Somatosensory control of the onset and retention of maternal responsiveness in primiparous Sprague-Dawley rats

Microglia depletion facilitates the display of maternal behavior and alters activation of the maternal brain network in nulliparous female rats

The peripartum period is accompanied by peripheral immune alterations to promote a successful pregnancy. We and others have also demonstrated significant neuroimmune changes that emerge during late pregnancy and persist postpartum, most prominently decreased microglia numbers within limbic brain regions. Here we hypothesized that microglial downregulation is important for the onset and display of maternal behavior. To test this, we recapitulated the peripartum neuroimmune profile by depleting microglia in non-mother (i.e., nulliparous) female rats who are typically not maternal but can be induced to behave maternally towards foster pups after repeated exposure, a process called maternal sensitization. BLZ945, a selective colony-stimulating factor 1 receptor (CSF1R) inhibitor, was administered systemically to nulliparous rats, which led to ~75% decrease in microglia number. BLZ- and vehicle-treated females then underwent maternal sensitization and tissue was stained for ∆fosB to examine activation across maternally relevant brain regions. We found BLZ-treated females with microglial depletion met criteria for displaying maternal behavior significantly sooner than vehicle-treated females and displayed increased pup-directed behaviors. Microglia depletion also reduced threat appraisal behavior in an open field test. Notably, nulliparous females with microglial depletion had decreased numbers of ∆fosB+ cells in the medial amygdala and periaqueductal gray, and increased numbers in the prefrontal cortex and somatosensory cortex, compared to vehicle. Our results demonstrate that microglia regulate maternal behavior in adult females, possibly by shifting patterns of activity in the maternal brain network.

Mammalian NREM and REM sleep: Why, when and how

This report proposes that fish use the spinal-rhombencephalic regions of their brain to support their activities while awake. Instead, the brainstem-diencephalic regions support the wakefulness in amphibians and reptiles. Lastly, mammals developed the telencephalic cortex to attain the highest degree of wakefulness, the cortical wakefulness. However, a paralyzed form of spinal-rhombencephalic wakefulness remains in mammals in the form of REMS, whose phasic signs are highly efficient in promoting maternal care to mammalian litter. Therefore, the phasic REMS is highly adaptive. However, their importance is low for singletons, in which it is a neutral trait, devoid of adaptive value for adults, and is mal-adaptive for marine mammals. Therefore, they lost it. The spinal-rhombencephalic and cortical wakeful states disregard the homeostasis: animals only attend their most immediate needs: foraging defense and reproduction. However, these activities generate allostatic loads that must be recovered during NREMS, that is a paralyzed form of the amphibian-reptilian subcortical wakefulness. Regarding the regulation of tonic REMS, it depends on a hypothalamic switch. Instead, the phasic REMS depends on an independent proportional pontine control.

Mother-Young Bonding: Neurobiological Aspects and Maternal Biochemical Signaling in Altricial Domesticated Mammals

Mother-young bonding is a type of early learning where the female and their newborn recognize each other through a series of neurobiological mechanisms and neurotransmitters that establish a behavioral preference for filial individuals. This process is essential to promote their welfare by providing maternal care, particularly in altricial species, animals that require extended parental care due to their limited neurodevelopment at birth. Olfactory, auditory, tactile, and visual stimuli trigger the neural integration of multimodal sensory and conditioned affective associations in mammals. This review aims to discuss the neurobiological aspects of bonding processes in altricial mammals, with a focus on the brain structures and neurotransmitters involved and how these influence the signaling during the first days of the life of newborns.

Peripartum effects of synthetic oxytocin: The good, the bad, and the unknown

The first clinical applications of oxytocin (OT) were in obstetrics as a hormone to start and speed up labor and to control postpartum hemorrhage. Discoveries in the 1960s and 1970s revealed that the effects of OT are not limited to its peripheral actions around birth and milk ejection. Indeed, OT also acts as a neuromodulator in the brain affecting fear memory, social attachment, and other forms of social behaviors. The peripheral and central effects of OT have been separately subject to extensive scrutiny. However, the effects of peripheral OT-particularly in the form of administration of synthetic OT (synOT) around birth-on the central nervous system are surprisingly understudied. Here, we provide a narrative review of the current evidence, suggest putative mechanisms of synOT action, and provide new directions and hypotheses for future studies to bridge the gaps between neuroscience, obstetrics, and psychiatry.

Psychological and neurobiological mechanisms underlying the decline of maternal behavior

The maternal behavior decline is important for the normal development of the young and the wellbeing of the mother. This paper reviews limited research on the factors and mechanisms involved in the rat maternal behavior decline and proposes a multi-level model. Framed in the parent-offspring conflict theory (an ultimate cause) and the approach-withdrawal model (a proximate cause), the maternal behavior decline is viewed as an active and effortful process, reflecting the dynamic interplay between the mother and her offspring. It is instigated by the waning of maternal motivation, coupled with the increased maternal aversion by the mother in responding to the changing sensory and motoric patterns of pup stimuli. In the decline phase, the neural circuit that mediates the inhibitory ("withdrawal") responses starts to increase activity and gain control of behavioral outputs, while the excitatory ("approach") maternal neural circuit is being inhibited or reorganized. Various hormones and certain monoamines may play a critical role in tipping the balance between the excitatory and inhibitory neural circuits to synchronize the mother-infant interaction.

Lateralized Expression of Cortical Perineuronal Nets during Maternal Experience is Dependent on MECP2

Cortical neuronal circuits along the sensorimotor pathways are shaped by experience during critical periods of heightened plasticity in early postnatal development. After closure of critical periods, measured histologically by the formation and maintenance of extracellular matrix structures called perineuronal nets (PNNs), the adult mouse brain exhibits restricted plasticity and maturity. Mature PNNs are typically considered to be stable structures that restrict synaptic plasticity on cortical parvalbumin+ (PV+) GABAergic neurons. Changes in environment (i.e., novel behavioral training) or social contexts (i.e., motherhood) are known to elicit synaptic plasticity in relevant neural circuitry. However, little is known about concomitant changes in the PNNs surrounding the cortical PV+ GABAergic neurons. Here, we show novel changes in PNN density in the primary somatosensory cortex (SS1) of adult female mice after maternal experience [called surrogate (Sur)], using systematic microscopy analysis of a whole brain region. On average, PNNs were increased in the right barrel field and decreased in the left forelimb regions. Individual mice had left hemisphere dominance in PNN density. Using adult female mice deficient in methyl-CpG-binding protein 2 (MECP2), an epigenetic regulator involved in regulating experience-dependent plasticity, we found that MECP2 is critical for this precise and dynamic expression of PNN. Adult naive Mecp2-heterozygous (Het) females had increased PNN density in specific subregions in both hemispheres before maternal experience, compared with wild-type (WT) littermate controls. The laterality in PNN expression seen in naive Het (NH) was lost after maternal experience in Sur Het (SH) mice, suggesting possible intact mechanisms for plasticity. Together, our results identify subregion and hemisphere-specific alterations in PNN expression in adult females, suggesting extracellular matrix plasticity as a possible neurobiological mechanism for adult behaviors in rodents.

Increased depression-related behavior during the postpartum period in inbred BALB/c and C57BL/6 strains

Pregnancy and lactation are characterized by dramatic changes in the endocrine system and brain in mammalian females. These changes, with stress before pregnancy, are potential risk factors for the development of postpartum depression (PPD). A valid animal model of PPD is needed to understand the neurobiological basis of the depressive state of females. To explore a mouse model of PPD, we first assessed anxiety-like and depression-related behaviors in nulliparous (virgin), nonlactating primiparous, and lactating primiparous females in four inbred strains of mice (C57BL/6J, C57BL/6JJcl, BALB/cAnNCrlCrlj, and BALB/cAJcl). Pups from the nonlactating female group were removed one day after parturition to examine the effects of physical interaction with pups on the postpartum behaviors. Second, we investigated the additional effects of prepregnancy stress (restraint stress for 6 h/day for 21 days) on postpartum behaviors in the BALB/cAJcl strain. We found that females of the two BALB/c substrains showed decreased locomotor activity and increased anxiety-like and depression-related behaviors compared with females of the two C57BL/6 substrains. Behavioral differences were also observed between the two substrains of each strain. Additionally, pregnancy- and lactation-dependent behavioral differences were found in some strains: lactating BALB/cAJcl females traveled shorter distance than the females of the other reproductive state groups, while nonlactating and lactating BALB/cAJcl and C57BL/6J females showed increased depression-related behavior compared with nulliparous females. Lactating BALB/cAJcl and C57BL/6JJcl females exhibited decreased sucrose preference or anhedonia-like behavior compared with nulliparous and nonlactating females, although these results did not reach statistical significance after correction for multiple testing. An additional independent experiment replicated the marked behavioral changes in lactating BALB/cAJcl females. Moreover, increased anxiety-like behavior was observed in lactating BALB/cAJcl females that experienced prepregnancy stress. These results suggest genetic contributions to the regulation of anxiety-like and depression-related behaviors in female mice. Furthermore, this study suggests that pregnancy and lactation cause decreased locomotor activity and increased depression-related behaviors, which was consistently found in our results, and that prepregnancy stress enhances anxiety-like behavior in the BALB/cAJcl strain. The inbred strain of female mice may be used as a potential model of PPD to further study the genetic and neurobiological mechanisms underlying the development of this disorder.

Cesarean Section or Natural Childbirth? Cesarean Birth May Damage Your Health

The increasing popularity of Cesarean birth has become a social concern in many countries. This paper reviews the literature on the effects of Cesarean section on children's psychological health. The results show that Cesarean birth may have adverse effects on children's sensory perception, sensory integration ability, neuropsychiatric development, and the infant-mother relationship. However, there remain deficiencies in extant research methods, research content, subject groupings, and interpretation of research results. Future research should improve research methods, broaden the research content, and refine the grouping of children born by Cesarean section. The exploration of neural mechanisms is also needed, as well as research directed toward suggesting effective interventions to reduce unnecessary Cesarean sections.

Effects of pre- and neonatal undernutrition on the kyphotic response and c-Fos activity in the caudal periaqueductal gray of primiparous lactating Wistar rats

In rodents, the most representative component of maternal behavior that meets the purpose of newborn nutrition is the kyphotic posture. During this posture, the mother maintains a unique environment for the protection, thermal regulation and breast-feeding of the progeny. The aim of this study was to investigate possible deficiencies in the kyphotic posture of adult lactating dams with pre- and neonatal undernutrition evoked by their own pups suckling in a home-cage situation. Wistar dams that had been previously exposed to perinatal undernutrition were mated at 90days of age, and pregnancy was confirmed by vaginal smears. Before testing if the perinatal underfed dam affected behavior, pups were removed (4h), and both the maternal response and the kyphotic posture were video-recorded (1h) and analyzed at 4 and 12days of lactation. Pre- and post-test litter weight gain was obtained. To immunostain the caudal periaqueductal gray, the litter was separated from their dams 24h before suckling stimulation. The results showed that underfed dams significantly reduced the duration of high kyphosis by choosing unconventional postures (prone and partial kyphosis). The body weight of the F1 offspring was significantly reduced, and the underfed F0 dams showed reduced c-Fos immunostaining at the caudal periaqueductal gray. The findings showed that early underfed dams have deficiencies in the mechanisms underlying the kyphosis, possibly because the pups' cues to evoke this posture were suboptimal and/or because the dam expressed deficient nursing. The results suggest that the abnormal kyphotic posture may affect the mother-litter bonds and have long-term effects on neonatal brain functions.

Maternal alterations induced by exposure to an unfamiliar home cage in early underfed dams

The expression of different behavioral components in the adult rat depends on a number of early influences, including age, hormones, manipulations of sensory cues, and perinatal undernutrition, all of which impact the development of brain areas underlying adaptive processes, maternal behavior, and the response to novelty. The current study investigates the effects of pre- and neonatal undernutrition on various components of maternal behavior of dams exposed to the challenge of an unfamiliar home cage on days 4, 8, and 12 of lactation. Food restriction was initiated from gestational day (G) G6 to G19 when dams received 50-70% of the normal balanced diet, followed by 100% from G20 to G21. After birth, pups were underfed by alternating every 12h between two lactating dams, one of which, had ligated nipples. Weaning was at 25days of age followed by an ad libitum diet until postpartum day 90, when females were mated, and subsequently tested for maternal behavior in an unfamiliar cage. The results indicated that in early underfed mothers the frequency of handling wood shavings and of, approaching, licking, crouching, and grasping pups for retrieval was significantly reduced. Moreover, self-grooming increased substantially in the underfed dams, but the frequency of rearing was reduced. Additionally, the body weight of pups nursed by early underfed dams was significantly lower than that of control pups. These findings suggest a relation between early food restriction and the deficient maternal care observed when these dams were challenged by exposure to an unfamiliar home cage.

Previous and recent maternal experiences modulate pups' incentive value relative to a male without affecting maternal behavior in postpartum estrous rats

This study extends the behavioral analysis of the postpartum estrus (PPE) which represents a unique period in the female rat's lifetime when maternal and sexual motivations co-exist. The aim of this study was to explore how previous and recent maternal experiences influence the maternal responses to pups when confronted with a male in a preference test or when they are presented independently in the home cage. To achieve this objective, we firstly compared the maternal behavior in the home cage and the preference for pups or a male in a Y-maze of primiparous and multiparous females approximately twelve hours after delivery. No differences were observed in the active and passive components of the maternal behavior of primiparous and multiparous rats; however second-time mothers made more efforts to gain access to the pups and tended to spend more time with them in the Y-maze than maternally inexperienced dams. In a second experiment, we assessed the influence of recent maternal experience with pups on PPE females' behavior by comparing pups vs. male preference and maternal behavior of females that had experienced continuous or limited (approximately two hours) interaction with their litters after parturition was completed. PPE rats subjected to reduced interaction with their pups preferred the male, while females continuously exposed to pups chose them over the male. This change in females' preference was not accompanied by significant alterations of maternal performance in the home cage, although anogenital licking tended to decrease in females with limited mother-litter interaction. Together, the results of these experiments indicate that previous and recent maternal experiences influence the motivational responses of PPE females, and that these effects are more evident when both motivations compete.

The maternal brain and its plasticity in humans

This article is part of a Special Issue "Parental Care". Early mother-infant relationships play important roles in infants' optimal development. New mothers undergo neurobiological changes that support developing mother-infant relationships regardless of great individual differences in those relationships. In this article, we review the neural plasticity in human mothers' brains based on functional magnetic resonance imaging (fMRI) studies. First, we review the neural circuits that are involved in establishing and maintaining mother-infant relationships. Second, we discuss early postpartum factors (e.g., birth and feeding methods, hormones, and parental sensitivity) that are associated with individual differences in maternal brain neuroplasticity. Third, we discuss abnormal changes in the maternal brain related to psychopathology (i.e., postpartum depression, posttraumatic stress disorder, substance abuse) and potential brain remodeling associated with interventions. Last, we highlight potentially important future research directions to better understand normative changes in the maternal brain and risks for abnormal changes that may disrupt early mother-infant relationships.

Common and divergent psychobiological mechanisms underlying maternal behaviors in non-human and human mammals

Maternal interactions with young occupy most of the reproductive period for female mammals and are absolutely essential for offspring survival and development. The hormonal, sensory, reward-related, emotional, cognitive and neurobiological regulators of maternal caregiving behaviors have been well studied in numerous subprimate mammalian species, and some of the importance of this body of work is thought to be its relevance for understanding similar controls in humans. We here review many of the important biopsychological influences on maternal behaviors in the two best studied non-human animals, laboratory rats and sheep, and directly examine how the conceptual framework established by some of the major discoveries in these animal "models" do or do not hold for our understanding of human mothering. We also explore some of the limits for extrapolating from non-human animals to humans. We conclude that there are many similarities between non-human and human mothers in the biological and psychological factors influencing their early maternal behavior and that many of the differences are due to species-characteristic features related to the role of hormones, the relative importance of each sensory system, flexibility in what behaviors are exhibited, the presence or absence of language, and the complexity of cortical function influencing caregiving behaviors.

Importance of mother-infant communication for social bond formation in mammals

Mother-infant bonding is a universal relationship of all mammalian species. Here, we describe the role of reciprocal communication between mother and infant in the formation of bonding for several mammalian species. Mother-infant bond formation is reinforced by various social cues or stimuli, including communicative signals, such as odor and vocalizations, or tactile stimuli. The mother also develops cross-modal sensory recognition of the infant, during bond formation. Many studies have indicated that the oxytocin neural system plays a pivotal role in bond formation by the mother; however, the underlying neural mechanisms for infants have not yet been clarified. The comparative understanding of cognitive functions of mother and infants may help us understand the biological significance of mother-infant communication in mammalian species.

Oxytocin and mutual communication in mother-infant bonding

Mother-infant bonding is universal to all mammalian species. In this review, we describe the manner in which reciprocal communication between the mother and infant leads to mother-infant bonding in rodents. In rats and mice, mother-infant bond formation is reinforced by various social stimuli, such as tactile stimuli and ultrasonic vocalizations (USVs) from the pups to the mother, and feeding and tactile stimulation from the mother to the pups. Some evidence suggests that mother and infant can develop a cross-modal sensory recognition of their counterpart during this bonding process. Neurochemically, oxytocin in the neural system plays a pivotal role in each side of the mother-infant bonding process, although the mechanisms underlying bond formation in the brains of infants has not yet been clarified. Impairment of mother-infant bonding, that is, deprivation of social stimuli from the mother, strongly influences offspring sociality, including maternal behavior toward their own offspring in their adulthood, implying a "non-genomic transmission of maternal environment," even in rodents. The comparative understanding of cognitive functions between mother and infants, and the biological mechanisms involved in mother-infant bonding may help us understand psychiatric disorders associated with mother-infant relationships.

The changing role of the medial preoptic area in the regulation of maternal behavior across the postpartum period: facilitation followed by inhibition

Maternal behavior in rats undergoes considerable plasticity in parallel to the developmental stage of the pups, resulting in distinct patterns of maternal behavior and care at different postpartum time points. The medial preoptic area (mPOA) of the hypothalamus is one critical neural substrate underlying the onset and early expression of maternal behavior in rats but little is known about its specific functional role in the evolving expression of maternal behavior across the postpartum period. The present study uses a reversible local neural inactivation method to examine the role of the mPOA in the regulation of maternal behavior throughout the postpartum period, particularly extending into the late postpartum, a little examined period. This approach avoids the compensatory plasticity in CNS that occurs after permanent lesions, and allows the repeated testing of same individuals. Early (PPD7-8) and late (PPD13-14) postpartum maternal behavior was evaluated in female rats following infusions of bupivacaine or vehicle into the mPOA or into control areas. As expected, mPOA inactivation severely but transiently disrupted early postpartum maternal behavior whereas infusion of vehicle or inactivation of adjacent control sites did not. Later in the postpartum period, however, transient mPOA inactivation facilitated the expression of maternal behaviors, highly contrasting the behavioral expression levels characteristic of late postpartum. Results strongly demonstrate that the mPOA is differentially engaged throughout postpartum in orchestrating appropriate maternal responses with the developmental stage of the pups.

Mother is not like mother: Concurrent pregnancy reduces lactating guinea pigs' responsiveness to pup calls

Offspring signalling can serve to communicate need to the parents and thus influence parental readiness to provide care. Offspring stimuli that affect parental care have been investigated extensively. Yet much less is known about the mechanisms leading to a decline in maternal motivation when conflicts of provisioning current and future offspring may arise. Here we tested responses by pregnant and non-pregnant female guinea pigs (Cavia aperea f. porcellus) to playback of pup calls during their period of lactation for dependent offspring. Most concurrently pregnant and lactating females did not respond to pup calls, whereas non-pregnant lactating females responded strongly. Our findings expand on previous studies by showing that female behavioural responsiveness to pup stimuli is strongly reduced by concurrent pregnancy and lactation. These instantaneous measurements of female responsiveness to young show more directly than standard measures like nursing performance or time to weaning how female motivation to care for current offspring is diminished by simultaneous gestation.

Maternal brain response to own baby-cry is affected by cesarean section delivery

A range of early circumstances surrounding the birth of a child affects peripartum hormones, parental behavior and infant wellbeing. One of these factors, which may lead to postpartum depression, is the mode of delivery: vaginal delivery (VD) or cesarean section delivery (CSD). To test the hypothesis that CSD mothers would be less responsive to own baby-cry stimuli than VD mothers in the immediate postpartum period, we conducted functional magnetic resonance imaging, 2-4 weeks after delivery, of the brains of six mothers who delivered vaginally and six who had an elective CSD. VD mothers' brains were significantly more responsive than CSD mothers' brains to their own baby-cry in the superior and middle temporal gyri, superior frontal gyrus, medial fusiform gyrus, superior parietal lobe, as well as regions of the caudate, thalamus, hypothalamus, amygdala and pons. Also, within preferentially active regions of VD brains, there were correlations across all 12 mothers with out-of-magnet variables. These include correlations between own baby-cry responses in the left and right lenticular nuclei and parental preoccupations (r = .64, p < .05 and .67, p < .05 respectively), as well as in the superior frontal cortex and Beck depression inventory (r = .78, p < .01). First this suggests that VD mothers are more sensitive to own baby-cry than CSD mothers in the early postpartum in sensory processing, empathy, arousal, motivation, reward and habit-regulation circuits. Second, independent of mode of delivery, parental worries and mood are related to specific brain activations in response to own baby-cry.

Nursing stimulation is more than tactile sensation: It is a multisensory experience

Novel sensory experiences, particularly those associated with epochal developmental events like nursing alter cortical representation, affecting memory, perception and behavior. Functional MRI was used here to test whether the sensoricortical map of the ventrum is modified during lactation. Three stimuli were used to drive cortical activation in primiparous rats: natural, artificial suckling stimulation and general mechanical rubbing of the skin of the ventrum. These stimuli significantly activated the somatosensory cortex of dams. Of the three stimuli, artificial and pup suckling robustly activated much of the cerebrum, most notably the visual, auditory and olfactory cortices. Surprisingly, activation occurred even in the absence of pups, with artificial suckling. This finding suggests that incoming information from a single modality was sufficient to drive activity of others. Enhanced sensitivity across the cortical mantle during nursing may help the dam to perceive, process, and remember stimuli critical to the care and protection of her young.

Regulation of anxiety during the postpartum period

Healthy mother-infant interactions are critical for the physical, cognitive, and psychological development of offspring. Such interactions rely on numerous factors, including a positive maternal emotional state. However, many postpartum women experience emotional dysregulation, often involving elevated anxiety. Neuroendocrine factors contributing to the onset of postpartum anxiety symptoms are mostly unknown, but irregularities in hypothalamic-pituitary-adrenal axis function, reduced prolactin and oxytocin signaling, or parturitional withdrawal of ovarian, placental and neural steroids could contribute to anxiety in susceptible women. Although the causes of initial onset are unclear, postpartum anxiety can be mitigated by recent contact with infants. Numerous neurochemical systems, including oxytocin, prolactin, GABA, and norepinephrine mediate this anxiolytic effect of infant contact. Insight into the etiology of postpartum anxiety disorders, and how contact with infants helps counter existing anxiety dysregulation, will surely facilitate the diagnosis and treatment of postpartum women at risk for, or experiencing, an anxiety disorder.

Arousal regulation, emotional flexibility, medial amygdala function, and the impact of early experience: comments on the paper of Lewis et al

The balance between optimal levels of emotional arousal and cognitive performance reflects the integration of several dopaminergically and adrenergically regulated neural systems. The amygdalar system is a key region for gating stimulation to cortical regions and the medial amygdala appears to play an especially key role in mediating the fear response. More generally, these arousal regulatory neural systems are key to frustration or stress impact prefrontal cortical function. Further, the threshold for when the level of stress is overwhelming and hence impairs cognitive function reflects minimally genetic and experiential influence. An important interface between Drs. Lewis and Davis's work is how early experience, especially through early parenting, may set the threshold of responsiveness for these arousal regulatory neural systems.

Demanding pups improve maternal behavioral impairments in sensitized and haloperidol-treated lactating female rats

The impairments in the maternal behavior of ovariectomized sensitized females, relative to lactating dams, resemble those deficits found in lactating females after treatment with the D1/D2 DA receptor antagonist haloperidol, which interferes with maternal motivation. Therefore, it could be speculated that these behavioral deficits found in sensitized females and haloperidol-treated dams are due to a reduced motivation to interact with pups. In support of this hypothesis, we have found that both sensitized and haloperidol-treated lactating females exhibited remarkably similar impairments in the expression of all active maternal behaviors relative to lactating dams. Furthermore, these deficits were overridden when they were allowed to interact with 12h-isolated pups (demanding pups). Interestingly, lactating dams also improved their maternal behavior in the presence of demanding pups, and clearly chose demanding more than non-demanding pups in a preference paradigm. These data support the idea that the behavioral deficits of sensitized and haloperidol-treated lactating females are due to a reduced behavioral activation in response to the incentive cues from pups compared to lactating dams, and not because of a motor inability to express maternal behavior. These findings ultimately suggest that pups modulate the activity of DA system involved in the regulation of maternal behavior.

Learning during motherhood: A resistance to stress

Hormonal and emotional responses to stress are diminished during pregnancy and the postpartum period. However, the effects of stress on learning during these stages of the female life span have not been examined. In previous studies, we have reported that exposure to an acute stressful event reduces classical eyeblink conditioning 24 h later in adult virgin female rats that are experiencing an ovarian cycle. Here we show that conditioning during late pregnancy was similarly reduced by stressful experience. However, conditioning in postpartum females was unaffected by stressor exposure. The resistance to stress during the postpartum period was evident as early as 2 days after parturition and persisted until the late postpartum period, just prior to weaning. Postpartum conditioning was unresponsive to numerous types of stressors, including brief inescapable tailshocks, swim stress, and exposure to a male intruder. The resistance to stress appears to be dependent on the presence of the offspring, because the impairment in conditioning returned when postpartum females were separated from their pups. Moreover, the resistance to stress occurred in virgin females that behaved maternally after being exposed to young pups for several days. Together, these data suggest that the presence of offspring and the nurturing and care-giving activities that they elicit protect females from the adverse effect of stress on processes involved in learning and memory.

Motivational aspects of maternal anxiolysis in lactating rats

RATIONALE AND OBJECTIVE

This study examines the role of maternal motivation on the reduced anxiety-like responses displayed by lactating rats in the plus maze test.

RESULTS

Maternal animals, both lactating and sensitized (ovariectomized females behaving maternal after a continuous exposure to pups), displayed anxiolytic-like responses in the plus maze test in contrast to ovariectomized non-maternal rats. However, the levels of experimental anxiety were lower in lactating than in sensitized females. Pups placed in the open arms of the maze further reduced the low levels of anxiety-like behavior of both sensitized and lactating rats. Low doses of haloperidol (0.05 and 0.1 mg/kg), a dopamine antagonist, which interfere with maternal motivation but has neither anxiolytic nor anxiogenic effect in the plus maze test, significantly increased the anxiety-like responses of lactating rats. The presence of the pups in the open arms of the maze overrode the behavioral effect of haloperidol on lactating dams' anxiety-related behavior.

CONCLUSIONS

These experiments show that maternity induces changes in the way the animals react to the environment, rendering them less anxious to aversive stimuli. The degree of experimental anxiolysis displayed by maternal animals varies according to their maternal motivation, which is modulated by the female's endocrine state, the pups and/or the dopaminergic system.

Reduced anxiety in postpartum rats requires recent physical interactions with pups, but is independent of suckling and peripheral sources of hormones

Changes in emotional behavior occur across the reproductive cycle in female rodents, with reduced anxiety found during the postpartum period, but relatively little is known about factors contributing to this decreased anxiety. Using increased duration of time spent in the open arms of an elevated plus-maze as an indicator of reduced anxiety, it was found in a series of experiments that (1) anxiety is significantly reduced in Long-Evans females during the first week of lactation, but not thereafter, (2) relatively recent contact with pups before testing (within 4 h) is necessary for their reduced anxiety, (3) dams that receive only distal sensory cues from pups for the 4 h prior to testing do not show reduced anxiety, (4) the absence of nipples, and therefore a lack of suckling by pups, has no effect on dams' anxiety, (5) cesarean delivery of pups 2 days prior to expected parturition did not alter later anxiety in dams, (6) hypophysectomy during mid-pregnancy or ovariectomy within 24 h after parturition also did not prevent reduced anxiety in dams, and (7) differences in anxiety between lactating and virgin females are greatest 4-8 min after being placed in the plus-maze. Therefore, exposure to their own peripheral hormones through mid-pregnancy is sufficient to prime female rats to show reduced anxiety, but only if they later have recent physical interaction with pups. Furthermore, because suckling and the peripheral hormones released during suckling appear to be unnecessary, decreased anxiety in maternal rats may instead be regulated by the transient intracerebral release of neuropeptides or neurotransmitters while dams receive other types of tactile inputs from their infants.

Maternal memory in adult, nulliparous rats: Effects of testing interval on the retention of maternal behavior

The retention of maternal behavior (i.e., maternal memory) was measured in adult, nulliparous rats induced to respond maternally by continuous exposure to foster pups. Specifically, the effects of the interval duration between the initial induction and the reinduction of maternal behavior were determined. Intact virgin rats were first exposed to foster young to induce maternal behavior. During the initial induction phase, females were required to be fully maternal on 2 consecutive test days. Animals were then assigned to one of three interval groups (10, 20, or 40 days). After being isolated from rat pups for these designated periods, females in each group were tested again for their latencies to induce maternal behavior. Whereas the initial median latencies to display full maternal behavior ranged from 4.5 to 5 days for each group, upon retesting, median latencies for each group declined to 1 to 4 days. The greatest reduction in latency was present in the 10-day group (80%), and the smallest reduction was detected in the 40-day group (20%). A significant negative linear correlation was found between test interval and percentage reduction in behavioral latency. Based upon this relationship and under these test conditions, "maternal memory" in the adult, nulliparous rat would be expected to be nondetectable after about an interval of 50 days between tests. The pattern of maternal memory acquisition and loss appears similar to that reported in parous animals. The present study highlights similarities and possible differences underlying the establishment of the retention of maternal behavior (i.e., maternal memory).

Complete maternal deprivation affects social, but not spatial, learning in adult rats

The effects of maternal deprivation on learning of social and spatial tasks were investigated in female adult rats. Pups were reared artificially and received "lickinglike" tactile stimulation (AR animals) or were reared with their mothers (MR animals). In adulthood, subjects were tested on paradigms of spatial learning and on paradigms involving learning of social cues. Results showed that maternal deprivation did not affect performance on spatial learning, but it did impair performance on the three social learning tasks. The AR animals made no distinction between a new and a previously presented juvenile conspecific. AR animals also responded less rapidly than MR animals at test for maternal behavior 2 weeks after a postpartum experience with pups. Finally, AR animals did not develop a preference for a food previously eaten by a familiar conspecific whereas MR animals did. This study indicates that animals reared without mother and siblings show no deficits in spatial tasks while showing consistent deficits in learning involving social interactions.

Early recognition of newborn goat kids by their mother: I. Nonolfactory discrimination

Mother sheep and goats develop an early bond with their neonate on the basis of olfactory recognition. We investigated whether goats were also able to show early (<24 hr postpartum) nonolfactory discrimination of their kids, as already reported in sheep. In a first experiment, we found that goats are not able to recognize their kid at 1 m away on the basis of olfactory cues alone. By contrast, they showed a significant preference for their own kid in a two-choice test as early as 4 hr postpartum, and prepartum maternal anosmia did not impede the ability of mothers to show discrimination. We conclude that goats, like sheep, are fully able to discriminate their neonate without the help of olfactory cues very early after parturition. The difference in the early spatial mother-young relationship between the two species due to the different behavior of the young (kids = hiders, lambs = followers), is not associated with marked differences in the dynamics or mechanisms controlling the development of recognition of the neonate by its mother.

Bicuculline infusion into the accessory olfactory bulb facilitates the induction of maternal behavior in rats

The GABAA antagonist bicuculline, intracranially infused into the accessory olfactory bulb (AOB), facilitated the expression of maternal behavior (MB) in virgin Wistar female rats. Behavioral effects were observed 24 hours after infusion and were injection dependent. Pheromonal stimuli, generated by the pups, are thought to exert an inhibitory effect on vomeronasal nuclei involved in MB in virgin rats. The present study investigated the possibility that a decrement in AOB output, resulting from long-term compensatory synaptic changes to chronic bicuculline infusion, would facilitate the expression of MB. The implications of our findings for the mechanisms involved in the induction of MB and the maternal experience effect are discussed.

Effects of stress during pregnancy on maternal behavior in mice

The effects of stress experienced during pregnancy and raising stressed offspring on maternal behavior were investigated in Swiss-Webster mice. Dams were either stressed or not stressed during pregnancy, and raised either prenatally stressed or nonstressed cross-fostered pups. Maternal behaviors such as grooming, nursing, pup retrieval and maternal aggression were assessed during the first 4 days after birth. Nonstressed dams raising stressed pups and stressed dams raising nonstressed pups groomed and nursed their pups significantly less than did control dams (stressed dams raising stressed pups and nonstressed dams raising nonstressed pups). Nonstressed dams raising stressed pups were also the slowest to retrieve both the first and last pup in retrieval tests. Nonstressed dams raising nonstressed pups were significantly less aggressive than other dams. In contrast, stressed dams raising stressed pups exhibited high levels of nursing and grooming, retrieved their pups rapidly and were very aggressive towards an intruder. These results indicate that raising stressed pups, or experiencing stress during a pregnancy can have significant effects on maternal behaviors. Stressed dams raising stressed pups exhibit maternal care comparable to that of nonstressed dams raising nonstressed pups at least for nesting/nurturing behaviors, and show increased levels of aggression and pup retrieval.

Plasticity in the maternal circuit: effects of experience and partum condition on brain astrocyte number in female rats

Female rats that have received a maternal experience undergo enhanced c-fos expression in a number of brain sites when reexposed to pups. The present 2 studies examined changes in the expression of another brain protein, glial fibrillary acidic protein (GFAP), which is a major unit of the astrocytic cytoskeleton. In both experiments, primiparous and multiparous female rats were given varying amounts of postpartum contact with pups and overdosed after varying intervals, with no pups. Brains were prepared for GFAP immunohistochemical analysis. In both studies, Day 5 postpartum multiparous subjects given additional postpartum contact with pups, when compared with pup-exposed primiparous subjects, were found to have significantly higher numbers of GFAP positive cells in the medial preoptic area of the hypothalamus, an area critical for the expression of maternal behavior, but not in control sites. In Experiment 2, an opposite effect of parity was found in the medial amygdala and habenula.

Neurobiology of mother-infant interactions: experience and central nervous system plasticity across development and generations

The optimal coordination between the new mammalian mother and her young involves a sequence of behaviors on the part of each that ensures that the young will be adequately cared for and show healthy physical, emotional, and social development. This coordination is accomplished by each member of the relationship having the appropriate sensitivities and responses to cues that characterize the other. Among many mammalian species, new mothers are attracted to their infants' odors and some recognize them based on their odors; they also respond to their infants' vocalizations, thermal properties, and touch qualities. Together these cues ensure that the mother will nurse and protect the offspring and provide them with the appropriate physical and stimulus environment in which to develop. The young, in turn, orient to the mother and show a suckling pattern that reflects a sensitivity to the mothers odor, touch, and temperature characteristics. This article explores the sensory, endocrine, and neural mechanisms that underlie this early mother-young relationship, from the perspective of, first, the mother and, then, the young, noting the parallels between them. It emphasizes the importance of learning and plasticity in the formation and maintenance of the mother-young relationship and mediation of these experience effects by the brain and its neurochemistry. Finally, it discusses ways in which the infants' early experiences with their mothers (or the absence of these experiences) may come to influence how they respond to their own infants when they grow up, providing a psychobiological mechanism for the inter-generational transmission of parenting styles and responsiveness.

Somatosensory contributions to c-fos activation within the caudal periaqueductal gray of lactating rats: effects of perioral, rooting, and suckling stimuli from pups

In lactating rats, the immediate-early gene c-fos was previously shown to be highly activated in several brain sites by physical interaction with either suckling or nonsuckling pups but not by distal stimuli from pups, a non-pup stimulus, or no stimulation. Further, even greater levels of Fos-immunoreactivity (ir) occurred following suckling versus nonsuckling contact with pups in only 1 of over 25 sites--the caudal periaqueductal gray (cPAG) at an intercollicular level, lesions of which severely reduced the typical suckling-induced kyphotic nursing posture. Herein we further evaluated the effects and site-specificity of various somatosensory cues received from pups during 60 min on Fos-ir in the PAG of day 7 postpartum rats after a 48-h dam-litter separation. Dams interacting with suckling versus nonsuckling pups showed relatively high numbers of Fos-ir cells in the intercollicular cPAG site identified earlier, but not in three other rostrocaudal planes of the PAG. Elimination of rooting on the dam's ventrum by use of fully anesthetized pups did not further diminish Fos-ir in maternally behaving, nonsuckled dams. Perioral anesthesia of dams prior to reunion with the litter prevented retrieval and licking of pups but not pup-initiated nursing behavior, the duration of which was positively correlated with Fos-ir levels within the intercollicular cPAG. Thus, various somatosensory stimuli from pups activate c-fos in a discrete region of the cPAG but only interactions that include suckling and its behavioral consequences elicit maximal expression, consistent with a role for this midbrain site in the sensorimotor control of kyphotic nursing in rats.

Trigeminal lesions and maternal behavior in Norway rats: III. Experience with pups facilitates recovery

In lactating rats, upper snout desensitization by trigeminal denervation severely impairs maternal behavior, with substantial recovery 12-24 hr later. The potential effects of interaction with pups on this recovery was investigated. Dams were subjected to bilateral infraorbital denervation or sham surgery on Day 5 postpartum and observed for 30 min with 6 foster pups 24 hr later, after separation from pups for 4 or 24 hr. Long separation reduced likelihood of retrieval and nursing behavior, infraorbital denervation reduced duration of pup licking, and these deprivations combined greatly increased latencies of pup sniffing and durations of facial self-grooming. Both conditions, and especially their combination, prolonged retrieval duration, the retrieval sequence being interrupted by other pup contact, such as licking. Thus, interaction with pups and adequate trigeminal stimulation maintain normal maternal behavior. These findings are possibly relevant to the normal decline of maternal responsiveness during late lactation and to other lesion-induced deficits.

Limbic system fos expression associated with paternal behavior

Axon-sparing lesions of the medial nucleus of the amygdala (MeA) decrease male parental behavior in the highly social prairie vole. To assess further the role of the amygdala in paternal behavior, male and female prairie voles were exposed to a pup or a non-social olfactory stimulus for 3 hours, and cells expressing Fos peptide were labelled using immunocytochemistry. Compared to controls, males exposed to a pup showed an increase in Fos expression in the MeA, as well as in several areas with connections to it: the accessory olfactory bulb, lateral septum, medial preoptic area, medial bed nucleus of the stria terminalis, nucleus reuniens and paraventricular nucleus of the thalamus. There was no increase in Fos immunoreactivity in the paraventricular nucleus of the hypothalamus or piriform cortex. The same pattern of Fos expression was found in female voles, with the exception of the thalamic paraventricular nucleus, where there was an increase in the pup-exposed group that was not statistically significant (P = 0.11). In addition, the magnitude of induction in females was markedly less than that in males in the medial preoptic area. These results provide further evidence that the MeA is involved in paternal behavior, and suggest certain other areas previously implicated in meternal behavior may also be involved in the control of paternal behavior in this species.

Neuropsychology of maternal behavior in the rat: c-fos expression during mother-litter interactions

This series of studies used the pattern of nuclear Fos-like immunoreactivity (Fos-lir) to map the functional pathways in the brain that mediate the onset and retention of maternal behavior. In the first two experiments, parturient rat dams were exposed to either pups or to other stimuli on Day 1 postpartum. Dams interacting with pups were either intact or sustained ventral somatosensory, olfactory, or combined desensitizations. Results showed that 1) all intact pup-interacting dams showed elevated levels of Fos-lir in the medial preoptic area (MPOA) and the medial and cortical amygdala as compared to control groups, and 2) olfactory and ventral somatosensory desensitization, either alone or in combination, did not decrease Fos-lir in the MPOA. However, olfactory desensitizations did decrease Fos-lir in the medial amygdala and the combined desensitizations significantly reduced Fos-lir in both the basolateral and central amygdala. In the third study, dams were either exposed to pups or to other stimuli and were subsequently reexposed to pups or to pup cues. Regardless of prior maternal experience, females who were able to interact with pups upon reexposure showed increased Fos-lir in the MPOA, the basolateral and central nuclei of the amygdala, and the nucleus accumbens when compared to females which did not interact with pups. Taken together, these studies suggest that the neuroanatomy of maternal behavior is a complex one, involving multiple systems that interconnect with the MPOA and that mediate the many behavioral processes activated when an animal responds maternally.

Noradrenergic involvement in the consolidation of maternal experience in postpartum rats

If postpartum rats are separated from pups following cesarean delivery, their maternal responsiveness declines such that in tests on day 10 they show maternal onset latencies that do not differ from those shown by virgin rats. If, however, dams are permitted a 1-h experience with pups within 36 h of cesarean delivery, rats exhibit a high level of responsiveness to foster pups on day 10 after c-section. The present research investigates the effect of the noradrenergic system in the long-term consolidation of a brief maternal experience in new mother rats. Groups of dams were cesarean delivered and were either given pups for a brief period 36 h after section (experienced) or received no experience (inexperienced). Immediately following the experience phase, dams were injected with different concentrations of the beta-adrenergic antagonist, propranolol (0, 0.5, 1.0, 5.0 mg/kg), or the adrenergic agonist, isoproterenol (0, 0.25 or 0.5 mg/kg). Ten days after cesarean delivery rats were given maternal induction tests. Rats receiving 60 min of experience and injected with propranolol exhibited significantly longer maternal onset latencies than did saline-injected rats, although their latencies were not as long as shown by the maternally inexperienced groups. In contrast, rats receiving 15 min of experience and injected with isoproterenol exhibited significantly shorter onset latencies than did saline-injected rats, whether or not they exhibited maternal behavior during the initial 15 min exposure period. These results suggest that the noradrenergic system is involved in the consolidation of a maternal experience.

Effects of transections to the vomeronasal nerves or to the main olfactory bulbs on the initiation and long-term retention of maternal behavior in primiparous rats

While the onset of maternal behavior at parturition is mediated by hormones, the maintenance of maternal behavior during the first few postpartum weeks depends on experiences acquired while the dam interacts with pups (Rosenblatt, 1990). In fact, if female rats are permitted as little as 2 h or maternal experience within 36 h after Cesarean delivery, they exhibit heightened maternal behavior during maternal induction tests 10 days later; in contrast, dams separated from young at the time of Cesarean delivery and not permitted a maternal experience fail to respond maternally in tests 10 days later (Orpen & Fleming, 1987). In this study we investigated the role of chemosensory input through the vomeronasal and main olfactory systems in this maternal experience effect. Six groups of primiparous females were tested for maternal behavior to foster pups presented 9-10 days after Cesarean delivery: three groups were permitted to interact with pups for a 2-h period 36 h after Cesarean delivery; and three groups were separated from pups until testing and were given no maternal experience. Within each experience condition, one group sustained bilateral section of the vomeronasal nerves, one sustained bilateral coronal cuts through the midsection of the main olfactory bulbs, and one group sustained small medial olfactory bulb cuts. The results showed that animals sustaining vomeronasal or olfactory transections, regardless of experience condition, exhibited significantly reduced latencies to maternal behavior in maternal induction tests. However, these chemosensory disruptions did not prevent an additional facilitation of maternal behavior produced by a prior maternal experience.