The oxytocin system and early‐life experience‐dependent plastic changes

Abstract Early‐life experience influences social and emotional behaviour in adulthood. Affiliative tactile stimuli in early life facilitate the development of social and emotional behaviour, whereas early‐life adverse stimuli have been shown to increase the risk of various diseases in later life. On the other hand, oxytocin has been shown to have organizational actions during early‐life stages. However, the detailed mechanisms of the effects of early‐life experience and oxytocin remain unclear. Here, we review the effects of affiliative tactile stimuli during the neonatal period and neonatal oxytocin treatment on the activity of the oxytocin–oxytocin receptor system and social or emotional behaviour in adulthood. Both affiliative tactile stimuli and early‐life adverse stimuli in the neonatal period acutely activate the oxytocin–oxytocin receptor system in the brain but modulate social behaviour and anxiety‐related behaviour apparently in an opposite direction in adulthood. Accumulating evidence suggests that affiliative tactile stimuli and exogenous application of oxytocin in early‐life stages induce higher activity of the oxytocin–oxytocin receptor system in adulthood, although the effects are dependent on experimental procedures, sex, dosages and brain regions examined. On the other hand, early‐life stressful stimuli appear to induce reduced activity of the oxytocin–oxytocin receptor system, possibly leading to adverse actions in adulthood. It is possible that activation of a specific oxytocin system can induce beneficial actions against early‐life maltreatments and thus could be used for the treatment of developmental psychiatric disorders.

behaviour, 10 prosocial temperament, 11 face recognition, 12 and autism spectrum disorder. 13 Oxytocin acts within the brain to induce various acute adaptive responses to environmental challenges. Oxytocin exerts neurotrophic actions in some cases. Oxytocin also has organizational effects via acting at certain developmental stages to modulate the developmental trajectory. These oxytocin actions are exerted mainly by activation of the oxytocin receptor located on neurons, astroglia 14 and microglia 15 in the nervous system and on cells in peripheral organs.
Various acute actions of oxytocin have been reported. Oxytocin facilitates the acquisition of social recognition 16 and increases a variety of social behaviours such as parental behaviours (pup retrieval, maternal aggression), mating-related behaviours, filial huddling, affiliative behaviour, emotional discrimination 17 and consolation-related behaviour toward distressed partners. 18 These actions have been shown to be meditated by enhancement of rewarding effects of social interaction 2 and by enhancement of attention to social signals. 19 Oxytocin also acutely attenuates stress-related or anxiety-related responses in autonomic, neuroendocrine or behavioural systems by acting on various brain regions, whereas oxytocin appears to facilitate stress responses in socially aversive situations. 20 In addition, oxytocin has analgesic actions possibly via acting on the spinal cord. 21,22 Oxytocin has been reported to reduce depression-related or anxiety-related behaviour after chronic stress. Oxytocin has negative energy homeostatic actions (decreased satiety, increased energy consumption and hyperthermia) 20,23,24 partly by inducing meal termination and by activating the sympathetic nervous system.
Oxytocin also induces reduction of drug addiction 25,26 by modulating the activity of the reward system. Oxytocin also has neuroprotective effects via anti-inflammation actions or switching GABA function from excitation to inhibition. [27][28][29] Oxytocin also has been shown to play a role in learning and memory including reversal learning. 30 All of these oxytocin actions of increasing social behaviours, modulating stress-related responses, inducing negative metabolic states and exerting neuroprotection, have been suggested to contribute to active coping towards various environmental challenges. 20 In addition to acute actions, oxytocin has been shown to be involved in neurogenesis and neurotropic actions. 31 Oxytocin administration stimulates neurogenesis in the hippocampus in adult male rats 32 and conditional ablation of the oxytocin receptor in the hippocampus impairs neurogenesis in mice. 33 Oxytocin has also been suggested to function during the developmental period. 34,35 There is accumulating evidence indicating that oxytocin acts in discrete and specific developmental periods to modulate the developmental trajectory of certain neural circuits to control behavioural and neuroendocrine stress responses. These organizational actions of oxytocin during the developmental period may be mediated by acute actions and by neurogenetic actions. On the other hand, both early-life experience and stimulation of the oxytocin system during early life have been suggested to induce long-term effects on the oxytocin system and behaviour. However, apparently inconsistent data concerning the effects of early-life experience and oxytocin on the oxytocin system have been reported. Detailed underlying mechanisms of early-life experience and oxytocin remain unclear. In the present review, we first discuss the development of the oxytocin-oxytocin receptor system and then effects of tactile stimuli, affiliative parental care, early-life adverse stimuli and early-life oxytocin on behaviours and the oxytocin system in adulthood. In the final section, future directions of research concerning organizational actions of oxytocin are discussed.

| Development of oxytocin neurons and the oxytocin receptor
The oxytocin-oxytocin receptor system appears to be established at a relatively late stage of development compared to the development of the vasopressin system. 36 Oxytocin mRNA is found at embryonic day 16.5 and the mature form of oxytocin peptide is detected only after birth in mice. 37 The number of neurons expressing oxytocin immunoreactivity increases during the pre-weaning period in both mice 35 and prairie voles. 38 On the other hand, oxytocin receptor mRNA is detected earlier than oxytocin and is expressed at embryonic day 11-12.5 in mice 37,39 and rats. 40 Oxytocin receptor binding is found at embryonic day 16.5 in mice 37,41 and rats 42 and increases during the course of development. In some brain regions, expression of the oxytocin receptor is not stable but transient during the postnatal period. These areas include the cingulate cortex, 42,43 caudate putamen, anterior thalamic nuclei, ventral tegmental area, 40 septum and auditory cortex. 44 Sexually dimorphic expression of the oxytocin receptor has been demonstrated in the ventral premammillary nucleus in male mice and in the medial preoptic area in female mice. 45 It is interesting that oxytocin binding in the ventrolateral part of the ventromedial hypothalamus increases after puberty. 42,45 The ventromedial hypothalamus has been shown to play roles in aggressive behaviours, 46,47 sexual behaviour and social defeat posture, 48 all of which develop after puberty.
Because the oxytocin-oxytocin receptor system develops from the perinatal period until adulthood, experiences especially in early life may influence the development of the oxytocin system to modulate neuroendocrine, autonomic and behavioural responses to environmental challenges in adulthood. In the next section, the effects of tactile stimuli during early-life stages on behaviours and the oxytocin system are discussed.

| Effects of affiliative stimuli including tactile stimulation (touch) in early life
Tactile stimulation plays an important role in non-verbal emotional communication. 49,50 Gentle stroking at a slow speed induces a pleasant sensation and social reward via activation of C-tactile fibres. 51 Social grooming or allo-grooming is induced toward distressed conspecifics 52 to buffer stress responses (social buffering) 53 and to produce analgesia in the recipients. Social grooming has been suggested to contribute to the formation of social groups.
Primates spend a considerable amount of time grooming other individuals to establish and maintain relationships among social members. 54 During the developmental period, tactile stimulation also plays an indispensable role. Nurturing touch during parental care has been proposed to facilitate the development of emotional and social behaviours and cognitive functions 55 and to establish an attachment relationship of bonding between infants and their guardians. 49 Deprivation of touch and social interactions in early childhood leads to irreversible deficits in emotional, social and cognitive behaviours, whereas tactile stimuli with a brush for isolated pups reduce maternal separation-induced distress in rats 56 and mandarin voles. 57 Gentle tactile stimuli during the developmental period establish an affiliative relationship, 58,59 possibly as a result of the activation of hypothalamic oxytocin neurons. [60][61][62] Warm sensations in addition to mechanosensation form an important part of tactile stimulation during parental care.
Maintenance of body temperature is important for survival during the developmental period when body size is small. Huddling behaviour effectively reduces heat loss. Rodents show thermalseeking huddling. It is interesting that oxytocin, which is increased in pups by contact with the mother or parental care, 63 facilitates the development of thermal-seeking huddling behaviour. 64 Oxytocin-deficient pups select cooler places compared to wildtype pups in a thermocline performance test. 65  Not only affiliative gentle mechanosensory stimuli 58-60,62 but also affiliative eye contact 68 acutely activate hypothalamic oxytocin neurons and facilitate oxytocin release. 69 Parental care including skin-to-skin contact with the mother modulates the amount of hypothalamic or serum oxytocin in rat pups. 70 Rat pups that received frequent licking from their mothers have been reported to show an increased plasma concentration of oxytocin at postnatal day 13,71 which is consistent with the view that maternal licking activates the oxytocin system in pups.
As described above, the oxytocin system is acutely activated by affiliative parental care. Parental affiliative stimuli during early life have also been shown to induce long-lasting influence on the oxytocin system. Early-life manipulations such as handling, maternal separation, nursery environments and paternal deprivation influence the amount of parental care that pups receive. In the next section, the long-lasting influence of parental care and its experimental manipulations of handling, maternal separation, cage environment changes and paternal deprivation on the oxytocin system is discussed. in offspring mice of dams that showed high corticosterone and low maternal care. 75 Increased expression of the oxytocin receptor has been suggested to be mediated, at least in part, via induction of oestrogen sensitivity to express the oxytocin receptor in rats. 76 Affiliative stimuli received during post-weaning period also induce long-lasting stimulating actions on the oxytocin system. Several studies have shown that postnatal maternal separation reduces oxytocin mRNA, oxytocin content or the number of oxytocin neurons in the hypothalamus and decreases the expression of the oxytocin receptor in some brain regions, although differences depending on the species, sex, experimental procedures and brain regions examined have been reported (Table 2).
Postnatal maternal separation for a short duration (e.g., 1-15 min day -1 ) has been shown to reduce the hypothalamic oxytocin content in suckling rats 78 and to reduce the number of hypothalamic oxytocin neurons in adult rats. 79,80 Reduction of oxytocin content has also been reported in the amygdala of adult male rats. 81 Maternal separation for a longer duration (e.g., 3 h day -1 ) has also been reported to induce a long-lasting decrease in oxytocin mRNA or oxytocin content in the hypothalamus and/or in other brain regions in mice, 82,83 rats 84,85 and Degus 86 (but see also Tsuda et al. 87 ). Maternal separation has also been shown to reduce oxytocin receptor binding in the lateral septum and the caudate putamen, whereas maternal separation increases oxytocin receptor binding in the ventromedial hypothalamus in rats in adulthood. 88,89 Interaction between maternal separation and postweaning stresses on expression of the oxytocin receptor has also been suggested. Maternal separation induces no changes in oxytocin receptor mRNA in the hippocampus, while a combination of maternal separation and forced swimming in young adulthood increases oxytocin receptor mRNA in the hippocampus of adult male mice. 90 The effects of maternal separation on oxytocin receptor expression may be dependent on brain regions and ages when stress is applied.

| Effects of limited bedding/nesting materials on the oxytocin system
Living conditions during the pre-weaning period have been reported to affect the oxytocin system in a sexually dimorphic fashion (Table 3) (Table 3).

| Effects of paternal deprivation on the oxytocin system
Patterns of parental care, such as biparental care, maternal care only (paternal deprivation) or alloparental care in monogamous animals, have also been shown to affect social behaviours and the oxytocin system in adulthood. In monogamous mandarin voles and prairie voles, both fathers and mothers show parental care toward their children. Paternal absence has been shown to reduce partner preference formation of the offspring in adulthood, 93 suggesting that the effect of maternal care alone on social behaviour of offspring is different from the effect of biparental care. Paternal deprivation has been shown to reduce oxytocin receptor mRNA and protein or to decrease oxytocin receptor binding in certain brain regions in prairie voles 93,94 and mandarin voles, 95,96 although contradictory effects of paternal deprivation on oxytocin neurons have also been reported (Table 4). Consistent with the view that the oxytocin system is involved in effects of paternal deprivation, paternal deprivationindued reduction in partner preferences has been shown to be TA B L E 1 Effects of parental care and tactitle stimuli on the oxytocin system in adulthood.

Treatments Species, sex Changes in the OXT system Reference
Early pronounced in prairie voles with a SNP that results in a lower density of the oxytocin receptor in the striatum. 97 As summarized above, parental care has been shown to produce long-lasting changes in the oxytocin system. Generally, oxytocin neuronal activity is activated after affiliative parental care and oxytocin receptor expression is increased in correlation with the amount of paternal care, although differential influences dependent on brain regions, experimental manipulation and sex have been reported.
Oxytocin neurons have been shown to be activated not only by affiliative stimuli, but also by stressful stimuli in adulthood. During the developmental period, stressful stimuli as well as parental care acutely influence activity of oxytocin neurons. Stressful stimuli in early life have also been shown to induce long-lasting changes in the oxytocin system, 26,98 as is the case for the long-lasting changes induced by parental care as described above. In the next section, the effects of adverse manipulations in early life (prenatal, postnatal and post-weaning periods) on the oxytocin system are discussed.

| Effects of sensory deprivation and stressful stimuli in early life on the oxytocin system
In animals in adulthood, various stressful stimuli, 99 regardless of whether given in a social context such as a social defeat paradigm 48 or in a non-social context such as a conditional fear paradigm, 20,100 have been shown to acutely activate hypothalamic oxytocin neurons and facilitate oxytocin release. It has also been shown in humans that psychological stress increases plasma oxytocin concentrations. 101 By contrast to the acute effects of stressful stimuli, chronic stressful stimuli might produce different actions. In female mandarin voles, chronic social defeat stress has been reported to induce anxiety-related and depression-related behaviour associated with a reduction of oxytocin fibres and oxytocin receptor mRNA in the nucleus accumbens. Administration of oxytocin into the nucleus accumbens has been shown to reverse the anxiogenic and depressive actions of chronic defeat stress. 102 During the developmental period, stressful stimuli influence oxytocin neurons in the hypothalamus. Social isolation stress TA B L E 2 Effects of maternal separation on the oxytocin system. during the lactating period has been reported to modulate oxytocin immunoreactivity in the hypothalamus of monogamous mandarin voles. 103 The oxytocin system has been shown to be influenced for a long time after early-life stressful stimuli including prenatal exposure to alcohol (Table 5). Prenatal stress has been shown to decrease the number of oxytocin neurons in the magnocellular paraventricular nucleus in the adult offspring of dam rats. 104 Postnatal sensory deprivation has also been shown to decrease activity of oxytocin neurons in the hypothalamus. Sensory deprivation by whisker deprivation and dark rearing have been reported to decrease oxytocin mRNA in the hypothalamus and to reduce excitatory synaptic transmission in the sensory cortex in mice. 105 Oxytocin administration has been shown to rescue this impairment, suggesting an important role of oxytocin in early experiencedependent cortical development. 105 Post-weaning social isolation has also been shown to reduce activity of oxytocin neurons in the hypothalamus of female rats in response to exposure to novel conspecifics, to impair social preference 106 and to reduce oxytocin receptor mRNA in the amygdala in mice. 107 Oxytocin receptor binding has also been shown to be reduced in the nucleus accumbens in rats, although hypothalamic oxytocin mRNA has also been reported be increased in rats 108 (Table 5).
As discussed above, numerous animal experiments have shown that various stressful treatments during early-life stages have longlasting inhibitory actions on the oxytocin system. In the next section, the effects of adverse environments in early life on the oxytocin system in humans are discussed.

| Effects of adverse environments in early-life stages on the oxytocin system in humans
In humans, prenatal or postnatal stressful stimuli have been shown to be associated with altered functions of brain regions including the superior frontal gyrus for cognitive processing, amygdala for emotional processing, precuneus for memory processing and putamen for reward-related learning processing. 109 Early-life adversities have been reported to be associated with vulnerability to a variety of diseases in adulthood including depression, 110  In adult women, an inverse association between severity of childhood abuse received and oxytocin concentrations in cerebrospinal fluid has been shown, 132 although a positive correlation between urine oxytocin and less severe maltreatment during childhood was also reported. 133 On the other hand, several studies have found no direct association between childhood abuse and methylation of the oxytocin receptor gene. 134,135 Maltreatment has also been shown to be associated with oxytocin receptor gene methylation indirectly via social instability. 134 Childhood abuse has been TA B L E 3 Effects of limited bedding/nesting materials on the oxytocin system. reported to induce anxiety and depression in a manner dependent on DNA methylation of the oxytocin receptor gene. 135 It has also been shown that oxytocin receptor gene methylation was transmitted from mothers to their newborns and that the intergenerational transmission of methylation levels was not observed in mothers who experienced maltreatment during childhood. 136 The findings suggest that experience of maltreatment disrupts possibly adaptive intergenerational epigenetic transmission.
Early-life experience has also been shown to modulate effects

| Long-term effects of early-life oxytocin stimulation
Oxytocin in early life has been shown to attenuate early-life stressinduced or genetically induced impairments of social behaviour or energy metabolisms, although detailed mechanisms of the organizational actions of oxytocin remain to be clarified.  newborns. 141 A part of the oxytocin released in mothers may be delivered into newborns because the placenta can partly transport oxytocin. 142 Perinatal oxytocin has been proposed to protect the newborn brain by inducing a hyperpolarizing response to GABA. [27][28][29] Plasma oxytocin concentrations are lower in caesareansection neonates and mothers than in vaginally born babies and mothers. 143 The effects of neonatal oxytocin administration to caesarean-section delivered pups have been reported in animals.

| Prenatal oxytocin stimulation
In mice, caesarean section has been shown to reduce preference to maternal bedding during the preweaning period and to impair social  preference and a lower dose of postnatal oxytocin administration resulted in recovery of impaired social recognition in adulthood. 144 Caesarean-section delivery has also been shown to be associated with a heightened risk for developing obesity. 145,146 Postnatal oxytocin administration (1 mg kg -1 , postnatals day [10][11][12][13][14] has been reported to increase 147 or decrease 148 body weight in rats. Further studies are necessary to clarify the role of early-life oxytocin in metabolism in adulthood. 149 Oxytocin has been widely used for stimulation of births.
Prenatal oxytocin administration to mothers has been shown to affect various brain functions in adulthood. In prairie voles, pups who received exogenous oxytocin via administration to their mothers were reported to show increased plasma oxytocin and other adults. 150 The effects of neonatally administered oxytocin on alloparental behaviour and partner preference behaviour in adulthood have also been reported be different dependent on oxytocin dosages in female prairie voles. 151 Neonatal oxytocin at some dosages has been reported to facilitate alloparental behaviour and partner preference in adulthood, whereas neonatal oxytocin at a higher dosage has been shown to impair these social behaviours.
For stimulation of labour in humans, low doses of oxytocin are applied over a long period. In humans, a population-based study showed that perinatal exogenous administration of oxytocin has no association with childhood emotional disorders, 152 and the relationship between oxytocin administration and postpartum depression also remains unclear. 153 However, a case-control study showed an association between a higher dose or longer duration of oxytocin administration and the odds of developing autism spectrum disorder, 154 and a positive relationship between oxytocin administration and a higher risk of postpartum depression has been reported. 155 Further investigations are necessary.

| Postnatal oxytocin stimulation
Milk contains oxytocin. 156 Orally administered oxytocin has been shown to be absorbed into the peripheral circulation, especially in neonates. 157 The oxytocin receptor exits in peripheral organs including the intestinal system. 158 A small amount of oxytocin may penetrate the blood-brain barrier into the brain. The effects of oxytocin in milk remain to be clarified.
On the other hand, postnatal oxytocin application at large dosages has been reported to rescue social behaviour, pain-related behaviour and memory in adulthood, which are impaired by genetic manipulations or behavioural treatments.
Postnatal daily administration of oxytocin (2 µg day -1 in the first 2 days or postnatal week) has been shown to increase pre-or post-synaptic transcript levels of neurexins and neuroligins 159  which facilitates oxytocin release. 164 Oxytocin administration in male prairie voles has also been shown to increase partner preference and to decrease anxiety-related behaviour, whereas the administration of an oxytocin receptor antagonist decreases parental behaviour in adulthood. 165 Chronic stress in early life caused by maternal separation (3 h day -1 from postnatal days 1-21) induces reduction in spatial memory and disturbance of long-term potentiation in the hippocampus, and this reduction has been shown to be attenuated by repetitive application of oxytocin in rats. 166 2.6.4 | Long-term effects of oxytocin stimulation on the brain Early-life experience influences the oxytocin system and oxytocin simulation in early life induces long-lasting effects on not only the oxytocin system, but also other brain circuits to affect emotional or social behaviours in adulthood. However, the underlying mechanisms of oxytocin actions remain largely unknown.
Oxytocin receptor activation upregulates the activity of potassium-chloride co-transporter2 (KCC2) to switch GABA function from depolarization to hyperpolarization in an early and narrow developmental time window of life. 29 Neonatal oxytocin has also been shown to modulate vasopressin V1a receptor binding in the bed nucleus of the stria terminalis, cingulate cortex, mediodorsal thalamus, medial preoptic area and lateral septum in prairie voles in adulthood. 167 Early-life oxytocin administration also influences alpha2 receptor agonist binding in rats of adulthood. 168 Expression of the oestrogen alpha receptor has also been reported to be changed by neonatal oxytocin administration in a manner dependent on sex, brain regions and oxytocin dosages in prairie voles. 169 Neonatal oxytocin administration also influences serotoninergic innervation in the anterior hypothalamus, cortical amygdala and ventromedial hypothalamus in male prairie voles. 170 Expression of brain-derived neurotrophic factor in the hippocampus of rats has also been shown to be modulated by oxytocin dependent on sex and age. 171 The effects of oxytocin on glutamatergic system in the hippocampus. 159 and in the prefrontal cortex 172 have also been shown. Neurogenetic actions by oxytocin may also be involved in the long-lasting actions of early-life oxytocin. Female pups appear to be more sensitive than male pups to the neonatal manipulation of oxytocin, 173 although sex disparities should be interpreted cautiously. 174 The finding that early-life experience modulates the brain differentially depending on sex and brain region are not surprising. Expression of the oxytocin receptor in some brain areas is sexually dimorphic, 45 and oestrogens or testosterone modulate the expression of oxytocin and the oxytocin receptor. 2 The oxytocin-oxytocin receptor system has differential roles depending on the brain region with respect to the control of behaviour and autonomic systems. 2,20 It is important to clarify region-specific roles of the oxytocin system using modern genetic tools that can manipulate activity of the oxytocin system in region-and/or function-specific manners. 4 Oxytocin is released from not only axon terminals, but also non-synaptic axonal varicosities, 175  It is also likely that discrepancies are caused, at least in part, by the complex nature of early-life experience. For example, maternal separation has several components including sensory deprivation from mothers (loss of touch, warmth, olfactory signals, visual signals of the mother and littermates) and also exposure to novel environments during separation, possible nutritional deprivation and temperature loss. Maternal separation modulates the parental care of mothers afterwards. Some of these components may be variable depending on laboratories. Simpler stimulation of a mono-sensory system and pathway-specific activity manipulation using molecular techniques may be useful as an early-life stimulus for clarifying the effects of early-life experience.

| FUTURE D IREC TI ON S OF RE S E ARCH AND CON CLUS IONS
Oxytocin administered not only during the neonatal period, but also during the adolescent period has been shown to have longlasting prosocial, anti-anxiety and anti-aggression actions. [180][181][182] During the adolescent period, the release of sexual hormones is drastically increased and social behaviours among peers are developed.
The roles of the endogenous oxytocin system, the activity of which is modulated by sexual hormones and which also facilitates social behaviour, during the adolescent period remain to be clarified. 183 Oxytocin has been considered for treatment of autism spectrum disorder 184 and post-traumatic stress disorder. 185 From translational points of view, it is important to clarify the long-term actions of oxytocin during early life and to find ways for activation of functionally specific oxytocin pathways to induce beneficial actions of the oxytocin system. It is possible that selective activation of a specific oxytocin system would rescue the adverse actions of early-life stressful experience and thus could be used for the treatment of developmental psychiatric disorders.

ACK N OWLED G EM ENTS
This work was financially supported by the Japan Society for the Promotion of Science, Grants-in Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan (20H03419 to TO, 20K07278 to YT), and by a grant for Fusion Oriented Research for disruptive Science and Technology from Japan Science and Technology Agency (JPMJFR2067to YT).

CO N FLI C T O F I NTE R E S T S
The authors declare that they have no conflicts of interest.