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

Influence of Early-Life Stress on the Excitability of Dynorphin Neurons in the Adult Mouse Dorsal Horn

While early-life adversity has been associated with a higher risk of developing chronic pain in adulthood, the cellular and molecular mechanisms by which chronic stress during the neonatal period can persistently sensitize developing nociceptive circuits remain poorly understood. Here, we investigate the effects of early-life stress (ELS) on synaptic integration and intrinsic excitability in dynorphin-lineage (DYN) interneurons within the adult mouse superficial dorsal horn (SDH), which are important for inhibiting mechanical pain and itch. The administration of neonatal limited bedding between postnatal days (P)2 and P9 evoked sex-dependent effects on spontaneous glutamatergic signaling, as female SDH neurons exhibited a higher amplitude of miniature excitatory postsynaptic currents (mEPSCs) after ELS, while mEPSC frequency was reduced in DYN neurons of the male SDH. Furthermore, ELS decreased the frequency of miniature inhibitory postsynaptic currents selectively in female DYN neurons. As a result, ELS increased the balance of spontaneous excitation versus inhibition (E:I ratio) in mature DYN neurons of the female, but not male, SDH network. Nonetheless, ELS weakened the total primary afferent-evoked glutamatergic drive onto adult DYN neurons selectively in females, without modifying afferent-evoked inhibitory signaling onto the DYN population. Finally, ELS failed to significantly change the intrinsic membrane excitability of mature DYN neurons in either males or females. Collectively, these data suggest that ELS exerts a long-term influence on the properties of synaptic transmission onto DYN neurons within the adult SDH, which includes a reduction in the overall strength of sensory input onto this important subset of inhibitory interneurons. PERSPECTIVE: This study suggests that chronic stress during the neonatal period influences synaptic function within adult spinal nociceptive circuits in a sex-dependent manner. These findings yield new insight into the potential mechanisms by which early-life adversity might shape the maturation of pain pathways in the central nervous system (CNS).

Intertemporal Improvement in Physicians' Perceptions of the Short-Term Adverse Outcomes of Neonatal Pain: Results of a Two-Time-Point National Survey

Pain in early life may seriously impact neonatal outcomes. This study aimed to evaluate whether the perceptions of physicians working in neonatal intensive care units (NICUs) of the short-term adverse outcomes associated with neonatal pain have changed over a 20-year period. Self-administered questionnaires were distributed to 117 and 145 neonatologists, pediatricians, and fellows working in level III NICUs in 2000 (T1) and 2019 (T2), respectively. The questionnaire consisted of four domains, including the central nervous, cardiovascular, and respiratory systems, as well as "other systems" (metabolic/endocrine system, growth, and general condition), with 21 total items overall. Although the proportion of positive (correct) responses to the total and system-specific domain scores was significantly higher at T2 than T1, the knowledge of certain short-term adverse outcomes was suboptimal even at T2. Adjustment for cofactors confirmed the independent association of the survey time-point with the total and system-specific domain scores. Moreover, NICU type was an independent significant factor associated with the adjusted total and central nervous system scores, while young doctors had a better knowledge of adverse cardiovascular effects. Conclusions: The perceptions of NICU physicians concerning the short-term outcomes associated with neonatal pain have significantly improved over the past 20 years, although remaining knowledge gaps mandate ongoing efforts to achieve an improvement in neonatal care.

Subjective experience of the environment determines serotoninergic antidepressant treatment outcome in male mice

BACKGROUND

The effects of the selective serotonin reuptake inhibitors (SSRIs), the first-line antidepressant treatment, have been proposed to be affected, at least in part, by the living environment. Since the quality of the environment depends not only on its objective features, but also on the subjective experience, we hypothesized that the latter plays a key role in determining SSRI treatment outcome.

METHODS

We chronically administered the SSRI fluoxetine to two groups of adult CD-1 male mice that reportedly show distinct subjective experiences of the environment measured as consistent and significantly different responses to the same emotional and social stimuli. These distinct socioemotional profiles were generated by rearing mice either in standard laboratory conditions (SN) or in a communal nest (CN) where three dams breed together their offspring, sharing caregiving behavior.

RESULTS

At adulthood, CN mice displayed higher levels of agonistic and anxiety-like behaviors than SN mice, indicating that they experience the environment as more socially challenging and potentially dangerous. We then administered fluoxetine, which increased offensive and anxious response in SN, while producing opposite effects in CN mice. BDNF regulation was modified by the treatment accordingly.

LIMITATIONS

Subjective experience in mice was assessed as behavioral response to the environment.

CONCLUSIONS

These results show that the subjective experience of the environment determines fluoxetine outcome. In a translational perspective, our findings suggest considering not only the objective quality, but also the subjective appraisal, of the patient's living environment for developing effective personalized therapeutic approaches in psychiatry.

The Bayliss-Starling Prize Lecture: The developmental physiology of spinal cord and cortical nociceptive circuits

When do we first experience pain? To address this question, we need to know how the developing nervous system processes potential or real tissue-damaging stimuli in early life. In the newborn, nociception preserves life through reflex avoidance of tissue damage and engagement of parental help. Importantly, nociception also forms the starting point for experiencing and learning about pain and for setting the level of adult pain sensitivity. This review, which arose from the Bayliss-Starling Prize Lecture, focuses on the basic developmental neurophysiology of early nociceptive circuits in the spinal cord, brainstem and cortex that form the building blocks of our first pain experience.

Exploration of the Relationships Between Stress and Distinct Pain and Sleep Disturbance Profiles in Patients Undergoing Chemotherapy

BACKGROUND

Unrelieved pain and sleep disturbance are common symptoms in oncology patients. Increased stress may be an underlying cause for both symptoms.

OBJECTIVES

The purposes of this study were to identify subgroups of outpatients with distinct pain and sleep disturbance profiles and to evaluate differences among these subgroups in demographic and clinical characteristics. Differences in global stress, cancer-specific stress, and cumulative life stress, as well as resilience and coping, were evaluated.

METHODS

Patients (N = 1343) completed self-report questionnaires for demographic and clinical characteristics and stress, resilience, and coping. Latent profile analysis was used to identify subgroups of patients with distinct pain and sleep disturbance profiles. Differences among the subgroups were determined using parametric and nonparametric tests.

RESULTS

Three distinct profiles were identified (ie, No Pain + Moderate Sleep Disturbance (SD) (27.6%), Moderate Pain + Moderate SD (38.6%), Severe Pain + High SD (33.8%)). Compared with the other 2 classes, Severe Pain + High SD class was younger, had fewer years of education, was more likely to be female, more likely to live alone, less likely to be employed, and had a higher level of comorbidity. This class had the highest stress scores and was more likely to report higher rates of adverse childhood experiences.

CONCLUSION

Over 70% of our sample reported clinically meaningful levels of both symptoms, and 33.8% reported relatively high rates of adverse childhood experiences.

IMPLICATION FOR PRACTICE

Clinicians need to perform routine assessments, particularly of adverse childhood experiences, and initiate appropriate referrals.

An Analgesic Technique for Orogastric Tube Insertion in Newborns: DOLATSONG, a Randomized Multicentric Controlled Trial

BACKGROUND

Gastric tube insertion is necessary to support early enteral feeding of newborns during their neonatal intensive care stay. This frequent and invasive procedure is known to be painful. Very few analgesic techniques (sweet solutions, sucking, swaddling, and skin-to-skin contact) are available to reduce the pain caused by orogastric tube insertion procedure.

OBJECTIVE

To determine whether a new orogastric tube insertion technique modifies the pain response in newborns, we hypothesize that inserting an orogastric tube through the nipple of a bottle reduces pain caused by this procedure.

DESIGN

Prospective, controlled, randomized, multicentered and open label study.

SETTINGS

Three neonatal intensive care units in France (2 level 3 units and 1 level 2B).

PARTICIPANTS

Full-term or premature newborns at 32 weeks of gestation or more, postnatal age between 48 hours and 21 days, not ventilated and requiring enteral feeding, were randomized into 2 groups: usual technique (n = 36) and experimental technique (n = 35).

METHODS

Our experimental technique was to insert the orogastric tube through a modified nipple of a bottle. This method was compared with the usual technique of inserting the tube directly into the newborn's mouth without a support to guide it accompanied by a nipple encouraging sucking with a nonnutritive solution. An association of nonnutritive sucking and orally administered 30% glucose was given to all children for analgesic purposes. Pain during the orogastric tube insertion was assessed on video recordings by 2 independent experts, using a heteroassessment behavioral scale for pain (DAN-Douleur Aiguë du Nouveau-né; APN-Acute Pain in Newborns). The primary outcome was an Acute Pain in Newborns score of less than 3 at the time of the procedure. Comparisons were made using Fisher exact test or Mann-Whitney U test. Factors associated with an Acute Pain in Newborns score of 3 and greater were explored using univariable and multivariable regression models.

RESULTS

All but 1 video recording in each group were analyzed. Among the 34 neonates in the experimental group, 71.4% (95% CI: [53.7-85.4]) had an Acute Pain in Newborns score of less than 3 during orogastric tube insertion versus 41% (95% CI: [27.9-61.9]) in the control group (P = .031). Gagging was frequent and nonsignificantly different between the 2 groups (69% in the control group, 51% in the experimental group, P = .13). In multivariable analysis, the experimental technique was an independent factor of pain prevention compared with the usual technique (odds ratio = 0.21 [0.06-0.71], P = .015).

CONCLUSIONS

This study suggests that a simple, inexpensive, and feasible technique of orogastric tube insertion through the nipple of a bottle limits pain associated with this procedure in newborns.

Involvement of oxidative stress in orofacial mechanical pain hypersensitivity following neonatal maternal separation in rats

Patients with persistent pain have sometimes history of physical abuse or neglect during infancy. However, the pathogenic mechanisms underlying orofacial pain hypersensitivity associated with early-life stress remain unclear. The present study focused on oxidative stress and investigated its role in pain hypersensitivity in adulthood following early-life stress. To establish an early-life stress model, neonatal pups were separated with their mother in isolated cages for 2 weeks. The mechanical head-withdrawal threshold (MHWT) in the whisker pad skin of rats received maternal separation (MS) was lower than that of non-MS rats at postnatal week 7. In MS rats, the expression of 8-hydroxy-deoxyguanosine, a marker of DNA oxidative damage, was enhanced, and plasma antioxidant capacity, but not mitochondrial complex I activity, decreased compared with that in non-MS rats. Reactive oxygen species (ROS) inactivation and ROS-sensitive transient receptor potential ankyrin 1 (TRPA1) antagonism in the whisker pad skin at week 7 suppressed the decrease of MHWT. Corticosterone levels on day 14 increased in MS rats. Corticosterone receptor antagonism during MS periods suppressed the reduction in antioxidant capacity and MHWT. The findings suggest that early-life stress potentially induces orofacial mechanical pain hypersensitivity via peripheral nociceptor TRPA1 hyperactivation induced by oxidative stress in the orofacial region.

Towards a central origin of nociceptive hypersensitivity in adult rats after a neonatal maternal separation

Early life adversities influence a nervous system still in development with long-term consequences for later life. These include nociceptive circuit alterations critical to shape an adaptive pain response to protect the organism from potential damage. Adult rats with a history of neonatal maternal separation (NMS) display visceral and somatic nociceptive hypersensitivity and inefficient analgesic responses to stress. In this study, we have characterized the consequences of NMS on wide dynamic range neurons (WDR) in the spinal cord of anaesthetized adult rats during the nociceptive processing of hot and cold noxious information. We found that WDR neurons of NMS rats display an excessive coding of mechanical and thermal information applied at the rat's hindpaws. This nicely explains the hypernociceptive behaviours seen after noxious mechanical, cold and hot peripheral stimulation. A peripheral change in the expression of molecular transducers for these stimuli (i.e., TRPV1, TRPM8 and TRPA1) does not seem to account for this general hyperexcitability. Instead, a decreased chloride-mediated inhibitory tone on WDR neurons may play a role as indicated by the abnormal elevation of the type 1 Na-K-Cl cotransporter transcripts. Altogether, we propose that long-term consequences of NMS are associated with reduced spinal cord inhibition favouring the expression of pain hypersensitivity. We cannot exclude that this phenomenon is also present at supraspinal sites, as other NMS-associated symptoms include excessive anxiety and impaired sociability.

Enhanced harm detection following maternal separation: Transgenerational transmission and reversibility by inhaled amiloride

Early-life adversities are associated with altered defensive responses. Here, we demonstrate that the repeated cross-fostering (RCF) paradigm of early maternal separation is associated with enhancements of distinct homeostatic reactions: hyperventilation in response to hypercapnia and nociceptive sensitivity, among the first generation of RCF-exposed animals, as well as among two successive generations of their normally reared offspring, through matrilineal transmission. Parallel enhancements of acid-sensing ion channel 1 (ASIC1), ASIC2, and ASIC3 messenger RNA transcripts were detected transgenerationally in central neurons, in the medulla oblongata, and in periaqueductal gray matter of RCF-lineage animals. A single, nebulized dose of the ASIC-antagonist amiloride renormalized respiratory and nociceptive responsiveness across the entire RCF lineage. These findings reveal how, following an early-life adversity, a biological memory reducible to a molecular sensor unfolds, shaping adaptation mechanisms over three generations. Our findings are entwined with multiple correlates of human anxiety and pain conditions and suggest nebulized amiloride as a therapeutic avenue.

Early-life adversity increases morphine tolerance and persistent inflammatory hypersensitivity through upregulation of δ opioid receptors in mice

ABSTRACT

Exposure to severely stressful events during childhood is associated with poor health outcomes in later life, including chronic pain and substance use disorder. However, the mediators and mechanisms are unclear. We investigated the impact of a well-characterized mouse model of early-life adversity, fragmented maternal care (FC) between postnatal day 2 and 9, on nociception, inflammatory hypersensitivity, and responses to morphine. Male and female mice exposed to FC exhibited prolonged basal thermal withdrawal latencies and decreased mechanical sensitivity. In addition, morphine had reduced potency in mice exposed to FC and their development of tolerance to morphine was accelerated. Quantitative PCR analysis in several brain regions and the spinal cords of juvenile and adult mice revealed an impact of FC on the expression of genes encoding opioid peptide precursors and their receptors. These changes included enhanced abundance of δ opioid receptor transcript in the spinal cord. Acute inflammatory hypersensitivity (induced by hind paw administration of complete Freund's adjuvant) was unaffected by exposure to FC. However, after an initial recovery of mechanical hypersensitivity, there was a reappearance in mice exposed to FC by day 15, which was not seen in control mice. Changes in nociception, morphine responses, and hypersensitivity associated with FC were apparent in males and females but were absent from mice lacking δ receptors or β-arrestin2. These findings suggest that exposure to early-life adversity in mice enhances δ receptor expression leading to decreased basal sensitivity to noxious stimuli coupled with accelerated morphine tolerance and enhanced vulnerability to persistent inflammatory hypersensitivity.

Effectiveness of parental education about pain in the neonatal period on knowledge, attitudes, and practices: A systematic review and meta-analysis

Background

Despite the availability of effective, safe, and feasible pain management strategies, infant pain remains undertreated. Parents can play a key role in advocating for or delivering pain management strategies if they are educated. To date, a quantitative synthesis of the effectiveness of parental education about pain management in the neonatal period has not been performed.

Objective

To systematically review the effectiveness of parental education during the neonatal period on pain management in infancy.

Methods

MEDLINE, EMBASE, PsycInfo, CINAHL, and the Cochrane Library were searched for relevant randomized controlled trials (RCTs) and non-randomized trials (NRTs) that evaluated parental education with respect to pain management during the neonatal period in any setting from inception to February 2021. Screening of article titles and abstracts and data extraction were performed in duplicate. The risk of bias was assessed using the Cochrane Risk Bias Tool 2.0 and the Risk of Bias in Non-randomized Studies of Interventions for RCTs and NRTs, respectively. As per the GRADE methodology, critically important and important outcomes were identified. Critically important outcomes included utilization of pain management strategies and infant pain. Important outcomes included parental knowledge about pain mitigation strategies, parental attitudes, compliance with painful procedures, procedure outcomes, and safety. Data were combined and presented as relative risk (RR) or mean or standardized mean difference (MD or SMD) with 95% confidence interval (CI).

Results

Of the six studies eligible for inclusion, four studies were RCTs and two studies were NRTs. Written information and/or video were used to deliver parental education during the neonatal period in hospital settings in all studies. Four studies (two RCTs and two NRTs) reported on critically important outcomes. The risk of bias was low for the two RCTs and moderate to serious for the two NRTs. Utilization of pain management strategies was assessed for heel lance in the first 48 hours of life in two studies and for vaccine injection at 2 to 6 months of life in two studies. Higher utilization rate for pain management strategies was reported in the pain education group in three studies (RR 1.15, 95% CI 1.04, 1.26; N=2712). There was no difference in the mean number of pain management strategies used in one NRT tracking utilization tracking utilization as continuous data (MD 0.20, 95% CI -0.01, 0.41; N=178). Parent-reported infant pain scores were lower in the pain education group in one RCT (MD -0.16, 95% CI -0.27, -0.06; N=1615). The quality of evidence for the outcome of utilization of pain management strategies was very low while for the outcome of infant pain the quality of evidence was moderate. Five studies (3 RCTs and 2 NRTs) reported on important outcomes. The risk of bias was low for two RCTs and high for one RCT and moderate to serious for the two NRTs. Parental knowledge about pain management strategies (SMD 0.54, 95% CI 0.26, 0.82), parental confidence in their ability to manage pain (SMD 0.24, 95% CI 0.14, 0.34), parental satisfaction with education (MD 1.18, 95% CI 0.84, 1.52) and parental satisfaction with pain management (RR 1.05. 95% CI 1.01, 1.08) were increased in the pain education group. None of the included studies reported on procedural outcomes. No adverse events with the pain education nor the use of pain management interventions were reported in one study.

Conclusions

Parental education in the neonatal period was effective in increasing utilization of pain management strategies during painful procedures. Reduction of pain in infants is based on one study of moderate quality. Furthermore, parental education increased parental knowledge about pain management strategies, confidence in their ability to manage infant pain, and satisfaction with the education and pain management. Parental pain education should be incorporated into postnatal care.

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.

Maternal High-Energy Diet during Pregnancy and Lactation Impairs Neurogenesis and Alters the Behavior of Adult Offspring in a Phenotype-Dependent Manner

Obesity is one of the biggest and most costly health challenges the modern world encounters. Substantial evidence suggests that the risk of metabolic syndrome or obesity formation may be affected at a very early stage of development, in particular through fetal and/or neonatal overfeeding. Outcomes from epidemiological studies indicate that maternal nutrition during pregnancy and lactation has a profound impact on adult neurogenesis in the offspring. In the present study, an intergenerational dietary model employing overfeeding of experimental mice during prenatal and early postnatal development was applied to acquire mice with various body conditions. We investigated the impact of the maternal high-energy diet during pregnancy and lactation on adult neurogenesis in the olfactory neurogenic region involving the subventricular zone (SVZ) and the rostral migratory stream (RMS) and some behavioral tasks including memory, anxiety and nociception. Our findings show that a maternal high-energy diet administered during pregnancy and lactation modifies proliferation and differentiation, and induced degeneration of cells in the SVZ/RMS of offspring, but only in mice where extreme phenotype, such as significant overweight/adiposity or obesity is manifested. Thereafter, a maternal high-energy diet enhances anxiety-related behavior in offspring regardless of its body condition and impairs learning and memory in offspring with an extreme phenotype.

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.