Priming of adult pain responses by neonatal pain experience: maintenance by central neuroimmune activity

Early pain experiences in dairy calves on pain sensitivity later in life

In humans, early painful experiences can increase pain sensitivity later in life, but little is known regarding this phenomenon in cattle. This study assessed if a painful event early in life affects later pain sensitivity in 40-d-old calves. Holstein calves (n = 26) were randomly assigned to control or treatment conditions, blocked within sex and birth weight. At 9.5 ± 1.8 d old, treatment calves had 1 horn bud disbudded using caustic paste, whereas control calves had 1 horn sham disbudded; in both cases, multimodal pain control was provided. All calves had the contralateral horn bud disbudded using a hot iron 4 wk later, again with multimodal pain control. Mechanical nociceptive responses were assessed weekly using an algometer applied adjacent to both horn buds and on the rump, beginning 3 d before the first disbudding and ending 30 d after the second disbudding. Following the second disbudding, both groups of calves showed evidence of increased sensitivity (i.e., algometer pressure declined 3.69 ± 0.60 √N to 2.13 ± 0.70 √N) on the contralateral bud, but there was no difference between the control group and the treatment groups. An interaction between treatment and time, likely driven by treatment differences, was found on the rump when tested 5 h after the second disbudding event. These responses are not consistent with the hypothesis that an early pain experience results in increased sensitivity to later painful experiences.

Deep learning to quantify care manipulation activities in neonatal intensive care units

Early-life exposure to stress results in significantly increased risk of neurodevelopmental impairments with potential long-term effects into childhood and even adulthood. As a crucial step towards monitoring neonatal stress in neonatal intensive care units (NICUs), our study aims to quantify the duration, frequency, and physiological responses of care manipulation activities, based on bedside videos and physiological signals. Leveraging 289 h of video recordings and physiological data within 330 sessions collected from 27 neonates in 2 NICUs, we develop and evaluate a deep learning method to detect manipulation activities from the video, to estimate their duration and frequency, and to further integrate physiological signals for assessing their responses. With a 13.8% relative error tolerance for activity duration and frequency, our results were statistically equivalent to human annotations. Further, our method proved effective for estimating short-term physiological responses, for detecting activities with marked physiological deviations, and for quantifying the neonatal infant stressor scale scores.

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.

Macrophage memories of early-life injury drive neonatal nociceptive priming

The developing peripheral nervous and immune systems are functionally distinct from those of adults. These systems are vulnerable to early-life injury, which influences outcomes related to nociception following subsequent injury later in life (i.e., "neonatal nociceptive priming"). The underpinnings of this phenomenon are unclear, although previous work indicates that macrophages are trained by inflammation and injury. Our findings show that macrophages are both necessary and partially sufficient to drive neonatal nociceptive priming, possibly due to a long-lasting remodeling in chromatin structure. The p75 neurotrophic factor receptor is an important effector in regulating neonatal nociceptive priming through modulation of the inflammatory profile of rodent and human macrophages. This "pain memory" is long lasting in females and can be transferred to a naive host to alter sex-specific pain-related behaviors. This study reveals a mechanism by which acute, neonatal post-surgical pain drives a peripheral immune-related predisposition to persistent pain following a subsequent injury.

Repetitive neonatal procedural pain affects stress-induced plasma corticosterone increase in young adult females but not in male rats

Exposure to repetitive painful procedures in the neonatal intensive care unit results in long-lasting effects, especially visible after a "second hit" in adulthood. As the nociceptive system and the hypothalamic-pituitary-adrenal (HPA) axis interact and are vulnerable in early life, repetitive painful procedures in neonates may affect later-life HPA axis reactivity. The first aim of the present study was to investigate the effects of repetitive neonatal procedural pain on plasma corticosterone levels after mild acute stress (MAS) in young adult rats. Second, the study examined if MAS acts as a "second hit" and affects mechanical sensitivity. Fifty-two rats were either needle pricked four times a day, disturbed, or left undisturbed during the first neonatal week. At 8 weeks, the animals were subjected to MAS, and plasma was collected before (t0), after MAS (t20), and at recovery (t60). Corticosterone levels were analyzed using an enzyme-linked immunosorbent assay, and mechanical sensitivity was assessed with von Frey filaments. Results demonstrate that repetitive neonatal procedural pain reduces stress-induced plasma corticosterone increase after MAS only in young adult females and not in males. Furthermore, MAS does not affect mechanical sensitivity in young adult rats. Altogether, the results suggest an age- and sex-dependent effect of repetitive neonatal procedural pain on HPA axis reprogramming.

The Pharmacological Treatment of Neuropathic Pain in Children

The International Association for the Study of Pain (IASP) defines neuropathic pain as pain caused by a lesion or disease of the somatosensory nervous system. It is characterized as a clinical condition in which diagnostic studies reveal an underlying cause of an abnormality in the peripheral or central nervous system. Many common causes of neuropathic pain in adults are rare in children. The purpose of this focused narrative review is, to 1) provide an overview of neuropathic pain in children, 2) highlight unique considerations related to the diagnosis and mechanisms of neuropathic pain in children, and 3) perform a comprehensive analysis of the pharmacological treatments available. We emphasize that data for routine use of pharmacological agents in children with neuropathic pain are largely inferred from adult literature with little research performed on pediatric populations, yet have clear evidence of harms to pediatric patients. Based on these findings, we propose risk mitigation strategies such as utilizing topical treatments whenever possible, assessing pain phenotyping to guide drug class choice, and considering pharmaceuticals in the broader context of the multidisciplinary treatment of pediatric pain. Furthermore, we highlight important directions for future research on pediatric neuropathic pain treatment.

Macrophage epigenetic memories of early life injury drive neonatal nociceptive priming

The developing peripheral nervous and immune systems are functionally distinct from adults. These systems are vulnerable to early life injury, which influences outcomes related to nociception following subsequent injury later in life (neonatal nociceptive priming). The underpinnings of this phenomenon are largely unknown, although previous work indicates that macrophages are epigenetically trained by inflammation and injury. We found that macrophages are both necessary and partially sufficient to drive neonatal nociceptive priming possibly due to a long-lasting epigenetic remodeling. The p75 neurotrophic factor receptor (NTR) was an important effector in regulating neonatal nociceptive priming through modulation of the inflammatory profile of rodent and human macrophages. This pain memory was long lasting in females and could be transferred to a naive host to alter sex-specific pain-related behaviors. This study reveals a novel mechanism by which acute, neonatal post-surgical pain drives a peripheral immune-related predisposition to persistent pain following a subsequent injury.

Effects of repetitive pinprick stimulation on preterm offspring: Alterations in nociceptive responses and inflammatory hypersensitivity in adulthood

INTRODUCTION

This study investigates the effects of repetitive pinprick stimulation on preterm offspring and its impact on nociceptive responses and inflammatory hypersensitivity in adulthood.

OBJECTIVES

The objective is to shed light on the potential long-term consequences of neonatal pain and prematurity on sensory processing.

METHODS

Term and preterm rats were subjected to repetitive pinprick (PP) stimulation or control (CC) during the neonatal period. Adult rats received CFA injection to induce inflammatory hypersensitivity, and mechanical hypersensitivity was measured. Gender differences in inflammatory hypersensitivity were also examined. Maternal behavior, litter weight, and offspring growth were monitored to assess any potential influences of the stimulation on these parameters.

RESULTS

In preterm rats, the PP stimulation did not affect baseline thresholds to mechanical stimuli, but increased mechanical hypersensitivity after CFA injection in adult rats. Females exhibited greater inflammatory hypersensitivity compared to males. Maternal behavior, litter weight, and offspring growth were not influenced by the stimulation. PP stimulation during the neonatal period led to changes in nociceptive responses in adulthood, potentially altering sensory processing.

CONCLUSION

PP stimulation in preterm rats during the neonatal period resulted in changes in nociceptive responses in adulthood, leading to increased inflammatory hypersensitivity. The study emphasizes how early development can significantly impact sensory processing and further highlights the potential long-term consequences of prematurity and neonatal pain on this processing.

Developmental impact of peripheral injury on neuroimmune signaling

A peripheral injury drives neuroimmune interactions at the level of the injury and throughout the neuraxis. Understanding these systems will be beneficial in the pursuit to target persistent pain that involves both neural and immune components. In this review, we discuss the impact of injury on the development of neuroimmune signaling, along with data that suggest a possible cellular immune memory. We also discuss the parallel effects of injury in the nervous system and immune related areas including bone marrow, lymph node and central nervous system-related cells. Finally, we relate these findings to patient populations and current research that evaluates human tissue.

Repetitive and compulsive behavior after Early-Life-Pain associated with reduced long-chain sphingolipid species

BACKGROUND

Pain in early life may impact on development and risk of chronic pain. We developed an optogenetic Cre/loxP mouse model of "early-life-pain" (ELP) using mice with transgenic expression of channelrhodopsin-2 (ChR2) under control of the Advillin (Avil) promoter, which drives expression of transgenes predominantly in isolectin B4 positive non-peptidergic nociceptors in postnatal mice. Avil-ChR2 (Cre +) and ChR2-flfl control mice were exposed to blue light in a chamber once daily from P1-P5 together with their Cre-negative mother.

RESULTS

ELP caused cortical hyperexcitability at P8-9 as assessed via multi-electrode array recordings that coincided with reduced expression of synaptic genes (RNAseq) including Grin2b, neurexins, piccolo and voltage gated calcium and sodium channels. Young adult (8-16 wks) Avil-ChR2 mice presented with nociceptive hypersensitivity upon heat or mechanical stimulation, which did not resolve up until one year of age. The persistent hypersensitivy to nociceptive stimuli was reflected by increased calcium fluxes in primary sensory neurons of aged mice (1 year) upon capsaicin stimulation. Avil-ChR2 mice behaved like controls in maze tests of anxiety, social interaction, and spatial memory but IntelliCage behavioral studies revealed repetitive nosepokes and corner visits and compulsive lickings. Compulsiveness at the behavioral level was associated with a reduction of sphingomyelin species in brain and plasma lipidomic studies. Behavioral studies were done with female mice.

CONCLUSION

The results suggest that ELP may predispose to chronic "pain" and compulsive psychopathology in part mediated by alterations of sphingolipid metabolism, which have been previously described in the context of addiction and psychiatric diseases.

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.

Early-Life Iron Deficiency Persistently Alters Nociception in Developing Mice

Clinical association studies have identified early-life iron deficiency (ID) as a risk factor for the development of chronic pain. While preclinical studies have shown that early-life ID persistently alters neuronal function in the central nervous system, a causal relationship between early-life ID and chronic pain has yet to be established. We sought to address this gap in knowledge by characterizing pain sensitivity in developing male and female C57Bl/6 mice that were exposed to dietary ID during early life. Dietary iron was reduced by ∼90% in dams between gestational day 14 and postnatal day (P)10, with dams fed an ingredient-matched, iron-sufficient diet serving as controls. While cutaneous mechanical and thermal withdrawal thresholds were not altered during the acute ID state at P10 and P21, ID mice were more sensitive to mechanical pressure at P21 independent of sex. During adulthood, when signs of ID had resolved, mechanical and thermal thresholds were similar between early-life ID and control groups, although male and female ID mice displayed increased thermal tolerance at an aversive (45 °C) temperature. Interestingly, while adult ID mice showed decreased formalin-induced nocifensive behaviors, they showed exacerbated mechanical hypersensitivity and increased paw guarding in response to hindpaw incision in both sexes. Collectively, these results suggest that early-life ID elicits persistent changes in nociceptive processing and appears capable of priming developing pain pathways. PERSPECTIVE: This study provides novel evidence that early-life ID evokes sex-independent effects on nociception in developing mice, including an exacerbation of postsurgical pain during adulthood. These findings represent a critical first step towards the long-term goal of improving health outcomes for pain patients with a prior history of ID.

New insights into irritable bowel syndrome pathophysiological mechanisms: contribution of epigenetics

Irritable bowel syndrome (IBS) is a complex multifactorial condition including alterations of the gut-brain axis, intestinal permeability, mucosal neuro-immune interactions, and microbiota imbalance. Recent advances proposed epigenetic factors as possible regulators of several mechanisms involved in IBS pathophysiology. These epigenetic factors include biomolecular mechanisms inducing chromosome-related and heritable changes in gene expression regardless of DNA coding sequence. Accordingly, altered gut microbiota may increase the production of metabolites such as sodium butyrate, a prominent inhibitor of histone deacetylases. Patients with IBS showed an increased amount of butyrate-producing microbial phila as well as an altered profile of methylated genes and micro-RNAs (miRNAs). Importantly, gene acetylation as well as specific miRNA profiles are involved in different IBS mechanisms and may be applied for future diagnostic purposes, especially to detect increased gut permeability and visceromotor dysfunctions. In this review, we summarize current knowledge of the role of epigenetics in IBS pathophysiology.

Microglial phagocytosis mediates long-term restructuring of spinal GABAergic circuits following early life injury

Peripheral injury during the early postnatal period alters the somatosensory system, leading to behavioural hyperalgesia upon re-injury in adulthood. Spinal microglia have been implicated as the cellular mediators of this phenomenon, but the mechanism is unclear. We hypothesised that neonatal injury (1) alters microglial phagocytosis of synapses in the dorsal horn leading to long-term structural changes in neurons, and/or (2) trains microglia, leading to a stronger microglial response after re-injury in adulthood. Using hindpaw surgical incision as a model we showed that microglial density and phagocytosis increased in the dorsal horn region innervated by the hindpaw. Dorsal horn microglia increased engulfment of synapses following injury, with a preference for those expressing the vesicular GABA transporter VGAT and primary afferent A-fibre terminals in neonates. This led to a long-term reduction of VGAT density in the dorsal horn and reduced microglial phagocytosis of VGLUT2 terminals. We also saw an increase in apoptosis following neonatal injury, which was not limited to the dorsal horn suggesting that larger circuit wide changes are happening. In adults, hindpaw incision increased microglial engulfment of predominantly VGAT synapses but did not alter the engulfment of A-fibres. This engulfment was not affected by prior neonatal injury, suggesting that microglial phagocytosis was not trained. These results highlight microglial phagocytosis in the dorsal horn as an important physiological response towards peripheral injury with potential long-term consequences and reveals differences in microglial responses between neonates and adults.

Neonatal injury modulates incisional pain sensitivity in adulthood: An animal study

Neonatal pain experiences including traumatic injury influences negatively on development of nociceptive circuit developments, resulting in persistent pain hypersensitivity in adults. However, the detailed mechanism is not yet well understood. In the present study, to clarify the pathogenesis of orofacial pain hypersensitivity associated with neonatal injury, the involvement of the voltage-gated sodium channel (Na_v) 1.8 and the C-C chemokine ligand 2 (CCL2)/C-C chemokine receptor 2 (CCR2) signaling in the trigeminal ganglion (TG) in facial skin incisional pain hypersensitivity was examined in 190 neonatal facial-injured and sham male rats. The whisker pad skin was incised on postnatal day 4 and week 7 (Incision-Incision group). Compared to the group without neonatal incision (Sham-Incision group), mechanical hypersensitivity in the whisker pad skin was enhanced in Incision-Incision group. The number of Na_v1.8-immunoreactive TG neurons and the amount of CCL2 expressed in the macrophages and satellite glial cells in the TG were increased on day 14 after re-incision in the Incision-Incision group, compared with Sham-Incision group. Blockages of Na_v1.8 in the incised region and CCR2 in the TG suppressed the enhancement of mechanical hypersensitivity in the Incision-Incision group. Administration of CCL2 into the TG enhanced mechanical hypersensitivity in the Sham-Sham, Incision-Sham and Sham-Incision group. Our results suggest that neonatal facial injury accelerates the TG neuronal hyperexcitability following orofacial skin injury in adult in association with Na_v1.8 overexpression via CCL2 signaling, resulting in the enhancement of orofacial incisional pain hypersensitivity in the adulthood.

Sensorial saturation improves infants' procedure-related pain behaviour in the cardiac intensive care unit: A quasi-experimental study

BACKGROUND

Painful procedures are unavoidable when providing critical care to infants in intensive care units. These adverse experiences during infancy can lead to later hyperalgesia and poor neurodevelopmental outcomes. Thus, appropriate interventions are required to relieve infant pain during these procedures.

OBJECTIVES

This study evaluated the effectiveness of sensorial saturation in reducing pain for infants during jugular central venous catheter removal procedures in intensive care units.

METHODS

This study involved a quasi-experimental, repeated-measures design. Data were collected from participants sequentially recruited from April to June 2019 (control period) and July to September 2019 (experimental period). Participants included 78 infants younger than 1 year with congenital heart disease. The control group (n = 38) received a general nursing intervention using swaddling, a common child-care practice that consists of wrapping infants to restrict movements, whereas the experimental group (n = 40) received sensorial saturation using oral sugar, body massage, and verbal interaction. Infants' physiological reactions to procedural pain were measured by changes in heart rate, oxygen saturation, and respiratory rate. Infants' procedural pain and behavioural indicators were measured using the Modified Behavioural Pain Scale. Data were analysed using descriptive statistics, independent t-tests, χ² tests, and repeated-measures analysis of variance.

RESULTS

Compared with the control group, the experimental group had lower heart rates (F = 53.15, p < .001), respiratory rates (F = 15.19, p < .001), and behavioural pain scores (F = 45.21, p < .001), both during and after the procedure.

CONCLUSIONS

Sensorial saturation can be used as a nursing intervention in infants. Given the many invasive procedures that are part of infant clinical care, sensorial saturation may be a safe analgesic alternative. The findings of this study could lead to the development of evidence-based clinical practice guidelines for the nonpharmacological management of acute pain in infants.

Stress-induced changes in nociceptive responding post-surgery in preclinical rodent models

Chronic post-surgical pain affects up to 85% of individuals depending on the type of surgery, the extent of inflammation, tissue and/or nerve damage. Pre-surgical stress is associated with greater pain intensity, prolonged recovery and is one of the main risk factors for the development of chronic post-surgical pain. Clinically valid animal models provide an important means of examining the mechanisms underlying the effects of stress on post-surgical pain and identifying potential novel therapeutic targets. This review discusses the current data from preclinical animal studies examining the effect of stress on post-surgical pain, the potential underlying mechanisms and gaps in the knowledge that require further investigation.

Decreased PPARgamma in the trigeminal spinal subnucleus caudalis due to neonatal injury contributes to incision-induced mechanical allodynia in female rats

Whisker pad skin incision in infancy causes the prolongation of mechanical allodynia after re-incision in adulthood. A recent study also proposed the importance of sex differences in pain signaling in the spinal cord. However, the sex difference in re-incision-induced mechanical allodynia in the orofacial region is not fully understood. In the rats that experienced neonatal injury in the whisker pad skin, the mechanical allodynia in the whisker pad was significantly prolonged after re-incision in adulthood compared to sham injury in infancy. No significant sex differences were observed in the duration of mechanical allodynia. The duration of mechanical allodynia in male rats was shortened by intracisternal administration of minocycline. However, minocycline had no effects on the duration of mechanical allodynia in female rats. In contrast, intracisternal administration of pioglitazone markedly suppressed mechanical allodynia in female rats after re-incision. Following re-incision, the number of peroxisome proliferator-activated receptor gamma (PPARgamma)-positive cells were reduced in the trigeminal spinal subnucleus caudalis (Vc) in female rats that experienced neonatal injury. Immunohistochemical analyses revealed that PPARgamma was predominantly expressed in Vc neurons. Pioglitazone increased the number of PPARgamma-positive Vc neurons in female rats whose whisker pad skin was incised in both infancy and adulthood stages. Pioglitazone also upregulated heme oxygenase 1 and downregulated NR1 subunit in the Vc in female rats after re-incision. Together, PPARgamma signaling in Vc neurons is a female-specific pathway for whisker pad skin incision-induced mechanical allodynia.

Early Life Pain Experience Changes Adult Functional Pain Connectivity in the Rat Somatosensory and the Medial Prefrontal Cortex

Early life pain (ELP) experience alters adult pain behavior and increases injury-induced pain hypersensitivity, but the effect of ELP on adult functional brain connectivity is not known. We have performed continuous local field potential (LFP) recording in the awake adult male rats to test the effect of ELP on functional cortical connectivity related to pain behavior. Primary somatosensory cortex (S1) and medial prefrontal cortex (mPFC) LFPs evoked by mechanical hindpaw stimulation were recorded simultaneously with pain reflex behavior for 10 d after adult incision injury. We show that, after adult injury, sensory evoked S1 LFP δ and γ energy and S1 LFP δ/γ frequency coupling are significantly increased in ELP rats compared with controls. Adult injury also induces increases in S1-mPFC functional connectivity, but this is significantly prolonged in ELP rats, lasting 4 d compared with 1 d in controls. Importantly, the increases in LFP energy and connectivity in ELP rats were directly correlated with increased behavioral pain hypersensitivity. Thus, ELP alters adult brain functional connectivity, both within and between cortical areas involved in sensory and affective dimensions of pain. The results reveal altered brain connectivity as a mechanism underlying the effects of ELP on adult pain perception.SIGNIFICANCE STATEMENT Pain and stress in early life has a lasting impact on pain behavior and may increase vulnerability to chronic pain in adults. Here, we record pain-related cortical activity and simultaneous pain behavior in awake adult male rats previously exposed to pain in early life. We show that functional connectivity within and between the somatosensory cortex and the medial prefrontal cortex (mPFC) is increased in these rats and that these increases are correlated with their behavioral pain hypersensitivity. The results reveal that early life pain (ELP) alters adult brain connectivity, which may explain the impact of childhood pain on adult chronic pain vulnerability.

Sciatic nerve block downregulates the BDNF pathway to alleviate the neonatal incision-induced exaggeration of incisional pain via decreasing microglial activation

Sciatic nerve block is under investigation as a possible therapeutic strategy for neonatal injury-induced exaggeration of pain responses to reinjury. Spinal microglial priming, brain-derived neurotrophic factor (BDNF) and Src homology-2 domain-containing protein tyrosine phosphatase-2 (SHP2) participate in exaggerated incisional pain induced by neonatal incision. However, effects of sciatic nerve block on exacerbated incisional pain and underlying mechanisms remain unclear. Here, we demonstrated that sciatic nerve block alleviates pain hypersensitivity and microglial activation in rats subjected to neonatal incision and adult incision (nIN-IN). Chemogenetic activation or inhibition of spinal microglia attenuates or mimics effects of sciatic nerve block on pain hypersensitivity, respectively. Moreover, α-amino-3-hydroxy- 5-methy- 4-isoxazole propionate (AMPA) receptor subunit GluA1 contributes to the exaggeration of incisional pain. The inhibition of BDNF or SHP2 blocks upregulations of downstream molecules in nIN-IN rats. Knockdown of SHP2 attenuates the increase of GluA1 induced by injection of BDNF in adult rats with only neonatal incision. The inhibition of microglia or ablation of microglial BDNF attenuates upregulations of SHP2 and GluA1. Additionally, sciatic nerve block downregulates the expression of these three molecules. Upregulation of BDNF, SHP2 or AMPA receptor attenuates sciatic nerve block-induced reductions of downstream molecules and pain hypersensitivity. Microglial activation abrogates reductions of these three molecules induced by sciatic nerve block. These results suggest that decreased activation of spinal microglia contributes to beneficial effects of sciatic nerve block on the neonatal incision-induced exaggeration of incisional pain via downregulating BDNF/SHP2/GluA1-containing AMPA receptor signaling. Thus, sciatic nerve block may be a promising therapy.

Long-term alterations in somatosensory functioning in survivors of childhood cancer

ABSTRACT

Cancer and its treatment can have lasting consequences on somatosensation, including pain, which is often underrecognized and undertreated. Research characterizing the impact of cancer on pain and sensory processing in survivors of childhood cancer is scarce. This study aimed to quantify generalized differences in pain and sensory processing in survivors of childhood cancer compared with reference data using a standardized thermal and mechanical quantitative sensory testing (QST) protocol. The association between demographic, clinical (eg, leukemia vs other cancers and treatment exposures), and psychosocial (eg, anxiety and pain catastrophizing) variables and sensitivity to pain and sensory stimuli were also evaluated. Participants were 56 survivors of various types of childhood cancer (52% male, Mage = 13.5 years, SD = 3.2, range = 8-17 years). On average, children were 7 years (SD = 4.1, range = 1.2-16.5) post treatment. Almost all participants (86%) had at least 1 abnormal QST parameter compared with age- and sex-matched reference data; however, few participants self-reported the presence of sensory abnormalities. Generally, participants exhibited reduced sensitivity across the QST parameters examined (Ps < 0.05, ds = 0.40-3.45). A significant minority (45%) also exhibited pain sensitization (P <0.001, d = 0.42). Several risk factors for changes in sensory processing were identified, including current age, history of leukemia, certain treatment exposures (eg, vincristine cumulative dose, major surgery, and bone marrow or stem cell transplant), time off treatment, and higher anxiety and pain catastrophizing scores. Overall, this study demonstrated that somatosensory changes are prevalent in survivors of childhood cancer years after the completion of treatment. Future research is needed to understand long-term implications of altered somatosensation in this complex population.

The Neuroimmune Interface and Chronic Pain Through the Lens of Production Animals

Communication between the central nervous system (CNS) and the immune system has gained much attention for its fundamental role in the development of chronic and pathological pain in humans and rodent models. Following peripheral nerve injury, neuroimmune signaling within the CNS plays an important role in the pathophysiological changes in pain sensitivity that lead to chronic pain. In production animals, routine husbandry procedures such as tail docking and castration, often involve some degree of inflammation and peripheral nerve injury and consequently may lead to chronic pain. Our understanding of chronic pain in animals is limited by the difficulty in measuring this pathological pain state. In light of this, we have reviewed the current understanding of chronic pain in production animals. We discuss our ability to measure pain and the implications this has on animal welfare and production outcomes. Further research into the neuroimmune interface in production animals will improve our fundamental understanding of chronic pain and better inform human clinical pain management and animal husbandry practices and interventions.

Genetic and epigenetic mechanisms influencing acute to chronic postsurgical pain transitions in pediatrics: Preclinical to clinical evidence

Background

Chronic postsurgical pain (CPSP) in children remains an important problem with no effective preventive or therapeutic strategies. Recently, genomic underpinnings explaining additional interindividual risk beyond psychological factors have been proposed.

Aims

We present a comprehensive review of current preclinical and clinical evidence for genetic and epigenetic mechanisms relevant to pediatric CPSP.

Methods

Narrative review.

Results

Animal models are relevant to translational research for unraveling genomic mechanisms. For example, Cacng2, p2rx7, and bdnf mutant mice show altered mechanical hypersensitivity to injury, and variants of the same genes have been associated with CPSP susceptibility in humans; similarly, differential DNA methylation (H1SP) and miRNAs (miR-96/7a) have shown translational implications. Animal studies also suggest that crosstalk between neurons and immune cells may be involved in nociceptive priming observed in neonates. In children, differential DNA methylation in regulatory genomic regions enriching GABAergic, dopaminergic, and immune pathways, as well as polygenic risk scores for enhanced prediction of CPSP, have been described. Genome-wide studies in pediatric CPSP are scarce, but pathways identified by adult gene association studies point to potential common mechanisms.

Conclusions

Bench-to-bedside genomics research in pediatric CPSP is currently limited. Reverse translational approaches, use of other -omics, and inclusion of pediatric/CPSP endophenotypes in large-scale biobanks may be potential solutions. Time of developmental vulnerability and longitudinal genomic changes after surgery warrant further investigation. Emergence of promising precision pain management strategies based on gene editing and epigenetic programing emphasize need for further research in pediatric CPSP-related genomics.

The development of descending serotonergic modulation of the spinal nociceptive network: a life span perspective

The nociceptive network, responsible for transmission of nociceptive signals that generate the pain experience, is not fully developed at birth. Descending serotonergic modulation of spinal nociception, an important part of the pain network, undergoes substantial postnatal maturation and is suggested to be involved in the altered pain response observed in human newborns. This review summarizes preclinical data of the development of descending serotonergic modulation of the spinal nociceptive network across the life span, providing a comprehensive background to understand human newborn pain experience and treatment. Sprouting of descending serotonergic axons, originating from the rostroventral medulla, as well as changes in receptor function and expression take place in the first postnatal weeks of rodents, corresponding to human neonates in early infancy. Descending serotonergic modulation switches from facilitation in early life to bimodal control in adulthood, masking an already functional 5-HT inhibitory system at early ages. Specifically the 5-HT₃ and 5-HT₇ receptors seem distinctly important for pain facilitation at neonatal and early infancy, while the 5-HT_1a, 5-HT_1b, and 5-HT₂ receptors mediate inhibitory effects at all ages. Analgesic therapy that considers the neurodevelopmental phase is likely to result in a more targeted treatment of neonatal pain and may improve both short- and long-term effects. IMPACT: The descending serotonergic system undergoes anatomical changes from birth to early infancy, as its sprouts and descending projections increase and the dorsal horn innervation pattern changes. Descending serotonergic modulation from the rostral ventral medulla switches from facilitation in early life via the 5-HT₃ and 5-HT₇ receptors to bimodal control in adulthood. A functional inhibitory serotonergic system mainly via 5-HT_1a, 5-HT_1b, and 5-HT_2a receptors at the spinal level exists already at the neonatal phase but is masked by descending facilitation.

Understanding early-life pain and its effects on adult human and animal emotionality: Translational lessons from rodent and zebrafish models

Critical for organismal survival, pain evokes strong physiological and behavioral responses in various sentient species. Clinical and preclinical (animal) studies markedly increase our understanding of biological consequences of developmental (early-life) adversity, as well as acute and chronic pain. However, the long-term effects of early-life pain exposure on human and animal emotional responses remain poorly understood. Here, we discuss experimental models of nociception in rodents and zebrafish, and summarize mounting evidence of the role of early-life pain in shaping emotional traits later in life. We also call for further development of animal models to probe the impact of early-life pain exposure on behavioral traits, brain disorders and novel therapeutic treatments.

Developmental Mechanisms of CPSP: Clinical Observations and Translational Laboratory Evaluations

Understanding mechanisms that underly the transition from acute to chronic pain and identifying potential targets for preventing or minimizing this progression have specific relevance for chronic postsurgical pain (CPSP). Though it is clear that multiple psychosocial, family, and environmental factors may influence CPSP, this review will focus on parallels between clinical observations and translational laboratory studies investigating the acute and long-term effects of surgical injury on nociceptive pathways. This includes data related to alterations in sensitivity at different points along nociceptive pathways from the periphery to the brain; age- and sex-dependent mechanisms underlying the transition from acute to persistent pain; potential targets for preventive interventions; and the impact of prior surgical injury. Ongoing preclinical studies evaluating age- and sex-dependent mechanisms will also inform comparative efficacy and preclinical safety assessments of potential preventive pharmacological interventions aimed at reducing the risk of CPSP. In future clinical studies, more detailed and longitudinal peri-operative phenotyping with patient- and parent-reported chronic pain core outcomes, alongside more specialized evaluations of somatosensory function, modulation, and circuitry, may enhance understanding of individual variability in postsurgical pain trajectories and improve recognition and management of CPSP.

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na+ and Ca2+ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K+ channels. Because they display limited central side effects, peripherally restricted Na+ and Ca2+ channel blockers and K+ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Nav1.3, Nav1.7, Nav1.8, Cav3.2, and HCN2 and activators of Kv7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K+ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na+ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca2+ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing "pain" as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Amygdalar Corticotropin-Releasing Factor Signaling Is Required for Later-Life Behavioral Dysfunction Following Neonatal Pain

Neonatal pain such as that experienced by infants in the neonatal intensive care unit is known to produce later-life dysfunction including heightened pain sensitivity and anxiety, although the mechanisms remain unclear. Both chronic pain and stress in adult organisms are known to influence the corticotropin-releasing factor (CRF) system in the Central Nucleus of the Amygdala, making this system a likely candidate for changes following neonatal trauma. To examine this, neonatal rats were subjected to daily pain, non-painful handling or left undisturbed for the first week of life. Beginning on postnatal day, 24 male and female rats were subjected to a 4-day fear conditioning and sensory testing protocol. Some subjects received intra-amygdalar administration of either Vehicle, the CRF receptor 1 (CRF₁) receptor antagonist Antalarmin, or the CRF receptor 2 (CRF₂) receptor antagonist Astressin 2B prior to fear conditioning and somatosensory testing, while others had tissue collected following fear conditioning and CRF expression in the CeA and BLA was assessed using fluorescent in situ hybridization. CRF₁ antagonism attenuated fear-induced hypersensitivity in neonatal pain and handled rats, while CRF₂ antagonism produced a general antinociception. In addition, neonatal pain and handling produced a lateralized sex-dependent decrease in CRF expression, with males showing a diminished number of CRF-expressing cells in the right CeA and females showing a similar reduction in the number of CRF-expressing cells in the left BLA compared to undisturbed controls. These data show that the amygdalar CRF system is a likely target for alleviating dysfunction produced by early life trauma and that this system continues to play a major role in the lasting effects of such trauma into the juvenile stage of development.

A Painful Beginning: Early Life Surgery Produces Long-Term Behavioral Disruption in the Rat

Early life surgery produces peripheral nociceptive activation, inflammation, and stress. Early life nociceptive input and inflammation have been shown to produce long-term processing changes that are not restricted to the dermatome of injury. Additionally stress has shown long-term effects on anxiety, depression, learning, and maladaptive behaviors including substance abuse disorder and we hypothesized that early life surgery would have long-term effects on theses complex behaviors in later life. In this study surgery in the rat hindpaw was performed to determine if there are long-term effects on anxiety, depression, audiovisual attention, and opioid reward behaviors. Male animals received paw incision surgery and anesthesia or anesthesia alone (sham) at postnatal day 6. At 10 weeks after surgery, open field center zone entries were decreased, a measure of anxiety (n = 20) (P = 0.03) (effect size, Cohen's d = 0.80). No difference was found in the tail suspension test as a measure of depression. At 16-20 weeks, attentional performance in an operant task was similar between groups at baseline and decreased with audiovisual distraction in both groups (P < 0.001) (effect size, η² = 0.25), but distraction revealed a persistent impairment in performance in the surgery group (n = 8) (P = 0.04) (effect size, η² = 0.13). Opioid reward was measured using heroin self-administration at 16-24 weeks. Heroin intake increased over time in both groups during 24-h free access (P < 0.001), but was greater in the surgery group (P = 0.045), with a significant interaction between time and treatment (P < 0.001) (effect size, Cohen f ² = 0.36). These results demonstrate long-term disruptions in complex behaviors from surgical incision under anesthesia. Future studies to explore sex differences in early life surgery and the attendant peripheral neuronal input, stress, and inflammation will be valuable to understand emerging learning deficits, anxiety, attentional dysfunction, and opioid reward and their mechanisms. This will be valuable to develop optimal approaches to mitigate the long-term effects of surgery in early life.

Methadone effectively attenuates acute and long-term consequences of neonatal repetitive procedural pain in a rat model

BACKGROUND

Painful procedures in early life cause acute pain and can alter pain processing at a spinal level lasting into adulthood. Current methods of analgesia seem unable to prevent both acute and long-term hypersensitivity associated with neonatal pain. The current study aims to prevent acute and long-term hypersensitivity associated with neonatal procedural pain using methadone analgesia in rat pups.

METHODS

Sprague-Dawley rat pups received either methadone (1 mg/kg) or saline prior to repetitive needle pricks into the left hind paw from the day of birth (postnatal day (P)0) to P7. Control littermates received a tactile stimulus. Mechanical sensitivity was assessed during the neonatal period (P0-P7), from weaning to adulthood (3-7 weeks) and following surgical re-injury of the same dermatome in adulthood.

RESULTS

Methadone administration completely reversed acute hypersensitivity from P0 to P7. In addition, neonatal methadone analgesia prevented prolonged hypersensitivity after re-injury in adulthood, without affecting sensitivity from weaning to adulthood.

CONCLUSIONS

The current study shows that neonatal methadone analgesia can attenuate acute as well as long-term hypersensitivity associated with neonatal procedural pain in a rat model.

IMPACT

Methadone treatment attenuates acute and long-term hypersensitivity associated with neonatal pain in a rat model. Clinical effectiveness studies are urgently warranted to assess acute and long-term analgesic effectivity of methadone.

Early Life Nociception is Influenced by Peripheral Growth Hormone Signaling

A number of cellular systems work in concert to modulate nociceptive processing in the periphery, but the mechanisms that regulate neonatal nociception may be distinct compared with adults. Our previous work indicated a relationship between neonatal hypersensitivity and growth hormone (GH) signaling. Here, we explored the peripheral mechanisms by which GH modulated neonatal nociception under normal and injury conditions (incision) in male and female mice. We found that GH receptor (GHr) signaling in primary afferents maintains a tonic inhibition of peripheral hypersensitivity. After injury, a macrophage dependent displacement of injury-site GH was found to modulate neuronal transcription at least in part via serum response factor (SRF) regulation. A single GH injection into the injured hindpaw muscle effectively restored available GH signaling to neurons and prevented acute pain-like behaviors, primary afferent sensitization, and neuronal gene expression changes. GH treatment also inhibited long-term somatosensory changes observed after repeated peripheral insult. Results may indicate a novel mechanism of neonatal nociception.SIGNIFICANCE STATEMENT Although it is noted that mechanisms of pain development in early life are unique compared with adults, little research focuses on neonatal-specific peripheral mechanisms of nociception. This gap is evident in the lack of specialized care for infants following an injury including surgeries. This report evaluates how distinct cellular systems in the periphery including the endocrine, immune and nervous systems work together to modulate neonatal-specific nociception. We uncovered a novel mechanism by which muscle injury induces a macrophage-dependent sequestration of peripheral growth hormone (GH) that effectively removes its normal tonic inhibition of neonatal nociceptors to promote acute pain-like behaviors. Results indicate a possible new strategy for treatment of neonatal postsurgical pain.

Altered sensory innervation and pain hypersensitivity in a model of young painful arthritic joints: short- and long-term effects

BACKGROUND

Early life experience can cause long-term alterations in the nociceptive processes underlying chronic pain, but the consequences of early life arthritic joint inflammation upon the sensory innervation of the joint is not known. Here, we measure pain sensitivity and sensory innervation in a young, juvenile and adult rodent model of arthritic joints and test the consequences of joint inflammation in young animals upon adult arthritic pain and joint innervation.

METHODS

Unilateral ankle joint injections of complete Freund's adjuvant (CFA) (6-20 µl) were performed in young, postnatal day (P)8, adolescent (P21) and adult (P40) rats. A separate cohort of animals were injected at P8, and again at P40. Hindpaw mechanical sensitivity was assessed using von Frey monofilaments (vF) for 10 days. Nerve fibres were counted in sections through the ankle joint immunostained for calcitonin gene-related peptide (CGRP) and neurofilament 200 kDa (NF200).

RESULTS

Ankle joint CFA injection increased capsular width at all ages. Significant mechanical pain hypersensitivity and increased number of joint CGRP + ve sensory fibres occurred in adolescent and adult, but not young, rats. Despite the lack of acute reaction, joint inflammation at a young age resulted in significantly increased pain hypersensitivity and CGRP+ fibre counts when the rats were re-inflamed as adults.

CONCLUSIONS

Joint inflammation increases the sensory nociceptive innervation and induces acute pain hypersensitivity in juvenile and adult, but not in young rats. However, early life joint inflammation 'primes' the joint such that adult inflammatory pain behaviour and nociceptive nerve endings in the joint are significantly increased. Early life joint inflammation may be an important factor in the generation and maintenance of chronic arthritic pain.

Subclinical post-traumatic stress symptomology and brain structure in youth with chronic headaches

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ACEs and PTSS did not significantly differ between patients and healthy controls.

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Patients demonstrated greater corticolimbic connectivity compared to controls.

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Greater PTSS and less corticolimbic connectivity increased headache frequency.

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Less corticolimbic connectivity may indicate greater disease progression.

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Patients may be more vulnerable to the effects of PTSS compared to controls.

Background/aims

Post-traumatic stress symptoms (PTSS) and chronic pain often co-occur at high rates in youth. PTSS may alter brain structure thereby contributing to headache chronicity. This study examined whether PTSS and altered limbic circuitry were associated with headache frequency in youth.

Methods

Thirty youth aged 10–18 years with chronic headaches and 30 age- and sex-matched controls underwent a 3T MRI scan. Volumes of the hippocampus and amygdala were obtained from T1-weighted images. Mean fractional anisotropy (FA, an index of white matter structure) axial and radial diffusivity values of the cingulum and uncinate fasciculus were extracted from diffusion-weighted images. Youth reported on their headaches daily, for one-month, and self-reported pubertal status, emotion regulation, adverse childhood experiences (ACEs) and PTSS using validated measures. Volumes of the hippocampus and amygdala and diffusivity values of the cingulum and uncinate were compared between patients and controls. Hierarchical linear regressions were used to examine the association between PTSS, subcortical volumes and/or diffusivity values and headache frequency.

Results

Mean FA values of the cingulum were higher in patients compared to controls (P = 0.02, Cohen’s d = 0.69). Greater PTSS (P = 0.04), smaller amygdala volumes (P = 0.01) and lower FA of the cingulum (P = 0.04) were associated with greater headache frequency, after accounting for age, puberty, pain duration, emotion regulation, and ACEs (Adjusted R² ≥ 0.15). Headache frequency was associated with increases in radial diffusivity (P = 0.002, Adjusted R² = 0.59), as opposed to axial diffusivity (n.s.).

Conclusions

PTSS, smaller amygdalar volume, and poorer cingulum structural connectivity were associated with headache frequency in youth, and may underlie headache chronicity.

Single-nucleus characterization of adult mouse spinal dynorphin-lineage cells and identification of persistent transcriptional effects of neonatal hindpaw incision

Neonatal tissue damage can have long-lasting effects on nociceptive processing in the central nervous system, which may reflect persistent injury-evoked alterations to the normal balance between synaptic inhibition and excitation in the spinal dorsal horn. Spinal dynorphin-lineage (pDyn) neurons are part of an inhibitory circuit which limits the flow of nociceptive input to the brain and is disrupted by neonatal tissue damage. To identify the potential molecular underpinnings of this disruption, an unbiased single-nucleus RNAseq analysis of adult mouse spinal pDyn cells characterized this population in depth and then identified changes in gene expression evoked by neonatal hindpaw incision. The analysis revealed 11 transcriptionally distinct subpopulations (ie, clusters) of dynorphin-lineage cells, including both inhibitory and excitatory neurons. Investigation of injury-evoked differential gene expression identified 15 genes that were significantly upregulated or downregulated in adult pDyn neurons from neonatally incised mice compared with naive littermate controls, with both cluster-specific and pan-neuronal transcriptional changes observed. Several of the identified genes, such as Oxr1 and Fth1 (encoding ferritin), were related to the cellular stress response. However, the relatively low number of injury-evoked differentially expressed genes also suggests that posttranscriptional regulation within pDyn neurons may play a key role in the priming of developing nociceptive circuits by early-life injury. Overall, the findings reveal novel insights into the molecular heterogeneity of a key population of dorsal horn interneurons that has previously been implicated in the suppression of mechanical pain and itch.

A role for neuroimmune signaling in a rat model of Gulf War Illness-related pain

More than a quarter of veterans of the 1990-1991 Persian Gulf War suffer from Gulf War Illness (GWI), a chronic, multi-symptom illness that commonly includes musculoskeletal pain. Exposure to a range of toxic chemicals, including sarin nerve agent, are a suspected root cause of GWI. Moreover, such chemical exposures induce a neuroinflammatory response in rodents, which has been linked to several GWI symptoms in rodents and veterans with GWI. To date, a neuroinflammatory basis for pain associated with GWI has not been investigated. Here, we evaluated development of nociceptive hypersensitivity in a model of GWI. Male Sprague Dawley rats were treated with corticosterone in the drinking water for 7 days, to mimic high physiological stress, followed by a single injection of the sarin nerve agent surrogate, diisopropyl fluorophosphate. These exposures alone were insufficient to induce allodynia. However, an additional sub-threshold challenge (a single intramuscular injection of pH 4 saline) induced long-lasting, bilateral allodynia. Such allodynia was associated with elevation of markers for activated microglia/macrophages (CD11b) and astrocytes/satellite glia (GFAP) in the lumbar dorsal spinal cord and dorsal root ganglia (DRG). Additionally, Toll-like receptor 4 (TLR4) mRNA was elevated in the lumbar dorsal spinal cord, while IL-1β and IL-6 were elevated in the lumbar dorsal spinal cord, DRG, and gastrocnemius muscle. Demonstrating a casual role for such neuroinflammatory signaling, allodynia was reversed by treatment with either minocycline, the TLR4 inhibitor (+)-naltrexone, or IL-10 plasmid DNA. Together, these results point to a role for neuroinflammation in male rats in the model of musculoskeletal pain related to GWI. Therapies that alleviate persistent immune dysregulation may be a strategy to treat pain and other symptoms of GWI.

Neonatal complete Freund's adjuvant-induced inflammation does not induce or alter hyperalgesic priming or alter adult distributions of C-fibre dorsal horn innervation

Introduction:

Inflammation during the neonatal period can exacerbate pain severity following reinjury in adulthood. This is driven by alterations in the postnatal development of spinal and supraspinal nociceptive circuitry. However, the contribution of alterations in peripheral nociceptor function remains underexplored.

Objectives:

We examined whether neonatal complete Freund's adjuvant (CFA)-induced inflammation induced or altered adult development of hyperalgesic priming (inflammation-induced plasticity in nonpeptidergic C fibres) or altered postnatal reorganization of calcitonin gene-related peptide (CGRP)-expressing and isolectin B4 (IB4)-binding C fibres in the spinal dorsal horn (DH).

Methods:

After intraplantar injection of CFA at postnatal day (P) 1, we assessed mechanical thresholds in adult (P60) rats before and after intraplantar carrageenan. One week later, intraplantar PGE₂-induced hypersensitivity persisting for 4 hours was deemed indicative of hyperalgesic priming. CGRP expression and IB4 binding were examined in adult rat DH after CFA.

Results:

P1 CFA did not alter baseline adult mechanical thresholds, nor did it change the extent or duration of carrageenan-induced hypersensitivity. However, this was slower to resolve in female than in male rats. Rats that previously received carrageenan but not saline were primed, but P1 hind paw CFA did not induce or alter hyperalgesic priming responses to PGE₂. In addition, CFA on P1 or P10 did not alter intensity or patterns of CGRP or IB4 staining in the adult DH.

Conclusion:

Complete Freund's adjuvant-induced inflammation during a critical period of vulnerability to injury during early postnatal development does not induce or exacerbate hyperalgesic priming or alter the broad distribution of CGRP-expressing or IB4-binding afferent terminals in the adult dorsal horn.

Spinal interleukin-6 contributes to central sensitisation and persistent pain hypersensitivity in a model of juvenile idiopathic arthritis

Pain is the most debilitating symptom in juvenile idiopathic arthritis. As pain correlates poorly to the extent of joint pathology, therapies that control joint inflammation are often inadequate as analgesics. We test the hypothesis that juvenile joint inflammation leads to sensitisation of nociceptive circuits in the central nervous system, which is maintained by cytokine expression in the spinal cord. Here, transient joint inflammation was induced in postnatal day (P)21 and P40 male Sprague-Dawley rats with a single intra-articular ankle injection of complete Freund's adjuvant. Hindpaw mechanical pain sensitivity was assessed using von Frey hair and weight bearing tests. Spinal neuron activity was measured using in vivo extracellular recording and immunohistochemistry. Joint and spinal dorsal horn TNFα, IL1β and IL6 protein expression was quantified using western blotting. We observed greater mechanical hyperalgesia following joint inflammation in P21 compared to P40 rats, despite comparable duration of swelling and joint inflammatory cytokine levels. This is mirrored by spinal neuron hypersensitivity, which also outlasted the duration of active joint inflammation. The cytokine profile in the spinal cord differed at the two ages: prolonged upregulation of spinal IL6 was observed in P21, but not P40 rats. Finally, spinal application of anti-IL-6 antibody (30 ng) reduced the mechanical hyperalgesia and neuronal activation. Our results indicate that persistent upregulation of pro-inflammatory cytokines in the spinal dorsal horn is associated with neuronal sensitisation and mechanical hyperalgesia in juvenile rats, beyond the progress of joint pathology. In addition, we provide proof of concept that spinal IL6 is a key target for treating persistent pain in JIA.

The Development of Adolescent Chronic Pain following Traumatic Brain Injury and Surgery: The Role of Diet and Early Life Stress

Pain is evolutionarily necessary for survival in that it reduces tissue damage by signaling the body to respond to a harmful stimulus. However, in many circumstances, acute pain becomes chronic, and this is often dysfunctional. Adolescent chronic pain is a growing epidemic with an unknown etiology and limited effective treatment options. Given that the relationship between acute pain and chronic pain is not straightforward, there is a need to better understand the factors that contribute to the chronification of pain. Since early life factors are critical to a variety of outcomes in the developmental and adolescent periods, they pose promise as potential mechanisms that may underlie the transition from acute to chronic pain. This review examines two early life factors: poor diet and adverse childhood experiences (ACEs); they may increase susceptibility to the development of chronic pain following surgical procedures or traumatic brain injury (TBI). Beyond their high prevalence, surgical procedures and TBI are ideal models to prospectively understand mechanisms underlying the transition from acute to chronic pain. Common themes that emerged from the examination of poor diet and ACEs as mechanisms underlying this transition included: prolonged inflammation and microglia activation leading to sensitization of the pain system, and stress-induced alterations to hypothalamic-pituitary-adrenal axis function, where cortisol is likely playing a role in the development of chronic pain. These areas provide promising targets for interventions, the development of diagnostic biomarkers, and suggest that biological treatment strategies should focus on regulating the neuroinflammatory and stress responses in an effort to modulate and prevent the development of chronic pain.

The effect of early burn injury on sensitivity to future painful stimuli in dairy heifers

Animals that experience painful procedures as neonates are more sensitive to pain later in life. We evaluated whether disbudding with a heated iron at 3 (n = 12), 35 (n = 9), or 56 (n = 20) d of age affected heifers’ pain responses to vaccine injections at 11 mo of age. Heifers responded to the injection procedure with struggling and changes in eye temperature and heart rate variability compared to a sham procedure the day before, and still had a heightened response 6 d later, regardless of disbudding age. However, some heart rate variability indices suggested increased sympathetic dominance in heifers disbudded at 35 d, compared to the other 2 age groups, independent of the injection procedure. We also found that heifers disbudded at 3 or 35 d had a higher mean heart rate after the injection procedure compared to those disbudded at 56 d. We conclude that: (1) heifers find injections aversive; and (2) there is some evidence that disbudding age influences autonomic nervous system activity later in life.

Neonatal Injury Evokes Persistent Deficits in Dynorphin Inhibitory Circuits within the Adult Mouse Superficial Dorsal Horn

Neonatal tissue damage induces long-term deficits in inhibitory synaptic transmission within the spinal superficial dorsal horn (SDH) that include a reduction in primary afferent-evoked, feedforward inhibition onto adult projection neurons. However, the subpopulations of mature GABAergic interneurons which are compromised by early-life injury have yet to be identified. The present research illuminates the persistent effects of neonatal surgical injury on the function of inhibitory SDH interneurons derived from the prodynorphin (DYN) lineage, a population that synapses directly onto lamina I spinoparabrachial neurons and is known to suppress mechanical pain and itch in adults. The results demonstrate that hindpaw incision at postnatal day 3 (P3) significantly decreased the strength of primary afferent-evoked glutamatergic drive onto DYN neurons within the adult mouse SDH while increasing the appearance of afferent-evoked inhibition onto the same population. Neonatal injury also dampened the intrinsic membrane excitability of mature DYN neurons, and reduced their action potential discharge in response to sensory input, compared with naive littermate controls. Furthermore, P3 incision decreased the efficacy of inhibitory DYN synapses onto adult spinoparabrachial neurons, which reflected a prolonged reduction in the probability of GABA release. Collectively, the data suggest that early-life tissue damage may persistently constrain the ability of spinal DYN interneurons to limit ascending nociceptive transmission to the adult brain. This is predicted to contribute to the loss of feedforward inhibition onto mature projection neurons, and the "priming" of nociceptive circuits in the developing spinal cord, following injuries during the neonatal period.SIGNIFICANCE STATEMENT Neonatal injury has lasting effects on pain processing in the adult CNS, including a reduction in feedforward inhibition onto ascending projection neurons in the spinal dorsal horn. While it is clear that spinal GABAergic interneurons are comprised of multiple subpopulations that play distinct roles in somatosensation, the identity of those interneurons which are compromised by tissue damage during early life remains unknown. Here we document persistent deficits in spinal inhibitory circuits involving dynorphin-lineage interneurons previously implicated in gating mechanical pain and itch. Notably, neonatal injury reduced the strength of dynorphin-lineage inhibitory synapses onto mature lamina I spinoparabrachial neurons, a major output of the spinal nociceptive network, which could contribute to the priming of pain pathways by early tissue damage.

The development of pain circuits and unique effects of neonatal injury

Pain is a necessary sensation that prevents further tissue damage, but can be debilitating and detrimental in daily life under chronic conditions. Neuronal activity strongly regulates the maturation of the somatosensory system, and aberrant sensory input caused by injury or inflammation during critical periods of early postnatal development can have prolonged, detrimental effects on pain processing. This review will outline the maturation of neuronal circuits responsible for the transmission of nociceptive signals and the generation of pain sensation-involving peripheral sensory neurons, the spinal cord dorsal horn, and brain-in addition to the influences of the neuroimmune system on somatosensation. This summary will also highlight the unique effects of neonatal tissue injury on the maturation of these systems and subsequent consequences for adult somatosensation. Ultimately, this review emphasizes the need to account for age as an independent variable in basic and clinical pain research, and importantly, to consider the distinct qualities of the pediatric population when designing novel strategies for pain management.

Implementation Effectiveness of a Parent-Directed YouTube Video ("It Doesn't Have To Hurt") on Evidence-Based Strategies to Manage Needle Pain: Descriptive Survey Study

BACKGROUND

Despite the availability of high-quality evidence and clinical practice guidelines for the effective management of pediatric pain, this evidence is rarely used in practice for managing children's pain from needle procedures such as vaccinations. Parents are generally unaware of pain management strategies they can use with their children.

OBJECTIVE

This study aimed to develop, implement, and evaluate the implementation effectiveness of a parent-directed YouTube video on evidence-based strategies to manage needle pain in children.

METHODS

This was a descriptive study. Analytics were extracted from YouTube to describe video reach. A Web-based survey was used to seek parent and health care professional (HCP) feedback about the video. The 2-minute 18-second video was launched on YouTube on November 4, 2013. In the video, a 4-year-old girl tells parents what they should and should not do to help needles hurt less. The key evidence-based messages shared in the video were distraction, deep breathing, and topical anesthetic creams. A group of parents (n=163) and HCPs (n=278) completed the Web-based survey. Measures of reach included number of unique views, country where the video was viewed, sex of the viewer, and length of watch time. The Web-based survey assessed implementation outcomes of the video, such as acceptability, appropriateness, penetration, and adoption.

RESULTS

As of November 4, 2018 (5 years after launch), the video had 237,132 unique views from 182 countries, with most viewers watching an average of 55.1% (76/138 seconds) of the video. Overall, both parents and HCPs reported strong acceptance of the video (ie, they liked the video, found it helpful, and felt more confident) and reported significant improvements in plans to use distraction, deep breathing, and topical anesthetic creams.

CONCLUSIONS

This parent-directed YouTube video was an acceptable and appropriate way to disseminate evidence about the procedure of pain management to a large number of parents.

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.

Interleukin-1 receptor antagonist ameliorates the pain hypersensitivity, spinal inflammation and oxidative stress induced by systemic lipopolysaccharide in neonatal rats

Perinatal inflammation-induced reduction in pain threshold may alter pain sensitivity to hyperalgesia or allodynia which may persist into adulthood. In this study, we investigated the anti-inflammatory protective effect of interleukin-1 receptor antagonist (IL-1ra), an anti-inflammatory cytokine, on systemic lipopolysaccharide (LPS)-induced spinal cord inflammation and oxidative stress, thermal hyperalgesia, and mechanical allodynia in neonatal rats. Intraperitoneal (i.p.) injection of LPS (2 mg/kg) or sterile saline was performed in postnatal day 5 (P5) rat pups, and IL-1ra (100 mg/kg) or saline was administered (i.p.) 5 min after LPS injection. Pain reflex behavior, spinal cord inflammation and oxidative stress were examined 24 h after LPS administration. Systemic LPS exposure led to a reduction of tactile threshold in the von Frey filament tests (mechanical allodynia) and pain response latency in the tail-flick test (thermal hyperalgesia) of P6 neonatal rats. Spinal cord inflammation was indicated by the increased numbers of activated glial cells including microglia (Iba1+) and astrocytes (GFAP+), and elevated levels of pro-inflammatory cytokine interleukin-1β (IL-1β), cyclooxygenase-2 (COX-2), and prostaglandin E2 (PGE2) 24 h after LPS treatment. LPS treatment induced spinal oxidative stress as evidenced by the increase in thiobarbituric acid reactive substances (TBARS) content in the spinal cord. LPS exposure also led to a significant increase in oligodendrocyte lineage population (Olig2+) and mature oligodendrocyte cells (APC+) in the neonatal rat spinal cord. IL-1ra treatment significantly reduced LPS-induced effects including hyperalgesia, allodynia, the increased number of activated microglia, astrocytes and oligodendrocytes, and elevated levels of IL-1β, COX-2, PGE2, and lipid peroxidation (TBARS) in the neonatal rat spinal cord. These data suggest that IL-1ra provides a protective effect against the development of pain hypersensitivity, spinal cord inflammation and oxidative stress in the neonatal rats following LPS exposure, which may be associated with the blockade of LPS-induced pro-inflammatory cytokine IL-1β.

Early life vincristine fails to prime developing pain pathways

Early life administration of vincristine (VNC), commonly used to treat pediatric leukemia, evokes peripheral neuropathy and mechanical pain hypersensitivity in rats that lasts into adolescence. However, the degree to which VNC-evoked neuropathic pain persists throughout adulthood has yet to be examined. It also remains unclear if pediatric VNC exposure can 'prime' developing nociceptive pathways and thereby exacerbate chronic pain following subsequent trauma later in life. To address these issues, rats received five total doses of VNC (60 μg/kg; or vehicle) on postnatal days (P) 11, 13, 17, 19 and 21 followed by a hindpaw surgical incision during adulthood. In addition, in order to model the clinical scenario where cancer relapse necessitates another round of chemotherapy, separate groups of rats that had been treated with VNC (or vehicle) as neonates were subsequently administered VNC as adults (five injections at 100 μg/kg). Intraepidermal nerve fiber density and baseline mechanical pain sensitivity were similar between the neonatal VNC and vehicle-treated littermate controls at 13-15 weeks of age, suggesting that the peripheral neuropathy, and resulting chronic pain, had resolved by adulthood. Importantly, there was no significant overall effect of early life VNC on the severity of post-operative pain following adult incision. Similarly, prior VNC exposure did not significantly influence the degree of mechanical pain hypersensitivity produced by adult VNC treatment. Collectively, these findings suggest that early life VNC administration does not increase the susceptibility to develop chronic pain as adults.

Does epigenetic 'memory' of early-life stress predispose to chronic pain in later life? A potential role for the stress regulator FKBP5

Animal behaviours are affected not only by inherited genes but also by environmental experiences. For example, in both rats and humans, stressful early-life events such as being reared by an inattentive mother can leave a lasting trace and affect later stress response in adult life. This is owing to a chemical trace left on the chromatin attributed to so-called epigenetic mechanisms. Such an epigenetic trace often has consequences, sometimes long-lasting, on the functioning of our genes, thereby allowing individuals to rapidly adapt to a new environment. One gene under such epigenetic control is FKBP5, the gene that encodes the protein FKPB51, a crucial regulator of the stress axis and a significant driver of chronic pain states. In this article, we will discuss the possibility that exposure to stress could drive the susceptibly to chronic pain via epigenetic modifications of genes within the stress axis such as FKBP5. The possibility that such modifications, and therefore, the susceptibility to chronic pain, could be transmitted across generations in mammals and whether such mechanisms may be evolutionarily conserved across phyla will also be debated. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Neonatal Injury Alters Sensory Input and Synaptic Plasticity in GABAergic Interneurons of the Adult Mouse Dorsal Horn

Neonatal tissue injury disrupts the balance between primary afferent-evoked excitation and inhibition onto adult spinal projection neurons. However, whether this reflects cell-type-specific alterations at synapses onto ascending projection neurons, or rather is indicative of global changes in synaptic signaling across the mature superficial dorsal horn (SDH), remains unknown. Therefore the present study investigated the effects of neonatal surgical injury on primary afferent synaptic input to adult mouse SDH interneurons using in vitro patch-clamp techniques. Hindpaw incision at postnatal day (P)3 significantly diminished total primary afferent-evoked glutamatergic drive to adult Gad67-GFP and non-GFP neurons, and reduced their firing in response to sensory input, in both males and females. Early tissue damage also shaped the relative prevalence of monosynaptic A- versus C-fiber-mediated input to mature GABAergic neurons, with an increased prevalence of Aβ- and Aδ-fiber input observed in neonatally-incised mice compared with naive littermate controls. Paired presynaptic and postsynaptic stimulation at an interval that exclusively produced spike timing-dependent long-term potentiation (t-LTP) in projection neurons predominantly evoked NMDAR-dependent long-term depression in naive Gad67-GFP interneurons. Meanwhile, P3 tissue damage enhanced the likelihood of t-LTP generation at sensory synapses onto the mature GABAergic population, and increased the contribution of Ca²⁺-permeable AMPARs to the overall glutamatergic response. Collectively, the results indicate that neonatal injury suppresses sensory drive to multiple subpopulations of interneurons in the adult SDH, which likely represents one mechanism contributing to reduced feedforward inhibition of ascending projection neurons, and the priming of developing pain pathways, following early life trauma.SIGNIFICANCE STATEMENT Mounting clinical and preclinical evidence suggests that neonatal tissue damage can result in long-term changes in nociceptive processing within the CNS. Although recent work has demonstrated that early life injury weakens the ability of sensory afferents to evoke feedforward inhibition of adult spinal projection neurons, the underlying circuit mechanisms remain poorly understood. Here we demonstrate that neonatal surgical injury leads to persistent deficits in primary afferent drive to both GABAergic and presumed glutamatergic neurons in the mature superficial dorsal horn (SDH), and modifies activity-dependent plasticity at sensory synapses onto the GABAergic population. The functional denervation of spinal interneurons within the mature SDH may contribute to the "priming" of developing pain pathways following early life injury.

Long-term effects of neonatal pain

Pain experienced during neonatal intensive care management can influence neurodevelopmental outcome and the somatosensory and/or emotional components of pain response in later life. Alterations in biological factors (e.g. peripheral and central somatosensory function and modulation, brain structure and connectivity) and psychosocial factors (e.g. gender, coping style, mood, parental response) that influence pain have been identified in children and young adults born very preterm or extremely preterm. Earlier gestational age at birth and cumulative pain exposure from tissue-breaking procedures and/or neonatal surgery influence the degree of change. In neonatal rodents, repeated needle insertion or hindpaw incision identify developmentally-regulated and activity-dependent long term alterations in nociceptive processing, and the efficacy of novel or current analgesic interventions can be compared. As prior neonatal experience and sex may influence current pain experience or the risk of persistent pain, these factors should be considered within the biopsychosocial assessment and formulation of pain in later life.