Early postnatal chronic inflammation produces long-term changes in pain behavior and N-methyl-D-aspartate receptor subtype gene expression in the central nervous system of adult mice

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.

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.

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.

Immune-Neuroendocrine Interactions: Evolution, Ecology, and Susceptibility to Illness

The integration between immune and neuroendocrine systems is crucial for maintaining homeostasis from invertebrates to humans. In the first, the phagocytic cell, i.e., the immunocyte, is the main actor, while in the latter, the principle player is the lymphocyte. Immunocytes are characterized by the presence of pro-opiomelanocortin (POMC) peptides, CRH, and other molecules that display a significant similarity to their mammalian counterparts regarding their functions, as both are mainly involved in fundamental functions such as immune (chemotaxis, phagocytosis, cytotoxicity, etc.) and neuroendocrine (stress) responses. Furthermore, the immune-neuroendocrine system provides vital answers to ecological and immunological demands in terms of economy and efficiency. Finally, susceptibility to disease emerges as the result of a continuous dynamic interaction between the world within and the world outside. New fields such as ecological immunology study the susceptibility to pathogens in an evolutionary perspective while the field of neuro-endocrine-immunology studies the susceptibility from a more immediate perspective.

Persistent changes in peripheral and spinal nociceptive processing after early tissue injury

It has become clear that tissue damage during a critical period of early life can result in long-term changes in pain sensitivity, but the underlying mechanisms remain to be fully elucidated. Here we review the clinical and preclinical evidence for persistent alterations in nociceptive processing following neonatal tissue injury, which collectively point to the existence of both a widespread hypoalgesia at baseline as well as an exacerbated degree of hyperalgesia following a subsequent insult to the same somatotopic region. We also highlight recent work investigating the effects of early trauma on the organization and function of ascending pain pathways at a cellular and molecular level. These effects of neonatal injury include altered ion channel expression in both primary afferent and spinal cord neurons, shifts in the balance between synaptic excitation and inhibition within the superficial dorsal horn (SDH) network, and a 'priming' of microglial responses in the adult SDH. A better understanding of how early tissue damage influences the maturation of nociceptive circuits could yield new insight into strategies to minimize the long-term consequences of essential, but invasive, medical procedures on the developing somatosensory system.

Interleukin 18 activates MAPKs and STAT3 but not NF-κB in hippocampal HT-22 cells

Interleukin (IL)-18 is a cytokine previously demonstrated to participate in neuroinflammatory processes. Since the components of the IL-18 receptor complex are expressed in neurons throughout the brain, IL-18 is also believed to directly influence neuronal function. Here we tested this hypothesis on mouse hippocampal neurons by measuring the effects of IL-18 on three pathways previously shown to be regulated by this cytokine in non-neuronal cells: the MAPK pathways, p38 and ERK1/2 MAPKs, STAT3 and NF-κB. Experiments were carried out in vitro using the immortalized hippocampal neuronal line HT-22 or in vivo following i.c.v. injection with recombinant mouse IL-18. We showed that IL-18 did not activate NF-κB in HT-22 cells whereas it induced a rapid (within 15min) activation of the MAPK pathways. Moreover, we demonstrated that IL-18 treatment enhanced P-STAT3 (Tyr705)/STAT3 ratio in the nucleus of HT-22 cells after 30-60min of exposure. A similar increase in P-STAT3 (Tyr705)/STAT3 ratio was observed in the whole hippocampus one hour after i.c.v. injection. These data demonstrate that IL-18 can act directly on neuronal cells affecting the STAT3 pathway; therefore, possibly regulating the expression of specific genes within the hippocampus. This effect may help to explain some of the IL-18-induced effects on synaptic plasticity and functionality within the hippocampal system.

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

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

Biological and neurodevelopmental implications of neonatal pain

Nociceptive pathways are functional following birth. In addition to physiological and behavioral responses, neurophysiological measures and neuroimaging evaluate nociceptive pathway function and quantify responses to noxious stimuli in preterm and term neonates. Intensive care and surgery can expose neonates to painful stimuli when the developing nervous system is sensitive to changing input, resulting in persistent impacts into later childhood. Early pain experience has been correlated with increased sensitivity to subsequent painful stimuli, impaired neurodevelopmental outcomes, and structural changes in brain development. Parallel preclinical studies have elucidated underlying mechanisms and evaluate preventive strategies to inform future clinical trials.

Central effects of a local inflammation in three commonly used mouse strains with a different anxious phenotype

As in humans, genetic background in rodents may influence a peculiar set of behavioural traits such as sensitivity to pain and stressors or anxiety-related behaviours. Therefore, we tested the hypothesis that mice with different genetic backgrounds [outbred (CD1), inbred (C57BL/6J) and hybrid (B6C3F1) adult male mice] display altered reactivity to pain, stress and anxiety related behaviours. We demonstrated that B6C3F1 mice displayed the more anxious phenotype with respect to C57BL/6J or CD1 animals, with the latter being the less anxious strain when tested in an open field and on an elevated plus maze. No difference was observed across strains in thermal sensitivity to a radiant heat source. Mice were then treated with a sub-plantar injection of the inflammatory agent Complete Freund's Adjuvant (CFA), 24h later they were hyperalgesic with respect to saline exposed animals, irrespective of strain. We then measured intra-strain differences and CFA-induced inter-strain effects on the expression of various genes with a recognized role in pain and anxiety: BDNF, IL-6, IL-1β, IL-18 and NMDA receptor subunits in the mouse thalamus, hippocampus and hypothalamus. The more anxious phenotype observed in B6C3F1 hybrid mice displayed lower levels of BDNF mRNA in the hippocampus and hypothalamus when compared to outbred CD1 and C57BL/6J inbred mice. CFA led to a general decrease in central gene expression of the evaluated targets especially in CD1 mice, while BDNF hypothalamic downregulation stands out as a common effect of CFA in all three strains evaluated.

The impact of the perioperative period on neurocognitive development, with a focus on pharmacological concerns

Mounting evidence from animal studies has implicated that all commonly used anaesthetics and sedatives may induce widespread neuronal cell death and result in long-term neurological abnormalities. These findings have led to serious questions regarding the safe use of these drugs in young children. In humans, recent findings from retrospective, epidemiological studies do not exclude the possibility of an association between surgery with anaesthesia early in life and subsequent learning abnormalities. These results have sparked discussions regarding the appropriate timing of paediatric surgery and the safe management of paediatric anaesthesia. However, important questions need to be addressed before findings from laboratory studies and retrospective clinical surveys can be used to guide clinical practice. This article summarises the currently available preclinical and clinical information regarding the impact of anaesthetics, sedatives, opioids, pain and stress, inflammation, hypoxia-ischaemia, co-morbidities and genetic predisposition on brain structure and long-term neurological function. Moreover, this article outlines the putative mechanisms of anaesthetic neurotoxicity, and the phenomenon's implications for clinical practice in this rapidly emerging field.

Neonatal bladder inflammation produces functional changes and alters neuropeptide content in bladders of adult female rats

Neonatal bladder inflammation has been demonstrated to produce hypersensitivity to bladder re-inflammation as an adult. The purpose of this study was to investigate the effects of neonatal urinary bladder inflammation on adult bladder function and structure. Female Sprague-Dawley rats were treated on postnatal days 14 to 16 with intravesical zymosan or anesthesia alone. At 12 to 16 weeks of age, micturition frequency and cystometrograms were measured. Similarly treated rats had their bladders removed for measurement of plasma extravasation after intravesical mustard oil, for neuropeptide analysis (calcitonin gene-related peptide or Substance P) or for detailed histological examination. Rats treated with zymosan as neonates exhibited increased micturition frequency, reduced micturition volume thresholds, greater extravasation of Evans blue after intravesical mustard oil administration, and greater total bladder content of calcitonin gene-related peptide and Substance P. In contrast, there were no quantitative histological changes in the thickness, fibrosis, or mast cells of bladder tissue due to neonatal zymosan treatments. Functional changes in urologic systems observed in adulthood, coupled with the increased neuropeptide content and neurogenic plasma extravasation in adult bladders, suggest that the neonatal bladder inflammation treatment enhanced the number, function, and/or neurochemical content of primary afferent neurons. These data support the hypothesis that insults to the urologic system in infancy may contribute to the development of adult bladder hypersensitivity.

PERSPECTIVE

Inflammation of the bladder early in life in the rat has multiple sequelae, including laboratory measures that suggest an alteration of the neurophysiological substrates related to the bladder. Some painful bladder syndromes in humans have similar characteristics and so may be due to similar mechanisms.

Early neonatal inflammation affects adult pain reactivity and anxiety related traits in mice: genetic background counts

Protracted or recurrent pain and inflammation in the early neonatal period may cause long-lasting changes in central neural function. However, more research is necessary to better characterize the long-term behavioral sequelae of such exposure in the neonatal period.

OBJECTIVES

(1) to study whether timing of postnatal exposure to persistent inflammation alters responsiveness to thermal pain in the adult animal; (2) to assess whether animals experiencing early postnatal chronic inflammation display altered anxiety related behavior; (3) to study the importance of genetic background. Newborn mice (outbred strain, CD1 and F1 hybrid strain, B6C3F1) received an injection of Complete Freund's Adjuvant (CFA) or saline on either postnatal day 1 or 14 (PND1; PND14) into the left hind paw. Pain to radiant heat and anxiety were examined in 12-week-old adult animals. Adult baseline PWL was significantly decreased in CD1 mice exposed to CFA on PND 1 and 14 as compared to their saline treated counterparts. B6C3F1 mice exposed to CFA on PND14 showed markedly reduced baseline PWL compared to the PND14 saline group. Persistent inflammation experienced by B6C3F1 mice on PND1 failed to affect baseline adult thermal responsiveness. Adult mice, CD1 and B6C3F1, displayed low anxiety traits only if they had been exposed to persistent inflammation on PND1 and not on PND14. Our research suggests a role for genetic background in modulating long-term behavioral consequences of neonatal persistent inflammation: the data support the hypothesis that pain experienced very early in life differentially affects adult behavioral and emotional responsiveness in outbred (CD1) and hybrid mice (B6C3F1).

The effects of tail biopsy for genotyping on behavioral responses to nociceptive stimuli

Removal of a small segment of tail at weaning is a common method used to obtain tissue for the isolation of genomic DNA to identify genetically modified mice. When genetically manipulated mice are used for pain research, this practice could result in confounding changes to the animals' responses to noxious stimuli. In this study, we sought to systematically investigate whether tail biopsy representative of that used in standard genotyping methods affects behavioral responses to a battery of tests of nociception. Wild-type littermate C57BL/6J and 129S6 female and male mice received either tail biopsies or control procedural handling at Day 21 after birth and were then tested at 6-9 weeks for mechanical and thermal sensitivity. C57BL/6J mice were also tested in the formalin model of inflammatory pain. In all tests performed (von Frey, Hargreaves, modified Randall Selitto, and formalin), C57BL/6J tail-biopsied animals' behavioral responses were not significantly different from control animals. In 129S6 animals, tail biopsy did not have a significant effect on behavioral responses in either sex to the von Frey and the modified Randall-Selitto tests of mechanical sensitivity. Interestingly, however, both sexes exhibited small but significant differences between tail biopsied and control responses to a radiant heat stimulus. These results indicate that tail biopsy for genotyping purposes has no effect on nocifensive behavioral responses of C57BL/6J mice, and in 129S6 mice, causes only a minor alteration in response to a radiant heat stimulus while other nocifensive behavioral responses are unchanged. The small effect seen is modality- and strain-specific.

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

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