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

Neonatal paw pricking alters adolescent behavior in a sex-dependent manner and sucrose partially remediates the effects

Neonatal exposure to noxious stimuli such as repeated heel lances can cause behavior changes. In the NICU sucrose given prior to procedures attenuates the immediate behavioral response to noxious stimuli but may not ameliorate the long-term consequences, and treatment with 24 % sucrose can brain structure and behavior in adult rodents. We used a rat model to determine whether paw pricks during the neonatal period alter social interaction and/or paw withdrawal thresholds (PWT) in adolescence, and if 7 % sucrose mitigates these effects. One male and one female pup per litter was assigned to each of six experimental groups (no paw prick (control), 1 paw prick (1PP), or 2PP, ± sucrose). Hind paws were pricked once or twice each day between postnatal day (P)3 and P10. Social behavior and PWT were tested in adolescence using the modified social interaction test and von Frey filaments, respectively. Social behavior was altered in the 2PP group; total time interacting was lower in 2PP rats, primarily due to less time sniffing a play partner. Sucrose did not mitigate effects of paw prick but trended to alter social behaviors in males; it decreased time in contact but increased social motivation (movement toward a play partner). PWTs were higher in 2PP animals, this was not altered by sucrose. Thus, rat pups exposed to paw pricks in the neonatal period have some altered behaviors in adolescence. The nature of the behavioral changes is sex-dependent, but sucrose did not mitigate these changes.

Repetitive daily oxytocin treatment reduces weight gain but not acute neonatal procedural pain

BACKGROUND

While the incidence of neonatal intensive care unit (NICU) admission steadily increases, neonatology lacks evidence of a safe, effective, and preventive analgesic for treating procedural pain. Given its role in nociception and promoting healthy neurodevelopment, the endogenous neuropeptide oxytocin (OT) emerges as a promising candidate.

METHODS

This study investigates the use of daily repeated subcutaneous OT (1 mg/kg) treatment in an established model of neonatal repetitive procedural pain and assesses the effectivity of OT treatment on mechanical sensitivity and body weight.

RESULTS

Contrary to our hypothesis repeated daily OT treatment did not prevent the development of mechanical hypersensitivity following needle pricks. Furthermore, treatment with OT diminished body weight gain in neonatal pups, a major side effect observed throughout the neonatal week. These results highlight the unique nature of the maturing nociceptive system that makes the identification and selection of analgesic options for the treatment of acute neonatal procedural pain a major challenge.

CONCLUSION

In conclusion, our preclinical results do not support the use of repeated OT for acute pain relief in the NICU, and the side effects on body weight gain raise concerns about the use of OT in the NICU.

IMPACT

Repeated daily OT treatment inhibits weight gain in neonatal rat pus. Repetitive daily OT administration does not prevent the development of mechanical hypersensitivity in a model of neonatal procedural pain. Future research must focus on the unique physiology of the developing nociceptive system to establish safe, effective and protective treatment of neonatal procedural pain.

On the relation of injury to pain-an infant perspective

ABSTRACT

Forty-five years ago, Patrick Wall published his John J Bonica lecture "On the relation of injury to pain."90 In this lecture, he argued that pain is better classified as an awareness of a need-state than as a sensation. This need state, he argued, serves more to promote healing than to avoid injury. Here I reframe Wall's prescient proposal to pain in early life and propose a set of different need states that are triggered when injury occurs in infancy. This paper, and my own accompanying Bonica lecture, is dedicated to his memory and to his unique contribution to the neuroscience of pain. The IASP definition of pain includes a key statement, "through their life experiences, individuals learn the concept of pain."69 But the relation between injury and pain is not fixed from birth. In early life, the links between nociception (the sense) and pain (the need state) are very different from those of adults, although no less important. I propose that injury evokes three pain need states in infancy, all of which depend on the state of maturity of the central nervous system: (1) the need to attract maternal help; (2) the need to learn the concept of pain; and (3) the need to maintain healthy activity dependent brain development.

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.

Sinomenine ameliorates fibroblast-like synoviocytes dysfunction by promoting phosphorylation and nuclear translocation of CRMP2

ETHNOPHARMACOLOGICAL RELEVANCE

Rheumatoid arthritis (RA) is a chronic autoimmune disease characterized by synovial inflammation and arthritic pain. Sinomenine (SIN), derived from the rhizome of Chinese medical herb Qing Teng (scientific name: Sinomenium acutum (Thunb.) Rehd. Et Wils), has a longstanding use in Chinese traditional medicine for treating rheumatoid arthritis. It has been shown to possess anti-inflammatory, analgesic, and immunosuppressive effects with minimal side-effects clinically. However, the mechanisms governing its effects in treatment of joint pathology, especially on fibroblast-like synoviocytes (FLSs) dysfunction, and arthritic pain remains unclear.

AIM

This study aimed to investigate the effect and underlying mechanism of SIN on arthritic joint inflammation and joint FLSs dysfunctions.

MATERIALS AND METHODS

Collagen-induced arthritis (CIA) was induced in rats and the therapeutic effects of SIN on joint pathology were evaluated histopathologically. Next, we conducted a series of experiments using LPS-induced FLSs, which were divided into five groups (Naïve, LPS, SIN 10, 20, 50 μg/ml). The expression of inflammatory factors was measured by qPCR and ELISA. The invasive ability of cells was detected by modified Transwell assay and qPCR. Transwell migration and cell scratch assays were used to assess the migration ability of cells. The distribution and content of relevant proteins were observed by immunofluorescence and laser confocal microscopy, as well as Western Blot and qPCR. FLSs were transfected with plasmids (CRMP2 T514A/D) to directly modulate the post-translational modification of CRMP2 protein and downstream effects on FLSs function was monitored.

RESULTS

SIN alleviated joint inflammation in rats with CIA, as evidenced by improvement of synovial hyperplasia, inflammatory cell infiltration and cartilage damage, as well as inhibition of pro-inflammatory cytokines release from FLSs induced by LPS. In vitro studies revealed a concentration-dependent suppression of SIN on the invasion and migration of FLSs induced by LPS. In addition, SIN downregulated the expression of cellular CRMP2 that was induced by LPS in FLSs, but increased its phosphorylation at residue T514. Moreover, regulation of pCRMP2 T514 by plasmids transfection (CRMP2 T514A/D) significantly influenced the migration and invasion of FLSs. Finally, SIN promoted nuclear translocation of pCRMP2 T514 in FLSs.

CONCLUSIONS

SIN may exert its anti-inflammatory and analgesic effects by modulating CRMP2 T514 phosphorylation and its nuclear translocation of FLSs, inhibiting pro-inflammatory cytokine release, and suppressing abnormal invasion and migration. Phosphorylation of CRMP2 at the T514 site in FLSs may present a new therapeutic target for treating inflammatory joint's destruction and arthritic pain in RA.

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.

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.

Repeated neonatal Needle-pricking stimulation alter neurodevelopment in adolescent rats

OBJECTIVE

To explore the repeated pain stimulation in neonatal rats affects their cognitive and memory abilities during puberty, and the proliferation expression of hippocampal neurons.

METHODS

Postnatal 1 day (P1) SD rats were randomly divided into two groups, and the skin of the needle group was pricked for seven days consecutively while the skin of the control group was stroked for the same period of time. The rats in both groups were weighed every week, and the Morris water maze experiment was performed from P44 to P49 to test the cognitive and memory abilities of the rats. On P50, the hippocampal tissue was extracted for observation of pathological features and the expressions of Ki-67 and caspase 3 were determined.

RESULTS

With the increase of the days, the body weight of the rats in the needle group increased slightly slower than that of the control group. The escape latency of the needle group was significantly higher than that of the control group in the water maze test at P45 and P48, and the number of times the rats crossing the platform in the needle group was lower than that of the control group. The HE staining of the hippocampal tissue showed that the cells in the needle group were disorganized, with irregular morphology. Under the electron microscope, the structure of neuron cells and organelles is changed in the hippocampal CA1 region of rats. It showed a decrease in the Ki-67 expression and an increase in caspase 3 in the needle group.

CONCLUSION

Repeated experience of needle-pricking stimulation in neonatal rats can cause cognitive impairment and memory loss in puberty, disrupt hippocampal organization, and diminish neuronal proliferation.

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.

Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities, and Networks-American Pain Society-American Academy of Pain Medicine Pain Taxonomy Diagnostic Criteria for Acute Needle Pain

Needle procedures are among the most common causes of pain and distress for individuals seeking health care. While needle pain is especially problematic for children needle pain and associated fear also has significant impact on adults and can lead to avoidance of appropriate medical care. Currently there is not a standard definition of needle pain. A taxonomy, or classification system, for acute needle pain would aid research efforts and enhance clinical care. To meet this need, the Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities, and Networks public-private partnership with the U.S. Food and Drug Administration, the American Pain Society, and the American Academy of Pain Medicine formed the Analgesic, Anesthetic, and Addiction Clinical Trial Translations, Innovations, Opportunities, and Networks-American Pain Society-American Academy of Pain Medicine Pain Taxonomy initiative. One of the goals of this initiative was to develop taxonomies for acute pain disorders, including needle pain. To accomplish this, a working group of experts in needle pain was convened. Based on available literature and expert opinion, the working group used a 5-dimenional structure (diagnostic criteria, common features, modulating factors, impact and/or functional consequences, and putative mechanisms) to develop an acute pain taxonomy that is specific needle pain. As part of this, a set of 4 diagnostic criteria, with 2 modifiers to account for the influence of needle associated fear, are proposed to define the types of acute needle pain. PERSPECTIVE: This article presents a taxonomy for acute needle pain. This taxonomy could help to standardize definitions of acute pain in clinical studies of patients undergoing needle procedures.

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.

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.

Enriched environment rescues neonatal pain induced cognitive deficits and the impaired hippocampal synaptic plasticity later in life

Although extensive and untreated pain that occurs during a critical developmental window may impair cognition later in life, environmental interventions at early might promote. However, the underlying mechanism is poorly understood. Our current study utilized a rat model of "repetitive needle pricks" from the day of birth (P0) to postnatal day 7 (P7) to mimic the painful experience of preterm neonates in the Neonatal intensive care unit (NICU). Enriched environment (EE) during development period (from P15 to P70) was implemented as a nonpharmacological intervention approach. Electrophysiological recording, behavioral tests and biochemical analysis were performed after the end of EE (between P71 and P80). Results showed neonatal repetitive pain resulted in a reduction in mechanical withdrawal thresholds by the von Frey test in P70 (P < 0.001). Furthermore, neonatal repetitive pain impaired spatial learning and memory (P < 0.05) and even led to dysfunction in fear memory (P < 0.01). In contrast, EE rescued neonatal pain induced cognitive deficits and normalized hippocampal long-term potentiation in rats exposed to neonatal pain (P < 0.05). The beneficial effect of EE might be the improvements in hippocampal synaptic plasticity via up-regulating neurotrophic factors and N-methyl-D-aspartate (NMDA) receptors in the hippocampus. Our findings provide evidence that early environmental intervention might be a safe strategy to overcome neurodevelopmental abnormalities in preterm infants who experienced multiple procedural painful events during the early critical period. This article is protected by copyright. All rights reserved.

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.

Selective Targeting of Serotonin 5-HT1a and 5-HT3 Receptors Attenuates Acute and Long-Term Hypersensitivity Associated With Neonatal Procedural Pain

Neonatal painful procedures causes acute pain and trigger long-term changes in nociceptive processing and anxiety behavior, highlighting the need for adequate analgesia during this critical time. Spinal serotonergic receptors 5-HT1a and 5-HT3 play an important role in modulating incoming nociceptive signals in neonates. The current study aims to attenuate acute and long-term hypersensitivity associated with neonatal procedural pain using ondansetron (a 5-HT3 antagonist) and buspirone (a 5-HT1a agonist) in a well-established rat model of repetitive needle pricking. Sprague-Dawley rat pups of both sexes received ondansetron (3 mg/kg), buspirone (3 mg/kg) or saline prior to repetitive needle pricks into the left hind-paw from postnatal day 0-7. Control animals received tactile stimulation or were left undisturbed. Acute, long-term, and post-operative mechanical sensitivity as well as adult anxiety were assessed. Neonatal 5-HT1a receptor agonism completely reverses acute hypersensitivity from P0-7. The increased duration of postoperative hypersensitivity after re-injury in adulthood is abolished by 5-HT3 receptor antagonism during neonatal repetitive needle pricking, without affecting baseline sensitivity. Moreover, 5-HT1a and 5-HT3 receptor modulation decreases adult state anxiety. Altogether, our data suggests that targeted pharmacological treatment based on the modulation of spinal serotonergic network via the 5-HT1a and 5-HT3 receptors in neonates may be of use in treatment of neonatal procedural pain and its long-term consequences. This may result in a new mechanism-based therapeutic venue in treatment of procedural pain in human neonates.

Anatomical changes in descending serotonergic projections from the rostral ventromedial medulla to the spinal dorsal horn following repetitive neonatal painful procedures

Excessive noxious stimulation during the critical neonatal period impacts the nociceptive network lasting into adulthood. As descending serotonergic projections from the rostral ventromedial medulla (RVM) to the spinal dorsal horn develop postnatally, this study aims to investigate the long-term effect of repetitive neonatal procedural pain on the descending serotonergic RVM-spinal dorsal horn network. A well-established rat model of repetitive noxious procedures is used in which neonatal rats received four noxious needle pricks or tactile stimulation with a cotton swab per day in the left hind paw from day of birth to postnatal day 7. Control animals were left undisturbed. When animals reached adulthood, tissue was collected for quantitative immunohistochemical analysis of serotonin (5-hydroxytryptamine, 5-HT) in the RVM and spinal dorsal horn. Both repetitive noxious and tactile procedures in the neonate decreased the 5-HT staining intensity in the adult ipsilateral, but not contralateral spinal dorsal horn. Repetitive neonatal noxious procedures resulted in an increased area covered with 5-HT staining in the adult RVM ipsilateral to the side of injury, whereas repetitive neonatal tactile stimulation resulted in increased 5-HT staining intensity in both the ipsi- and contralateral RVM. The number of 5-HT cells in adult RVM is unaffected by neonatal conditions. This detailed anatomical study shows that not only neonatal noxious procedures, but also repetitive tactile procedures result in long-lasting anatomical changes of the descending serotonergic system within the RVM and spinal dorsal horn. Future studies should investigate whether these anatomical changes translate to functional differences in descending serotonergic modulation after neonatal adverse experiences.

Multisite Simultaneous Neural Recording of Motor Pathway in Free-Moving Rats

Neural interfaces typically focus on one or two sites in the motoneuron system simultaneously due to the limitation of the recording technique, which restricts the scope of observation and discovery of this system. Herein, we built a system with various electrodes capable of recording a large spectrum of electrophysiological signals from the cortex, spinal cord, peripheral nerves, and muscles of freely moving animals. The system integrates adjustable microarrays, floating microarrays, and microwires to a commercial connector and cuff electrode on a wireless transmitter. To illustrate the versatility of the system, we investigated its performance for the behavior of rodents during tethered treadmill walking, untethered wheel running, and open field exploration. The results indicate that the system is stable and applicable for multiple behavior conditions and can provide data to support previously inaccessible research of neural injury, rehabilitation, brain-inspired computing, and fundamental neuroscience.

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.

Brain Gene Expression Pattern Correlated with the Differential Brain Activation by Pain and Touch in Humans

Genes involved in pain and touch sensations have been studied extensively, but very few studies have tried to link them with neural activities in the brain. Here, we aimed to identify genes preferentially correlated to painful activation patterns by linking the spatial patterns of gene expression of Allen Human Brain Atlas with the pain-elicited neural responses in the human brain, with a parallel, control analysis for identification of genes preferentially correlated to tactile activation patterns. We identified 1828 genes whose expression patterns preferentially correlated to painful activation patterns and 411 genes whose expression patterns preferentially correlated to tactile activation pattern at the cortical level. In contrast to the enrichment for astrocyte and inhibitory synaptic transmission of genes preferentially correlated to tactile activation, the genes preferentially correlated to painful activation were mainly enriched for neuron and opioid- and addiction-related pathways and showed significant overlap with pain-related genes identified in previous studies. These findings not only provide important evidence for the differential genetic architectures of specific brain activation patterns elicited by painful and tactile stimuli but also validate a new approach to studying pain- and touch-related genes more directly from the perspective of neural responses in the human brain.

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.

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.

Effectiveness of Gentle Human Touch for Pain Control During Examination for Retinopathy of Pre-maturity: A Randomized Controlled Trial

Background: Retinopathy of pre-maturity (ROP) is a disorder of the retinal blood vessels in pre-term infants with low birth weight. It is a leading cause of blindness in children. During ROP screening, the use of mydriatic drops and eyelid openers causes pain and discomfort. Pain management strategies include medications and behavioral interventions. The objectives of this study was to investigate the effects of Gentle Human Touch on pain in pre-term infants undergoing screening for ROP. Methods: In this randomized controlled trial, 82 infants in the neonatal intensive care unit at Children's Hospital of Nanjing Medical University who met the ROP screening criteria were randomly assigned to experimental and control groups using the random number table. The infants in the experimental group continuously received Gentle Human Touch during screening, while those in the control group were screened according to the routine procedure. All neonates were administered local eye anesthesia before the screening. The degree of pain was assessed using the Pre-mature Infant Pain Profile score. A double-channel near-infrared spectroscopy device was used to monitor regional cerebral oxygen saturation (rScO2), while oxygen saturation (SaO2) and heart rate were measured using pulse oximetry. The Pre-mature Infant Pain Profile score was the primary outcome, while heart rate, SaO2, and rScO2 were the secondary outcomes. Results: The gestational age, corrected gestational age, birth weight, and Apgar score at examination and the basal heart rate, SaO2, and rScO2 showed no significant intergroup differences (P > 0.05 for all). Both groups demonstrated significant decreases in SaO2 and rScO2 in response to the examination (P < 0.05 for all). During the examination, the Pre-mature Infant Pain Profile score (14.82 ± 3.22 vs. 9.29 ± 2.89, respectively; P < 0.05) was significantly higher in the control group than in the experimental group, while rScO2 (57.61 ± 3.51 vs. 54.76 ± 4.54%, respectively; P < 0.05) and SaO2 (91.89 ± 6.43 vs. 85.68 ± 8.31%; P < 0.05) were significantly higher in the experimental group than in the control group. There was no significant difference in heart rate changes between the two groups before and after the examination (182.60 ± 3.50 vs. 170.80 ± 3.50 time/min; P > 0.05). Conclusions: The findings of this study suggest that Gentle Human Touch can effectively alleviate pain during ROP screening in pre-mature infants. Clinical Trial Registration: ISRCTN10976481, Registered 06 March 2020, Retrospectively registered.

Qualitative review on N-methyl-D-aspartate receptor expression in rat spinal cord during the postnatal development: Implications for central sensitization and pain

The N‐methyl‐D‐aspartate receptor (NMDAR) is an important mediator of central sensitization and nociception in the rat spinal dorsal horn. The NMDAR subunits and splice variants determine the properties of the receptor. Understanding the expression of NMDAR subunits in spinal cord during the neonatal development is important as it may have consequences for the process of central sensitization and nociception in later life. In this review, a systematic literature search was conducted using three databases: Medline, Embase, and PubMed. A quality assessment was performed on predetermined entities of bias. Thirteen articles were identified to be relevant. The results show that NMDAR subunits and splice variants are dynamically expressed during postnatal development in the spinal dorsal horn. During the first 2 weeks, the expression of less excitable GluN2A subunit and more sensitive GluN2B subunit increases while the expression of high excitable GluN2C subunit decreases. During the 2nd week of postnatal development GluN1 subunits with exon 21 spliced in but exon 22 spliced out are predominantly expressed, increasing phosphorylation, and transport to the membrane. The data suggest that in rats, the nociceptive system is most susceptible to central sensitization processes during the first two postnatal weeks. This may have important consequences for nociception and pain responses in later life. From this, we conclude that targeted therapy directed toward specific NMDAR subunits is a promising candidate for mechanism‐based treatment of pain in neonates.

Repetitive Pain in Neonatal Male Rats Impairs Hippocampus-Dependent Fear Memory Later in Life

Preterm infants in neonatal intensive care units are inevitably subjected to numerous painful procedures. However, little is known about the consequences of early pain experience on fear memory formation later in life. We hypothesized that exposure to repetitive pain in early life triggered hippocampal synaptic plasticity and resulted in memory deficiency in prepubertal and adult rats. From the day of birth (P0) to postnatal day 7 (P7), neonatal male rat pups were randomly assigned to either needle pricks or tactile touches repetitively every 6 h. Trace fear conditioning was performed on rats on P24-P26 and P87-P89. On P24 and P87, rats were sacrificed for molecular and electrophysiological studies. On P24-26 and P87-89, rats that experienced neonatal needle treatment showed a significant reduction in freezing time in the contextual fear conditioning (P < 0.05) and trace fear conditioning tests (P < 0.05). Moreover, repetitive neonatal procedural pain caused a significant decrease in the magnitude of hippocampal long-term potentiation induced by high-frequency stimulation. Furthermore, rats that experienced neonatal needle treatment demonstrated sustained downregulation of NR1, NR2A, NR2B, and GluR1 expression in the hippocampus. Therefore, neonatal pain is related to deficits in hippocampus-related fear memory later in life and might be caused by impairments in hippocampal synaptic plasticity.

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

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

Acute pain measured with the modified Bernese Pain Scale for Neonates is influenced by individual contextual factors

BACKGROUND

Individual contextual factors like gestational age (GA) or previous painful experiences have an influence on neonates' pain responses and may lead to inaccurate pain assessment when not appropriately considered.

OBJECTIVES

We set out to determine the influence of individual contextual factors on variability in pain response in neonates, measured with the modified Bernese Pain Scale for Neonates (BPSN), and, if necessary, to incorporate relevant individual factors into a revised version of the BPSN.

METHODS

We videotaped 154 full-term and preterm neonates of different GAs during 1-5 capillary heel sticks in their first 14 days of life. For each heel stick, we produced three video sequences: baseline, heel stick, and recovery. The randomized sequences were rated on the BPSN by five blinded nurses. Individual contextual factors were retrospectively extracted from patient charts and from the video recordings. We analysed the data in single and multiple linear mixed models.

RESULTS

Premature birth (b = -0.721), caffeine (b = -0.302), and the behavioural states quiet and awake (b = -0.283), active and asleep (b = -0.158), and quiet and asleep (b = -0.498) were associated with changes in behavioural pain scores. Premature birth (b = -0.232), mechanical ventilation (b = -0.196), and duration of the heel stick procedure (b = 0.0004) were associated with changes in physiological pain scores. Premature birth (b = -0.907), Caffeine (b = -0.402), the behavioural states quiet and awake (b = -0.274), and quiet and asleep (b = -0.459), and duration of the heel stick procedure (b = 0.001) were associated with changes in the modified BPSN total scores.

CONCLUSIONS

Postmenstrual age, behavioural state, caffeine, and ventilation status have an influence on neonates' pain response and should be incorporated in the revised BPSN to enhance clinical pain assessment in neonates with different GAs.

SIGNIFICANCE

We identified individual contextual factors associated with dampened pain response in neonates and will incorporate them into a revised version of the Bernese Pain Scale for Neonates to provide clinicians with a tool they can use to more accurately assess and manage pain in this vulnerable population.

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.

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.

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.

Neuroimmune modulation of pain across the developmental spectrum

Today's treatment for chronic pain is inadequate, and novel targets need to be identified. This requires a better understanding of the mechanisms involved in pain sensitization and chronification. In this review, we discuss how peripheral inflammation, as occurs during an infection, modulates the central pain system. In rodents, neonatal inflammation leads to increased pain sensitivity in adulthood by priming immune components both peripherally and centrally. The excitability of neurons in the spinal cord is also altered by neonatal inflammation and may add to pain sensitization later in life. In adult humans, inflammation modulates pain sensitivity as well, partly by affecting the activity in brain areas that process and regulate pain signals. Low-grade inflammation is common in clinical populations both peripherally and centrally, and priming of the immune system has also been suggested in some pain populations. The nociceptive and immune systems are primed by infections and inflammation. The early life programming of nociceptive responses following exposure to infections or inflammation will define individual differences in adult pain perception. Immune-to-brain mechanisms and neuroimmune pathway need further investigation as they may serve both as predictors and therapeutic targets in chronic pain.

Repeated neonatal needle-prick stimulation increases inflammatory mechanical hypersensitivity in adult rats

BACKGROUND AND AIMS

Newborn infants are vulnerable to procedural stress and pain exposure on the first weeks of life that represents a critical period for the development of nociceptive, sensory, emotional, and social functions. We evaluated the nociceptive behavior of adult male and female rats that were submitted to nociceptive experience in the neonatal period and the maternal behavior in the postnatal period.

METHODS

The animals were submitted to repetitive needle pricking from the second to the fifteenth postnatal day (PND 2-15). Maternal behavior and litter weight were evaluated during this period. Mechanical sensitivity to pain was assessed in offsprings during the adulthood by exposing them to inflammatory stimuli, including formalin test or the Freund's complete adjuvant (CFA) injection followed by the electronic von Frey test at 0, 3, 6 and 24 h later.

RESULTS

Maternal behavior and litter weight were not altered by pinprick stimuli during PND 2-15. Additionally, pinprick stimulation reduced the paw withdrawal threshold in CFA-injected animals compared to control. In the formalin test, there was a difference between the genders. Female rats are statically more sensitive to formalin stimulation and showed an increased licking time in both the first and second phases and increased number of flinches in second phase.

CONCLUSIONS

Experiencing early life repetitive pain exposure increased inflammatory pain sensitivity in adult offspring rats and female rats are more sensitive to chemical stimulation.

IMPLICATIONS

Future investigations of the mechanisms involved in this effect may contribute to the improvement of the understanding of inflammatory pain sensitivity differences.

Paediatric Pain Medicine: Pain Differences, Recognition and Coping Acute Procedural Pain in Paediatric Emergency Room

Paediatric pain and its assessment and management are challenging for medical professionals, especially in an urgent care environment. Patients in a paediatric emergency room (PER) often undergo painful procedures which are an additional source of distress, anxiety, and pain. Paediatric procedural pain is often underestimated and neglected because of various myths, beliefs, and difficulties in its evaluation and treatment. However, it is very different from other origins of pain as it can be preventable. It is known that neonates and children can feel pain and that it has long-term effects that last through childhood into adulthood. There are a variety of pain assessment tools for children and they should be chosen according to the patient’s age, developmental stage, communication skills, and medical condition. Psychological factors such as PER environment, preprocedural preparation, and parental involvement should also be considered. There are proven methods to reduce a patient’s pain and anxiety during different procedures in PER. Distraction techniques such as music, videogames, virtual reality, or simple talk about movies, friends, or hobbies as well as cutaneous stimulation, vibration, cooling sprays, or devices are effective to alleviate procedural pain and anxiety. A choice of distraction technique should be individualized, selecting children who could benefit from nonpharmacological pain treatment methods or tools. Nonpharmacological pain management may reduce dosage of pain medication or exclude pharmacological pain management. Most nonpharmacological treatment methods are cheap, easily accessible, and safe to use on every child, so it should always be a first choice when planning a patient’s care. The aim of this review is to provide a summary of paediatric pain features, along with their physiology, assessment, management, and to highlight the importance and efficacy of nonpharmacological pain management in an urgent paediatric care setting.