Excitatory synapses in the rat superficial dorsal horn are strengthened following peripheral inflammation during early postnatal development

Current Evidence for Biological Biomarkers and Mechanisms Underlying Acute to Chronic Pain Transition across the Pediatric Age Spectrum

Chronic pain is highly prevalent in the pediatric population. Many factors are involved in the transition from acute to chronic pain. Currently, there are conceptual models proposed, but they lack a mechanistically sound integrated theory considering the stages of child development. Objective biomarkers are critically needed for the diagnosis, risk stratification, and prognosis of the pathological stages of pain chronification. In this article, we summarize the current evidence on mechanisms and biomarkers of acute to chronic pain transitions in infants and children through the developmental lens. The goal is to identify gaps and outline future directions for basic and clinical research toward a developmentally informed theory of pain chronification in the pediatric population. At the outset, the importance of objective biomarkers for chronification of pain in children is outlined, followed by a summary of the current evidence on the mechanisms of acute to chronic pain transition in adults, in order to contrast with the developmental mechanisms of pain chronification in the pediatric population. Evidence is presented to show that chronic pain may have its origin from insults early in life, which prime the child for the development of chronic pain in later life. Furthermore, available genetic, epigenetic, psychophysical, electrophysiological, neuroimaging, neuroimmune, and sex mechanisms are described in infants and older children. In conclusion, future directions are discussed with a focus on research gaps, translational and clinical implications. Utilization of developmental mechanisms framework to inform clinical decision-making and strategies for prevention and management of acute to chronic pain transitions in children, is highlighted.

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

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

The development of pain circuits and unique effects of neonatal injury

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

Neonatal vincristine administration modulates intrinsic neuronal excitability in the rat dorsal root ganglion and spinal dorsal horn during adolescence

Our recent work has shown that the early-life administration of vincristine (VNC), commonly used to treat pediatric cancers, evokes mechanical pain hypersensitivity in rats that emerges during adolescence and persists into adulthood. However, the underlying mechanisms remain unclear, as nothing is known about how neonatal VNC treatment influences peripheral and central nociceptive processing at the cellular level. Here, we used in vitro intracellular microelectrode and whole-cell patch-clamp recordings to evaluate the consequences of early-life VNC administration on the intrinsic membrane properties of adolescent dorsal root ganglion and spinal superficial dorsal horn neurons. The results demonstrate that VNC treatment increased the prevalence and rate of repetitive firing in both large- and medium-diameter sensory neurons, while reducing repetitive firing in small-diameter neurons, in comparison with vehicle-treated littermate controls. By contrast, passive membrane properties and peripheral conduction velocities were similar between experimental groups across all classes of primary afferents. Within the adolescent superficial dorsal horn, neonatal VNC exposure significantly enhanced the intrinsic membrane excitability of lamina I spinoparabrachial neurons, as evidenced by a decrease in rheobase and elevation of repetitive firing frequency compared with controls. Meanwhile, putative interneurons within lamina I exhibited a reduction in repetitive action potential discharge after early-life chemotherapy. Collectively, these findings suggest that neonatal VNC treatment evokes cell type-specific changes in intrinsic excitability at multiple levels of the ascending pain pathway. Overall, this work lays an essential foundation for the future exploration of the ionic mechanisms that drive chemotherapy-induced chronic pain in children and adolescents.

Excitability of Rat Superficial Dorsal Horn Neurons Following a Neonatal Immune Challenge

Previous studies have shown that neonatal exposure to a mild inflammatory challenge, such as lipopolysaccharide (LPS, Salmonella enteriditis) results in altered pain behaviors later in life. To further characterize the impact of a neonatal immune challenge on pain processing, we examined the excitability of superficial dorsal horn (SDH) neurons following neonatal LPS exposure and subsequent responses to noxious stimulation at three time-points during early postnatal development. Wistar rats were injected with LPS (0.05 mg/kg i.p.) or saline on postnatal days (PNDs) 3 and 5, and later subjected to the formalin test at PNDs 7, 13, and 22. One hour after formalin injection into the plantar hindpaw, animals were euthanized (Ketamine, 100 mg/kg i.p.) and transverse slices from the lumbosacral spinal cord were prepared. Whole-cell patch-clamp recordings were made from SDH neurons (KCH₃SO₄-based internal, 22–24°C) on the ipsi- and contralateral sides of the spinal cord. Depolarising current steps were injected into SDH neurons to categorize action potential (AP) discharge. In both saline- and LPS-treated rats we observed age-related increases the percentage of neurons exhibiting tonic-firing, with concurrent decreases in single-spiking, between PND 7 and 22. In contrast, neonatal exposure to LPS failed to alter the proportions of AP discharge patterns at any age examined. We also assessed the subthreshold currents that determine AP discharge in SDH neurons. The rapid outward potassium current, I_Ar decreased in prevalence with age, but was susceptible to neonatal LPS exposure. Peak I_Ar current amplitude was greater in ipsilateral vs. contralateral SDH neurons from LPS-treated rats. Spontaneous excitatory synaptic currents (sEPSCs) were recorded to assess network excitability. Age-related increases were observed in sEPSC frequency and time course, but not peak amplitude, in both saline- and LPS-treated rats. Furthermore, sEPSC frequency was higher in ipsilateral vs. contralateral SDH neurons in LPS-treated animals. Taken together, these data suggest a neonatal immune challenge does not markedly affect the intrinsic properties of SDH neurons, however, it can increase the excitability of local spinal cord networks via altering the properties of rapid A-type currents and excitatory synaptic connections. These changes, made in neurons within spinal cord pain circuits, have the capacity to alter nociceptive signaling in the ascending pain pathway.

Growth hormone regulates the sensitization of developing peripheral nociceptors during cutaneous inflammation

Cutaneous inflammation alters the function of primary afferents and gene expression in the affected dorsal root ganglia (DRG). However, specific mechanisms of injury-induced peripheral afferent sensitization and behavioral hypersensitivity during development are not fully understood. Recent studies in children suggest a potential role for growth hormone (GH) in pain modulation. Growth hormone modulates homeostasis and tissue repair after injury, but how GH affects nociception in neonates is not known. To determine whether GH played a role in modulating sensory neuron function and hyperresponsiveness during skin inflammation in young mice, we examined behavioral hypersensitivity and the response properties of cutaneous afferents using an ex vivo hairy skin-saphenous nerve-DRG-spinal cord preparation. Results show that inflammation of the hairy hind paw skin initiated at either postnatal day 7 (P7) or P14 reduced GH levels specifically in the affected skin. Furthermore, pretreatment of inflamed mice with exogenous GH reversed mechanical and thermal hypersensitivity in addition to altering nociceptor function. These effects may be mediated through an upregulation of insulin-like growth factor 1 receptor (IGFr1) as GH modulated the transcriptional output of IGFr1 in DRG neurons in vitro and in vivo. Afferent-selective knockdown of IGFr1 during inflammation also prevented the observed injury-induced alterations in cutaneous afferents and behavioral hypersensitivity similar to that after GH pretreatment. These results suggest that GH can block inflammation-induced nociceptor sensitization during postnatal development leading to reduced pain-like behaviors, possibly by suppressing the upregulation of IGFr1 within DRG.

Rewiring of Developing Spinal Nociceptive Circuits by Neonatal Injury and Its Implications for Pediatric Chronic Pain

Significant evidence now suggests that neonatal tissue damage can evoke long-lasting changes in pain sensitivity, but the underlying cellular and molecular mechanisms remain unclear. This review highlights recent advances in our understanding of how injuries during a critical period of early life modulate the functional organization of synaptic networks in the superficial dorsal horn (SDH) of the spinal cord in a manner that favors the excessive amplification of ascending nociceptive signaling to the brain, which likely contributes to the generation and/or maintenance of pediatric chronic pain. These persistent alterations in synaptic function within the SDH may also contribute to the well-documented "priming" of developing pain pathways by neonatal tissue injury.

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.

Contrasting alterations to synaptic and intrinsic properties in upper-cervical superficial dorsal horn neurons following acute neck muscle inflammation

Background

Acute and chronic pain in axial structures, like the back and neck, are difficult to treat, and have incidence as high as 15%. Surprisingly, most preclinical work on pain mechanisms focuses on cutaneous structures in the limbs and animal models of axial pain are not widely available. Accordingly, we developed a mouse model of acute cervical muscle inflammation and assessed the functional properties of superficial dorsal horn (SDH) neurons.

Results

Male C57/Bl6 mice (P24-P40) were deeply anaesthetised (urethane 2.2 g/kg i.p) and the rectus capitis major muscle (RCM) injected with 40 μl of 2% carrageenan. Sham animals received vehicle injection and controls remained anaesthetised for 2 hrs. Mice in each group were sacrificed at 2 hrs for analysis. c-Fos staining was used to determine the location of activated neurons. c-Fos labelling in carrageenan-injected mice was concentrated within ipsilateral (87% and 63% of labelled neurons in C1 and C2 segments, respectively) and contralateral laminae I - II with some expression in lateral lamina V. c-Fos expression remained below detectable levels in control and sham animals. In additional experiments, whole cell recordings were obtained from visualised SDH neurons in transverse slices in the ipsilateral C1 and C2 spinal segments. Resting membrane potential and input resistance were not altered. Mean spontaneous EPSC amplitude was reduced by ~20% in neurons from carrageenan-injected mice versus control and sham animals (20.63 ± 1.05 vs. 24.64 ± 0.91 and 25.87 ± 1.32 pA, respectively). The amplitude (238 ± 33 vs. 494 ± 96 and 593 ± 167 pA) and inactivation time constant (12.9 ± 1.5 vs. 22.1 ± 3.6 and 15.3 ± 1.4 ms) of the rapid A type potassium current (I_Ar), the dominant subthreshold current in SDH neurons, were reduced in carrageenan-injected mice.

Conclusions

Excitatory synaptic drive onto, and important intrinsic properties (i.e., I_Ar) within SDH neurons are reduced two hours after acute muscle inflammation. We propose this time point represents an important transition period between peripheral and central sensitisation with reduced excitatory drive providing an initial neuroprotective mechanism during the early stages of the progression towards central sensitisation.

Alleviation of behavioral hypersensitivity in mouse models of inflammatory pain with two structurally different casein kinase 1 (CK1) inhibitors

BACKGROUND

The phylogenetically highly conserved CK1 protein kinases consisting of at least seven isoforms form a distinct family within the eukaryotic protein kinases. CK1 family members play crucial roles in a wide range of signaling activities. However, the functional role of CK1 in somatosensory pain signaling has not yet been fully understood. The aim of this study was to clarify the role of CK1 in the regulation of inflammatory pain in mouse carrageenan and complete Freund's adjuvant (CFA) models.

RESULTS

We have used two structurally different CK1 inhibitors, TG003 and IC261. TG003, which was originally identified as a cdc2-like kinase inhibitor, had potent inhibitory effects on CK1 isoforms in vitro and in cultured cells. Intrathecal injection of either TG003 (1-100 pmol) or IC261 (0.1-1 nmol) dose-dependently decreased mechanical allodynia and thermal hyperalgesia induced by carrageenan or CFA. Bath-application of either TG003 (1 μM) or IC261 (1 μM) had only marginal effects on spontaneous excitatory postsynaptic currents (sEPSCs) recorded in the substantia gelatinosa neurons of control mice. However, both compounds decreased the frequency of sEPSCs in both inflammatory pain models.

CONCLUSIONS

These results suggest that CK1 plays an important pathophysiological role in spinal inflammatory pain transmission, and that inhibition of the CK1 activity may provide a novel strategy for the treatment of inflammatory pain.

Sex differences in pain responses at maturity following neonatal repeated minor pain exposure in rats

There is mounting evidence of long-lasting changes in pain sensitivity in school-age children who were cared for in a neonatal intensive care unit. Such care involves multiple pain exposures, 70% of which are accounted for by heel lance to monitor physiological well-being. The authors sought to model the repeated brief pain resulting from heel lance by administering repeated paw needle stick to neonatal rat pups. Repeated needle stick during the first 8 days of life was sex-specific in altering responses to mechanical and inflammatory stimuli, but not to a thermal stimulus, at maturity. Specifically, neonatal paw needle stick males exhibited significantly greater mechanical sensitivity in response to von Frey hair testing, whereas neonatal paw needle stick females exhibited significantly greater pain behavior scores following hindpaw formalin injection. This is the first study to show such sex-dependent changes in pain responsiveness at maturity in animals having experienced repeated neonatal needle stick pain. These findings support existing evidence that there are long-term sensory sequelae following neonatal pain experiences in rats and further suggest that there are sex-linked differences in the nature of the consequences. If these relationships hold in humans, these findings suggest that even mild painful insults early in life are not without sensory consequences.

Neonatal repetitive needle pricking: plasticity of the spinal nociceptive circuit and extended postoperative pain in later life

Repetitive exposure of neonates to noxious events is inherent to their health status monitoring in neonatal intensive care units (NICU). Altered basal nociception in the absence of an injury in later life has been demonstrated in ex-NICU children, but the impact on pain hypersensitivity following an injury in later life is unknown. Also, underlying mechanisms for such long-term changes are relatively unknown. The objective of this study is to investigate acute and long-term effects of neonatal repetitive painful skin-breaking procedures on nociception and to investigate plasticity of the nociceptive circuit. The repetitive needle prick animal model was used in which neonatal rats received four needle pricks into the left hind paw per day during the first postnatal week and control animals received nonpainful tactile stimuli. Repetitive needle pricking during the first week of life induced acute hypersensitivity to mechanical stimuli. At the age of 8 weeks, increased duration of postoperative hypersensitivity to mechanical stimuli after ipsilateral hind paw incision was shown in needle prick animals. Basal nociception from 3 to 8 weeks of age was unaffected by neonatal repetitive needle pricking. Increased calcitonin gene-related peptide expression was observed in the ipsilateral and contralateral lumbar spinal cord but not in the hind paw of needle prick animals at the age of 8 weeks. Innervation of tactile Aβ-fibers in the spinal cord was not affected. Our results indicate both acute and long-term effects of repetitive neonatal skin breaking procedures on nociception and long-term plasticity of spinal but not peripheral innervation of nociceptive afferents.

Are all spinal segments equal: intrinsic membrane properties of superficial dorsal horn neurons in the developing and mature mouse spinal cord

Neurons in the superficial dorsal horn (SDH; laminae I-II) of the spinal cord process nociceptive information from skin, muscle, joints and viscera. Most of what we know about the intrinsic properties of SDH neurons comes from studies in lumbar segments of the cord even though clinical evidence suggests nociceptive signals from viscera and head and neck tissues are processed differently. This ‘lumbar-centric' view of spinal pain processing mechanisms also applies to developing SDH neurons. Here we ask whether the intrinsic membrane properties of SDH neurons differ across spinal cord segments in both the developing and mature spinal cord. Whole cell recordings were made from SDH neurons in slices of upper cervical (C2-4), thoracic (T8-10) and lumbar (L3-5) segments in neonatal (P0-5) and adult (P24-45) mice. Neuronal input resistance (R(IN)), resting membrane potential, AP amplitude, half-width and AHP amplitude were similar across spinal cord regions in both neonates and adults (∼100 neurons for each region and age). In contrast, these intrinsic membrane properties differed dramatically between neonates and adults. Five types of AP discharge were observed during depolarizing current injection. In neonates, single spiking dominated (∼40%) and the proportions of each discharge category did not differ across spinal regions. In adults, initial bursting dominated in each spinal region, but was significantly more prevalent in rostral segments (49% of neurons in C2-4 vs. 29% in L3-5). During development the dominant AP discharge pattern changed from single spiking to initial bursting. The rapid A-type potassium current (I(Ar)) dominated in neonates and adults, but its prevalence decreased (∼80% vs. ∼50% of neurons) in all regions during development. I(Ar) steady state inactivation and activation also changed in upper cervical and lumbar regions during development. Together, our data show the intrinsic properties of SDH neurons are generally conserved in the three spinal cord regions examined in both neonate and adult mice. We propose the conserved intrinsic membrane properties of SDH neurons along the length of the spinal cord cannot explain the marked differences in pain experienced in the limbs, viscera, and head and neck.

Developmental differences in peripheral glabrous skin mechanosensory nerve receptive field and intracellular electrophysiologic properties: phenotypic characterization in infant and juvenile rats

Developmental differences in peripheral neuron characteristics and functionality exist. Direct measurement of active and passive electrophysiologic and receptive field characteristics of single mechanosensitive neurons in glabrous skin was performed and phenotypic characterization of fiber subtypes was applied to analyze developmental differences in peripheral mechanosensitive afferents. After Institutional approval, male Sprague-Dawley infant (P7: postnatal day 7) and juvenile (P28) rats were anesthetized and single cell intracellular electrophysiology was performed in the dorsal root ganglion (DRG) soma of mechanosensitive cells with receptive field (RF) in the glabrous skin of the hindpaw. Passive and active electrical properties of the cells and RF size and characteristics determined. Fiber subtype classification was performed and developmental differences in fiber subtype properties analyzed. RF size was smaller at P7 for both low and high threshold mechanoreceptor (LTMR and HTMR) with no differences between A- and C-HTMR (AHTMR and CHTMR). The RF size was also correlated to anatomic location on glabrous skin, toes having smaller RF. Conduction velocity (CV) was adequate at P28 for AHTMR and CHTMR classification, but not at P7. Only width of the action potential at half height (D50) was significantly different between HTMR at P7, while D50, CV and amplitude of the AP were significant for HTMR at P28. RF size is determined in part by the RF distribution of the peripheral neuron. Developmental differences in RF size occur with larger RF sizes occurring in younger animals. This is consistent with RF size differences determined by measuring RF in the spinal cord, except the peripheral RF is much smaller, more refined, and in some cases pinpoint. Developmental differences make CV alone unreliable for neuron classification. Utilizing integration of all measured parameters allows classification of neurons into subtypes even at the younger ages. This will prove important in understanding changes that occur in the peripheral sensory afferents in the face of ongoing development and injury early in life.

Neonatal tissue damage facilitates nociceptive synaptic input to the developing superficial dorsal horn via NGF-dependent mechanisms

Tissue injury during a critical period of early life can facilitate spontaneous glutamatergic transmission within developing pain circuits in the superficial dorsal horn (SDH) of the spinal cord. However, the extent to which neonatal tissue damage strengthens nociceptive synaptic input to specific subpopulations of SDH neurons, as well as the mechanisms underlying this distinct form of synaptic plasticity, remains unclear. Here we use in vitro whole-cell patch clamp recordings from rodent spinal cord slices to demonstrate that neonatal surgical injury selectively potentiates high-threshold primary afferent input to immature lamina II neurons. In addition, the increase in the frequency of miniature excitatory postsynaptic currents after hindpaw incision was prevented by neonatal capsaicin treatment, suggesting that early tissue injury enhances glutamate release from nociceptive synapses. This occurs in a widespread manner within the developing SDH, as incision elevated miniature excitatory postsynaptic current frequency in both GABAergic and presumed glutamatergic lamina II neurons of Gad-GFP transgenic mice. The administration of exogenous nerve growth factor into the rat hindpaw mimicked the effects of early tissue damage on excitatory synaptic function, while blocking trkA receptors in vivo abolished the changes in both spontaneous and primary afferent-evoked glutamatergic transmission following incision. These findings illustrate that neonatal tissue damage can alter the gain of developing pain pathways by activating nerve growth factor-dependent signaling cascades, which modify synaptic efficacy at the first site of nociceptive processing within the central nervous system.

Central sensitization: implications for the diagnosis and treatment of pain

Nociceptor inputs can trigger a prolonged but reversible increase in the excitability and synaptic efficacy of neurons in central nociceptive pathways, the phenomenon of central sensitization. Central sensitization manifests as pain hypersensitivity, particularly dynamic tactile allodynia, secondary punctate or pressure hyperalgesia, aftersensations, and enhanced temporal summation. It can be readily and rapidly elicited in human volunteers by diverse experimental noxious conditioning stimuli to skin, muscles or viscera, and in addition to producing pain hypersensitivity, results in secondary changes in brain activity that can be detected by electrophysiological or imaging techniques. Studies in clinical cohorts reveal changes in pain sensitivity that have been interpreted as revealing an important contribution of central sensitization to the pain phenotype in patients with fibromyalgia, osteoarthritis, musculoskeletal disorders with generalized pain hypersensitivity, headache, temporomandibular joint disorders, dental pain, neuropathic pain, visceral pain hypersensitivity disorders and post-surgical pain. The comorbidity of those pain hypersensitivity syndromes that present in the absence of inflammation or a neural lesion, their similar pattern of clinical presentation and response to centrally acting analgesics, may reflect a commonality of central sensitization to their pathophysiology. An important question that still needs to be determined is whether there are individuals with a higher inherited propensity for developing central sensitization than others, and if so, whether this conveys an increased risk in both developing conditions with pain hypersensitivity, and their chronification. Diagnostic criteria to establish the presence of central sensitization in patients will greatly assist the phenotyping of patients for choosing treatments that produce analgesia by normalizing hyperexcitable central neural activity. We have certainly come a long way since the first discovery of activity-dependent synaptic plasticity in the spinal cord and the revelation that it occurs and produces pain hypersensitivity in patients. Nevertheless, discovering the genetic and environmental contributors to and objective biomarkers of central sensitization will be highly beneficial, as will additional treatment options to prevent or reduce this prevalent and promiscuous form of pain plasticity.

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.

Activity-dependent modulation of glutamatergic signaling in the developing rat dorsal horn by early tissue injury

Tissue injury in early life can produce distinctive effects on pain processing, but little is known about the underlying neural mechanisms. Neonatal inflammation modulates excitatory synapses in spinal nociceptive circuits, but it is unclear whether this results directly from altered afferent input. Here we investigate excitatory and inhibitory synaptic transmission in the rat superficial dorsal horn following neonatal hindlimb surgical incision using in vitro patch-clamp recordings and test the effect of blocking peripheral nerve activity on the injury-evoked changes. Surgical incision through the skin and muscle of the hindlimb at postnatal day 3 (P3) or P10 selectively increased the frequency, but not amplitude, of glutamatergic miniature excitatory postsynaptic currents (mEPSCs) recorded 2-3 days after injury, without altering miniature inhibitory postsynaptic current frequency or amplitude at this time point. Meanwhile, incision at P17 failed to affect excitatory or inhibitory synaptic function at 2-3 days postinjury. The elevated mEPSC frequency was accompanied by increased inward rectification of evoked alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR)-mediated currents, but no change in AMPAR/N-methyl-D-aspartate receptor ratios, and was followed by a persistent reduction in mEPSC frequency by 9-10 days postinjury. Prolonged blockade of primary afferent input from the time of injury was achieved by administration of bupivacaine hydroxide or tetrodotoxin to the sciatic nerve at P3. The increase in mEPSC frequency evoked by P3 incision was prevented by blocking sciatic nerve activity. These results demonstrate that increased afferent input associated with peripheral tissue injury selectively modulates excitatory synaptic drive onto developing spinal sensory neurons and that the enhanced glutamatergic signaling in the dorsal horn following neonatal surgical incision is activity dependent.

Substance P enhances excitatory synaptic transmission on spinally projecting neurons in the rostral ventromedial medulla after inflammatory injury

It has been proposed, but not directly tested, that persistent inflammatory nociception enhances excitatory glutamatergic inputs to neurons in the rostral ventromedial medulla (RVM), altering the activity and function of these neurons. This study used whole cell patch-clamp methods to record evoked excitatory postsynaptic currents (eEPSCs) in spinally projecting RVM neurons from rats injected with saline or complete Freund's adjuvant (CFA) 3-4 days earlier and to examine the role of substance P (SP) in modulating excitatory synaptic transmission. Input-output relationships demonstrated that CFA treatment facilitated fast excitatory glutamatergic inputs to type 1 and type 2 nonserotonergic spinally projecting RVM neurons, but not to type 3 neurons. The facilitation in type 1 and 2 neurons was dependent on neurokinin-1 (NK1) and N-methyl-d-aspartate (NMDA) receptors and prevented by the PKC inhibitor GF109203X. In a subset of neurons from naïve rats, SP mimicked the effects of CFA and increased the potency and efficacy of glutamatergic synaptic transmission. The facilitation was prevented by 10 microM GF109203X, but not by 10 microM KN93, a CaMKII inhibitor. SP (0.3-3 microM) by itself produced concentration-dependent inward currents in most nonserotonergic, but not serotonergic neurons. The present study is the first demonstration, at the cellular level, that persistent inflammatory nociception leads to a sustained facilitation of fast excitatory glutamatergic inputs to RVM neurons by an NK1 and NMDA receptor-dependent mechanism that involves PKC. Further, it demonstrates that the facilitation is restricted to specific populations of RVM neurons that by inference may be pain facilitatory neurons.