Repeated neonatal pain influences maternal behavior, but not stress responsiveness in rat offspring

Dyadic social interaction paradigm reveals selective role of ovarian estrogen in the caring behavior and socially transferred pain in female mice

When a naïve observer meets with a familiar conspecific in pain, mice may have a myriad of social (sniffing, allolicking, allogrooming, huddling) and non-social (self-grooming) behaviors under dyadic social interaction (DSI) paradigm. Unlike male, female observers express more allolicking behavior toward injury site of a familiar female in pain, but with less body allogrooming. In current study, we investigated roles of natural estrus cycle phases and ovarian estrogen in these behaviors and results showed that: (1) there was no changes in above behaviors in terms of latency, time and bouts across different natural estrus cycle phases in intact female. (2) however, ovariectomy (OVX) changed estrus cycle phases, lowered circulating level of ovarian estrogen, reduced time and bouts of allolicking behavior and increased time of self-grooming without affecting other behaviors. Moreover, OVX in observers decreased social buffering effect of DSI on spontaneous pain-related behavior in demonstrator relative to naïve and sham controls. (3) treatment of OVX-female with β-estradiol (E2) or progesterone (PROG) as replacement therapies, only E2 reversed impairment of allolicking behavior. (4) Additionally, socially transferred pain could be identified in intact female across all estrus cycle phases post-DSI, but disappeared in OVX-female, which could be reversed completely by E2 but not by PROG. (5) Finally, serum levels of estrogen, PROG, oxytocin, arginine vasopressin (AVP), prolactin, norepinephrine and 5-HT were examined by ELISA after E2, results showed only AVP level was significantly increased. These results suggest both injury site-targeted caring behavior and socially transferred pain are selectively dependent on ovarian estrogen.

Emotional contagion and prosocial behaviour in fish: An evolutionary and mechanistic approach

In this review, we consider the definitions and experimental approaches to emotional contagion and prosocial behaviour in mammals and explore their evolutionary conceptualisation for studying their occurrence in the evolutionarily divergent vertebrate group of ray-finned fish. We present evidence for a diverse set of fish phenotypes that meet definitional criteria for prosocial behaviour and emotional contagion and discuss conserved mechanisms that may account for some preserved social capacities in fish. Finally, we provide some considerations on how to address the question of interdependency between emotional contagion and prosocial response, highlighting the importance of recognition processes, decision-making systems, and ecological context for providing evolutionary explanations.

Neonatal nociceptive stimulation results in pain sensitization, reduction of hippocampal 5-HT1A receptor, and p-CREB expression in adult female rats

Painful invasive procedures are often performed on newborns admitted to intensive care units (ICU). The acute and long-term effects caused by these stimuli can be investigated in animal models, such as newborn rats. Previous studies have shown that animals subjected to nociceptive stimuli in the neonatal period show sex-specific behavioral changes such as signs of anxiety or depression. Under the same conditions, neonatal stimuli also provoke an increase in the rate of neurogenesis and cell activation in the hippocampal dentate gyrus. So, this study aims to identify the possible roles of central monoamines, receptor expression (5-HT1A), and signaling factors (p-CREB) underlying the long-term effects of neonatal nociceptive stimulation. For this, noxious stimulation was induced by intra-plantar injection of Complete Freund´s adjuvant (CFA) on the postnatal day 1 (P1) or 8 (P8). Control animals were not stimulated. On P75 the behavioral tests were conducted (hotplate and elevated plus maze), followed by sacrifice and molecular studies. Our results showed that neonatal nociceptive stimulation alters pain sensitization specially in females, while stimulation on P1 increases pain threshold, P8-stimulated animals respond with reduced pain threshold (P < 0.001). Hippocampal expression of 5-HT1A receptor and p-CREB were reduced in P8 F group (P < 0.001) in opposition to the increased utilization rate of dopamine and serotonin in this group (P < 0.05). This study shows sex- and age-specific responses of signaling pathways within the hippocampus accompanied by altered behavioral repertoire, at long-term after neonatal painful stimulation.

Alleviating Neonatal Intensive Care Unit Stress: A Chinese Medicine Approach in Neonatal Rats

Background

Premature infants are exposed to numerous stressors in neonatal intensive care unit (NICU) during a crucial period for brain development; this period exerts long-term influences on cognitive and behavioral development.

Aims

To evaluate the effect of NICU-related stress on neonatal rat pups and explore the effect of Chinese medicine treatment (CMT).

Methods

Sixty male rat pups were randomly assigned to three groups: the control group, the NICU group (NICU-related stress), and the CMT group (NICU-related stress plus CMT). All stressors and interventions were administered from 0 to 7 days after birth. Body weight, serum corticosterone levels, and behavior in the open field (OF) test, elevated plus maze (EPM) test, sucrose preference test, and Morris water maze (MWM) test were recorded, and blood samples were collected at five different time points (T0, T1, T2, T3, and T4).

Results

The body weights of rats in the CMT and control groups were heavier than those in the NICU group in both early life and adulthood (P < 0.05). Serum corticosterone levels significantly differed with time (except T0 vs. T1 and T3 vs. T4) but did not significantly differ among the three groups (F = 0.441, P = 0.894). Regardless of age, spatial memory and anxiety-like and depression-like behavior did not differ among the three groups.

Conclusion

NICU-related stress exerted a long-term effect on rat growth and development but did not affect spatial memory, anxiety-like behavior, depression-like behavior, or serum corticosterone levels. CMT alleviated the impact of NICU-related stress on rats and promoted the growth and development of neonatal rats.

Early Life Painful Procedures: Long-Term Consequences and Implications for Farm Animal Welfare

Farm animals routinely undergo painful husbandry procedures early in life, including disbudding and castration in calves and goat kids, tail docking and castration in piglets and lambs, and beak trimming in chicks. In rodents, inflammatory events soon after birth, when physiological systems are developing and sensitive to perturbation, can profoundly alter phenotypic outcomes later in life. This review summarizes the current state of research on long-term phenotypic consequences of neonatal painful procedures in rodents and farm animals, and discusses the implications for farm animal welfare. Rodents exposed to early life inflammation show a hypo-/hyper-responsive profile to pain-, fear-, and anxiety-inducing stimuli, manifesting as an initial attenuation in responses that transitions into hyperresponsivity with increasing age or cumulative stress. Neonatal inflammation also predisposes rodents to cognitive, social, and reproductive deficits, and there is some evidence that adverse effects may be passed to offspring. The outcomes of neonatal inflammation are modulated by injury etiology, age at the time of injury and time of testing, sex, pain management, and rearing environment. Equivalent research examining long-term phenotypic consequences of early life painful procedures in farm animals is greatly lacking, despite obvious implications for welfare and performance. Improved understanding of how these procedures shape phenotypes will inform efforts to mitigate negative outcomes through reduction, replacement, and refinement of current practices.

The effects of neonatal procedural pain and maternal isolation on hippocampal cell proliferation and reelin concentration in neonatal and adult male and female rats

Preterm births accounted for over 10% of all U.S. live births in 2019 and the rate is rising. Neonatal stressors, especially procedural pain, experienced by preterm infants in the neonatal intensive care unit (NICU) have been associated with neurodevelopmental impairments. Parental care can alleviate stress during stressful or painful procedures; however, infants in the NICU often receive reduced parental care compared with their peers. Animal studies suggest that decreased maternal care similarly impairs neurodevelopment but also influences the effects of neonatal pain. It is important to mimic both stressors in animal models of neonatal stress exposure. In this study, researchers investigated the individual and combined impact of neonatal pain and maternal isolation on reelin protein levels and cellular proliferation in the hippocampal dentate gyrus of 8 days old and adult rats. Exposure to either stressor individually, but not both, increased reelin levels in the dentate gyrus of adult females without significantly altering reelin levels in adult males. However, cell proliferation levels at either age were unaffected by the early-life stressors. These results suggest that each early-life stressor has a unique effect on markers of brain development and more research is needed to further investigate their distinct influences.

Neonatal Stress, Health, and Development in Preterms: A Systematic Review

CONTEXT

An overview of the full range of neonatal stressors and the associated clinical, laboratory, and imaging outcomes regarding infants' health and development may contribute to the improvement of neonatal care.

OBJECTIVE

To systematically review existing literature on the associations between all kinds of neonatal stressors and the health and development of preterm infants.

DATA SOURCES

Data sources included Embase, Medline, PsycINFO, the Cumulative Index to Nursing and Allied Health Literature, and reference lists.

STUDY SELECTION

Studies were eligible if they included a measure of neonatal stress during the NICU stay, reported clinical, laboratory, and/or imaging outcomes regarding health and/or development on discharge from the NICU or thereafter, included preterm infants, and were written in English or Dutch.

DATA EXTRACTION

Two reviewers independently screened the sources and extracted data on health and development. Study quality was assessed by using the Newcastle-Ottawa Quality Assessment Scale.

RESULTS

We identified 20 articles that reported on neonatal stress associated negatively with clinical outcomes, including cognitive, motor, and emotional development, and laboratory and imaging outcomes, including epigenetic alterations, hypothalamic-pituitary-adrenal axis functioning, and structural brain development. We found no evidence regarding associations with growth, cardiovascular health, parent-infant interaction, the neonatal immune system, and the neonatal microbiome.

LIMITATIONS

The studies were all observational and used different definitions of neonatal stress.

CONCLUSIONS

Neonatal stress has a profound impact on the health and development of preterm infants, and physicians involved in their treatment and follow-up should be aware of this fact.

Biological Functions of Rat Ultrasonic Vocalizations, Arousal Mechanisms, and Call Initiation

This review summarizes all reported and suspected functions of ultrasonic vocalizations in infant and adult rats. The review leads to the conclusion that all types of ultrasonic vocalizations subserving all functions are vocal expressions of emotional arousal initiated by the activity of the reticular core of the brainstem. The emotional arousal is dichotomic in nature and is initiated by two opposite-in-function ascending reticular systems that are separate from the cognitive reticular activating system. The mesolimbic cholinergic system initiates the aversive state of anxiety with concomitant emission of 22 kHz calls, while the mesolimbic dopaminergic system initiates the appetitive state of hedonia with concomitant emission of 50 kHz vocalizations. These two mutually exclusive arousal systems prepare the animal for two different behavioral outcomes. The transition from broadband infant isolation calls to the well-structured adult types of vocalizations is explained, and the social importance of adult rat vocal communication is emphasized. The association of 22 kHz and 50 kHz vocalizations with aversive and appetitive states, respectively, was utilized in numerous quantitatively measured preclinical models of physiological, psychological, neurological, neuropsychiatric, and neurodevelopmental investigations. The present review should help in understanding and the interpretation of these models in biomedical research.

A nutty idea: Exploring a novel method using a hazelnut cocoa spread to temporarily increase maternal care behavior within a rat litter

Maternal care plays a crucial role for infant development. In humans, skin-to-skin care is often used for preterm infants to reduce acute stress and improve long-term developmental outcome. Here we propose a new translational approach to study the effects of acute increases in maternal care on offspring development in rodents. First, pilot studies determined that dams spent more time licking pups covered in hazelnut cocoa spread (Nutella^® )) than any other studied substance. For the main experiment, Sprague-Dawley rats were bred in-house, and Nutella^® was applied four times a day to treatment pups (2 males/females per litter) of seven litters from PN2 to 5. Control pups (2 males/females per litter) were handled similarly without applying any substance. Maternal licking and grooming behavior was observed for 30 min following each Nutella^® application. Nutella^® -covered pups received significantly more licking and grooming compared to control pups within each litter. There was no difference in body weight or basal serum corticosterone levels between Nutella^® -covered and control pups on PN5, suggesting that the Nutella^® application and subsequent increased maternal licking was not stressful for the pups. Future studies will investigate whether this increase in maternal care can protect pups from stress- or pain-induced rises in corticosterone.

Perinatal depression: Heterogeneity of disease and in animal models

Perinatal depression (PND) can have either an antepartum or postpartum onset. Although the greatest risk factor for PND is previous depression history,de novoPND occurs with the majority of cases occurring in the postpartum. Timing of depression can impact etiology, prognosis, and response to treatment. Thus, it is crucial to study the impact of the heterogeneity of PND for better health outcomes. In this review, we outline the differences between antepartum and postpartum depression onset of PND. We discuss maternal physiological changes that differ between pregnancy and postpartum and how these may differentially impact depression susceptibility. We highlight changes in the maternal steroid and peptide hormone levels, immune signalling, serotonergic tone, metabolic factors, brain morphology, and the gut microbiome. Finally, we argue that studying the heterogeneity of PND in clinical and preclinical models can lead to improved knowledge of disease etiopathology and treatment outcomes.

Neonatal pain and reduced maternal care alter adult behavior and hypothalamic-pituitary-adrenal axis reactivity in a sex-specific manner

Preterm infants often spend a significant amount of time in the neonatal intensive care unit (NICU) where they are exposed to many stressors including pain and reduced maternal care. These early-life stressful experiences can have negative consequences on brain maturation during the neonatal period; however, less is known about the long-term cognitive and affective outcomes. Thus, this study was conducted to investigate the impact of neonatal pain and reduced maternal care on adult behavior and HPA axis reactivity in an animal model. Male and female rats underwent a series of repetitive needle pokes and/or reduced maternal care (through a novel tea ball infuser encapsulation method) during the first 4 days of life and were then assessed in a battery of behavioral tests as adults. We found that early-life pain enhanced spatial learning independent of the animal's sex, but altered HPA recovery from an acute stressor in females only. Moreover, reduced maternal care altered long-term spatial memory and reversal learning in males. These findings indicate that neonatal stressors have unique sex-dependent long-term biobehavioral effects in rodents. Continued examination of the behavioral consequences of these stressors may help explain varying vulnerability and resiliency in preterm infants who experienced early stress in the NICU.

Neonatal pain: A journey spanning three decades

From 1980 into present day, 2020, the evolution of neonatal pain research is told as a journey by one researcher, Celeste Johnston. At the beginning of her work, there was essentially no interest or work in the area. She was fortunate to be led into the area by a clinical problem: how to determine the amount of pain babies in the NICU were experiencing. That question resulted in over three decades of work with neonates. Measuring pain was the first challenge and is one that remains a focus of current research. Initially, the only choices for treating pain in neonates were either opioids or anesthetics, each with problems. Research on sweet taste and more recently on skin‐to‐skin contact has offered effective and safe options for procedural pain. Although progress has been made in the incidence of pain management in infants, it still is far less than it could be. Steps along the way of measurement, treatment, and knowledge utilization are chronicled by this researcher.

Early Neonatal Pain-A Review of Clinical and Experimental Implications on Painful Conditions Later in Life

Modern health care has brought our society innumerable benefits but has also introduced the experience of pain very early in life. For example, it is now routine care for newborns to receive various injections or have blood drawn within 24 h of life. For infants who are sick or premature, the pain experiences inherent in the required medical care are frequent and often severe, with neonates requiring intensive care admission encountering approximately fourteen painful procedures daily in the hospital. Given that much of the world has seen a steady increase in preterm births for the last several decades, an ever-growing number of babies experience multiple painful events before even leaving the hospital. These noxious events occur during a critical period of neurodevelopment when the nervous system is very vulnerable due to immaturity and neuroplasticity. Here, we provide a narrative review of the literature pertaining to the idea that early life pain has significant long-term effects on neurosensory, cognition, behavior, pain processing, and health outcomes that persist into childhood and even adulthood. We refer to clinical and pre-clinical studies investigating how early life pain impacts acute pain later in life, focusing on animal model correlates that have been used to better understand this relationship. Current knowledge around the proposed underlying mechanisms responsible for the long-lasting consequences of neonatal pain, its neurobiological and behavioral effects, and its influence on later pain states are discussed. We conclude by highlighting that another important consequence of early life pain may be the impact it has on later chronic pain states-an area of research that has received little attention.

The Effect of Behavioural Indicators of Calf Discomfort Following Routine Procedures on Cow Maternal Care

Simple Summary

Animals are believed to show empathy when witnessing others in pain. Painful experiences alter arousal and induce specific forms of behaviour to reduce pain. It is unclear if observers respond simply to the level of arousal or can detect differences in the expression of pain indicators. We explored whether beef cows adjust their care-giving behaviours dependent upon their calf’s expression of pain following ear tagging and (in males) castration. Greatest empathy is expected when witnessing aversive experiences in kin, and beef cows show vigorous maternal care shortly after calving. We therefore hypothesise that cows show increased maternal care towards calves that display the most behavioural evidence of pain. Observations were made for 20 min before and 30 min after tagging (and castration) of 58 calves. Increased arousal in the calf was associated with increased maternal care behaviour by the cow. However, the expression of pain indicator behaviours did not influence the cow’s behaviour. Males that had experienced castration did not receive more care than females. Cows seem to be insensitive to behaviours of their calf previously shown to indicate pain but this may be due to the subtlety of behavioural expression in young calves or the recent experience of calving.

Abstract

Pain causes altered arousal and specific behaviours that are rare at other times (pain indicators). We examined whether beef cows (mean age 6 years) are sensitive to pain indicators in addition to arousal following ear tagging and rubber ring castration (males only, n = 31) of their calf (n = 58). Behavioural pain indicators and activity were recorded continuously for 20 min before and 2 h after handling. The 30 min period posthandling captured the peak of behavioural change during this window. Cow maternal and maintenance behaviours were recorded for 20 min before and for 30 min after calf handling. Principal component (PC) analysis identified two dimensions (48% of the variance) in calf behaviour. Arousal and two active pain indicators loaded onto PC1 were shown by linear mixed models to positively affect some maternal behaviours. Lethargy, passive pain indicators and further active pain indicators loaded onto PC2 did not affect cow behaviour. Castration did not result in male calves receiving more maternal attention. Cows alter maternal behaviour in response to calf activity but not behaviours previously shown to indicate pain. Maternal care peaks soon after calving, but recent calving stress or the subtlety of neonatal behavioural expression may constrain cows in responding to the individual behavioural expression of their calf. Future studies exploring empathy should be aware of the constraints on behavioural expression associated with the neonatal and periparturient period.

The Effects of Acute Neonatal Pain on Expression of Corticotropin-Releasing Hormone and Juvenile Anxiety in a Rodent Model

Premature infants in the neonatal intensive care unit (NICU) may be subjected to numerous painful procedures without analgesics. One necessary, though acutely painful, procedure is the use of heel lances to monitor blood composition. The current study examined the acute effects of neonatal pain on maternal behavior as well as amygdalar and hypothalamic activation, and the long-term effects of neonatal pain on later-life anxiety-like behavior, using a rodent model. Neonatal manipulations consisted of either painful needle pricks or non-painful tactile stimulation in subjects’ left plantar paw surface which occurred four times daily during the first week of life [postnatal day (PND)1–PND7]. Additionally, maternal behaviors in manipulated litters were compared against undisturbed litters via scoring of videotaped interactions to examine the long-term effects of pain on dam-pup interactions. Select subjects underwent neonatal brain collection (PND6) and fluorescent in situ hybridization (FISH) for corticotropin-releasing hormone (CRH) and the immediate early gene c-fos. Other subjects were raised to juvenile age (PND24 and PND25) and underwent innate anxiety testing utilizing an elevated plus maze (EPM) protocol. FISH indicated that neonatal pain influenced amygdalar CRH and c-fos expression, predominately in males. No significant increase in c-fos or CRH expression was observed in the hypothalamus. Additionally, neonatal pain altered anxiety behaviors independent of sex, with neonatal pain subjects showing the highest frequency of exploratory behavior. Neonatal manipulations did not alter maternal behaviors. Overall, neonatal pain drives CRH expression and produces behavioral changes in anxiety that persist until the juvenile stage.

Inflammatory neonatal pain disrupts maternal behavior and subsequent fear conditioning in a rodent model

Infants spending extended time in the neonatal intensive care unit are at greater risk of developing a variety of mental health problems later in life, possibly due to exposure to painful/stressful events. We used a rodent model of inflammatory neonatal pain to explore effects on fear conditioning, somatosensory function and maternal behavior. Hindpaw injections of 2% λ-carrageenan on postnatal days 1 and 4 produced an attenuation in conditioned freezing during the postweaning period, similar to our previous work with acute pain, but did not cause lasting impacts on contextual freezing nor somatosensory function. Additionally, we assessed maternal behavior to observe dam-pup interactions during the neonatal period. Results showed dams of litters which experienced pain spent similar amounts of time with pups as undisturbed controls. However, the specific behaviors differed per condition. Dams of pain litters exhibited less time licking/grooming, but more time nursing than controls. These results suggest changes in maternal care following pain could be a contributing factor underlying the long-term effects of neonatal trauma. Furthermore, our laboratory has previously shown acute, but not inflammatory pain, disrupted conditioned freezing; the current experiment observed the long-term effects of neonatal inflammatory pain on conditioned fear using a weak conditioning protocol.

Adverse Behavioral Changes in Adult Mice Following Neonatal Repeated Exposure to Pain and Sucrose

Sucrose is recommended for the treatment of pain during minor procedures in preterm infants in the neonatal intensive care unit (NICU) and is currently used worldwide as the standard of care. We recently reported that adult mice repetitively exposed to sucrose compared to water during the first week of life, irrespective of exposure to an intervention, had significantly smaller brain volumes in large white matter, cortical and subcortical structures (e.g., hippocampus, striatum, fimbria). These structures are important for stress regulation and memory formation. Here, we report the effects of repeated neonatal exposure to pain and sucrose on adult behavior in mice. Neonatal C57BL/6J mice (N = 160, 47% male) were randomly assigned to one of two treatments (sucrose, water) and one of three interventions (needle-prick, tactile, handling). Pups received 10 interventions daily from postnatal day 1 (P1) to P6. A single dose of 24% sucrose or water was given orally 2 min before each intervention. At adulthood (P60-85) mice underwent behavioral testing to assess spatial memory, anxiety, motor function, pain sensitivity, and sugar preference. We found that mice that had received sucrose and handling only, had poorer short-term memory in adulthood compared to water/handling controls (p < 0.05). When exposed to pain, mice treated with repetitive sucrose or water did not differ on memory performance (p = 0.1). A sugar preference test showed that adult mice that received sucrose before an intervention as pups consumed less sugar solution compared to controls or those that received water before pain (p < 0.05). There were no significant group differences in anxiety, motor, or pain sensitivity. In a mouse model that closely mimics NICU care, we show for the first time that memory in adulthood was poorer for mice exposed to pain during the first week of life, irrespective of sucrose treatment, suggesting that sucrose does not protect memory performance when administered for pain. In the absence of pain, early repetitive sucrose exposure induced poorer short-term memory, highlighting the importance of accurate pain assessment.

Neonatal pain and reduced maternal care: Early-life stressors interacting to impact brain and behavioral development

Advances in neonatal intensive care units (NICUs) have drastically increased the survival chances of preterm infants. However, preterm infants are still exposed to a wide range of stressors during their stay in the NICU, which include painful procedures and reduced maternal contact. The activation of the hypothalamic-pituitary-adrenal (HPA) axis, in response to these stressors during this critical period of brain development, has been associated with many acute and long-term adverse biobehavioral outcomes. Recent research has shown that Kangaroo care, a non-pharmacological analgesic based on increased skin-to-skin contact between the neonate and the mother, negates the adverse outcomes associated with neonatal pain and reduced maternal care, however the biological mechanism remains widely unknown. This review summarizes findings from both human and rodent literature investigating neonatal pain and reduced maternal care independently, primarily focusing on the role of the HPA axis and biobehavioral outcomes. The physiological and positive outcomes of Kangaroo care will also be discussed in terms of how dampening of the HPA axis response to neonatal pain and increased maternal care may account for positive outcomes associated with Kangaroo care.

Emotional contagion of distress in young pigs is potentiated by previous exposure to the same stressor

This study tested whether emotional contagion occurs when piglets directly observe a penmate in distress (restraint) and whether there is an effect of previous experience on the response to subsequent restraint or exposure to conspecific distress. Piglets (49.7 ± 0.7 days) were exposed in pairs to two stress phases (SP1 and SP2) in an arena divided into two pens by a wire mesh wall. During SP1, one of the pigs of a pair was either restrained (Stress treatment) or sham-restrained (Control treatment), while the other pig was considered observer. During SP2, the previous observer was restrained, while its penmate took the observer role. Heart rate variability, locomotion, vocalizations, body/head/ear and tail postures were monitored. During SP1, observer pigs responded to conspecific distress with increased indicators of attention (looking at, proximity to and snout contacts with the distressed pigs) and increased indicators of fear (reduced locomotion, increased freezing). During SP2, the observer pigs that had been restrained previously reacted more strongly (through higher proximity, decreased locomotion, increased freezing) to observing the penmate in restraint than pigs without the previous negative experience. This study suggests that young pigs are susceptible to emotional contagion and that this contagion is potentiated by previous exposure to the same stressor. These findings have implications for pig welfare in practical animal husbandry systems.

Fetal and Neonatal HPA Axis

Stress is an integral part of life. Activation of the hypothalamus-pituitary-adrenal (HPA) axis in the adult can be viewed as mostly adaptive to restore homeostasis in the short term. When stress occurs during development, and specifically during periods of vulnerability in maturing systems, it can significantly reprogram function, leading to pathologies in the adult. Thus, it is critical to understand how the HPA axis is regulated during developmental periods and what are the factors contributing to shape its activity and reactivity to environmental stressors. The HPA axis is not a passive system. It can actively participate in critical physiological regulation, inducing parturition in the sheep for instance or being a center stage actor in the preparation of the fetus to aerobic life (lung maturation). It is also a major player in orchestrating mental function, metabolic, and cardiovascular function often reprogrammed by stressors even prior to conception through epigenetic modifications of gametes. In this review, we review the ontogeny of the HPA axis with an emphasis on two species that have been widely studied-sheep and rodents-because they each share many similar regulatory mechanism applicable to our understanding of the human HPA axis. The studies discussed in this review should ultimately inform us about windows of susceptibility in the developing brain and the crucial importance of early preconception, prenatal, and postnatal interventions designed to improve parental competence and offspring outcome. Only through informed studies will our public health system be able to curb the expansion of many stress-related or stress-induced pathologies and forge a better future for upcoming generations.

The Val66Met brain-derived neurotrophic factor gene variant interacts with early pain exposure to predict cortisol dysregulation in 7-year-old children born very preterm: Implications for cognition

Early stress in the form of repetitive neonatal pain, in infants born very preterm, is associated with long-term dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis and with poorer cognitive performance. Brain-derived neurotrophic factor (BDNF) which is important in synaptic plasticity and cognitive functions is reduced by stress. Therefore the BDNF Val66Met variant, which affects secretion of BDNF, may interact with early exposure to pain-related stress in children born very preterm, to differentially affect HPA regulation that in turn may be associated with altered cognitive performance. The aims of this study were to investigate whether in children born very preterm, the BDNF Val66Met variant modulates the association between neonatal pain-related stress and cortisol levels at age 7years, and if cortisol levels were related to cognitive function. Furthermore, we examined whether these relationships were sex-specific. Using a longitudinal cohort design, N=90 children born very preterm (24-32weeks gestation) were followed from birth to age 7years. Cortisol was assayed from hair as an index of cumulative stress and from saliva to measure reactivity to a cognitive challenge. BDNF Val66Met variant was genotyped at 7years using real-time polymerase chain reaction (PCR). Using generalized linear modeling, in boys with the Met allele, greater neonatal pain-related stress (adjusted for clinical risk factors) predicted lower hair cortisol (p=0.006) and higher reactivity salivary cortisol (p=0.002). In both boys and girls with the Met allele, higher salivary cortisol reactivity was correlated with lower IQ (r=-0.60; p=0.001) and poorer visual-motor integration (r=-0.48; p=0.008). Our findings show associations between lower BDNF availability (presence of the Met allele) and vulnerability to neonatal pain/stress in boys, but not girls. This exploratory study suggests new directions for research into possible mechanisms underlying how neonatal pain/stress is related to cognitive performance in children born very preterm.

Ewes Direct Most Maternal Attention towards Lambs that Show the Greatest Pain-Related Behavioural Responses

Although neonatal farm animals are frequently subjected to painful management procedures, the role of maternal behaviour in pain coping, has not been much studied. We investigated whether ewes were able to distinguish between lambs in pain and those that were not, and whether their behaviour altered depending on the severity of lamb pain. Eighty male lambs were allocated to one of 4 pain treatments within 24 hours of birth. Lambs were either handled only (C), bilaterally castrated with tight rubber rings (RR), as for RR but with the application of a Burdizzo clamp immediately proximal to the ring (Combined) or subjected to short scrotum castration (SSC) where the testicles were retained within the abdomen and only the scrotum removed. The behaviour of the ewe, treated lamb and untreated sibling where present (n = 54) were recorded for 30 minutes after treatment. Castration treatment increased the expression of abnormal standing and lying postures, specific pain-related behaviours (head-turning, stamping/kicking, easing quarters, tail wagging) and composite pain scores (P<0.001 for all). The greatest expression of pain-related behaviours was shown by lambs in the RR group, which were the only group to show rolling responses indicative of severe pain, followed by the SSC group. Ewes expressed more licking/sniffing responses to the RR and SSC lambs than towards the Combined and C lambs (P<0.05), and oriented most to RR lambs and least to C lambs (P<0.001). Ewes with two lambs also directed more attention towards the treated than the untreated lamb (P<0.001). The quantity of maternal care directed towards the lamb was positively correlated with the expression of active pain behaviours. The data demonstrate that ewes are able to discriminate between lambs in pain and those that are not, and that their response is increased with a greater severity of pain.

Sex-related long-term behavioral and hippocampal cellular alterations after nociceptive stimulation throughout postnatal development in rats

Early noxious stimuli may alter the neurogenesis rate in the dentate gyrus and the behavioral repertoire of adult rats. This study evaluated the long-term effects of noxious stimulation, imposed in different phases of development, on nociceptive and anxiety-like behaviors, hippocampal activation, cell proliferation, hippocampal BDNF and plasma corticosterone levels in 40 day-old male and female adolescents. Noxious stimulation was induced by intra-plantar injection of Complete Freund's adjuvant (CFA), on postnatal days (P) 1 (group P1), 8 (P8) or 21 (P21). Control animals were not stimulated in any way. On P21 a subset of animals from each group received BrdU and was perfused on P40 for identification of proliferating cells in the granule cell layer of the dentate gyrus. Another subset of rats was subjected to behavioral testing on P40 and one week later, to magnetic resonance imaging (MRI) acquisition. Noxious stimulation evoked hypoalgesia in adolescents, mainly in females (P < 0.02), reflected by greater latency to withdraw the paw and less paw lickings in the hot plate test than controls (P < 0.001). It also resulted in more time spent in the open arms, e.g., less anxiety-like behavior than controls (P < 0.01), especially in females (P < 0.01, compared with males). Proliferative cell rate in the dentate gyrus was the highest in P8 males and females (P < 0.001), with males exhibiting more proliferation than females on P1 and P8, which was directly related to the hippocampal levels of BDNF and inversely related to plasma corticosterone. Sex differences were also detected in manganese-enhanced MRI signal, which was more prominent in P1 females than males (P < 0.01). This study represents the first step of investigation on the cellular basis of the sex-dependent long-term consequences of nociceptive stimuli in newborns.

Neonatal pain in very preterm infants: long-term effects on brain, neurodevelopment and pain reactivity

Effects of early life psychosocial adversity have received a great deal of attention, such as maternal separation in experimental animal models and abuse/neglect in young humans. More recently, long-term effects of the physical stress of repetitive procedural pain have begun to be addressed in infants hospitalized in neonatal intensive care. Preterm infants are more sensitive to pain and stress, which cannot be distinguished in neonates. The focus of this review is clinical studies of long-term effects of repeated procedural pain-related stress in the neonatal intensive care unit (NICU) in relation to brain development, neurodevelopment, programming of stress systems, and later pain sensitivity in infants born very preterm (24-32 weeks' gestational age). Neonatal pain exposure has been quantified as the number of invasive and/or skin-breaking procedures during hospitalization in the NICU. Emerging studies provide convincing clinical evidence for an adverse impact of neonatal pain/stress in infants at a time of physiological immaturity, rapidly developing brain microstructure and networks, as well as programming of the hypothalamic-pituitary-adrenal axis. Currently it appears that early pain/stress may influence the developing brain and thereby neurodevelopment and stress-sensitive behaviors, particularly in the most immature neonates. However, there is no evidence for greater prevalence of pain syndromes compared to children and adults born healthy at full term. In addressing associations between pain/stress and outcomes, careful consideration of confounding clinical factors related to prematurity is essential. The need for pain management for humanitarian care is widely advocated. Non-pharmacological interventions to help parents reduce their infant's stress may be brain-protective.

Are there long-term consequences of pain in newborn or very young infants?

Physiologic studies indicate that very early pain or stress experiences have more than immediate consequences for infants. Assessment and care of pain are complex subjects made even more complex and challenging when the individual experiencing pain is a very young infant. This review provides evidence that significant and long-lasting physiological consequences may follow painful insults in the very young, including changes in the central nervous system and changes in responsiveness of the neuroendocrine and immune systems to stress at maturity.

Developmental hippocampal neuroplasticity in a model of nicotine replacement therapy during pregnancy and breastfeeding

RATIONALE

The influence of developmental nicotine exposure on the brain represents an important health topic in light of the popularity of nicotine replacement therapy (NRT) as a smoking cessation method during pregnancy.

OBJECTIVES

In this study, we used a model of NRT during pregnancy and breastfeeding to explore the consequences of chronic developmental nicotine exposure on cerebral neuroplasticity in the offspring. We focused on two dynamic lifelong phenomena in the dentate gyrus (DG) of the hippocampus that are highly sensitive to the environment: granule cell neurogenesis and long-term potentiation (LTP).

METHODS

Pregnant rats were implanted with osmotic mini-pumps delivering either nicotine or saline solutions. Plasma nicotine and metabolite levels were measured in dams and offspring. Corticosterone levels, DG neurogenesis (cell proliferation, survival and differentiation) and glutamatergic electrophysiological activity were measured in pups.

RESULTS

Juvenile (P15) and adolescent (P41) offspring exposed to nicotine throughout prenatal and postnatal development displayed no significant alteration in DG neurogenesis compared to control offspring. However, NRT-like nicotine exposure significantly increased LTP in the DG of juvenile offspring as measured in vitro from hippocampal slices, suggesting that the mechanisms underlying nicotine-induced LTP enhancement previously described in adult rats are already functional in pups.

CONCLUSIONS

These results indicate that synaptic plasticity is disrupted in offspring breastfed by dams passively exposed to nicotine in an NRT-like fashion.

Sex Differences in Pain Responses at Maturity Following Neonatal Repeated Minor Pain Exposure in Rats

There is mounting evidence of long-lasting changes in pain sensitivity in school-age children who were cared for in a neonatal intensive care unit. Such care involves multiple pain exposures, 70% of which are accounted for by heel lance to monitor physiological well-being. The authors sought to model the repeated brief pain resulting from heel lance by administering repeated paw needle stick to neonatal rat pups. Repeated needle stick during the first 8 days of life was sex-specific in altering responses to mechanical and inflammatory stimuli, but not to a thermal stimulus, at maturity. Specifically, neonatal paw needle stick males exhibited significantly greater mechanical sensitivity in response to von Frey hair testing, whereas neonatal paw needle stick females exhibited significantly greater pain behavior scores following hindpaw formalin injection. This is the first study to show such sex-dependent changes in pain responsiveness at maturity in animals having experienced repeated neonatal needle stick pain. These findings support existing evidence that there are long-term sensory sequelae following neonatal pain experiences in rats and further suggest that there are sex-linked differences in the nature of the consequences. If these relationships hold in humans, these findings suggest that even mild painful insults early in life are not without sensory consequences.

Effects of stress across the lifespan

Stress is a known precipitant for metabolic and neurological diseases, with sensitive periods identified across the developmental continuum from conception to old age. However, the effects of stress may vary depending on the point or points along the developmental trajectory when adversity strikes. Past research has emphasized the consequences of stress on fully developed physiological systems in the brain and periphery, but more recent studies have explored the impact of stress on systems at different stages of maturation, with differential effects being revealed. This review provides an overview of the diverse effects of stress at critical developmental stages and the potential outcomes that may be associated with experiencing environmental adversity during ontogeny.

Avian maternal response to chick distress

The extent to which an animal is affected by the pain or distress of a conspecific will depend on its capacity for empathy. Empathy most probably evolved to facilitate parental care, so the current study assessed whether birds responded to an aversive stimulus directed at their chicks. Domestic hens were exposed to two replicates of the following conditions in a counterbalanced order: control (C; hen and chicks undisturbed), air puff to chicks (APC; air puff directed at chicks at 30 s intervals), air puff to hen (APH; air puff directed at hen at 30 s intervals) and control with noise (CN; noise of air puff at 30 s intervals). During each test, the hens' behaviour and physiology were measured throughout a 10 min pre-treatment and a 10 min treatment period. Hens responded to APH and APC treatments with increased alertness, decreased preening behaviour and a reduction in eye temperature. No such changes occurred during any control period. Increased heart rate and maternal vocalization occurred exclusively during the APC treatment, even though chicks produced few distress vocalizations. The pronounced and specific reaction observed indicates that adult female birds possess at least one of the essential underpinning attributes of empathy.

Long-term impact of neonatal injury in male and female rats: Sex differences, mechanisms and clinical implications

Over the last several decades, the relative contribution of early life events to individual disease susceptibility has been explored extensively. Only fairly recently, however, has it become evident that abnormal or excessive nociceptive activity experienced during the perinatal period may permanently alter the normal development of the CNS and influence future responses to somatosensory input. Given the significant rise in the number of premature infants receiving high-technology intensive care over the last 20 years, ex-preterm neonates may be exceedingly vulnerable to the long-term effects of repeated invasive interventions. The present review summarizes available clinical and laboratory findings on the lasting impact of exposure to noxious stimulation during early development, with a focus on the structural and functional alterations in nociceptive circuits, and its sexually dimorphic impact.

Artificial rearing of rat pups reveals the beneficial effects of mother care on neonatal inflammation and adult sensitivity to pain

Repeated pain during brain development can have long-term consequences in both humans and animals. We previously showed that maternal care provided to pups experiencing pain reduced adult pain sensitivity. This study tested whether sensory stimulation was responsible for this effect. Rat pups were either mother-reared controls (MR-CON) or artificially reared (AR) with minimal (AR-MIN) or maximal (AR-MAX) stimulation provided daily. In each rearing condition, pups were either uninjected or injected from postnatal day (PND) 4 to 14 with saline (0.9%) or formalin (0.2-0.4%). Pain behavior and paw inflammation were scored. Thermal sensitivity and responses to formalin were tested in adulthood (PND 70). AR neonates, irrespective of sensory stimulation received, exhibited a pain response (p < 0.001), even with a mild formalin dose. Maternal rearing reduced inflammation during the second week of life compared with AR pups (p < 0.05). Early pain exposure did not modify adult pain sensitivity. However, rearing altered adult pain sensitivity such that uninjected MR-CON rats had lower pain sensitivities than uninjected AR rats (p < 0.05). This suggests that the beneficial effects of maternal rearing can be obliterated if additional stimulation/stress occurs during the early neonatal period. In addition, this suggests that optimal level of maternal stimulation exists that determines adult pain sensitivity.

Neonatal pain, parenting stress and interaction, in relation to cognitive and motor development at 8 and 18 months in preterm infants

Procedural pain in the neonatal intensive care unit triggers a cascade of physiological, behavioral and hormonal disruptions which may contribute to altered neurodevelopment in infants born very preterm, who undergo prolonged hospitalization at a time of physiological immaturity and rapid brain development. The aim of this study was to examine relationships between cumulative procedural pain (number of skin-breaking procedures from birth to term, adjusted for early illness severity and overall intravenous morphine exposure), and later cognitive, motor abilities and behavior in very preterm infants at 8 and 18 months corrected chronological age (CCA), and further, to evaluate the extent to which parenting factors modulate these relationships over time. Participants were N=211 infants (n=137 born preterm 32 weeks gestational age [GA] and n=74 full-term controls) followed prospectively since birth. Infants with significant neonatal brain injury (periventricular leucomalacia, grade 3 or 4 intraventricular hemorrhage) and/or major sensori-neural impairments, were excluded. Poorer cognition and motor function were associated with higher number of skin-breaking procedures, independent of early illness severity, overall intravenous morphine, and exposure to postnatal steroids. The number of skin-breaking procedures as a marker of neonatal pain was closely related to days on mechanical ventilation. In general, greater overall exposure to intravenous morphine was associated with poorer motor development at 8 months, but not at 18 months CCA, however, specific protocols for morphine administration were not evaluated. Lower parenting stress modulated effects of neonatal pain, only on cognitive outcome at 18 months.

Naturally occurring variations in maternal care modulate the effects of repeated neonatal pain on behavioral sensitivity to thermal pain in the adult offspring

Repeated pain during a critical period of development can have long-term behavioral and physiological consequences in both human and animals. We previously showed that rat mothers caring for pups subjected to mild pain in neonatal life increased pup licking and grooming behavior. Therefore, we tested whether naturally occurring variations in maternal behavior would modulate the effects of repeated mild inflammatory pain on behavioral responses to pain and stress in the adult male offspring. Rat pups were either uninjected (UI) or injected twice daily between PND3 and PND14 with either saline (0.9%) or formalin (0.2-0.4%) in the footpad of the hindpaw. Maternal behavior (pup licking and grooming) was recorded under basal conditions and after reunion with the litter post injection to determine maternal phenotype (High, Middle, Low licking). Adult offspring (PND60) were tested for their thermal sensitivity, inflammatory pain responses after formalin injection and neuroendocrine responses to formalin injection. Maternal phenotype significantly altered pain sensitivity after thermal stimulation, but not formalin injection. Offspring from the High licking mothers displayed increased withdrawal latencies compared to offspring from Low mothers, regardless of neonatal treatment. Pain responses after formalin injection were higher in offspring receiving formalin as neonates compared to saline-treated or uninjected rats, demonstrating a long lasting increased sensitivity to inflammatory pain. Neuroendocrine responses to pain stress were not affected by neonatal treatment. These data suggest that changes in maternal behavior can influence some modalities of pain sensitivity and that repeated mild inflammatory pain in neonatal period causes hypersensitivity to formalin in the adult offspring.

Dysfunctional nurturing behavior in rat dams with limited access to nesting material: a clinically relevant model for early-life stress

BACKGROUND

Early-life emotional stress may be associated with affective and cognitive disorders later in life, yet satisfactory animal models for studying the underlying mechanisms are limited. Because maternal presence and behavior critically influence molecular and behavioral stress responses in offspring, we sought to create a model of dysfunctional, fragmented maternal nurturing behavior that would, in turn, provoke chronic early-life stress in the offspring.

METHODS

Sprague-Dawley rat dams' nursing and nurturing behaviors were altered by limiting their ability to create satisfactory nests during postpartum days 2-9. Maternal behavior was observed throughout the diurnal cycle, and the frequency and duration of nurturing behaviors were scored. In addition, potential stress and anxiety of the dams were assessed using behavioral, molecular and hormonal measures.

RESULTS

Both the quantity and the quality of dams' care of their pups were profoundly influenced by restriction of nesting materials in their cages: licking/grooming activities decreased and the frequency of leaving the pups increased, resulting in fragmented interactions between the dams and pups. The abnormal activity patterns of the dams were accompanied by increased anxiety-like behavior in the open field, but not in the elevated plus maze tests. Additionally, dams' plasma corticosterone levels and adrenal weights were augmented, suggesting chronic stress of these dams. By the end of the limited-nesting, stress-inducing period, hypothalamic corticotropin releasing hormone (CRH) mRNA expression was reduced in the limited-nesting dams, while arginine-vasopressin (AVP) mRNA levels were not significantly affected.

CONCLUSION

Limiting dams' ability to construct a nest for their pups leads to an abnormal repertoire of nurturing behaviors, possibly as a result of chronic stress and mild anxiety of the dams. Because the fragmented and aberrant maternal behavior provoked chronic stress in the pups, the limited-nesting paradigm provides a useful tool for studying the mechanisms and consequences of such early-life stress experience in the offspring.

Effects of brief and long maternal separations on the HPA axis activity and the performance of rats on context and tone fear conditioning

Previous studies show that early life events result in neurobehavioural alterations that may be either beneficial or detrimental to the stress response. Given the close relationship between corticosterone secretion and mnemonic processes, the purpose of the present study was to investigate the effects of brief (BMS, 15 min) and long maternal separations (LMS, 180 min) on memory tasks in adult rats, assessed by context and tone fear conditioning. At adulthood, males were evaluated for behavioural and hormonal reaction to the training environment, being tested for context fear conditioning; tone fear conditioning; and learning curve in the context fear conditioning, in which rats were daily re-exposed to the context, followed by a brief footshock and in the last day of the experiment (day 5) animals were exposed to the context. Corticosterone and ACTH plasma levels were determined in naïve rats (basal) or 5, 25 or 45 min after each test. Peak ACTH and corticosterone levels were similar among the groups after context fear conditioning; however, levels of CTL rats remained elevated for a longer time. In the learning curve of context fear conditioning, both BMS and LMS rats exhibited less freezing behaviour than CTL rats, without differences in hormone secretion. There was neither an association between activity of the HPA axis and performance on memory tasks nor different activational properties of the tasks on the HPA axis between BMS and LMS rats, i.e., both manipulations lead to similar performance in hippocampus-dependent and independent memory tasks.

Maternal stress and behavior modulate relationships between neonatal stress, attention, and basal cortisol at 8 months in preterm infants

There is evidence that the developmental trajectory of cortisol secretion in preterm infants is altered, with elevated basal cortisol levels observed postnatally through at least 18 months corrected age (CA). This alteration is possibly due to neonatal pain-related stress. High cortisol levels might contribute to greater risk of impaired neurodevelopment. Since maternal factors are important for the regulation of infant stress responses, we investigated relationships between infant (neonatal pain-related stress, attention, cortisol) and maternal (stress, interactive behaviors) factors at age 8 months CA. We found that interactive maternal behaviors buffered the relationship between high neonatal pain-related stress exposure and poorer focused attention in mothers who self-reported low concurrent stress. Furthermore, in preterm infants exposed to high concurrent maternal stress and overwhelming interactive maternal behaviors, higher basal cortisol levels were associated with poor focused attention. Overall, these findings suggest that maternal factors can influence the cognitive resilience at 8 months of preterm infants exposed to early life stress.

Alterations in stress-associated behaviors and neurochemical markers in adult rats after neonatal short-lasting local inflammatory insult

Recently, there has been a growing interest in long-term consequences of neonatal pain because modern neonatal intensive care units routinely employ procedures that cause considerable pain and may be followed by local inflammation and hyperalgesia lasting for several hours or even days. To address this question, we developed a rat model of short lasting (<2 days) early local inflammatory insult produced by a single injection of 0.25% carrageenan (CAR) into the plantar surface of a hindpaw. Previously, we demonstrated that rats receiving this treatment within the first week after birth grow into adults with a global reduction in responsiveness to acute pain. Here, we report that these animals also manifest a low anxiety trait associated with reduced emotional responsiveness to stress. This conclusion is based in the following observations: (a) rats in our model display reduced anxiety on an elevated plus-maze; (b) in the forced swim test, these rats exhibit behavioral characteristics associated with stronger ability for stress coping; and (c) these animals have reduced basal and stress-induced plasma levels of such stress-related neuroendocrine markers as corticotropin-releasing factor, vasopressin, and adrenocorticotrophic hormone. In addition, we used DNA microarray and real-time reverse-transcriptase polymerase chain reaction to profile long-term changes in gene expression in the midbrain periaqueductal gray (PAG; a region involved in both stress and pain modulation) in our animal model. Among the affected genes, serotonergic receptors were particularly well represented. Specifically, we detected increase in the expression of 5-HT1A, 5-HT1D, 5-HT2A, 5-HT2C and 5-HT4 receptors. Several of these receptors are known to be involved in the anxiolytic and analgesic activity of the PAG. Finally, to determine whether neonatal inflammatory insult induces elevation in maternal care, which may play a role in generating long-term behavioral alterations seen in our model, we examined maternal behavior for 3 days following CAR injection. Indeed, we observed a substantial increase in maternal attention to the pups at the time of inflammation, but this increase was not without its cost: a period of significant maternal neglect afterward.