infants cry excessively and exhibit sleep difficulties for no apparent reason, causing parental stress and even triggering impulsive child maltreatment in a small number of cases.1-8 While several sleep training methods or parental education programs may provide long-term improvement of infant cry and sleep problems, there is yet to be a conclusive recommendation for on-site behavioral interventions.9-13 Previously we have reported that brief carrying of infants transiently reduces infant cry via the transport response, a coordinated set of vagal activation and behavioral calming conserved in altricial mammals.14-18 In this study, we disentangled complex infant responses to maternal holding and transport by combining subsecond-scale, event-locked physiological analyses with dynamic mother-infant interactions. Infant cry was attenuated either by maternal carrying or by reciprocal motion provided by a moving cot, but not by maternal holding. Five-minute carrying promoted sleep for crying infants even in the daytime when these infants were usually awake, but not for non-crying infants. Maternal laydown of sleeping infants into a cot exerted bimodal effects, either interrupting or deepening the infants' sleep. During laydown, sleeping infants were alerted most consistently by the initiation of maternal detachment, then calmed after the completion of maternal detachment in a successful laydown. Finally, the sleep outcome after laydown was associated with the sleep duration before the laydown onset. These data propose a "5-min carrying, 5- to 8- min sitting" scheme for attending to infant cry and sleep difficulties, which should be further substantiated in future studies. VIDEO ABSTRACT.

In vivo electroporation to physiologically identified deep brain regions in postnatal mammals.

Genetic manipulation is widely used to research the central nervous system (CNS). The manipulation of molecular expression in a small number of neurons permits the detailed investigation of the role of specific molecules on the function and morphology of the neurons. Electroporation is a broadly used technique for gene transfer in the CNS. However, the targeting of gene transfer using electroporation in postnatal animals was restricted to the cortex, hippocampus, or the region facing the ventricle in previous reports. Electroporation targeting of deep brain structures, such as the thalamus, has been difficult. We introduce a novel electroporation technique that enables gene transfer to a physiologically identified deep brain region using a glass pipette. We recorded neural activity in young-adult mice to identify the location of the lateral geniculate nucleus (LGN) of the thalamus, using a glass pipette electrode containing the plasmid DNA encoding enhanced green fluorescent protein (EGFP). The location of the LGN was confirmed by monitoring visual responses, and the plasmid solution was pressure-injected into the recording site. Voltage pulses were delivered through the glass pipette electrode. Several EGFP-labeled somata and dendrites were observed in the LGN after a few weeks, and labeled axons were found in the visual cortex. The EGFP-expressing structures were observed in detail sufficient to reconstruct their morphology in three dimensions. We further confirmed the applicability of this technique in cats. This method should be useful for the transfer of various genes into cells in physiologically identified brain regions in rodents and gyrencephalic mammals.

Netrin-4 regulates thalamocortical axon branching in an activity-dependent fashion.

Axon branching is remodeled by sensory-evoked and spontaneous neuronal activity. However, the underlying molecular mechanism is largely unknown. Here, we demonstrate that the netrin family member netrin-4 (NTN4) contributes to activity-dependent thalamocortical (TC) axon branching. In the postnatal developmental stages of rodents, ntn4 expression was abundant in and around the TC recipient layers of sensory cortices. Neuronal activity dramatically altered the ntn4 expression level in the cortex in vitro and in vivo. TC axon branching was promoted by exogenous NTN4 and suppressed by depletion of the endogenous protein. Moreover, unc-5 homolog B (Unc5B), which strongly bound to NTN4, was expressed in the sensory thalamus, and knockdown of Unc5B in thalamic cells markedly reduced TC axon branching. These results suggest that NTN4 acts as a positive regulator for TC axon branching through activity-dependent expression.

Assessment of vitreous drug concentration in the porcine eye following intracameral injection or irrigation with moxifloxacin.

INTRODUCTION: Posterior capsule rupture causes instant vitreous contamination, resulting in endophthalmitis. However, transfer of intracameral moxifloxacin (MFLX) to the vitreous has not been examined in detail. We investigated vitreous antibiotic concentrations following intracameral MFLX in both ruptured and intact posterior capsular eyes. METHODS: Experiment 1: Intraocular lenses were inserted into 21 extracted porcine eyes by one of the following three methods: (1) Irrigation: Throughout surgery, 33-fold diluted MFLX irrigation solution (150 µg/mL) was used; (2) Bag and chamber flushing: After surgery, the anterior chamber and area behind the intraocular lenses were irrigated with 30-fold diluted MFLX (167 µg/mL) using a 5 mL syringe; (3) Simple injection: Tenfold diluted MFLX (50 µg in 0.1 mL) was injected intracamerally at the conclusion of surgery. The eyeballs were frozen and the anterior, central, and posterior portions of the vitreous were cubed. After defrosting, concentrations were measured using high-performance liquid chromatography. Experiment 2: The same procedure was conducted for 18 eyes in which the posterior capsule had been ruptured. RESULTS: Experiment 1: Transfer of intracameral MFLX to the anterior vitreous was approximately 1% (1.56-2.20 µg/mL) regardless of the administration method. Experiment 2: MFLX reached a high concentration in the vitreous with irrigation solution administration (maximum 30.22 µg/mL). The concentrations reached by simple injection or flushing were significantly less than those obtained by irrigation. CONCLUSION: With an intact posterior capsule, intracameral MFLX exhibited limited effects on vitreous concentration. Despite the fact that the risk of infection clearly increases in cases of ruptured capsule, no special infection prevention protocol has been proposed. It was confirmed that irrigation solution caused vitreous contamination in ruptured eyes within only a short irrigation time. In this case, intracameral administration did not necessarily achieve preventive concentrations for endophthalmitis, but it appears that an effective drug concentration can be achieved in the vitreous by the administration of irrigation solution.

Critical period of experience-driven axon retraction in the pharmacologically inhibited visual cortex.

Monocular deprivation (MD) during the critical period reduces the visual cortical response to the deprived eye and causes the geniculocortical axons serving the deprived eye to retract. When MD is combined with a pharmacological inhibition of the visual cortex, the cortical neurons weaken their response to an open eye and the input axons serving the open eye retract. To determine whether the 2 types of ocular dominance (OD) plasticity reflect an experience-driven modification of neural circuits sharing the same developmental time course, we analyzed the OD plasticity in an inhibited visual cortex using cats at different ages. MD did not affect the OD distribution in the inhibited cortex of adults, confirming that the OD plasticity in the inhibited cortex represents a developmental plasticity. In developing animals, the OD plasticity in the inhibited cortex was observed at the late phase of the critical period (P40-46) but not at the early phase (P22-26). We found a retraction of input axons serving an open eye at the late phase, whereas those at the early phase were comparable to the axons of normal animals. Therefore, the maturation of visual circuits might include an experience-driven rearrangement of thalamocortical projections during the late phase of development.