Primary and secondary hyperalgesia can be differentiated by postnatal age and ERK activation in the spinal dorsal horn of the rat pup

Distinct Age-Dependent C Fiber-Driven Oscillatory Activity in the Rat Somatosensory Cortex

When skin afferents are activated, the sensory signals are transmitted to the spinal cord and eventually reach the primary somatosensory cortex (S1), initiating the encoding of the sensory percept in the brain. While subsets of primary afferents mediate specific somatosensory information from an early age, the subcortical pathways that transmit this information undergo striking changes over the first weeks of life, reflected in the gradual emergence of specific sensory behaviors. We therefore hypothesized that this period is associated with differential changes in the encoding of incoming afferent volleys in S1. To test this, we compared S1 responses to A fiber skin afferent stimulation and A + C skin afferent fiber stimulation in lightly anaesthetized male rats at postnatal day (P)7, P14, P21, and P30. Differences in S1 activity following A and A + C fiber stimulation changed dramatically over this period. At P30, A + C fiber stimulation evoked significantly larger γ, β, and α energy increases compared with A fiber stimulation alone. At younger ages, the changes in S1 oscillatory activity evoked by the two afferent volleys were not significantly different. Silencing TRPV1+ C fibers with QX-314 significantly reduced the γ and β S1 oscillatory energy increases evoked by A + C fibers, at P30 and P21, but not at younger ages. Thus, C fibers differentially modulate S1 oscillatory activity only from the third postnatal week, well after the functional maturation of the somatosensory cortex. This age-related change in afferent evoked S1 oscillatory activity may underpin the maturation of sensory discrimination in the developing brain.

Trajectories of post-surgical pain in infants admitted to neonatal intensive care

BACKGROUND

The aim of this study was (a) to statistically identify distinct trajectories of pain following surgery in infants less than 6 months of age, and (b) to compare these trajectories to descriptions of chronic pain in infants in the neonatal intensive care unit (NICU).

METHODS

This was a retrospective medical record review of infants admitted to a NICU between 2008 and 2018 following surgery. All infants who underwent one major procedure to the abdomen or thorax and returned to the NICU following surgery were included. Pain was assessed regularly using a validated Pain Assessment Tool. Group-based trajectory analysis was used to determine the trajectory of recovery from pain following surgery.

RESULTS

A total of 726 infants were included in the study. A two-group trajectory model, defined as typical and atypical pain trajectories, was selected. The typical group (n = 467) consisted of infants who had significantly fewer days (1.5 ± 2.3 vs 5.3 ± 5.5, p < .001) and recorded instances of pain (2.0 ± 3.4 vs 9.7 ± 10.5, p < .001) compared to infants in the atypical group (n = 259). The incidence of iatrogenic neonatal abstinence syndrome was greater in the atypical than the typical group (11% vs 5%, p = .001).

CONCLUSIONS

This study has revealed two distinct pain trajectories in infants after surgery. While recovery from pain occurs within days in the typical group, the atypical group demonstrates pain for a significantly longer period, often weeks and months following surgery. This latter group adheres closely to current descriptions of chronic pain in infants.

Aqueous and methanol extracts of Paullinia pinnata L. (Sapindaceae) improve inflammation, pain and histological features in CFA-induced mono-arthritis: Evidence from in vivo and in vitro studies

ETHNOPHARMACOLOGICAL RELEVANCE

Paullinia pinnata L. (Sapindaceae) is an African woody vine, traditionally used for the treatment of itch and pain-related conditions such as rheumatoid arthritis.

AIM

This work evaluates, in vitro and in vivo, the anti-inflammatory and analgesic effects of aqueous (AEPP) and methanol (MEPP) extracts from Paullinia pinnata leaves.

METHODS

AEPP and MEPP (100, 200 and 300 mg/kg/day) were administered orally in monoarthritic rats induced by a unilateral injection of 50 μl of Complete Freund's Adjuvant (CFA) in the ankle joint. During the 14 days of treatment, pain and inflammation were evaluated alternatively in both ankle and paw of the CFA-injected leg. Malondialdehyde (MDA) and glutathione (GSH) levels were assessed in serum and spinal cord. Histology of soft tissue of the ankle was also analyzed. For in vitro studies, AEPP and MEPP (10, 30 and 100 μg/ml) were evaluated against nitric oxide (NO) production by macrophages that were either non-stimulated or stimulated with LPS, 8-Br-AMPc and the mixture of both substances after 8 h exposure. These extracts were also evaluated on TNF-α and IL-1β production in cells stimulated with LPS for 8 h. Finally, the ability of the extracts to bind to neuroactive receptors was evaluated in vitro using competitive binding assays with >45 molecular targets.

RESULTS

AEPP and MEPP significantly reduced by 20-98% (p < 0.001) the inflammation and pain sensation in both the ankle and paw. AEPP significantly increased glutathione levels (p < 0.05) in serum. Both extracts reduced MDA production in serum and spinal cord (p < 0.001), and significantly improved tissue reorganization in treated arthritic rats. P. pinnata extracts did not affect NO production in non-stimulated macrophages but significantly reduced it by 47-88% in stimulated macrophages. AEPP and MEPP also significantly inhibited TNF-α (35-68%) and IL-1β (31-36%) production in LPS stimulated macrophages. No cytotoxic effect of plant extracts was observed. MEPP showed concentration-dependent affinity for Sigma 2 receptors with an IC₅₀ of 50 μg/ml.

CONCLUSION

These results demonstrate the analgesic and anti-inflammatory effects of P. pinnata extracts on monoarthritis and further support its traditional use for pain and inflammation. These activities are at least partly due to the ability of these extracts to inhibit the production of NO, TNF-α, IL-1β and to likely modulate Sigma 2 receptors.

PI3K/Akt Pathway is Required for Spinal Central Sensitization in Neuropathic Pain

Phosphatidylinositol-3-kinase (PI3K) has been identified in the expression of central sensitization after noxious inflammatory stimuli. However, its contribution in neuropathic pain remains to be determined. Here we address the role of PI3K signaling in central sensitization in a model of neuropathic pain, and propose a novel potential drug target for neuropathic pain. Chronic constriction injury (CCI) rat model was used in the study as the model for neuropathic pain. Western blotting, whole-cell patch clamp, and von Frey assay were performed to study biochemical, electrical, and behavioral changes in CCI rats, respectively. A steroid metabolite of the fungi (wortmannin) was used to block PI3K signaling and its effects on CCI rats were tested. PI3K/Akt signaling increased in the spinal cord L4-L6 sections in the CCI rats. CCI also facilitated miniature excitatory postsynaptic potential of dorsal horn substantia gelatinosa neurons, increased phosphorylation of glutamate receptor subunit GluA1 and synapsin at the synapse, and induced mechanic allodynia. Wortmannin reversed biochemical, electrical, and behavioral changes in CCI rats. This study is the first to show PI3K/Akt signaling is required for spinal central sensitization in the CCI neuropathic pain model.

Beyond Acute Pain: Understanding Chronic Pain in Infancy

This topical review presents the current challenges in defining chronic pain in infants, summarizes evidence from animal and human infant studies regarding the biological processes necessary for chronic pain signaling, and presents observational/experiential evidence from clinical experts. A literature search of four databases (CINAHL, EMBASE, PsycINFO, and MEDLINE) was conducted, along with hand searches of reference lists. Evidence from animal studies suggest that important neurophysiological mechanisms, such as the availability of key neurotransmitters needed for maintenance of chronic pain, may be immature or absent in the developing neonate. In some cases, human infants may be significantly less likely to develop chronic pain. However, evidence also points to altered pain perception, such as allodynia and hyperalgesia, with significant injury. Moreover, clinicians and parents in pediatric intensive care settings describe groups of infants with altered behavioral responses to repeated or prolonged painful stimuli, yet agreement on a working definition of chronic pain in infancy remains elusive. While our understanding of infant chronic pain is still in the rudimentary stages, a promising avenue for the future assessment of chronic pain in infancy would be to develop a clinical tool that uses both neurophysiological approaches and clinical perceptions already presented in the literature.

Prevention and Management of Procedural Pain in the Neonate: An Update

The prevention of pain in neonates should be the goal of all pediatricians and health care professionals who work with neonates, not only because it is ethical but also because repeated painful exposures have the potential for deleterious consequences. Neonates at greatest risk of neurodevelopmental impairment as a result of preterm birth (ie, the smallest and sickest) are also those most likely to be exposed to the greatest number of painful stimuli in the NICU. Although there are major gaps in knowledge regarding the most effective way to prevent and relieve pain in neonates, proven and safe therapies are currently underused for routine minor, yet painful procedures. Therefore, every health care facility caring for neonates should implement (1) a pain-prevention program that includes strategies for minimizing the number of painful procedures performed and (2) a pain assessment and management plan that includes routine assessment of pain, pharmacologic and nonpharmacologic therapies for the prevention of pain associated with routine minor procedures, and measures for minimizing pain associated with surgery and other major procedures.

Targeting p38 Mitogen-activated Protein Kinase to Reduce the Impact of Neonatal Microglial Priming on Incision-induced Hyperalgesia in the Adult Rat

BACKGROUND

Neonatal surgical injury triggers developmentally regulated long-term changes that include enhanced hyperalgesia and spinal microglial reactivity after reinjury. To further evaluate priming of response by neonatal hindpaw incision, the authors investigated the functional role of spinal microglial p38 mitogen-activated protein kinase after reincision in adult rodents.

METHODS

Plantar hindpaw incision was performed in anesthetized adult rats, with or without previous incision on postnatal day 3. Numbers and distribution of phosphorylated-p38 (1, 3, 24 h) and phosphorylated extracellular signal-regulated kinase (15 min, 24 h) immunoreactive cells in the lumbar dorsal horn were compared after adult or neonatal plus adult incision. Withdrawal thresholds evaluated reversal of incision-induced hyperalgesia by p38 inhibition with intrathecal SB203850.

RESULTS

Neonatal injury significantly increased phosphorylated-p38 expression 3 h after adult incision (55 ± 4 vs. 35 ± 4 cells per section, mean ± SEM, n = 6 to 7, P < 0.01). Increased expression was restricted to microglia, maintained across lumbar segments, and also apparent at 1 and 24 h. Preincision intrathecal SB203850 prevented the enhanced mechanical hyperalgesia in adults with previous neonatal injury and was effective at a lower dose (0.2 vs. 1 mg/kg, n = 8, P < 0.05) and for a longer duration (10 vs. 3 days). Lumbar neuronal phosphorylated extracellular signal-regulated kinase expression reflected the distribution of hindpaw primary afferents, but was not significantly altered by previous incision.

CONCLUSIONS

Neonatal incision primes spinal neuroglial signaling, and reincision in adult rats unmasks centrally mediated increases in functional microglial reactivity and persistent hyperalgesia. After early life injury, p38 inhibitors may have specific benefit as part of multimodal analgesic regimes to reduce the risk of persistent postsurgical pain.

Overview of neurodevelopment and pain research, possible treatment targets

Pain is a common presenting and often persistent symptom for children with rheumatological disease. Pain is not clearly related to disease severity in children with inflammatory juvenile idiopathic arthritis, and presentations of non-inflammatory musculoskeletal pain are common but there is limited evidence to guide management. Pain assessment must extend beyond measures of pain severity to more fully evaluate characteristics of pain, functional impact and psychosocial effects and family interactions. Evaluation of mechanisms of joint pain in adults has identified potential treatment targets, but additional studies are required as the acute and long-term impacts of pain and injury change during postnatal development. Genotyping, sensory evaluation and neuroimaging may better characterize chronic musculoskeletal pain, identify high-risk groups and/or provide additional outcome measures to monitor disease and treatment progress. An integrated approach to management is required to effectively select and target interventions, reduce pain and disability and improve long-term outcome.

Interactions between glia, the immune system and pain processes during early development

Pain is a serious problem for infants and children and treatment options are limited. Moreover, infants born prematurely or hospitalized for illness likely have concurrent infection that activates the immune system. It is now recognized that the immune system in general and glia in particular influence neurotransmission and that the neural bases of pain are intimately connected to immune function. We know that injuries that induce pain activate immune function and suppressing the immune system alleviates pain. Despite this advance in our understanding, virtually nothing is known of the role that the immune system plays in pain processing in infants and children, even though pain is a serious clinical issue in pediatric medicine. This brief review summarizes the existing data on immune-neural interactions in infants, providing evidence for the immaturity of these interactions.

Acute and chronic pain in children

Pain in neonates and children differs to that in adults. One of the many challenges associated with the diagnosis and management of pain in early life is that neonates are non-verbal and therefore incapable of communicating their pain effectively to their caregivers. Early life pain is characterised by lowered thermal and mechanical thresholds, and exaggerated and often inappropriate behavioural reactions to pain. These differing behavioural reactions are underpinned by increased excitability/decreased inhibition within the spinal dorsal horn. This itself is the result of immaturity in the anatomical expression of key neurotransmitters and neuromodulators within spinal pain circuits, as well as decreased inhibitory input to these circuits from brainstem centres, and an immature relationship between neuronal and non-neuronal cells which affects pain response. These differences between early and adult pain impact upon not just acute reactions to pain, but also the incidence, severity and duration of chronic pain. In this chapter, chronic pain in childhood is discussed, as are the structural and functional differences that underpin differences in acute pain processing between adults and children. The ability of pain that occurs in early life to alter life-long pain responding is also addressed.

Postnatal temporal, spatial and modality tuning of nociceptive cutaneous flexion reflexes in human infants

Cutaneous flexion reflexes are amongst the first behavioural responses to develop and are essential for the protection and survival of the newborn organism. Despite this, there has been no detailed, quantitative study of their maturation in human neonates. Here we use surface electromyographic (EMG) recording of biceps femoris activity in preterm (<37 weeks gestation, GA) and term (≥ 37 weeks GA) human infants, less than 14 days old, in response to tactile, punctate and clinically required skin-breaking lance stimulation of the heel. We show that all infants display a robust and long duration flexion reflex (>4 seconds) to a single noxious skin lance which decreases significantly with gestational age. This reflex is not restricted to the stimulated limb: heel lance evokes equal ipsilateral and contralateral reflexes in preterm and term infants. We further show that infant flexion withdrawal reflexes are not always nociceptive specific: in 29% of preterm infants, tactile stimulation evokes EMG activity that is indistinguishable from noxious stimulation. In 40% of term infants, tactile responses are also present but significantly smaller than nociceptive reflexes. Infant flexion reflexes are also evoked by application of calibrated punctate von Frey hairs (vFh), 0.8-17.2 g, to the heel. Von Frey hair thresholds increase significantly with gestational age and the magnitude of vFh evoked reflexes are significantly greater in preterm than term infants. Furthermore flexion reflexes in both groups are sensitized by repeated vFh stimulation. Thus human infant flexion reflexes differ in temporal, modality and spatial characteristics from those in adults. Reflex magnitude and tactile sensitivity decreases and nociceptive specificity and spatial organisation increases with gestational age. Strong, relatively non-specific, reflex sensitivity in early life may be important for driving postnatal activity dependent maturation of targeted spinal cord sensory circuits.

Evaluation of spinal toxicity and long-term spinal reflex function after intrathecal levobupivaciane in the neonatal rat

BACKGROUND

Neuraxial anesthesia is utilized in children of all ages. Local anesthetics produce dose-dependent toxicity in certain adult models, but the developing spinal cord may also be susceptible to drug-induced apoptosis. In postnatal rodents, we examined the effects of intrathecal levobupivacaine on neuropathology and long-term sensorimotor outcomes.

METHODS

Postnatal day 3 (P3) or P7 rat pups received intrathecal levobupivacaine 2.5 mg/kg (0.5%) or saline. Mechanical withdrawal thresholds and motor block were assessed. Spinal cord tissue analysis included apoptosis counts (activated caspase-3, Fluoro-Jade C) at 24 h, glial reactivity at 7 days, and histopathology in cord and cauda equina at 24 h and 7 days. Long-term spinal function in young adults (P35) was assessed by hind limb withdrawal thresholds, electromyography responses to suprathreshold stimuli, and gait analysis.

RESULTS

Intrathecal levobupivacaine produced spinal anesthesia at P3 and P7. No increase in apoptosis or histopathological change was seen in the cord or cauda equina. In the P3 saline group, activated caspase-3 (mean±SEM per lumbar cord section 6.1±0.3) and Fluoro-Jade C (12.1±1.2) counts were higher than at P7, but were not altered by levobupivacaine (P=0.62 and P=0.11, two-tailed Mann-Whitney test). At P35, mechanical withdrawal thresholds, thermal withdrawal latency, and electromyographic reflex responses did not differ across P3 or P7 levobupivacaine or saline groups (one way ANOVA with Bonferroni comparisons). Intrathecal bupivacaine at P3 did not alter gait.

CONCLUSION

Single dose intrathecal levobupivacaine 0.5% did not increase apoptosis or produce spinal toxicity in neonatal rat pups. This study provides preclinical safety data relevant to neonatal use of neuraxial local anesthesia.

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.

Skin incision-induced receptive field responses of mechanosensitive peripheral neurons are developmentally regulated in the rat

Maturation of the nervous system results in changes in both central and peripheral processing. To better understand responses to injury in the young, developmental differences in the acute response to incision were investigated in both tactile and nociceptive myelinated peripheral mechanosensitive afferent neurons in vivo. Neuronal intrasomal recordings were performed in juvenile and infant rats in 34 L5 dorsal root ganglia, and each neuron was phenotypically defined. Neurons had a mechanosensitive receptive field in the glabrous skin on the plantar surface of the hind paw, which was characterized at baseline and for up to 45 min after incision. Fundamental maturational differences in the effect of incision were clear: in high-threshold nociceptive mechanoreceptors, the mechanical threshold decreased immediately and the receptive field size increased rapidly in juvenile rats but not in infant rats. Additionally, a divergence in changes in the instantaneous response frequency of tactile afferents occurred between the two ages. These differences may help explain maturational differences in responses to peripheral injury and suggest that differences in central nervous system responses may be partially mitigated by spatially confined and frequency-dependent differences resulting from tactile and nociceptive mechanosensitive input.

Lamina I NK1 expressing projection neurones are functional in early postnatal rats and contribute to the setting up of adult mechanical sensory thresholds

BACKGROUND

A small proportion of lamina I neurons of the spinal cord project upon the hindbrain and are thought to engage descending pathways that modulate the behavioural response to peripheral injury. Early postnatal development of nociception in rats is associated with exaggerated and diffuse cutaneous reflexes with a gradual refinement of responses over the first postnatal weeks related to increased participation of inhibitory networks. This study examined the postnatal development of lamina I projection neurons from postnatal day 3 (P3) until P48.

RESULTS

At P3, a subset of lamina I neurons were found to express the neurokinin 1 (NK1) receptor. Using fluorogold retrograde tracing, we found that the NK1 positive neurons projected upon the parabrachial nucleus (PB) within the hindbrain. Using c-fos immunohistochemistry, we showed that lamina I and PB neurons in P3 rats responded to noxious stimulation of the periphery. Finally, ablation of lamina I neurons with substance-P saporin conjugates at P3 resulted in increased mechanical sensitivity from P45 onwards compared to control animals of the same age.

CONCLUSIONS

These results suggest that the lamina I pathway is present and functional at least from P3 and required for establishing and fine-tuning mechanical sensitivity in adult rats.

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

Adult brain connectivity is shaped by the balance of sensory inputs in early life. In the case of pain pathways, it is less clear whether nociceptive inputs in infancy can have a lasting influence upon central pain processing and adult pain sensitivity. Here, we show that adult pain responses in the rat are 'primed' by tissue injury in the neonatal period. Rats that experience hind-paw incision injury at 3 days of age, display an increased magnitude and duration of hyperalgesia following incision in adulthood when compared with those with no early life pain experience. This priming of spinal reflex sensitivity was measured by both reductions in behavioural withdrawal thresholds and increased flexor muscle electromyographic responses to graded suprathreshold hind-paw stimuli in the 4 weeks following adult incision. Prior neonatal injury also 'primed' the spinal microglial response to adult injury, resulting in an increased intensity, spatial distribution and duration of ionized calcium-binding adaptor molecule-1-positive microglial reactivity in the dorsal horn. Intrathecal minocycline at the time of adult injury selectively prevented both the hyperalgesia and early microglial reactivity associated with prior neonatal injury. The enhanced neuroimmune response seen in neonatally primed animals could also be demonstrated in the absence of peripheral tissue injury by direct electrical stimulation of tibial nerve fibres, confirming that centrally mediated mechanisms contribute to these long-term effects. These data suggest that early life injury may predispose individuals to enhanced sensitivity to painful events.

Cocaine- and amphetamine-regulated transcript (CART) peptide activates ERK pathways via NMDA receptors in rat spinal cord dorsal horn in an age-dependent manner

Activation of extracellular signal-regulated kinase (ERK) cascade in the spinal cord dorsal horn may contribute to pain hypersensitivity. Our recent study showed that cocaine- and amphetamine-regulated transcript peptide fragment 55-102 (CARTp) increased the levels of phosphoserine 896 and phosphoserine 897 on the N-methyl-d-aspartate (NMDA) receptor NR1 subunit (pNR1-ser896 and pNR1-ser897) via protein kinase A (PKA) and protein kinase C (PKC) signaling pathways leading to increases in NMDA receptor function in spinal cord dorsal horn neurons. Because NMDA receptor, PKC, and PKA signaling pathways may participate in ERK activation, we examined the effects of CARTp on ERK activation in spinal cord dorsal horn neurons in vitro. Western blot analysis showed a significant increase in the level of phosphorylated (activated) ERK (pERK) in the dorsal part of the spinal cord slices after incubation of the slices with CARTp (300nM). Co-administration of CARTp with an NMDA receptor antagonist, MK801 or AP5, or an ERK inhibitor PD98059 blocked the increase in the level of pERK. Interestingly, the increase in the level of pERK by CARTp was observed in postnatal week 3 (W3) and postnatal week 4 (W4), but not in postnatal week 2 (W2) rats. The age-related responses were also noted by CARTp-induced increases in the levels of pNR1-ser896 and pNR1-ser897. In the in vitro electrophysiological study, CARTp increased the amplitude of NMDA-mediated depolarizations in spinal substantia gelatinosa neurons of W3 and W4 rats, but not W2 rats. The results suggest that CARTp activation of ERK signals via the NMDA receptor in the spinal cord dorsal horn was age-dependent.

Dorsal root ganglion neurons respond with prolonged extracellular signal-regulated protein kinase phosphorylation following noxious heat and cold stimulation

In the present study the time course of extracellular signal-regulated protein kinase phosphorylation (pERK1/2 appearance) in lumbar sensory dorsal root ganglia (DRG) was determined following a 5-min noxious heat or a noxious cold stimulus to the hind paw of the rat. The thermal stimuli were chosen to activate transient receptor potential (TRP) channels, but not to induce tissue damage. A quantitative analysis of phospho-ERK1/2 was performed by protein extraction and Western blot analysis. Western blot analysis showed that following the heat stimulus, phosphorylation of ERK1/2 increased 2-3-fold between 10 and 30min in the DRG on the ipsilateral side. High levels were maintained from 30min up to 16h. Following the cold stimulus to the paw, pERK1/2 immediately increased 2-fold within 2min in the DRG on the ipsilateral side, it declined within 2h and reached a second peak at 4h. In the DRGs on the contralateral side of the paw's heat or cold immersion the pERK1/2 remained low at all time points investigated. Fluorescence immunohistochemistry of the DRG following the thermal stimuli revealed an increased cytoplasmic staining for pERK1/2 in neurons. The present results show that following a 5-min nociceptive thermal stimulus sensory neurons respond with a characteristic long-lasting phosphorylation of ERK1/2.

Central sensitization: a generator of pain hypersensitivity by central neural plasticity

Central sensitization represents an enhancement in the function of neurons and circuits in nociceptive pathways caused by increases in membrane excitability and synaptic efficacy as well as to reduced inhibition and is a manifestation of the remarkable plasticity of the somatosensory nervous system in response to activity, inflammation, and neural injury. The net effect of central sensitization is to recruit previously subthreshold synaptic inputs to nociceptive neurons, generating an increased or augmented action potential output: a state of facilitation, potentiation, augmentation, or amplification. Central sensitization is responsible for many of the temporal, spatial, and threshold changes in pain sensibility in acute and chronic clinical pain settings and exemplifies the fundamental contribution of the central nervous system to the generation of pain hypersensitivity. Because central sensitization results from changes in the properties of neurons in the central nervous system, the pain is no longer coupled, as acute nociceptive pain is, to the presence, intensity, or duration of noxious peripheral stimuli. Instead, central sensitization produces pain hypersensitivity by changing the sensory response elicited by normal inputs, including those that usually evoke innocuous sensations.

PERSPECTIVE

In this article, we review the major triggers that initiate and maintain central sensitization in healthy individuals in response to nociceptor input and in patients with inflammatory and neuropathic pain, emphasizing the fundamental contribution and multiple mechanisms of synaptic plasticity caused by changes in the density, nature, and properties of ionotropic and metabotropic glutamate receptors.

Hindpaw incision in early life increases the hyperalgesic response to repeat surgical injury: critical period and dependence on initial afferent activity

Pain in early life can enhance the response to subsequent injury, but effects are influenced by both the nature and timing of neonatal injury. Using plantar hindpaw incision, we investigated how postnatal age influences the response to repeat surgical injury two weeks later. The degree and time course of behavioural changes in mechanical withdrawal threshold were measured, and injury-related hyperalgesia was further quantified by flexion reflex electromyographic responses to suprathreshold mechanical stimuli 24 h following incision. Plantar hindpaw incision produces acute mechanical hyperalgesia in neonatal and adult rats, but incision in neonatal pups has an additional effect on the response to subsequent injury. With initial incision at postnatal day (P) 3 or 6, the degree of hyperalgesia following repeat incision 2 weeks later was greater than in animals having a single incision at the same age. At older ages (initial incision at P10, P21 or P40) responses did not differ in repeat and single incision groups. To test the role of primary afferent activity, levobupivacaine sciatic block was performed prior to P6 plantar incision, and controls received saline or subcutaneous levobupivacaine. Repeat peri-operative, but not a single pre-operative sciatic block, prevented the enhanced response to repeat incision two weeks later. Our results show that the first postnatal week represents a critical period when incision increases hyperalgesia following repeat surgery two weeks later, and effects are initiated by peripheral afferent activity. This has potential therapeutic implications for the type and duration of peri-operative analgesia used for neonatal surgery.

TrkB signaling is required for both the induction and maintenance of tissue and nerve injury-induced persistent pain

Activation of primary afferent nociceptors produces acute, short-lived pain, and tissue or nerve injury induces long-term enhancement of nociceptive processing, manifested as hypersensitivity to thermal and mechanical stimulation. Here we used a chemical-genetic and pharmacological approach to study the contribution of the receptor tyrosine kinase, type 2 (TrkB) to the generation and maintenance of injury-induced persistent pain. We performed the studies in wild-type mice and transgenic (TrkB(F616A)) mice that express mutant but fully functional TrkB receptors. By injecting a small molecule derivative of the protein kinase inhibitor protein phosphatase 1 (1NM-PP1), it is possible to produce highly selective inhibition of TrkB autophosphorylation in adult mice, without interfering with the activity of other protein kinases. We report that oral administration of 1NM-PP1, at doses that blocked phosphorylation of TrkB in the spinal cord, had no effect in behavioral tests of acute heat, mechanical, or chemical pain sensitivity. However, the same pretreatment with 1NM-PP1 prevented the development of tissue- or nerve injury-induced heat and mechanical hypersensitivity. Established hypersensitivity was transiently reversed by intraperitoneal injection of 1NM-PP1. Although interfering with TrkB signaling altered neither acute capsaicin nor formalin-induced pain behavior, the prolonged mechanical hypersensitivity produced by these chemical injuries was prevented by 1NM-PP1 inhibition of TrkB signaling. We conclude that TrkB signaling is not only an important contributor to the induction of heat and mechanical hypersensitivity produced by tissue or nerve injury but also to the persistence of the pain.

Models and mechanisms of hyperalgesia and allodynia

Hyperalgesia and allodynia are frequent symptoms of disease and may be useful adaptations to protect vulnerable tissues. Both may, however, also emerge as diseases in their own right. Considerable progress has been made in developing clinically relevant animal models for identifying the most significant underlying mechanisms. This review deals with experimental models that are currently used to measure (sect. II) or to induce (sect. III) hyperalgesia and allodynia in animals. Induction and expression of hyperalgesia and allodynia are context sensitive. This is discussed in section IV. Neuronal and nonneuronal cell populations have been identified that are indispensable for the induction and/or the expression of hyperalgesia and allodynia as summarized in section V. This review focuses on highly topical spinal mechanisms of hyperalgesia and allodynia including intrinsic and synaptic plasticity, the modulation of inhibitory control (sect. VI), and neuroimmune interactions (sect. VII). The scientific use of language improves also in the field of pain research. Refined definitions of some technical terms including the new definitions of hyperalgesia and allodynia by the International Association for the Study of Pain are illustrated and annotated in section I.

Congenital hypothyroidism alters formalin-induced pain response in neonatal rats

The present study designed to investigate the development of nociceptive circuits upon formalin-induced pain in congenital hypothyroid pups during the first three postnatal weeks. Following induction of maternal hypothyroidism, the offspring pups were received right intraplantar injection of different formalin concentrations at 7, 15, and 23 days of age. Significant reduction in weight gain was observed in PTU-treated offspring from postnatal days 15 up to 23 (P<0.001). No difference was observed between normal and hypothyroid PND7 pups in total pain intensity score with 0.3% solution of formalin. However, normal pups showed higher total pain score (P<0.01) during the first phase of 1% formalin injection. PND15 normal pups showed a biphasic pain response with a concentration of 2% formalin injection. Obvious persistence of higher pain intensity was observed in hypothyroid pups after interphase through the 2nd phase (P2) and recovery phase (P3), (P<0.001). PND23 hypothyroid rats showed slightly biphasic pattern of pain behavior with persistence of lower pain intensity during P2 (2.5% formalin, P<0.05), (10% formalin, P<0.001) without any further decline during P3 (P<0.01, P<0.001 respectively). In general, the number of flexes+shakes in hypothyroid pups was higher than normal pups in both the early and late phases of the test. Licking activity was intensively expressed only in normal pups during phase 2 at the age of 23 days. In contrast to acute pain, hypothyroidism results to pain hypersensitivity in two weeks old rats whereas weaned rats were hyposensitive to tonic nociceptive stimulation without showing the subsequent recovery phase.

Comparison of the immediate effects of surgical incision on dorsal horn neuronal receptive field size and responses during postnatal development

BACKGROUND

Pain behavior in response to skin incision is developmentally regulated, but little is known about the underlying neuronal mechanisms. The authors hypothesize that the spatial activation and intensity of dorsal horn neuron responses to skin incision differ in immature and adult spinal cord.

METHODS

Single wide-dynamic-range dorsal horn cell spike activity was recorded for a minimum of 2 h from anesthetized rat pups aged 7 and 28 days. Cutaneous pinch and brush receptive fields were mapped and von Frey hair thresholds were determined on the plantar hind paw before and 1 h after a skin incision was made.

RESULTS

Baseline receptive field areas for brush and pinch were larger and von Frey thresholds lower in the younger animals. One hour after the incision, brush and pinch receptive field area, spontaneous firing, and evoked spike activity had significantly increased in the 7-day-old animals but not in the 28-day-old animals. Von Frey hair thresholds decreased at both ages.

CONCLUSIONS

Continuous recording from single dorsal horn cells both before and after injury shows that sensitization of receptive fields and of background and afferent-evoked spike activity at 1 h is greater in younger animals. This difference is not reflected in von Frey mechanical thresholds. These results highlight the importance of studying the effects of injury on sensory neuron physiology. Injury in young animals induces a marked and rapid increase in afferent-evoked activity in second-order sensory neurons, which may be important when considering long-term effects and analgesic interventions.

Electrophysiologic characteristics of large neurons in dorsal root ganglia during development and after hind paw incision in the rat

BACKGROUND

Withdrawal thresholds in the paw are lower in younger animals, and incision further reduces these thresholds. The authors hypothesized that these differences result in part from changes in intrinsic electrophysiologic properties of large neurons.

METHODS

Using isolated whole dorsal root ganglion, current clamping was performed to determine the electrophysiologic properties of large neurons before and after incision in animals aged 1 and 4 weeks. Mechanical withdrawal thresholds were used to follow paw sensitivity.

RESULTS

After paw incision, withdrawal thresholds decreased to a similar degree at both ages, but returned to control threshold at 72 h only in the 1-week-old animals. The resting membrane potential was less negative and the rheobase and the resistance of the membrane were lower at baseline in the 1-week-old animals (P < 0.05). After incision, the membrane potential became more depolarized and the rheobase was less in both ages. These changes remained 72 h after the incision in both ages.

CONCLUSION

These findings suggest that lower mechanical thresholds in the younger animals may be partially attributed to the intrinsic electrophysiologic properties of the larger-diameter afferent neurons. The lack of resolution of the electrophysiologic changes in the young despite the resolution of the withdrawal response suggests that continued input from large fibers into the central nervous system may occur at this age despite the apparent resolution of behavioral changes. Further studies are needed to determine the etiology of these differences, their impact in the central nervous system, and whether theses changes can be prevented.

Differential activation of p38 and extracellular signal-regulated kinase in spinal cord in a model of bee venom-induced inflammation and hyperalgesia

Background

Honeybee's sting on human skin can induce ongoing pain, hyperalgesia and inflammation. Injection of bee venom (BV) into the intraplantar surface of the rat hindpaw induces an early onset of spontaneous pain followed by a lasting thermal and mechanical hypersensitivity in the affected paw. The underlying mechanisms of BV-induced thermal and mechanical hypersensitivity are, however, poorly understood. In the present study, we investigated the role of mitogen-activated protein kinase (MAPK) in the generation of BV-induced pain hypersensitivity.

Results

We found that BV injection resulted in a quick activation of p38, predominantly in the L4/L5 spinal dorsal horn ipsilateral to the inflammation from 1 hr to 7 d post-injection. Phosphorylated p38 (p-p38) was expressed in both neurons and microglia, but not in astrocytes. Intrathecal administration of the p38 inhibitor, SB203580, prevented BV-induced thermal hypersensitivity from 1 hr to 3 d, but had no effect on mechanical hypersensitivity. Activated ERK1/2 was observed exclusively in neurons in the L4/L5 dorsal horn from 2 min to 1 d, peaking at 2 min after BV injection. Intrathecal administration of the MEK inhibitor, U0126, prevented both mechanical and thermal hypersensitivity from 1 hr to 2 d. p-ERK1/2 and p-p38 were expressed in neurons in distinct regions of the L4/L5 dorsal horn; p-ERK1/2 was mainly in lamina I, while p-p38 was mainly in lamina II of the dorsal horn.

Conclusion

The results indicate that differential activation of p38 and ERK1/2 in the dorsal horn may contribute to the generation and development of BV-induced pain hypersensitivity by different mechanisms.

Pain in children: recent advances and ongoing challenges

Significant advances in the assessment and management of acute pain in children have been made, and are supported by an increase in the availability and accessibility of evidence-based data. However, methodological and practical issues in the design and performance of clinical paediatric trials limit the quantity, and may influence the quality, of current data, which lags behind that available for adult practice. Collaborations within research networks, which incorporate both preclinical and clinical studies, may increase the feasibility and specificity of future trials. In early life, the developing nervous system responds differently to pain, analgesia, and injury, resulting in effects not seen in later life and which may have long-term consequences. Translational laboratory studies further our understanding of developmental changes in nociceptor pathway structure and function, analgesic pharmacodynamics, and the impact of different forms of injury. Chronic pain in children has a negative impact on quality of life, resulting in social and emotional consequences for both the child and the family. Despite age-related differences in many chronic pain conditions, such as neuropathic pain, management in children is often empirically based on data from studies in adults. There is a major need for further clinical research, training of health-care providers, and increased resources, to improve management and outcomes for children with chronic pain.

Extracellular signal-regulated kinase (ERK) and protein kinase B (AKT) pathways involved in spinal cord stimulation (SCS)-induced vasodilation

BACKGROUND AND AIMS

SCS is used to improve peripheral circulation in selected patients with ischemia of the extremities. However the mechanisms are not fully understood. The present study investigated whether blockade of ERK and AKT activation modulated SCS-induced vasodilation.

METHODS

A unipolar ball electrode was placed on the left dorsal column at the lumbar 2-3 spinal segments in rats. Cutaneous blood flows from left and right hind foot pads were recorded with laser Doppler flow perfusion monitors. SCS was applied through a ball electrode at 60% or 90% of MT. U0126, an inhibitor of ERK kinase, or LY294002, an inhibitor of PI3K upstream of AKT, was applied to the lumbar 3-5 spinal segments (n=7, each group).

RESULTS

U0126 (100 nM, 5 microM and 250 microM) significantly attenuated SCS-induced vasodilation at 60% (100 nM: P<0.05; 5 microM and 250 microM: P<0.01, respectively) and 90% of MT (100 nM and 5 microM: P<0.05; 250 microM: P<0.01, respectively). LY294002 at 100 microM also attenuated SCS-induced vasodilation at 60% and 90% of MT (P<0.05).

CONCLUSIONS

These data suggest that ERK and AKT pathways are involved in SCS-induced vasodilation.

Characterization of spinal alpha-adrenergic modulation of nociceptive transmission and hyperalgesia throughout postnatal development in rats

BACKGROUND AND PURPOSE

The selective alpha(2)-adrenergic agonist dexmedetomidine is used clinically for analgesia and sedation, but effects in early life are not well characterized. Investigation of age-related effects of dexmedetomidine is important for evaluating responses to exogenously administered analgesics and provides insight into postnatal function of noradrenergic pathways.

EXPERIMENTAL APPROACH

We examined effects of epidural dexmedetomidine in anaesthetized rat pups (3, 10 and 21 postnatal days) using a quantitative model of nociception and C-fibre induced hyperalgesia. Electromyographic recordings of withdrawal responses to hindpaw mechanical stimuli measured effects of dexmedetomidine upon the baseline reflex and the response to mustard oil application on the hindpaw (primary hyperalgesia) or hindlimb (secondary hyperalgesia). In addition, we compared epidural with systemic administration, examined effects of spinal transection and evaluated heart rate changes following dexmedetomidine.

KEY RESULTS

Epidural dexmedetomidine dose-dependently prevented mustard oil-induced hyperalgesia at all ages but dose requirements were lower in the youngest pups. Higher doses also suppressed the baseline nociceptive reflex when given epidurally, but had no effect when given systemically. Analgesic efficacy was the same for primary and secondary hyperalgesia, and was not diminished by spinal cord transection.

CONCLUSIONS AND IMPLICATIONS

Our laboratory studies predict that spinally mediated alpha(2)-agonist analgesia would be effective throughout postnatal development, dose requirements would be lower in early life and selective anti-hyperalgesic effects could be achieved with epidural administration at doses lower than associated with antinociceptive or cardiovascular effects. Clinical trials of alpha(2) agonists in neonates and infants should consider developmentally regulated changes.