Maternal separation leads to persistent reductions in pain sensitivity in female rats

Opioidergic tuning of social attachment: reciprocal relationship between social deprivation and opioid abuse

Individuals misusing opioids often report heightened feelings of loneliness and decreased ability to maintain social connections. This disruption in social functioning further promotes addiction, creating a cycle in which increasing isolation drives drug use. Social factors also appear to impact susceptibility and progression of opioid dependence. In particular, increasing evidence suggests that poor early social bond formation and social environments may increase the risk of opioid abuse later in life. The brain opioid theory of social attachment suggests that endogenous opioids are key to forming and sustaining social bonds. Growing literature describes the opioid system as a powerful modulator of social separation distress and attachment formation in rodents and primates. In this framework, disruptions in opioidergic signaling due to opioid abuse may mediate social reward processing and behavior. While changes in endogenous opioid peptides and receptors have been reported in these early-life adversity models, the underlying mechanisms remain poorly understood. This review addresses the apparent bidirectional causal relationship between social deprivation and opioid addiction susceptibility, investigating the role of opioid transmission in attachment bond formation and prosocial behavior. We propose that early social deprivation disrupts the neurobiological substrates associated with opioid transmission, leading to deficits in social attachment and reinforcing addictive behaviors. By examining the literature, we discuss potential overlapping neural pathways between social isolation and opioid addiction, focusing on major reward-aversion substrates known to respond to opioids.

Impact of neonatal pain and opiate administration in animal models: A meta-analysis concerning pain threshold

BACKGROUND AND AIM

Neonatal intensive care treatment, including frequently performed painful procedures and administration of analgesic drugs, can have different effects on the neurodevelopment. This systematic review and meta-analysis aimed to investigate the influence of pain, opiate administration, and pre-emptive opiate administration on pain threshold in animal studies in rodents, which had a brain development corresponding to preterm and term infants.

METHODS

A systematic literature search of electronic data bases including CENTRAL (OVID), CINAHL (EBSCO), Embase.com, Medline (OVID), Web of Science, and PsycInfo (OVID) was conducted. A total of 42 studies examining the effect of pain (n = 38), opiate administration (n = 9), and opiate administration prior to a painful event (n = 5) in rodents were included in this analysis.

RESULTS

The results revealed that pain (g = 0.42, 95%CI 0.16-0.67, p = 0.001) increased pain threshold leading to hypoalgesia. Pre-emptive opiate administration had the opposite effect, lowering pain threshold, when compared to pain without prior treatment (g = -1.79, 95%CI -2.71-0.86, p = 0.0001). Differences were found in the meta regression for type of stimulus (thermal: g = 0.66, 95%CI 0.26-1.07, p = 0.001; vs. mechanical: g = 0.13, 95%CI -0.98-1.25, p = 0.81) and gestational age (b = -1.85, SE = 0.82, p = 0.027). In addition, meta regression indicated an association between higher pain thresholds and the amount of cumulative pain events (b = 0.06, SE = 0.03, p = 0.05) as well as severity of pain events (b = 0.94, SE = 0.28, p = 0.001).

CONCLUSION

Neonatal exposure to pain results in higher pain thresholds. However, caution is warranted in extrapolating these findings directly to premature infants. Further research is warranted to validate similar effects in clinical contexts and inform evidence-based practices in neonatal care.

The role of histamine H1 receptor in the anterior cingulate cortex on nociception level following acute restraint stress in male rats

Considering the importance of pain and stress, we decided to investigate the intra-anterior cingulate cortex (ACC) microinjection of histamine and mepyramine alone and concurrently on acute pain induced by hot plate following restraint stress in male rats. 24-gauge, 10 mm stainless steel guide cannula was implanted over the ACC in the incised scalp of 4 groups. Restraint stress in healthy rats produced a significant increase (p < .05) in the pain threshold. The simultaneous microinjection of 4 μg/side histamine and 8 μg/side mepyramine as a histaminergic system inverse agonist in healthy nonrestraint animals did not affect the pain threshold. Although Histamine decreased the threshold of pain meaningfully, mepyramine elevated it in a significant manner (p < .05). In the restrained animals, intra-ACC microinjection of histamine produced no significant impact on the pain threshold. However, intra-ACC microinjection of mepyramine before histamine, significantly (p < .01) altered the result and enhanced the threshold of pain. The results of our study demonstrated that histaminergic neurons have an important role in the processing of pain in the ACC following restraint stress.

Current Evidence for Biological Biomarkers and Mechanisms Underlying Acute to Chronic Pain Transition across the Pediatric Age Spectrum

Chronic pain is highly prevalent in the pediatric population. Many factors are involved in the transition from acute to chronic pain. Currently, there are conceptual models proposed, but they lack a mechanistically sound integrated theory considering the stages of child development. Objective biomarkers are critically needed for the diagnosis, risk stratification, and prognosis of the pathological stages of pain chronification. In this article, we summarize the current evidence on mechanisms and biomarkers of acute to chronic pain transitions in infants and children through the developmental lens. The goal is to identify gaps and outline future directions for basic and clinical research toward a developmentally informed theory of pain chronification in the pediatric population. At the outset, the importance of objective biomarkers for chronification of pain in children is outlined, followed by a summary of the current evidence on the mechanisms of acute to chronic pain transition in adults, in order to contrast with the developmental mechanisms of pain chronification in the pediatric population. Evidence is presented to show that chronic pain may have its origin from insults early in life, which prime the child for the development of chronic pain in later life. Furthermore, available genetic, epigenetic, psychophysical, electrophysiological, neuroimaging, neuroimmune, and sex mechanisms are described in infants and older children. In conclusion, future directions are discussed with a focus on research gaps, translational and clinical implications. Utilization of developmental mechanisms framework to inform clinical decision-making and strategies for prevention and management of acute to chronic pain transitions in children, is highlighted.

Gut instinct: Sex differences in the gut microbiome are associated with changes in adolescent nociception following maternal separation in rats

Adolescent chronic pain is a growing public health epidemic. Our understanding of its etiology is limited; however, several factors can increase susceptibility, often developing in response to an acute pain trigger such as a surgical procedure or mild traumatic brain injury (mTBI), or an adverse childhood experience (ACE). Additionally, the prevalence and manifestation of chronic pain is sexually dimorphic, with double the rates in females than males. Despite this, the majority of pre-clinical pain research focuses on males, leaving a gap in mechanistic understanding for females. Given that emerging evidence has linked the gut microbiome and the brain-gut-immune axis to various pain disorders, we aimed to investigate sex-dependent changes in taxonomic and functional gut microbiome features following an ACE and acute injury as chronic pain triggers. Male and female Sprague Dawley rat pups were randomly assigned to either a maternal separation (MS) or no stress paradigm, then further into a sham, mTBI, or surgery condition. Chronically, the von Frey test was used to measure mechanical nociception, and fecal samples were collected for 16S rRNA sequencing. Animals in the surgery group had an increase in pain sensitivity when compared to mTBI and sham groups, and this was complemented by changes to the gut microbiome. In addition, significant sex differences were identified in gut microbiome composition, which were exacerbated in response to MS. Overall, we provide preliminary evidence for sex differences and ACE-induced changes in bacterial composition that, when combined, may be contributing to heterogeneity in pain outcomes.

The Foundations for Chronic Low Back Pain Management may Start in Early Life. Exploring the Role of Caregiver Parental Leave on Future Low Back Pain in the Offspring

Chronic low back pain is difficult to treat and despite increased spending on health services, clinical outcomes for people with low back pain have not improved. Innovative, large scale initiatives seem necessary to stem the cost of low back pain. Psychological health contributes to the development and persistence of chronic low back pain and psychological interventions are important in the management of low back pain. Given the contribution of psychological health to low back pain development and management, it raises the question; can we support psychological health in later life by bolstering emotional development in early life, and reduce the burden of this common condition? Positive early life experiences, including those induced by extended paid parental leave, could bolster emotional development and support the psychological health necessary to manage low back pain in later life. We present the current state of evidence demonstrating the potential value of increasing support for parent-child relationships in early life to reduce the burden of low back pain in future generations. The current evidence is limited to cross-sectional associations, but strong preclinical data clearly shows the potential negative impacts of maternal separation on rodent pup health that compels consideration in human populations. PERSPECTIVE: The benefits stemming from enhanced child development include stable emotional foundations, possibly improving psychological health and low back pain management in the future. This perspective raises questions for future studies - within the context of low back pain, what ingredients bolster stable psychological health? And are these ingredients influenced by parental leave?

Maternal separation with neonatal pain influences later-life fear conditioning and somatosenation in male and female rats

Early life adversity, including that which occurs in a medical setting, has been increasingly shown to have lasting consequences on both physical and mental health. In order to understand the lasting effects of early-life adversity, such as that might occur in the neonatal intensive care unit (NICU), several rodent models have been developed including maternal separation, neonatal handling, and repeated needle prick pain. However, in the clinical scenario, these stressors are often combined. Thus, the current study seeks to observe the lasting impacts of both neonatal pain and maternal separation in a rodent model. Rats were separated from their dam for 6 h per day during the first 7 days of life, during which they were subjected to repeated needle prick pain or handling. A separate group was left undisturbed. All rats were subsequently tested for threat processing using a 3-day Pavlovian fear conditioning model and for somatosensation using measures of mechanical and thermal thresholds. Results indicated that rats subjected to maternal separation and pain had enhanced fear conditioning in adolescence as well as displaying a modest age-independent tactile hypersensitivity compared to undisturbed controls. These data show that experiencing combined neonatal pain and maternal separation may create a latent vulnerability to subsequent stressors.

Preserved Thermal Pain in 3xTg-AD Mice With Increased Sensory-Discriminative Pain Sensitivity in Females but Affective-Emotional Dimension in Males as Early Sex-Specific AD-Phenotype Biomarkers

The increase of the aging population, where quite chronic comorbid conditions are associated with pain, draws growing interest across its investigation and the underlying nociceptive mechanisms. Burn injuries associated problems might be of relevance in the older adult’s daily life, but in people with dementia, exposure to high temperatures and heat sources poses a significantly increased risk of burns. In this brief report, the hind paws and tail pain withdrawal reflexes and the emotional responses to thermal nociception in 3xTg-AD mice were characterized for the first time in the plantar test and compared to their non-transgenic (NTg) counterparts. We studied a cohort of male and female 3xTg-AD mice at asymptomatic (2 months), early (6 months), middle (9 months), and advanced (12 and 15 months) stages of the disease and as compared to sex- and age-matched NTg control mice with normal aging. At 20 and 40W intensities, the sensorial-discriminative thresholds eliciting the withdrawal responses were preserved from asymptomatic to advanced stages of the disease compared to NTg counterparts. Moreover, 3xTg-AD females consistently showed a greater sensory-discriminative sensitivity already at premorbid ages, whereas increased emotionality was shown in males. False-negative results were found in “blind to sex and age” analysis, warning about the need to study sexes independently. The current results and previous report in cold thermal stimulation provide two paradigms unveiling sex-specific early AD-phenotype nociceptive biomarkers to study the mechanistic underpinnings of sex-, age- and AD-disease-dependent thermal pain sensitivity.

Spatial Learning Is Impaired in Male Pubertal Rats Following Neonatal Daily but Not Randomly Spaced Maternal Deprivation

Severe early life stress has long been associated with neuropsychological disorders in adulthood, including depression, schizophrenia, post-traumatic stress disorder, and memory dysfunction. To some extent, all of these conditions involve dysregulation of the hypothalamic-pituitary-adrenal (HPA) axis and reduced negative feedback inhibition of cortisol release in adulthood. However, the time course for mental health and hormonal outcomes across life stages and the attributes of early life stress that direct the behavioral and biological alterations is not fully understood. We designed our studies to compare outcomes of the two most common maternal deprivation schedules on cognitive ability prior to adulthood. We exposed rat pups to daily or randomly spaced maternal separation bouts within the first 3 weeks of life and examined cognitive performance, neurotrophic signaling, and stress and immune system markers during puberty. We found that the daily separation schedule impaired spatial learning while the randomly spaced schedule did not alter maze performance relative to normally reared control animals. Animals that underwent daily separation showed a tendency for reduced body weight compared to the randomly spaced condition, but there were no differences in adrenal weight. Thymus weight normalized by body weight was increased following daily separation compared to random separation and control conditions. Plasma corticosterone levels measured after behavior testing did not differ amongst experimental groups and there was no impact of TrKB receptor inhibition. Combined, the results show that different early life stress schedules produce different behavioral and biological outcomes when measured at puberty. Combined with prior findings from more mature animals, the results presented here suggest that daily neonatal stress produces varied alterations in spatial cognition at different life stages with a transient learning deficit at puberty preceding a more persistent and a progressive memory impairment through adulthood and into aging.

The burden of early life stress on the nociceptive system development and pain responses

For a long time, the capacity of the newborn infant to feel pain was denied. Today it is clear that the nociceptive system, even if still immature, is functional enough in the newborn infant to elicit pain responses. Unfortunately, pain is often present in the neonatal period, in particular in the case of premature infants which are subjected to a high number of painful procedures during care. These are accompanied by a variety of environmental stressors, which could impact the maturation of the nociceptive system. Therefore, the question of the long-term consequences of early life stress is a critical question. Early stressful experience, both painful and non-painful, can imprint the nociceptive system and induce long-term alteration in brain function and nociceptive behavior, often leading to an increase sensitivity and higher susceptibility to chronic pain. Different animal models have been developed to understand the mechanisms underlying the long-term effects of different early life stressful procedures, including pain and maternal separation. This review will focus on the clinical and preclinical data about early life stress and its consequence on the nociceptive system.

Neonatal morphine exposure and maternal deprivation alter nociceptive response and central biomarkers' levels throughout the life of rats

In the present study, we investigated the effect of repeated neonatal morphine exposure and/or maternal deprivation(MD) on the nociceptive response and central biomarkers' BDNF, IL-1β, and IL-4 levels at postnatal days 16(PND16), 30(PND30), and 60(PND60). At birth, the litters were standardized to contain 8 pups/dam (n = 58). From PND1 to PND10, the pups of the deprived groups were separated daily from their mothers for 3 h and divided into 5 groups: control(C), saline(S), morphine(M), deprived-saline(DS), and deprived-morphine(DM). The pups received subcutaneous injections of saline/morphine (5 μg) in the mid-scapular area between PND8 and PND14. Nociceptive responses were assessed by hot plate(HP) and tail-flick(TFL) tests and biomarker levels by ELISA. Thermal hyperalgesia(HP) was found in all assessments for the M, DS, and DM groups, and a decrease in nociceptive threshold(TFL) was found in the DS group at PND16; M and DM groups at PND30; and M, DS, and DM groups at PND60. There were interactions between treatment/deprivation/timepoint in all central biomarkers' levels. The current study indicates that neonatal exposure to morphine and MD, which occurs in the pediatric ICU, can alter the nociceptive and neuroinflammatory responses.

Environmental enrichment and pain sensitivity; a study in maternally separated rats

Rodents are highly dependent on maternal care after birth. Maternal separation (MS) is an animal model for studying neglect and abuse. Depriving the pup of such care renders the animal with Hypothalamic-Pituitary-adrenal (HPA) dysfunction and these animals are more susceptible to anxiety and stress as well as poor cognition. These effects are due to abnormal brain development in these animals. We have tried to investigate how maternal separation can affect pain sensation and whether a non-pharmacological intervention such as enriched environment (EE) can restore an abnormal pain sensation. Animals were put into four groups MS, control (CTRL) and MS + EE and CTRL + EE groups that underwent EE after weaning until adulthood. These groups were tested for pain sensitivity with hot plate and tail flick for sensory pain and formalin for affect pain. The results showed that MS rats are more sensitive to pain in the hot plate test and formalin test, however, no significant difference was seen between groups for tail flick test. When MS rats experience EE their pain sensitivity is restored at the behavioral level. Further research is required to see how EE restores pain sensation in MS rats.

Physical exercise enhances vulnerability to migraine headache associated with CGRP up-expression in trigeminal nucleus caudalis of stressed rats

OBJECTIVES

There is conflicting evidence on the effect of physical exercise on migraine development. Present study investigated the impact of treadmill exercise on migraine - associated symptoms and changes in calcitonin gene-related peptide (CGRP) expression in rats with and without maternal deprivation stress (MD).

METHODS

Two days after birth, the male Wistar pups were randomly divided into four groups (n = 6) as follows: intact, exercise, MD, and MD plus exercise. The animals in the MD groups were separated from their dams 4 h per day for 2 weeks. At 8 weeks of age, the rats were exercised on a motor-driven treadmill for 4 weeks. Then, nitroglycerin (NTG) (5 mg/kg/IP) was used to induce migraine and pain-related symptoms were recorded for 90 min. NTG-related thermal hyperalgesia was measured by tail flick and hot plate methods. Finally, immunofluorescence staining of CGRP in trigeminal subnucleus caudalis (Vc) was performed.

RESULTS

NTG - produced a significant headache symptoms and thermal hypersensitivity, which were aggravated following physical exercise in stressed or unstressed groups. Besides, NTG administration increased CGRP expression in the Vc of rats. Such effect was overpowered by treadmill running only in rats exposed to MD stress.

CONCLUSION

These findings highlight the worsening effects of treadmill exercise for migraine in rats with and without MD stress. However, inflammatory response can further exacerbate in stressed rats.

Effects of sleep modulation during pregnancy in the mother and offspring: Evidences from preclinical research

Disturbed sleep during gestation may lead to adverse outcomes for both mother and child. Animal research plays an important role in providing insights into this research field by enabling ethical and methodological requirements that are not possible in humans. Here, we present an overview and discuss the main research findings related to the effects of prenatal sleep deprivation in animal models. Using systematic review approaches, we retrieved 42 articles dealing with some type of sleep alteration. The most frequent research topics in this context were maternal sleep deprivation, maternal behaviour, offspring behaviour, development of sleep-wake cycles in the offspring, hippocampal neurodevelopment, pregnancy viability, renal physiology, hypertension and metabolism. This overview indicates that the number of basic studies in this field is growing, and provides biological plausibility to suggest that sleep disturbances might be detrimental to both mother and offspring by promoting increased risk at the behavioural, hormonal, electrophysiological, metabolic and epigenetic levels. More studies on the effects of maternal sleep deprivation are needed, in light of their major translational perspective.

What are the consequences of Methylphenidate exposure for maternally separated rats?

Methylphenidate (MPH) abuse is prevalent among youth. Drug abuse results in pain perception. We sought to investigate whether Maternal separation (MS) prone to MPH addiction. The next question was whether MPH abusers with MS differ in pain perception. We investigated the impact of MS on addiction and drug reward as well as pain perception following 5 days of MPH injection in males and females rats. Initially, rats underwent MS protocol of 3 hr daily for 21 days. Conditioned place preference (CPP) test was an attempt to investigate whether MS rats experience more reward with MPH. The protocol consisted of 10 min habituation on Day 1, conditioning on Day 2-Day 6 (5 mg per kg MPH injection in drug compartment and saline in saline compartment with 4 hr gap between injections) and 10 min test on Day 7. Furthermore, using another group, differences in pain perception were investigated after 5 days of daily MPH injection with 5 mg per kg. Sensory pain sensitivity was tested on PND 39 using tail flick and hotplate in MS and control groups with and without MPH exposure. Results indicated that female rats are equally prone to addiction in CPP. On the other hand, MS males experience a higher reward in CPP. In tail flick test, female MS rats exposed to MPH show a lower sensory pain threshold with similar MPH exposure. Experiencing MPH similarly declined hotplate pain perception in MS and controls in the females. Males, on the other hand, did not show any difference in sensory pain tests. According to results one can argue MS is detrimental. MS males experience more reward with MPH, females are equally addiction prone and MS females experience more pain in tail flick. On the other hand pain threshold can decline in hotplate test for both control and MS females that received MPH.

Pain in neuropsychiatry: Insights from animal models

Pain is the most common symptom reported in clinical practice, meaning that it is associated with many pathologies as either the origin or a consequence of other illnesses. Furthermore, pain is a complex emotional and sensorial experience, as the correspondence between pain and body damage varies considerably. While these issues are widely acknowledged in clinical pain research, until recently they have not been extensively considered when exploring animal models, important tools for understanding pain pathophysiology. Interestingly, chronic pain is currently considered a risk factor to suffer psychiatric disorders, mainly stress-related disorders like anxiety and depression. Conversely, pain appears to be altered in many psychiatric disorders, such as depression, anxiety and schizophrenia. Thus, pain and psychiatric disorders have been linked in epidemiological and clinical terms, although the neurobiological mechanisms involved in this pathological bidirectional relationship remain unclear. Here we review the evidence obtained from animal models about the co-morbidity of pain and psychiatric disorders, placing special emphasis on the different dimensions of pain.

Repetitive noxious stimuli during early development affect acute and long-term mechanical sensitivity in rats

BACKGROUND

Prematurely born infants are frequently exposed to painful procedures in the neonatal intensive care unit, causing changes to the development of the nervous system lasting into adulthood. The current study aims to study acute and long-term consequences of neonatal repetitive noxious stimulation.

METHODS

Rat pups received either 4 or 10 unilateral needle pricks per day, while control littermates received 4 or 10 tactile stimuli in the first postnatal week. Behavioural sensitivity was assessed in the neonatal phase, in adulthood, and after re-injury of the same dermatome in adulthood.

RESULTS

An increase in the number of repetitive painful stimuli, from 4 to 10 needle pricks per day, resulted in increased mechanical hypersensitivity during the neonatal period. In adulthood, repetitive painful stimuli resulted in hyposensitivity to mechanical stimuli, while thermal sensitivity was unaffected. After re-injury of the same dermatome in adulthood, the number of repetitive noxious stimuli did not affect mechanical hypersensitivity. Both needle prick groups showed an increased duration of postoperative hypersensitivity compared to control.

CONCLUSION

This study shows that repetitive noxious stimulation during the early postnatal period affects acute and long-term mechanical sensitivity. Therefore, the amount of nociceptive stimuli should be minimized or adequately treated in a clinical setting.

Maternal deprivation alters nociceptive response in a gender-dependent manner in rats

The present study aimed at investigating both the early and long-term effects of maternal deprivation as well as gender on neuromotor reflexes, anxiety behavior and thermal nociceptive responses. A total of 64 Wistar rats pups (32 males, 32 females) were utilized and were deprived of their mother for 3 h/daily, from postnatal day 1 (P1) until P10. Successively, animals were divided into 2 groups: control group (C) - pups no subjected to intervention; and the maternal-deprived group (MD): pups subjected to maternal deprivation. The neuromotor reflexes were evaluated through the righting reflex and negative geotaxis tests; the exploratory behavior by open field test (OFT); the anxiety-like behavior by elevated plus-maze test (EPM); the thermal nociceptive responses byhot plate (HP) and tail-flick (TFL) tests. All the animals subjected to maternal deprivation showed a delayed reflex response at P8 in the negative geotaxis test. In contrast, the OFT at P20 identified an effect of gender on the outer crossings and grooming as well as an interaction between gender and maternal deprivation on latency. Additionally, effect of maternal deprivation in the open and closed arms as well as gender effect in the protected head-dipping (PHD) and non-protected head-dipping (NPHD) were observed at P20 (EPM). In contrast, there were a gender effect on latency and an interaction between gender and maternal deprivation on rearing at P42. Moreover, in nociceptive tests was observed an analgesic effect induced by maternal deprivation; however, in the TFL test, only deprived females showed this effect. Surprisingly, only control animals presented an ontogeny nociceptive effect in the HP testat P21 and P43, which may be related to an increase in the inhibitory nociceptive pathways throughout life. In this way, we suggest maternal deprivation to be able to anticipate the maturation of the inhibitory nociceptive pathway. In conclusion, maternal deprivation induced a delayed reflex response at P8 and altered the anxiety and nociceptive behaviors according to the time after exposure to this stressor, in a gender-specific manner.

Repeated touch and needle-prick stimulation in the neonatal period increases the baseline mechanical sensitivity and postinjury hypersensitivity of adult spinal sensory neurons

Neonatal abnormal noxious and tactile stimulations facilitate the activity of spinal neurons, which leads to an altered somatosensory and pain phenotype in adulthood.

Noxious stimulation at critical stages of development has long-term consequences on somatosensory processing in later life, but it is not known whether this developmental plasticity is restricted to nociceptive pathways. Here, we investigate the effect of repeated neonatal noxious or innocuous hind paw stimulation on adult spinal dorsal horn cutaneous mechanical sensitivity. Neonatal Sprague-Dawley rats of both sexes received 4 unilateral left hind paw needle pricks (NPs, n = 13) or 4 tactile (cotton swab touch) stimuli, per day (TC, n = 11) for the first 7 days of life. Control pups were left undisturbed (n = 17). When adult (6-8 weeks), lumbar wide-dynamic-range neuron activity in laminae III-V was recorded using in vivo extracellular single-unit electrophysiology. Spike activity evoked by cutaneous dynamic tactile (brush), pinch and punctate (von Frey hair) stimulation, and plantar receptive field areas were recorded, at baseline and 2 and 5 days after left plantar hind paw incision. Baseline brush receptive fields, von Frey hair, and pinch sensitivity were significantly enhanced in adult NP and TC animals compared with undisturbed controls, although effects were greatest in NP rats. After incision, injury sensitivity of adult wide-dynamic-range neurons to both noxious and dynamic tactile hypersensitivity was significantly greater in NP animals compared with TC and undisturbed controls. We conclude that both repeated touch and needle-prick stimulation in the neonatal period can alter adult spinal sensory neuron sensitivity to both innocuous and noxious mechanical stimulation. Thus, spinal sensory circuits underlying touch and pain processing are shaped by a range of early-life somatosensory experiences.

Targeted sensory enrichment interventions protect against behavioral and neuroendocrine consequences of early life stress

Both basic and clinical research support the use of tactile stimulation to rescue several neurobiobehavioral consequences that follow early life stress. Here, using a translational rodent model of the neonatal intensive care unit (NICU), we tested the individual prophylactic potential of a variety of sensory interventions including tactile (brushing pups with a paint brush to mimic maternal licking), auditory (a simulated lactating rat dam heart beat), and olfactory (a series of aroma therapy scents) stimulation. The NICU model was developed to mimic not only the reduced parental contact that sick infants receive (by isolating rat pups from their litters), but also the nosocomial infections and medical manipulations associated with this experience (by utilizing a dual lipopolysaccharide injection schedule). Each of the neurobiobehavioral consequences observed were dissociable between isolation and inflammation, or required a combined presentation ('two hits') of the neonatal stressors. Sprague-Dawley rats exposed to these early life stressors presented with sex-specific disruptions in both separation-induced ultrasonic vocalization (USV) distress calls (males & females) and juvenile social play USVs (males only). All three sensory enhancement interventions were associated with the rescue of potentiated distress calls while olfactory stimulation was protective of social vocalizations. Female rats exposed to early life stress experienced precocious puberty and shifts in the hypothalamic GnRh axis; sensory enrichment counter-acted the advanced pubertal onset. Animals that underwent the NICU protocol also displayed maturational acceleration in terms of the loss of the rooting reflex in addition to hyperalgesia, a reduced preference for a novel conspecific, blunted basal plasma corticosterone and reduced hippocampal glucocorticoid receptor expression. These alterations closely simulated the clinical effects of early life adversity in terms of disruptions in the hypothalamic pituitary "stress" axis, social communication and engagement, tactile system processing, and accelerated maturation. Moreover, sensory enrichment attenuated many of these behavioral and neurophysiological alterations, and even slowed maturation. Overall, this supports the translatability of our novel rodent model and its potential utility in understanding how brain maturation and quality of early life experiences may interact to shape the integrity of stress and sensory system development. Future work must determine the appropriate modalities and parameters (e.g. patterning, timing) for effective sensory enrichment interventions.

Is the Nociception Mechanism Altered in Offspring of Morphine-Abstinent Rats?

To investigate the effect of parental drug abuse on children, nociception, electrophysiological alteration, mRNA expression of opioid receptors, and expression of certain intracellular proteins in offspring of morphine-abstinent rats were studied. Adult male and female animals received water-soluble morphine for 21 days. Ten days after the last morphine administration, animals were placed for mating in 4 groups as follows: healthy (drug naive) female and male, morphine-abstinent female and healthy male, morphine-abstinent male and healthy female, morphine-abstinent male and morphine-abstinent female. Their adult male offspring were tested for nociception, neuronal discharge in nucleus accumbens (NAC) and prefrontal cortex (PFC). Our results showed that nociception in male offspring of all morphine-abstinent parent(s) groups was significantly reduced, compared with the control group. In the offspring of morphine-abstinent parent(s) groups, sensitivity to the antinociceptive effect of morphine was enhanced in chronic as well as in acute phases of the formalin test. Neuronal electrical activity reduced in the offspring of the morphine-exposed parent(s) in NAC as well as PFC regions. Moreover, our findings show that opioid receptors' expressions (µ, κ, and δ) increased in NAC of the litter of morphine-abstinent parent(s), compared with the control group. In addition, the expression of κ receptors was remarkably increased in the PFC in morphine-abstinent parent group, relative to the control group. The phosphorylated levels of extracellular regulated kinase 1/2 and cyclic adenosine monophosphate responsive element binding protein were significantly higher in the offspring of the morphine-abstinent parent(s) than the control group in the NAC. Our results indicated that endogenous opioid is altered in offspring of the morphine-exposed parent(s) and that heritage has a major role.

PERSPECTIVE

This study showed that nociception was reduced in offspring of morphine-abstinent rat(s) and also these litters had a low level of neuronal firing rate, and enhanced opioid receptors expression, especially in the NAC. Because these offspring are more sensitive to the analgesic effect of morphine, clinicians should consider this issue to manage the dosage of morphine for treating pain in children with an abstinent parent(s).

Maternal separation stress leads to resilience against neuropathic pain in adulthood

Early life stress (ELS) leads to a permanent reprogramming of biochemical stress response cascades that may also be relevant for the processing of chronic pain states such as neuropathy. Despite clinical evidence, little is known about ELS-related vulnerability for neuropathic pain and the possibly underlying etiology.

In the framework of experimental studies aimed at investigating the respective relationships we used the established ELS model of maternal separation (MS). Rat dams and neonates were separated for 3 h/day from post-natal day 2–12. At adulthood, noxious mechanical and thermal thresholds were assessed before and during induction of neuropathic pain by chronic constriction injury (CCI). The potential involvement of spinal glutamatergic transmission, glial cells, pro-inflammatory cytokines and growth factors was studied by using qPCR.

MS per se did not modify pain thresholds. But, when exposed to neuropathic pain, MS rats exhibited a marked reduction of thermal sensitivity and a delayed development of mechanical allodynia/hyperalgesia when compared to control animals. Also, MS did not alter glucocorticoid receptor mRNA levels, but prevented the CCI-induced down-regulation of NR1 and NR2 sub-units of the NMDA receptor and of the glutamate transporter EAAT3 as observed at 21 days post-surgery. Additionally, CCI-provoked up-regulation of glial cell markers was either prevented (GFAP for astrocytes) or dampened (Iba1 for microglia) by MS. Pro-inflammatory cytokine mRNA expression was either not affected (IL-6) or reduced (IL-1β) by MS shortly after CCI. The growth factors GDNF and NGF were only slightly downregulated 4 days after CCI in the MS-treated animals. The changes in glutamatergic signaling, astroglial and cytokine activation as well as neurotrophin expression could, to some extent, explain these changes in pain behavior. Taken together, the results obtained in the described experimental conditions support the mismatch theory of chronic stress where an early life stress, rather than predisposing individuals to certain pathologies, renders them resilient.

Neonatal Injury Results in Sex-Dependent Nociceptive Hypersensitivity and Social Behavioral Deficits During Adolescence, Without Altering Morphine Response

Neonatal injury is associated with persistent changes in sensory function and altered nociceptive thresholds that give rise to aberrant pain sensitivity in later life. Although these changes are well documented in adult rodents, little is known about the consequences of neonatal injury during adolescence. Because adolescence is a critical developmental period during which persistent pain conditions can arise, we examined the effect of neonatal injury on nociception, social behavior, and response to morphine in adolescent Sprague Dawley rats. Male and female rats exposed to plantar incision injury at postnatal day 3 displayed mechanical hypersensitivity that resolved by 24 hours after incision. When these animals reached adolescence (postnatal day 28-40), neonatally-injured male rats showed ipsilaterally restricted mechanical, heat, and cold hypersensitivity, as well as social behavioral deficits. In contrast, these effects were not seen in female rats. Neonatal injury did not alter acute morphine antinociception or the development of analgesic tolerance in either sex. Morphine-induced conditioned place preference, behavioral sensitization, and physical withdrawal were also not affected by neonatal incision. Thus, early-life injury results in sex-dependent pain-related hypersensitivity and social behavior deficits during adolescence, without altering the response to opioids.

PERSPECTIVE

Neonatal surgery has greater effects on adolescent male than female rats, resulting in pain-related hypersensitivity and social behavioral deficits. Neonatal surgery does not alter the antinociceptive effects of morphine or abuse liability.

Metabolic and affective consequences of fatherhood in male California mice

Physiological and affective condition can be modulated by the social environment and parental state in mammals. However, in species in which males assist with rearing offspring, the metabolic and affective effects of pair bonding and fatherhood on males have rarely been explored. In this study we tested the hypothesis that fathers, like mothers, experience energetic costs as well as behavioral and affective changes (e.g., depression, anxiety) associated with parenthood. We tested this hypothesis in the monogamous, biparental California mouse (Peromyscus californicus). Food intake, blood glucose and lipid levels, blood insulin and leptin levels, body composition, pain sensitivity, and depression-like behavior were compared in males from three reproductive groups: virgin males (VM, housed with another male), non-breeding males (NB, housed with a tubally ligated female), and breeding males (BM, housed with a female and their first litter). We found statistically significant (P<0.007, when modified for Adaptive False Discovery Rate) or nominally significant (0.007<P<0.05) differences among reproductive groups in relative testis mass, circulating glucose, triglyceride, and insulin concentrations, pain sensitivity, and anxiety-like behaviors. A priori contrasts indicated that VM produced significantly more fecal pellets than BM in the tail-suspension test, had significantly higher glucose levels than NB, and had significantly lower average testis masses than did NB and BM. A priori contrasts also indicated that VM had a nominally longer latency to the pain response than NB and that VM had nominally higher insulin levels than did NB. For breeding males, litter size (one to three pups) was a nominally significant positive predictor of body mass, food consumption, fat mass, and plasma leptin concentration. These results indicate that cohabitation with a female and/or fatherhood influences several metabolic, morphological, and affective measures in male California mice. Overall, the changes we observed in breeding males were minor, but stronger effects might occur in long-term breeding males and/or under more challenging environmental conditions.

Experiencing neonatal maternal separation increased pain sensitivity in adult male mice: Involvement of oxytocinergic system

Early-life stress adversely affects the development of the brain, and alters a variety of behaviors such as pain in later life. In present study, we investigated how early-life stress (maternal separation or MS) can affect the nociceptive response later in life. We particularly focused on the role of oxytocin (OT) in regulating nociception in previously exposed (MS during early postnatal development) mice that were subjected to acute stress (restraint stress or RS). Further, we evaluated whether such modulation of pain sensation in MS mice are regulated by shared mechanisms of the OTergic and opioidergic systems. To do this, we assessed the underlying systems mediating the nociceptive response by administrating different antagonists (for both opioid and OTergic systems) under the different experimental conditions (control vs MS, and control plus RS vs MS plus RS). Our results showed that MS increased pain sensitivity in both tail-flick and hot-plate tests while after administration of OT (1μg/μl/mouse, i.c.v) pain threshold was increased. Atosiban, an OT antagonist (10μg/μl/mouse, i.c.v) abolished the effects of OT. While acute RS increased the pain threshold in control (and not MS) mice, treating MS mice with OT normalized the pain response to RS. This latter effect was reversed by atosiban and/or naltrexone, an opioid antagonist (0.5μg/μl/mouse, i.c.v) suggesting that OT enhances the effect of endogenous opioids. OTergic system is involved in mediating the nociception under acute stress in mice subjected to early-life stress and OTergic and opioidergic systems interact to modulate pain sensitivity in MS mice.

Maternal separation induces neuroinflammation and long-lasting emotional alterations in mice

Early life experiences play a key role in brain function and behaviour. Adverse events during childhood are therefore a risk factor for psychiatric disease during adulthood, such as mood disorders. Maternal separation is a validated mouse model for maternal neglect, producing negative early life experiences that result in subsequent emotional alteration. Mood disorders have been found to be associated with neurochemical changes and neurotransmitter deficits such as reduced availability of monoamines in discrete brain areas. Emotional alterations like depression result in reduced serotonin availability and enhanced kynurenine metabolism through the action of indoleamine 2, 3-dioxygenase in response to neuroinflammatory factors. This mechanism involves regulation of the neurotransmitter system by neuroinflammatory agents, linking mood regulation to neuroinmunological reactions. In this context, the aim of this study was to investigate the effects of maternal separation with early weaning on emotional behaviour in mice. We investigated neuroinflammatory responses and the state of the tryptophan-kynurenine metabolic pathway in discrete brain areas following maternal separation. We show that adverse events during early life increase risk of long-lasting emotional alterations during adolescence and adulthood. These emotional alterations are particularly severe in females. Behavioural impairments were associated with microglia activation and disturbed tryptophan-kynurenine metabolism in brain areas related to emotional control. This finding supports the preeminent role of neuroinflammation in emotional disorders.

Experiencing neonatal maternal separation increased the seizure threshold in adult male mice: Involvement of the opioid system

Experiencing early-life stress has been considered as a potent risk factor for the development of many of brain disorders, including seizures. Intervening mechanisms through which neonatal maternal separation (MS) alters the seizure susceptibility in adulthood have not been well studied. In the current study, by applying 180 min of MS stress (PND 2-14), we determined the seizure susceptibility and considered the role of the opioid system. Maternal separation increased the seizure threshold, and administration of anticonvulsant/proconvulsant doses of morphine (1 and 30 mg/kg, respectively) reversed the impact of MS. Using tail flick and hot plate tests, we exposed animals to 30 min Restraint stress (RS) and found that MS decreased the pain threshold, suggesting the hyporesponsiveness of the opioid system. These results supported the abnormal seizure activity observed in the MS mice and suggested that abnormalities in the opioid system following MS alter seizure susceptibility in later life.

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.

The consequences of pain in early life: injury-induced plasticity in developing pain pathways

Pain in infancy influences pain reactivity in later life, but how and why this occurs is poorly understood. Here we review the evidence for developmental plasticity of nociceptive pathways in animal models and discuss the peripheral and central mechanisms that underlie this plasticity. Adults who have experienced neonatal injury display increased pain and injury-induced hyperalgesia in the affected region but mild injury can also induce widespread baseline hyposensitivity across the rest of the body surface, suggesting the involvement of several underlying mechanisms, depending upon the type of early life experience. Peripheral nerve sprouting and dorsal horn central sensitization, disinhibition and neuroimmune priming are discussed in relation to the increased pain and hyperalgesia, while altered descending pain control systems driven, in part, by changes in the stress/HPA axis are discussed in relation to the widespread hypoalgesia. Finally, it is proposed that the endocannabinoid system deserves further attention in the search for mechanisms underlying injury-induced changes in pain processing in infants and children.

Exercise reverses the effects of early life stress on orexin cell reactivity in male but not female rats

Early life stress (ELS) is a known antecedent for the development of mood disorders such as depression. Orexin neurons drive arousal and motivated behaviors in response to stress. We tested the hypothesis that ELS alters orexin system function and leads to an altered stress-induced behavioral phenotype in adulthood. We also investigated if voluntary exercise during adolescent development could reverse the ELS-induced changes. Male and female Wistar rats were subjected to maternal separation stress on postnatal days (PND) 2-14. A subset of animals was given access to running wheels in late adolescence (1hr/day, PND40-70). In adulthood, rats were exposed to restraint stress and then tested on the open field (OF) and elevated plus maze (EPM). Brains were processed for Fos-protein and orexin or tyrosine hydroxylase immunohistochemistry. Restraint stress stimulated Fos-protein expression in perifornical area orexin cells, the paraventricular hypothalamic nucleus, and paraventricular thalamic nuclei, but this neuronal response was dampened in male and female rats exposed to ELS. ELS also reduced exploration in the OF, without affecting EPM behavior. These neural and behavioral changes are consistent with a depressive-like phenotype. Adolescent exercise reversed the orexin and behavioral deficits in ELS males. Exercise was not protective in females, although this may be due to sex differences in running behavior. Our findings highlight the inherent plasticity of the orexin system—a trait that may lead to a state of pathological rewiring but could also be treated using non-pharmacological approaches. We also highlight a need to better understand the sex-specific changes in orexin circuits and stress-related pathology.

Lithium ameliorates autistic-like behaviors induced by neonatal isolation in rats

Neonatal isolation is a widely accepted model to study the long-term behavioral changes produced by the early life events. However, it remains unknown whether neonatal isolation can induce autistic-like behaviors, and if so, whether pharmacological treatment can overcome it. Here, we reported that newborn rats subjected to individual isolations from their mother and nest for 1 h per day from postnatal days 1–9 displayed apparent autistic-like symptoms including social deficits, excessive repetitive self-grooming behavior, and increased anxiety- and depressive-like behaviors tested in young adult (postnatal days 42–56) compared to normal reared controls. Furthermore, these behavioral changes were accompanied by impaired adult hippocampal neurogenesis and reduced the ratio of excitatory/inhibitory synaptic transmissions, as reflected by an increase in spontaneous inhibitory postsynaptic current (sIPSC) and normal spontaneous excitatory postsynaptic current (sEPSC) in the hippocampal CA1 pyramidal neuron. More importantly, chronic administration of lithium, a clinically used mood stabilizer, completely overcame neonatal isolation-induced autistic-like behaviors, and restored adult hippocampal neurogenesis as well as the balance between excitatory and inhibitory activities to physiological levels. These findings indicate that neonatal isolation may produce autistic-like behaviors, and lithium may be a potential therapeutic agent against autism spectrum disorders (ASD) during development.

The effect of early environmental manipulation on locomotor sensitivity and methamphetamine conditioned place preference reward

Early life stress leads to several effects on neurological development, affecting health and well-being later in life. Instances of child abuse and neglect are associated with higher rates of depression, risk taking behavior, and an increased risk of drug abuse later in life. This study used repeated neonatal separation of rat pups as a model of early life stress. Rat pups were either handled and weighed as controls or separated for 180 min per day during postnatal days 2-8. In adulthood, male and female rats were tested for methamphetamine conditioned place preference reward and methamphetamine induced locomotor activity. Tissue samples were collected and mRNA was quantified for the norepinephrine transporter in the prefrontal cortex and the dopamine transporter in the nucleus accumbens. Results indicated rats given methamphetamine formed a conditioned place preference, but there was no effect of early separation or sex. Separated males showed heightened methamphetamine-induced locomotor activity, but there was no effect of early separation for females. Overall females were more active than males in response to both saline and methamphetamine. No differences in mRNA levels were observed across any conditions. These results suggest early neonatal separation affects methamphetamine-induced locomotor activity in a sex-dependent manner but has no effects on methamphetamine conditioned place preference.

Vaginal hypersensitivity and hypothalamic-pituitary-adrenal axis dysfunction as a result of neonatal maternal separation in female mice

Early life stress can permanently alter functioning of the hypothalamic-pituitary-adrenal (HPA) axis, which regulates the stress response and influences the perception of pain. Chronic pelvic pain patients commonly report having experienced childhood neglect or abuse, which increases the likelihood of presenting with comorbid chronic pain and/or mood disorders. Animal models of neonatal stress commonly display enhanced anxiety-like behaviors, colorectal hypersensitivity, and disruption of proper neuro-immune interactions in adulthood. Here, we tested the hypothesis that early life stress impacts vaginal sensitivity by exposing mice to neonatal maternal separation (NMS) for 3h/day during the first two (NMS14) or three (NMS21) postnatal weeks. As adults, female mice underwent vaginal balloon distension (VBD), which was also considered an acute stress. Before or after VBD, mice were assessed for anxiety-like behavior, hindpaw sensitivity, and changes in gene and protein expression related to HPA axis function and regulation. NMS21 mice displayed significantly increased vaginal sensitivity compared to naïve mice, as well as significantly reduced anxiety-like behavior at baseline, which was heightened following VBD. NMS21 mice exhibited significant thermal and mechanical hindpaw hypersensitivity at baseline and following VBD. NMS14 mice displayed no change in anxiety-like behavior and only exhibited significantly increased hindpaw mechanical and thermal sensitivity following VBD. Centrally, a significant decrease in negative regulation of the HPA axis was observed in the hypothalamus and hippocampus of NMS21 mice. Peripherally, NMS and VBD affected the expression of inflammatory mediators in the vagina and bladder. Corticotropin-releasing factor (CRF) receptor and transient receptor potential (TRP) channel protein expression was also significantly, and differentially, affected in vagina, bladder, and colon by both NMS and VBD. Together these data indicate that NMS affects both central and peripheral aspects of the HPA axis, which may drive changes in vaginal sensitivity and the development of comorbid chronic pain and mood disorders.

Maternal behavior in basic science: translational research and clinical applicability

Clinical aspects of the mother-infant relationship and related issues are well studied and very relevant to medical practice. Nevertheless, some approaches in this context cannot plausibly be investigated in humans due to their ethical implications and to the potential harm to the mother's and child's health. Studies on maternal behavior in animals have evident importance to some clinical fields, such as psychiatry and psychology, particularly considering topics, including mother-infant relationship, postpartum depression, cognitive and behavioral development of children, and associated issues. Hence, this theoretical article draws attention to the clinical applicability of studies about maternal behavior in animals to psychobiology, taking into account a translational perspective.

Maternal deprivation is associated with sex-dependent alterations in nociceptive behavior and neuroinflammatory mediators in the rat following peripheral nerve injury

Early-life stress is associated with an increased risk of developing affective disorders and chronic pain conditions. This study examined the effect of maternal deprivation (MD) on nociceptive responding prior to and following peripheral nerve injury (L5-L6 spinal nerve ligation [SNL]). Because neuroimmune signaling plays an important role in pain and affective disorders, associated alterations in glial and cytokine expression were assessed in key brain regions associated with emotional and nociceptive responding, the hippocampus and prefrontal cortex. MD female, but not male, rats exhibited thermal hypoalgesia and mechanical allodynia compared with control (non-MD) counterparts. SNL resulted in mechanical and cold allodynia in MD and control rats of both sexes. However, MD females exhibited enhanced SNL-induced allodynic responding compared with non-MD counterparts. Interleukin 6 (IL-6) expression was reduced in the prefrontal cortex of MD-SNL males when compared with non-SNL counterparts. Glial fibrillary acidic protein and IL-1β expression in the hippocampus of MD-SNL males was increased compared with non-MD controls. MD-SNL females exhibited reduced tumor necrosis factor alpha in the prefrontal cortex with a concomitant increase in IL-6 and tumor necrosis factor alpha expression in the hippocampus, compared with either MD or SNL alone. In conclusion, MD female, but not male, rats exhibit enhanced nociceptive responding following peripheral nerve injury, effects that may relate to the distinct neuroinflammatory profile observed in female versus male rats.

PERSPECTIVE

This study demonstrates that females rats exposed to early-life stress exhibit enhanced neuropathic pain responding, effects that are associated with alterations in neuroinflammatory mediators. Increased understanding of the interactions among early-life stress, gender, and pain may lead to the identification of novel therapeutic targets for the treatment of chronic pain disorders.

Opioid receptors: distinct roles in mood disorders

The roles of opioid receptors in pain and addiction have been extensively studied, but their function in mood disorders has received less attention. Accumulating evidence from animal research reveals that mu, delta and kappa opioid receptors (MORs, DORs and KORs, respectively) exert highly distinct controls over mood-related processes. DOR agonists and KOR antagonists have promising antidepressant potential, whereas the risk-benefit ratio of currently available MOR agonists as antidepressants remains difficult to evaluate, in addition to their inherent abuse liability. To date, both human and animal studies have mainly examined MORs in the etiology of depressive disorders, and future studies will address DOR and KOR function in established and emerging neurobiological aspects of depression, including neurogenesis, neurodevelopment, and social behaviors.

Early-life stress produces muscle hyperalgesia and nociceptor sensitization in the adult rat

Chronic pain in adults has been associated with early-life stress. To examine the pronociceptive effect of early-life stress, we evaluated cutaneous and muscle nociception and activity in muscle nociceptors in an animal model of neonatal stress, limited bedding, in the rat. In this neonatal limited bedding (NLB) model, litters are exposed to limited bedding between postnatal days 2 and 9, and controls to standard bedding. In adult NLB-treated rats, mechanical nociceptive threshold in skeletal muscle was significantly lower (~22%) than in controls. Furthermore, administration of prostaglandin E(2) in skin as well as muscle produced markedly prolonged hyperalgesia, an effect prevented by spinal intrathecal injection of oligodeoxynucleotide antisense to protein kinase Cε (PKCε), a second messenger in nociceptors that has been implicated in the induction and maintenance of chronic pain. In electrophysiological studies, mechanical threshold of muscle nociceptors was reduced by ~31% and conduction velocity significantly increased (~28%). These findings indicate that neonatal stress induces a persistent hyperalgesia and nociceptor sensitization manifest in the adult and that the second messenger PKCε may be a target against which therapies might be directed to treat a chronic pain syndrome that is associated with early-life traumatic stress.

Resistance to early-life stress in mice: effects of genetic background and stress duration

Early-life stress can induce marked behavioral and physiological impairments in adulthood including cognitive deficits, depression, anxiety, and gastrointestinal dysfunction. Although robust rat models of early-life stress exist there are few established effective paradigms in the mouse. Genetic background and protocol parameters used are two critical variables in such model development. Thus we investigated the impact of two different early-life stress protocols in two commonly used inbred mouse strains. C57BL/6 and innately anxious BALB/c male mice were maternally deprived 3 h daily, either from postnatal day 1 to 14 (protocol 1) or 6 to 10 (protocol 2). Animals were assessed in adulthood for cognitive performance (spontaneous alternation behavior test), anxiety [open-field, light/dark box (L/DB), and elevated plus maze (EPM) tests], and depression-related behaviors (forced swim test) in addition to stress-sensitive physiological changes. Overall, the results showed that early-life stressed mice from both strains displayed good cognitive ability and no elevations in anxiety. However, paradoxical changes occurred in C57BL/6 mice as the longer protocol (protocol 1) decreased anxiety in the L/DB and increased exploration in the EPM. In BALB/c mice there were also limited effects of maternal separation with both separation protocols inducing reductions in stress-induced defecation and protocol 1 reducing the colon length. These data suggest that, independent of stress duration, mice from both strains were on the whole resilient to the maladaptive effects of early-life stress. Thus maternal separation models of brain–gut axis dysfunction should rely on either different stressor protocols or other strains of mice.

Adolescent opiate exposure in the female rat induces subtle alterations in maternal care and transgenerational effects on play behavior

The non-medical use of prescription opiates, such as Vicodin(®) and MSContin(®), has increased dramatically over the past decade. Of particular concern is the rising popularity of these drugs in adolescent female populations. Use during this critical developmental period could have significant long-term consequences for both the female user as well as potential effects on her future offspring. To address this issue, we have begun modeling adolescent opiate exposure in female rats and have observed significant transgenerational effects despite the fact that all drugs are withdrawn several weeks prior to pregnancy. The purpose of the current set of studies was to determine whether adolescent morphine exposure modifies postpartum care. In addition, we also examined juvenile play behavior in both male and female offspring. The choice of the social play paradigm was based on previous findings demonstrating effects of both postpartum care and opioid activity on play behavior. The findings revealed subtle modifications in the maternal behavior of adolescent morphine-exposed females, primarily related to the amount of time females' spend nursing and in non-nursing contact with their young. In addition, male offspring of adolescent morphine-exposed mothers (MOR-F1) demonstrate decreased rough and tumble play behaviors, with no significant differences in general social behaviors (i.e., social grooming and social exploration). Moreover, there was a tendency toward increased rough and tumble play in MOR-F1 females, demonstrating the sex-specific nature of these effects. Given the importance of the postpartum environment on neurodevelopment, it is possible that modifications in maternal-offspring interactions, related to a history of adolescent opiate exposure, plays a role in the observed transgenerational effects. Overall, these studies indicate that the long-term consequences of adolescent opiate exposure can impact both the female and her future offspring.

Randomized crossover trial of kangaroo care to reduce biobehavioral pain responses in preterm infants: a pilot study

Kangaroo care (KC), skin-to-skin contact between mother and infant, is a promising method for blunting pain responses. This crossover pilot tested KC effects on biobehavioral responses to heel stick in preterm infants (30-32 weeks' gestational age, 2-9 days old) measured by Premature Infant Pain Profile (PIPP) and salivary and serum cortisol. Mother-infant dyads were randomly assigned to KC heel stick (KCH) first or incubator heel stick (IH) first. Study 1 (80-min study, N = 18) tested the effect of 80 min of KC before and throughout the heel stick procedure versus incubator care. Study 2 (30-min study, N = 10) tested 30 min of KC before and throughout the heel stick versus incubator care. KCH and IH began during a premeasurement phase and continued through four data collection phases: baseline, heel warming, heel stick, and recovery. PIPP responses were measured every 30 s during data collection; salivary cortisol was measured at the end of baseline and recovery; and serum cortisol was measured during heel stick. Study 1 showed no differences between KCH and IH. Study 2 showed lower PIPP scores at four time points during recovery (p < .05 to p < .001), lower salivary cortisol at the end of recovery (p < .05), and lower serum cortisol during heel stick for the KCH condition (p < .05) as well as clinically lower PIPP scores in the KCH condition during heel stick. Thirty minutes of KC before and throughout the heel stick reduced biobehavioral responses to pain in preterm infants.