Search

Although chronic pain states have been associated with impaired cognitive functions, including memory and cognitive flexibility, the cognitive effects of osteoarthritis (OA) pain remain to be clarified. The aim of this study was to measure cognitive function in the mono-iodoacetate (MIA) rat model of chronic OA-like knee pain.We used young adult male Lister hooded rats, which are well suited for cognitive testing. Rats received either a unilateral knee injection of MIA (3mg/50µL) or saline as control. Joint pain at rest was assessed for up to 12 weeks using weight-bearing asymmetry, and referred pain at a distal site using determination of hindpaw withdrawal thresholds (PWT). The watermaze delayed-matching-to-place (DMP) test of rapid place learning, novel object recognition (NOR) memory assay and an operant response-shift and -reversal task were used to measure memory and behavioural flexibility. Open field locomotor activity, startle response, and prepulse inhibition (PPI) were also measured for comparison.MIA-injected rats showed markedly reduced weight-bearing on the injured limb, as well as pronounced cartilage damage and synovitis, but interestingly no changes in PWT. Rearing was reduced, but otherwise locomotor activity was normal and no changes in startle and PPI were detected. MIA-injected rats had intact watermaze DMP performance, suggesting no substantial change in hippocampal function, and there were no changes in NOR memory or performance on the operant task of behavioural flexibility. Our finding that OA-like pain does not alter hippocampal function, unlike other chronic pain conditions, is consistent with human neuroimaging findings.

Systemic responses to immune challenge are immature at birth. However exposure to experimental inflammogens are able to produce an immunologic response which is characterised by swelling and oedema but, unlike in adults, does not result in sensory hypersensitivity. We sought to investigate whether the lack of nociceptive hypersensitivity was as a result of altered hemapoietic immune cell recruitment to the site of inflammation and/or differences in the cytokine and chemokine profile released by tissue invading cells. Postnatal (day of birth) and young adult (40-days old) Sprague-Dawley rats were used. Inflammation was induced by s.c. injection of Complete Freunds Adjuvant (CFA) unilaterally into the one hind paw. Mechanical withdrawal thresholds were measured before and after injection (2-168hrs). In adults a significant hyperalgesia was evoked which was absent in neonates. Immunohistochemical analysis of invading immune cells present in the perfusion fixed skin showed that although total cell numbers in the paw were the same in both age groups, neonates recruited more cells positive for both cell surface markers CD68 and Mannose-receptor (MR) whereas adults recruited significantly more cells positive for MR alone. There were no differences in neutrophil recruitment (as measured with H&E staining). TaqMan qPCR demonstrated that the temporal profile of cytokine production in the skin differed between ages with neonates responding faster than adults and that neonates produced significantly more IL-1b and IL-27 then adults who expressed significantly more IL-6 and IL-10. This study illustrates that in neonates the cell recruitment and cytokine profiles are markedly different to those seen in adults; this may in part explain why behavioural responses to inflammation are suppressed relative to adults.

Supplemental Digital Content is Available in the Text. Comorbid anxiety and osteoarthritis pain is associated with increased opioid intake. Our translational model identifies reduced opioid analgesia and endogenous opioid dysfunction as potential mechanisms., Introduction: Negative affect, including anxiety and depression, is prevalent in chronic pain states such as osteoarthritis (OA) and associated with greater use of opioid analgesics, potentially contributing to present and future opioid crises. Objectives: We tested the hypothesis that the interaction between anxiety, chronic pain, and opioid use results from altered endogenous opioid function. Methods: A genetic model of negative affect, the Wistar–Kyoto (WKY) rat, was combined with intra-articular injection of monosodium iodoacetate (MIA; 1 mg) to mimic clinical presentation. Effects of systemic morphine (0.5–3.5 mg·kg−1) on pain behaviour and spinal nociceptive neuronal activity were compared in WKY and normo-anxiety Wistar rats 3 weeks after MIA injection. Endogenous opioid function was probed by the blockade of opioid receptors (0.1–1 mg·kg−1 systemic naloxone), quantification of plasma β-endorphin, and expression and phosphorylation of spinal mu-opioid receptor (MOR). Results: Monosodium iodoacetate–treated WKY rats had enhanced OA-like pain, blunted morphine-induced analgesia, and greater mechanical hypersensitivity following systemic naloxone, compared with Wistar rats, and elevated plasma β-endorphin levels compared with saline-treated WKY controls. Increased MOR phosphorylation at the master site (serine residue 375) in the spinal cord dorsal horn of WKY rats with OA-like pain (P = 0.0312) indicated greater MOR desensitization. Conclusions: Reduced clinical analgesic efficacy of morphine was recapitulated in a model of high anxiety and OA-like pain, in which endogenous opioid tone was altered, and MOR function attenuated, in the absence of previous exogenous opioid ligand exposure. These findings shed new light on the mechanisms underlying the increased opioid analgesic use in high anxiety patients with chronic pain.

Recognition of the need for alternative analgesic regimens for managing neonatal and childhood pain has led to a rich literature concerning the ways in which early life pain differs from that at older ages. As in adults, opiates are often considered the gold-standard analgesic class of drugs, of which morphine is the prototypical agent. There is a wealth of data detailing clinical observations, measurements, and interventions with regard to the use of opioids in treating pain in children. Studies in the early part of this century have highlighted that, in humans, age is an important factor that influences the morphine requirement of neonates following surgery, and dose requirements are influenced by both pharmacokinetic and pharmacodynamic factors. Laboratory studies have extended our understanding of changes within the peripheral and central nervous systems that underlie alterations in nociception in early life. This chapter will review what is currently known about the actions of opioids upon nociceptive and nociresponsive elements of the nervous system in early life, how they differ from adult responses, and ask whether manipulating endogenous opioid systems in early life may have consequences on neurodevelopment.

Background Neuroinflammation is a critical feature of sensitisation of spinal nociceptive processing in chronic pain states. We hypothesised that the resolvin pathways, a unique endogenous control system, may ameliorate aberrant spinal processing of somatosensory inputs associated with chemotherapy-induced neuropathic pain (CINP). Method The paclitaxel (PCX) model of CINP was established in male Sprague-Dawley rats and compared to control rats (n = 23 and 22, respectively). Behavioural pain responses were measured, and either single unit electrophysiological recordings of dorsal horn wide dynamic range (WDR) neurones were performed, or mRNA microarray analysis of the dorsal horn of the spinal cord was undertaken. Results PCX rats exhibited significant changes in behavioural responses to mechanical and cold stimuli. A higher proportion of WDR neurones in PCX rats were polymodal (generating post-discharge following a non-noxious mechanical stimulus, responding to non-noxious cold and exhibiting spontaneous activity) compared to control (p < 0.05). Microarray analysis revealed changes in proinflammatory pathways (Tlr, Tnfrsf1a, Nlrp1a, Cxcr1, Cxcr5, Ccr1, Cx3cr1) and anti-inflammatory lipid resolvin pathways (Alox5ap, Cyp2j4 and Ptgr1) compared to control (p < 0.05). Ingenuity pathway analysis predicted changes in glutamatergic and astrocyte signaling in the PCX group. Activation of the resolvin system via the spinal administration of aspirin-triggered resolvin D1 (AT-RvD1) markedly inhibited (73 ± 7% inhibition) normally non-noxious mechanically (8 g) evoked responses of WDR neurones only in PCX rats, whilst leaving responses to noxious mechanically induced stimuli intact. Inhibitory effects of AT-RvD1were comparable in magnitude to spinal morphine (84 ± 4% inhibition). Conclusion The PCX model of CINP was associated with mechanical allodynia, altered neuronal responses and dysregulation of pro- and anti-inflammatory signalling in the spinal dorsal horn. The resolvin AT-RvD1 selectively inhibited low weight mechanical-evoked responses of WDR neurones in PCX rats, but not in controls. Our data support the targeting of spinal neuroinflammation via the activation of the resolvin system as a new therapeutic approach for CINP.

Introduction: \ud Inflammation during the neonatal period can exacerbate pain severity following reinjury in adulthood. This is driven by alterations in the postnatal development of spinal and supraspinal nociceptive circuitry. However, the contribution of alterations in peripheral nociceptor function remains underexplored.\ud Objectives: \ud We examined whether neonatal complete Freund's adjuvant (CFA)-induced inflammation induced or altered adult development of hyperalgesic priming (inflammation-induced plasticity in nonpeptidergic C fibres) or altered postnatal reorganization of calcitonin gene-related peptide (CGRP)-expressing and isolectin B4 (IB4)-binding C fibres in the spinal dorsal horn (DH).\ud Methods: \ud \ud After intraplantar injection of CFA at postnatal day (P) 1, we assessed mechanical thresholds in adult (P60) rats before and after intraplantar carrageenan. One week later, intraplantar PGE2-induced hypersensitivity persisting for 4 hours was deemed indicative of hyperalgesic priming. CGRP expression and IB4 binding were examined in adult rat DH after CFA.\ud Results: \ud P1 CFA did not alter baseline adult mechanical thresholds, nor did it change the extent or duration of carrageenan-induced hypersensitivity. However, this was slower to resolve in female than in male rats. Rats that previously received carrageenan but not saline were primed, but P1 hind paw CFA did not induce or alter hyperalgesic priming responses to PGE2. In addition, CFA on P1 or P10 did not alter intensity or patterns of CGRP or IB4 staining in the adult DH.\ud Conclusion: \ud Complete Freund's adjuvant-induced inflammation during a critical period of vulnerability to injury during early postnatal development does not induce or exacerbate hyperalgesic priming or alter the broad distribution of CGRP-expressing or IB4-binding afferent terminals in the adult dorsal horn.

The descending pain modulatory system (DPMS) constitutes a network of widely distributed brain regions whose integrated function is essential for effective modulation of sensory input to the central nervous system and behavioural responses to pain. Animal studies demonstrate that young rodents have an immature DPMS, but comparable studies have not been conducted in human infants. In Goksan et al. (2015) we used functional MRI (fMRI) to show that pain-related brain activity in newborn infants is similar to that observed in adults. Here, we investigated whether the functional network connectivity strength across the infant DPMS influences the magnitude of this brain activity. FMRI scans were collected while mild mechanical noxious stimulation was applied to the infant’s foot. Greater pre-stimulus functional network connectivity across the DPMS was significantly associated with lower noxious-evoked brain activity (p = 0.0004, r = -0.86, n = 13), suggesting that in newborn infants the DPMS may regulate the magnitude of noxious-evoked brain activity.

Key points Traditional, widely used in vivo electrophysiological techniques for the investigation of spinal processing of somatosensory information fail to account for the diverse functions of each lamina.To overcome this oversimplification, we have used multi‐electrode arrays, in vivo, to simultaneously record neuronal activity across all laminae of the spinal dorsal horn.Multi‐electrode arrays are sensitive enough to detect lamina‐ and region‐specific encoding of different subtypes of afferent fibres and to detect short‐lived changes in synaptic plasticity as measured by the application of cutaneous electrical stimulation of varying intensity and frequency.Differential encoding of innocuous and noxious thermal and mechanical stimuli were also detected across the laminae with the technique, as were the effects of the application of capsaicin.This new approach to the study of the dorsal spinal cord produces significantly more information per experiment, permitting accelerated research whilst also permitting the effects of pharmacological tools to modulate network responses. Abstract The dorsal horn (DH) of the spinal cord is a complex laminar structure integrating peripheral signals into the central nervous system. Spinal somatosensory processing is commonly measured electrophysiologically in vivo by recording the activity of individual wide‐dynamic‐range neurons in the deep DH and extrapolating their behaviour to all cells in every lamina. This fails to account for the specialized processes that occur in each lamina and the considerable heterogeneity in cellular phenotype within and between laminae. Here we overcome this oversimplification by employing linear multi‐electrode arrays (MEAs) in the DH of anaesthetized rats to simultaneously measure activity across all laminae. The MEAs, comprising 16 channels, were inserted into the lumbar dorsal horn and peripheral neurons activated electrically via transcutaneous electrodes and ethologically with von Frey hairs (vFHs) or an aluminium heating block. Ascending electrical stimuli showed fibre thresholds with distinct dorsoventral innervation profiles. Wind up was observed across the DH during the C‐fibre and post‐discharge latencies following 0.5 Hz stimulation. Intrathecal application of morphine (5 ng/50 μl) significantly reduced Aδ‐ and C‐fibre‐evoked activity in deep and superficial DH. Light vFHs (≤10 g) predominantly activated intermediate and deep laminae whereas noxious vFHs (26 g) also activated the superficial laminae. Noxious heat (55°C) induced significantly greater activity in the superficial and deep laminae than the innocuous control (30°C). The application of these arrays produced the first description of the processing of innocuous and noxious stimuli throughout the intact DH., Key points Traditional, widely used in vivo electrophysiological techniques for the investigation of spinal processing of somatosensory information fail to account for the diverse functions of each lamina.To overcome this oversimplification, we have used multi‐electrode arrays, in vivo, to simultaneously record neuronal activity across all laminae of the spinal dorsal horn.Multi‐electrode arrays are sensitive enough to detect lamina‐ and region‐specific encoding of different subtypes of afferent fibres and to detect short‐lived changes in synaptic plasticity as measured by the application of cutaneous electrical stimulation of varying intensity and frequency.Differential encoding of innocuous and noxious thermal and mechanical stimuli were also detected across the laminae with the technique, as were the effects of the application of capsaicin.This new approach to the study of the dorsal spinal cord produces significantly more information per experiment, permitting accelerated research whilst also permitting the effects of pharmacological tools to modulate network responses.

The synthetic peptide PnPP-19 comprehends 19 amino acid residues and it represents part of the primary structure of the toxin δ-CNTX-Pn1c (PnTx2-6), isolated from the venom of the spider Phoneutria nigriventer. Behavioural tests suggest that PnPP-19 induces antinociception by activation of CB1, μ and δ opioid receptors. Since the peripheral and central antinociception induced by PnPP-19 involves opioid activation, the aim of this work was to identify whether this synthetic peptide could directly activate opioid receptors and investigate the subtype selectivity for μ-, δ- and/or κ-opioid receptors. Furthermore, we also studied the modulation of calcium influx driven by PnPP-19 in dorsal root ganglion neurons, and analyzed whether this modulation was opioid-mediated. PnPP-19 selectively activates μ-opioid receptors inducing indirectly inhibition of calcium channels and hereby impairing calcium influx in dorsal root ganglion (DRG) neurons. Interestingly, notwithstanding the activation of opioid receptors, PnPP-19 does not induce β-arrestin2 recruitment. PnPP-19 is the first spider toxin derivative that, among opioid receptors, selectively activates μ-opioid receptors. The lack of β-arrestin2 recruitment highlights its potential for the design of new improved opioid agonists. ispartof: TOXINS vol:10 issue:1 ispartof: location:Switzerland status: published

Primary somatosensory cortex (S1) contains a nociceptive map that localizes potential tissue damage on the body and encodes stimulus intensity. An objective and specific biomarker of pain however is currently lacking and is urgently required for use in non-verbal clinical populations as well as in the validation of pre-clinical pain models. Here we describe studies to see if the responses of the primary somatosensory cortex (S1) in juvenile rats are different to those in the adult. We recorded EEG responses from S1 of lightly-anaesthetised Sprague-Dawley rats at either postnatal day 21 or postnatal day 40 during the presentation of noxious (550C) or innocuous (300C) thermal stimuli applied to the plantar surface of the left hindpaw. The total EEG power across the recording period was the same in both ages after stimulation but the frequency distribution was significantly effected by age: noxious heat evoked a significant increase in theta band (4-8 Hz) activity in adults only (P

The mechanisms underlying the transition from acute nociceptive pain to centrally maintained chronic pain are not clear. We have studied the contributions of the peripheral and central nervous systems during the development of osteoarthritis (OA) pain. Male Sprague-Dawley rats received unilateral intra-articular injections of monosodium iodoacetate (MIA 1 mg) or saline, and weight-bearing (WB) asymmetry and distal allodynia measured. Subgroups of rats received intra-articular injections of, QX-314 (membrane impermeable local anaesthetic) + capsaicin, QX-314, capsaicin or vehicle on days 7, 14 or 28 post-MIA and WB and PWT remeasured. On days 7&14 post-MIA, but not day 28, QX-314 + capsaicin signficantly attenuated changes in WB induced by MIA, illustrating a crucial role for TRPV1 expressing nociceptors in early OA pain. The role of top-down control of spinal excitability was investigated. The mu-opioid receptor agonist DAMGO was microinjected into the rostroventral medulla, to activate endogenous pain modulatory systems, in MIA and control rats and reflex excitability measured using electromyography. DAMGO (3 ng) had a significantly larger inhibitory effect in MIA treated rats than in controls. These data show distinct temporal contribtuions of TRPV1 expressing nociceptors and opioidergic pain control systems at later timepoints.

Summary The increase in expression of endogenous opioidergic targets coincides with functional maturation of pain signalling in the third postnatal week of the rat., Significant opioid-dependent changes occur during the fourth postnatal week in supraspinal sites (rostroventral medulla [RVM], periaqueductal grey [PAG]) that are involved in the descending control of spinal excitability via the dorsal horn (DH). Here we report developmentally regulated changes in the opioidergic signalling within the PAG and DH, which further increase our understanding of pain processing during early life. Microinjection of the μ-opioid receptor (MOR) agonist DAMGO (30 ng) into the PAG of Sprague-Dawley rats increased spinal excitability and lowered mechanical threshold to noxious stimuli in postnatal day (P)21 rats, but had inhibitory effects in adults and lacked efficacy in P10 pups. A tonic opioidergic tone within the PAG was revealed in adult rats by intra-PAG microinjection of CTOP (120 ng, MOR antagonist), which lowered mechanical thresholds and increased spinal reflex excitability. Spinal adminstration of DAMGO inhibited spinal excitability in all ages, yet the magnitude of this was greater in younger animals than in adults. The expression of MOR and related peptides were also investigated using TaqMan real-time polymerase chain reaction and immunohistochemistry. We found that pro-opiomelanocortin peaked at P21 in the ventral PAG, and MOR increased significantly in the DH as the animals aged. Enkephalin mRNA transcripts preceded the increase in enkephalin immunoreactive fibres in the superficial dorsal horn from P21 onwards. These results illustrate that profound differences in the endogenous opioidergic signalling system occur throughout postnatal development.

ObjectivesBlockade of transient receptor potential vanilloid 1 (TRPV1) with systemic antagonists attenuates osteoarthritis (OA) pain behaviour in rat models, but on-target-mediated hyperthermia has halted clinical trials. The present study investigated the potential for targeting TRPV1 receptors within the OA joint in order to produce analgesia.MethodsThe presence of TRPV1 receptors in human synovium was detected using western blotting and immunohistochemistry. In a rat model of OA, joint levels of an endogenous ligand for TRPV1, 12-hydroxy-eicosatetraenoic acid (12-HETE), were quantified using liquid chromatography-tandem mass spectrometry (LC-MS/MS). Effects of peripheral administration of the TRPV1 receptor antagonist JNJ-17203212 on afferent fibre activity, pain behaviour and core body temperature were investigated. Effects of a spinal administration of JNJ-17203212 on dorsal horn neuronal responses were studied.ResultsWe demonstrate increased TRPV1 immunoreactivity in human OA synovium, confirming the diseased joint as a potential therapeutic target for TRPV1-mediated analgesia. In a model of OA pain, we report increased joint levels of 12-HETE, and the sensitisation of joint afferent neurones to mechanical stimulation of the knee. Local administration of JNJ-17203212 reversed this sensitisation of joint afferents and inhibited pain behaviour (weight-bearing asymmetry), to a comparable extent as systemic JNJ-17203212, in this model of OA pain, but did not alter core body temperature. There was no evidence for increased TRPV1 function in the spinal cord in this model of OA pain.ConclusionsOur data provide a clinical and mechanistic rationale for the future investigation of the therapeutic benefits of intra-articular administration of TRPV1 antagonists for the treatment of OA pain.

Background: Peripheral nerve injuries can trigger neuropathic pain in adults but cause little or no pain when they are sustained in infancy or early childhood. This is confirmed in rodent models where neonatal nerve injury causes no pain behaviour. However, delayed pain can arise in man some considerable time after nerve damage and to examine this following early life nerve injury we have carried out a longer term follow up of rat pain behaviour into adolescence and adulthood. Results: Spared nerve injury (SNI) or sham surgery was performed on 10 day old (P10) rat pups and mechanical nociceptive reflex thresholds were analysed 3, 7, 14, 21, 28, 38 and 44 days post surgery. While mechanical thresholds on the ipsilateral side are not significantly different from controls for the first 2–3 weeks post P10 surgery, after that time period, beginning at 21 days post surgery (P31), the SNI group developed following early life nerve injury significant hypersensitivity compared to the other groups. Ipsilateral mechanical nociceptive threshold was 2-fold below that of the contralateral and sham thresholds at 21 days post surgery (SNI-ipsilateral 28 (±5) g control groups 69 (±9) g, pConclusions: We report a novel consequence of early life nerve injury whereby mechanical hypersensitivity only emerges later in life. This delayed adolescent onset in mechanical pain thresholds is accompanied by neuroimmune activation and NMDA dependent central sensitization of spinal nociceptive circuits. This delayed onset in mechanical pain sensitivity may provide clues to understand the long term effects of early injury such as late onset phantom pain and the emergence of complex adolescent chronic pain syndromes.

Background: Clinical studies of osteoarthritis (OA) suggest central sensitization may contribute to the chronic pain experienced. This preclinical study used the monosodium iodoacetate (MIA) model of OA joint pain to investigate the potential contribution of spinal sensitization, in particular spinal glial cell activation, to pain behaviour in this model. Experimental OA was induced in the rat by the intra-articular injection of MIA and pain behaviour (change in weight bearing and distal allodynia) was assessed. Spinal cord microglia (Iba1 staining) and astrocyte (GFAP immunofluorescence) activation were measured at 7, 14 and 28 days post MIA-treatment. The effects of two known inhibitors of glial activation, nimesulide and minocycline, on pain behaviour and activation of microglia and astrocytes were assessed. Results: Seven days following intra-articular injection of MIA, microglia in the ipsilateral spinal cord were activated (p < 0. 05, compared to contralateral levels and compared to saline controls). Levels of activated microglia were significantly elevated at day 14 and 21 post MIA-injection. At day 28, microglia activation was significantly correlated with distal allodynia (p < 0.05). Ipsilateral spinal GFAP immunofluorescence was significantly (p < 0.01) increased at day 28, but not at earlier timepoints, in the MIA model, compared to saline controls. Repeated oral dosing (days 14-20) with nimesulide attenuated pain behaviour and the activation of microglia in the ipsilateral spinal cord at day 21. This dosing regimen also significantly attenuated distal allodynia (p < 0.001) and numbers of activated microglia (p < 0.05) and GFAP immunofluorescence (p < 0.001) one week later in MIA-treated rats, compared to vehicle-treated rats. Repeated administration of minocycline also significantly attenuated pain behaviour and reduced the number of activated microglia and decreased GFAP immunofluorescence in ipsilateral spinal cord of MIA treated rats. Conclusions: Here we provide evidence for a contribution of spinal glial cells to pain behaviour, in particular distal allodynia, in this model of osteoarthritic pain. Our data suggest there is a potential role of glial cells in the central sensitization associated with OA, which may provide a novel analgesic target for the treatment of OA pain.

Brainstem–spinal cord connections play an essential role in adult pain processing, and the modulation of spinal pain network excitability by brainstem nuclei is known to contribute to hyperalgesia and chronic pain. Less well understood is the role of descending brainstem pathways in young animals when pain networks are more excitable and exposure to injury and stress can lead to permanent modulation of pain processing. Here we show that up to postnatal day 21 (P21) in the rat, the rostroventral medulla of the brainstem (RVM) exclusively facilitates spinal pain transmission but that after this age (P28 to adult), the influence of the RVM shifts to biphasic facilitation and inhibition. Graded electrical microstimulation of the RVM at different postnatal ages revealed a robust shift in the balance of descending control of both spinal nociceptive flexion reflex EMG activity and individual dorsal horn neuron firing properties, from excitation to inhibition, beginning after P21. The shift in polarity of descending control was also observed following excitotoxic lesions of the RVM in adult and P21 rats. In adults, RVM lesions decreased behavioural mechanical sensory reflex thresholds, whereas the same lesion in P21 rats increased thresholds. These data demonstrate, for the first time, the changing postnatal influence of the RVM in spinal nociception and highlight the central role of descending brainstem control in the maturation of pain processing.

Background The significant postnatal maturation of gamma-aminobutyric acid signaling in the developing brain is likely to have important implications for infant pain processing. Gamma-aminobutyric acid receptor activation evokes analgesia and sedation in the adult, but the impact of immature gamma-aminobutyric acid signaling on modulators of the gamma-aminobutyric acid type A receptor, such as the benzodiazepines, is not known in infants. Methods Nociceptive processing was measured using behavioral and electrophysiological recordings of hind limb flexor withdrawal threshold and magnitude to mechanical and thermal stimulation of the hind paw. The effects of midazolam (0.1-10 mg/kg subcutaneously, 0.1 mg/kg intrathecally) or saline treatment were compared in rats aged 3, 10, 21, and 40 days (adult). The sedative action of midazolam was assessed at each age using righting reflex latencies. Results Midazolam dose-dependently decreased mechanical reflex thresholds and increased mechanical and thermal reflex magnitudes in neonates. In older rat pups and adults, midazolam had the reverse effect, increasing thresholds and decreasing reflex magnitude. These differences were mediated supraspinally; intrathecal administration of midazolam did not affect flexion reflexes at any age. Midazolam had no sedative action in the youngest rats; sedation increased gradually through postnatal development. Conclusions The results show a striking reversal in the effects of midazolam on nociception and sedation in rats between postnatal days 3 and 10. Midazolam fails to sedate young rats and sensitizes their flexor reflex activity. The sedative and desensitizing effects of midazolam are not observed until later in life after maturation in supraspinal centers. The results indicate a need to better understand the pharmacology of drugs used routinely in neonatal intensive care.

Neuropathic pain behaviour is not observed in neonatal rats and tactile allodynia does not develop in the spared nerve injury (SNI) model until rats are 4 weeks of age at the time of surgery. Since activated spinal microglia are known to play a key role in neuropathic pain, we have investigated whether the microglial response to nerve injury in young rats differs from that in adults. Here we show that dorsal horn microglial activation, visualised with IBA-1 immunostaining, is significantly less in postnatal day (P) 10 rat pups than in adults, 7 days after SNI. This was confirmed by qPCR analysis of IBA-1 mRNA and mRNA of other microglial markers, integrin-alpha M, MHC-II DMalpha and MHC-II DMbeta. Dorsal horn IBA-1+ve microglia could be activated, however, by intraspinal injections of lipopolysaccharide (LPS) or N-methyl-d-aspartate (NMDA) at P10, although the increase in the levels of mRNA for all microglial markers was less than in the adult rat. In addition, P10 rats developed a small but significant mechanical allodynia in response to intrathecal LPS. Intrathecal injection of cultured ATP-activated microglia, known to cause mechanical allodynia in adult rats, had no behavioural effect at P10 and only began to cause allodynia if injections were performed at P16. The results clearly demonstrate immaturity of the microglial response triggered by nerve injury in the first postnatal weeks which may explain the absence of tactile allodynia following peripheral nerve injury in young rats.

The rostral ventromedial medulla (RVM) regulates a range of involuntary behaviours but is most often associated with nociception via the action of pronociceptive ON cells and antinociceptive OFF cells. The phasic responses of ON and OFF cells determine whether or not incoming noxious signals provoke a withdrawal reflex, and previous studies have suggested that reflex RVM activity patterns actively shape motor output. Here we challenged the model by using juvenile rats, which are known to exhibit markedly different reflex responses compared with adults. By recording single-cell activity in the RVM and the electromyography responses of hindlimb flexor muscles to noxious thermal stimulation we found that the juvenile reflex had a shorter onset latency, was larger in amplitude and exhibited a decreased rise time compared with the adult reflex. The responses of ON and OFF cells faithfully tracked the shorter onset latency of the reflex by also responding earlier and, thus, still preceded the reflex. However, neither the reflex amplitude nor the ongoing response profile was predicted by the firing rate of RVM cells in either age group. Instead we found a close correspondence between RVM activity and the reflex only during the initiation of the response. Furthermore, the short rise time of the juvenile reflex was reflected in higher rates of change of both ON and OFF cell firing. Our data suggest that the RVM is associated only with the initiation of reflexes and does not shape ongoing muscle activity, which is more likely to be subserved by downstream spinal processes.

Background: Neonatal pain and injury can alter long-term sensory thresholds. Descending rostroventral medulla (RVM) pathways can inhibit or facilitate spinal nociceptive processing in adulthood. As these pathways undergo significant postnatal maturation, the authors evaluated long-term effects of neonatal surgical injury on RVM descending modulation. Methods: Plantar hind paw or forepaw incisions were performed in anesthetized postnatal day (P)3 Sprague-Dawley rats. Controls received anesthesia only. Hind limb mechanical and thermal withdrawal thresholds were measured to 6 weeks of age (adult). Additional groups received pre- and post-incision sciatic nerve levobupivacaine or saline. Hind paw nociceptive reflex sensitivity was quantified in anesthetized adult rats using biceps femoris electromyography, and the effect of RVM electrical stimulation (5–200 μA) measured as percentage change from baseline. Results: In adult rats with previous neonatal incision (n = 9), all intensities of RVM stimulation decreased hind limb reflex sensitivity, in contrast to the typical bimodal pattern of facilitation and inhibition with increasing RVM stimulus intensity in controls (n = 5) (uninjured vs. neonatally incised, P < 0.001). Neonatal incision of the contralateral hind paw or forepaw also resulted in RVM inhibition of hind paw nociceptive reflexes at all stimulation intensities. Behavioral mechanical threshold (mean ± SEM, 28.1 ± 8 vs. 21.3 ± 1.2 g, P < 0.001) and thermal latency (7.1 ± 0.4 vs. 5.3 ± 0.3 s, P < 0.05) were increased in both hind paws after unilateral neonatal incision. Neonatal perioperative sciatic nerve blockade prevented injury-induced alterations in RVM descending control. Conclusions: Neonatal surgical injury alters the postnatal development of RVM descending control, resulting in a predominance of descending inhibition and generalized reduction in baseline reflex sensitivity. Prevention by local anesthetic blockade highlights the importance of neonatal perioperative analgesia.

GABAergic signalling exerts powerful inhibitory control over spinal tactile and nociceptive processing, but during development GABA can be depolarizing and the functional consequences of this upon neonatal pain processing is unknown. Here we show a postnatal switch in tonic GABA(A) receptor (GABA(A)R) modulation of cutaneous tactile and nociceptive reflexes from excitation to inhibition, but only in the intact spinal cord. Neonatal and 21-day-old (P21) rats were intrathecally treated with one of the GABA(A)R antagonists bicuculline and gabazine, with both compounds dose-dependently decreasing hindpaw mechanical and thermal withdrawal thresholds in P21 rats but increasing them in P3 neonates. Intrathecal gabazine also produced an increase in the cutaneous evoked electromyography (EMG) response of the biceps femoris in P21 rates but lowering the response in neonates. Injections of 3H-gabazine in the L4-L5 region at P3 confirmed that gabazine binding was restricted to the lumbar spinal cord. Spinalization of P3 neonates at the upper thoracic level prior to drug application reversed the behavioural and EMG responses to GABA antagonists so that they resembled those of P21 rats. The effects of spinalization were consistent with gabazine facilitation of ventral root potentials observed in isolated neonatal spinal cord. These data show a marked postnatal developmental switch in GABAergic control of neonatal nociception that is mediated by supraspinal structures and illustrate the importance of studying developmental circuits in the intact nervous system.

5-Hydroxytryptamine (5-HT) is an important neurotransmitter mediating many aspects of cognition and behaviour. One psychology in which 5-HT plays an important role is impulsive responding. Recently, we have demonstrated that variation in an aspect of impulsive behaviour, namely delayed gratification, has a clear genetic contribution. Here, we examined the neurobiological relevance of a recently discovered single nucleotide polymorphism (SNP) in the murine gene tryptophan hydroxylase (mTph2) by analysing extracellular levels of 5-HT in medial prefrontal cortex (mPFC) and ventral striatum (VS), key brain regions for impulsive behaviours. The allelic variants were associated with systematic effects on baseline 5-HT efflux in the mPFC and VS. We then went on to examine whether the mTph2 allelic variants gave rise to differences in impulsive behaviour. However, the mTph2 genotype, and therefore presumably baseline brain levels of 5-HT, did not predict impulsive choice, as indexed by sensitivity to delayed reinforcement. Consequently, the data do not support a role for the mTph2 C1473G polymorphism on this aspect of impulsive behaviour. Instead, they indicate that perturbations of the 5-HT system via heritable traits may have differential consequences for qualitatively distinct aspects of impulsive behaviour.

Application of somatostatin to the striatum of the anaesthetized rat has previously been shown to elicit large increases in extracellular levels of dopamine and GABA via a glutamate-dependent mechanism. These actions have been ascribed to the SSTR2 receptor. Here we describe experiments designed to investigate whether these effects occur in C57Bl6 mice and if they elicit rotational behaviours associated with increased dopamine in the striatum. Application of somatostatin resulted in increased concentrations of dopamine in striatum, hippocampus and amygdala of anaesthetized mice. Unilateral striatal infusions of the peptide by retrodialysis increased locomotion. Application of N-methyl-D-aspartate and AMPA to the freely-moving mouse striatum resulted in increased dopamine release; however, only AMPA caused increased locomotion. These results further confirm that somatostatin can play a role in the control of locomotor function by modulating striatal dopamine release.

The peptide somatostatin can modulate the functional output of the basal ganglia. The exact sites and mechanisms of this action, however, are poorly understood, and the physiological context in which somatostatin acts is unknown. Somatostatin acts as a neuromodulator via a family of five 7-transmembrane G protein-coupled receptors, SSTR1-5, one of which, SSTR2, is known to be functional in the striatum. We have investigated the role of SSTR2 in basal ganglia function using mice in which Sstr2 has been inactivated and replaced by the lacZ reporter gene. Analysis of Sstr2lacZ expression in the brain by beta-galactosidase histochemistry demonstrated a widespread pattern of expression. By comparison to previously published in situ hybridization and immunohistochemical data, Sstr2lacZ expression was shown to accurately recapitulate that of Sstr2 and thus provided a highly sensitive model to investigate cell-type-specific expression of Sstr2. In the striatum, Sstr2 expression was identified in medium spiny projection neurons restricted to the matrix compartment and in cholinergic interneurons. Sstr2 expression was not detected in any other nuclei of the basal ganglia except for a sparse number of nondopaminergic neurons in the substantia nigra. Microdialysis in the striatum showed Sstr2-null mice were selectively refractory to somatostatin-induced dopamine and glutamate release. In behavioural tests, Sstr2-null mice showed normal levels of locomotor activity and normal coordination in undemanding tasks. However, in beam-walking, a test of fine motor control, Sstr2-null mice were severely impaired. Together these data implicate an important neuromodulatory role for SSTR2 in the striatum.

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.

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.

1. We have used in vivo microdialysis in anaesthetized rats to investigate whether levels of striatal somatostatin (SRIF) can be increased in response to application of the ionotropic glutamate receptor agonists AMPA and NMDA. 2. Application of both AMPA and NMDA (10, 50, 100 and 500 microM) for 20 min periods produced concentration-dependent increases in the extracellular levels of SRIF. A 500 microM dose of each compound was shown to be the most potent concentration tested, increasing levels of SRIF by 32 fold (NMDA) and 35 fold (AMPA). At lower concentrations (10 microM) NMDA failed to evoke significant amounts of SRIF while AMPA increased levels of the peptide 2.3 fold. 3. Application of the respective receptor antagonists APV (NMDA receptor) and DNQX (AMPA receptor) abolished the abilities of the agonists to evoke release of SRIF. Interestingly DNQX abolished the ability of NMDA to evoke release of the peptide as well. 4. The ability of both AMPA and NMDA to evoke increases in the levels of extracellular SRIF further illustrates the reciprocal relationship that exists between SRIF and glutamate in the striatum which impacts particularly on dopaminergic functioning in this region.

1 Somatostatin (SRIF) is a cyclic tetradecapeptide present in medium-sized aspiny interneurones in the rat striatum. We have previously shown that exogenous SRIF potently stimulates striatal dopamine (DA) release via a glutamate-dependent mechanism. We now report the ability of the selective sst2 receptor agonist, BIM-23027, to mimic this effect of SRIF. 2 In vivo microdialysis studies were performed in anaesthetized male Wistar rats. In most experiments, compounds were administered by retrodialysis into the striatum for 15 min periods, 90 min and 225 min after sampling commenced, with levels of neurotransmitters being measured by HPLC with electrochemical and fluorescence detection. 3 BIM-23027 (50 and 100 nM) stimulated DA release with extracellular levels increasing by up to 18 fold. 4 Prior retrodialysis of BIM-23027 (50 nM) abolished the effects of subsequent administration of SRIF (100 nM). 5 The agonist effects of both BIM-23027 and SRIF were abolished by the selective sst2 receptor antagonist, L-Tyr8-CYN-154806 (100 nM). 6 The AMPA/kainate receptor antagonist, DNQX (100 microM), abolished the agonist effects of BIM-23027 as previously shown for SRIF. 7 This study provides evidence that the sst2 receptor mediates the potent dopamine-releasing actions observed with SRIF in the rat striatum. Dopamine release evoked by both peptides appears to be mediated indirectly via a glutamatergic pathway. Other subtype-specific somatostatin receptor ligands were unable to elicit any effects and therefore we conclude that no other somatostatin receptor types are involved in mediating the dopamine-releasing actions of SRIF in the striatum.

The sensitization of spinal dorsal horn neurones leads to prolonged enhancement of pain behaviour and can be evoked by intense C-fibre stimulation, tissue inflammation and peripheral nerve injury. Activation of central immune cells plays a key role in establishing pain hypersensitivity but the exact nature of the afferent input that triggers the activation of microglia and other glial cells within the CNS, remains unclear. Here intense but non-damaging, electrical stimulation of intact adult rat C-fibres for 5 min at 10 Hz induced central sensitization characterized by significant decreases in mechanical withdrawal thresholds 3, 24 and 48 h later. This maintained (>3 h) hypersensitivity was not observed following topical skin application of capsaicin. C-fibre evoked sensitization was accompanied by significant microglial activation, shown by increased Iba-1 immunoreactivity throughout the dorsal horn at 24 and 48 h and significant upregulation of markers of microglial activation: IL-6 and Mcp-1 at 3h and Mmp3, CSF-1 and CD163 at 24 and 48 h. C-fibre stimulation caused no nerve damage at ultrastructural and molecular levels. Lower intensity stimulation that did not activate C-fibres or sham stimulation did not increase Iba-1 immunoreactivity or induce behavioural sensitivity. Pre-treatment with minocycline (40 mg/kg, i.p.) prevented the C-fibre evoked sensitization and microglial activation. Identical C-fibre stimulation in 10-day old rat pups failed to activate microglia or change behaviour. These results demonstrate that a brief period of low frequency C-fibre stimulation, in the absence of nerve damage, is sufficient to activate microglia resulting in behavioural hyperalgesia.

The developmental regulation of the underlying neurobiology of pain pathways has become an increasing focus of clinical and basic research in recent years. These developmental changes involve alterations in neural processing from the molecular level to synaptic connections and neural networks. Recent reviews have focused upon the changes that take place within the dorsal horn of the spinal cord where neural connectivity has been shown to alter significantly throughout the early postnatal period. In this chapter, we will also discuss developmental changes in the brainstem and higher centers and how these changes may influence the experience of pain in the neonate. © 2008 Elsevier Inc. All rights reserved.

Administration of the neurotrophin nerve growth factor (NGF) to rats and humans has been shown to induce both thermal and mechanical hyperalgesia and is used as a model of inflammatory pain. Here we describe a mouse model of secondary hyperalgesia after NGF application. NGF was injected into the biceps femoris muscle unilaterally, and at various intervals afterwards the electromyographic (EMG) activity from the same muscle was recorded in response to mechanical von Frey hair stimulation of the plantar surface of the hind paw in isoflurane-anesthetized mice. Secondary cutaneous hyperalgesia in the hind paw reached a peak 60 minutes after injection and returned to baseline levels after an additional 60 minutes. This was followed by a second increase in EMG magnitude at 24 hours after injection that was still present after 5 days. The effects of NGF were dose-dependent, and a dose of 2 μg/g NGF had the maximal observed effect. No increase in EMG magnitude occurred on the untreated side. This study describes a quantitative mouse model of prolonged secondary cutaneous hyperalgesia after NGF-induced muscle inflammation that can be used for genetic manipulations of putative central molecular pathways that underlie secondary hyperalgesia. Perspective This study describes the development of a novel model of NGF-induced secondary hyperalgesia. The development of this model will allow further investigations into the processes that underlie the development of secondary hyperalgesia and pain associated with the musculature.

The peptide somatostatin can modulate the functional output of the basal ganglia. The exact sites and mechanisms of this action, however, are poorly understood, and the physiological context in which somatostatin acts is unknown. Somatostatin acts as a neuromodulator via a family of five 7-transmembrane G protein-coupled receptors, SSTR1-5, one of which, SSTR2, is known to be functional in the striatum. We have investigated the role of SSTR2 in basal ganglia function using mice in which Sstr2 has been inactivated and replaced by the lacZ reporter gene. Analysis of Sstr2lacZ expression in the brain by beta-galactosidase histochemistry demonstrated a widespread pattern of expression. By comparison to previously published in situ hybridization and immunohistochemical data, Sstr2lacZ expression was shown to accurately recapitulate that of Sstr2 and thus provided a highly sensitive model to investigate cell-type-specific expression of Sstr2. In the striatum, Sstr2 expression was identified in medium spiny projection neurons restricted to the matrix compartment and in cholinergic interneurons. Sstr2 expression was not detected in any other nuclei of the basal ganglia except for a sparse number of nondopaminergic neurons in the substantia nigra. Microdialysis in the striatum showed Sstr2-null mice were selectively refractory to somatostatin-induced dopamine and glutamate release. In behavioural tests, Sstr2-null mice showed normal levels of locomotor activity and normal coordination in undemanding tasks. However, in beam-walking, a test of fine motor control, Sstr2-null mice were severely impaired. Together these data implicate an important neuromodulatory role for SSTR2 in the striatum.

Integrating science communication training into an undergraduate research project encourages greater academic involvement in public engagement, maximizes audience size, and provides high-quality research data.

Somatostatin is a neuromodulator and neurotransmitter in the central nervous system. Administration of somatostatin to the spinal cord or brain areas involved in nociception has been shown to result in analgesia. Little information is available about the somatostatin receptor types which may be involved in mediating the neuromodulatory and analgesic effects of the peptide. To define the neuronal systems expressing the sst2(a) receptor in brain areas associated with analgesia, immunohistochemical co-localisation studies were carried out in the periaqueductal grey (PAG) and spinal cord using an antibody specific for the sst2(a) receptor. To further define sst2(a) receptor expressing neurones, sst2(a) receptor immunohistochemistry was combined with retrograde tracing using fluorogold. In the PAG, sst2(a) receptor expressing neurones were found to co-express calbindin D28k (36%), the glutamate transporter EAAC-1 (25%), and GABA transporter GAT-1 (∼10%). A total of 65% of sst2(a) positive neurones projected to the thalamus. In the spinal cord, the sst2(a) receptor shows cellular co-localisation with EAAC-1 and GAT-1. Immunohistochemistry and receptor autoradiography using [ 125 I ] BIM 23027 after dorsal rhizotomy of the lumbar dorsal roots, L4 and L5, suggests that the somatostatin sst2(a) receptor is not present on primary afferent neurones. Dorsal hemisections of the mid thoracic cord did not alter the immunohistochemical signal for the somatostatin sst2(a) receptor, providing further evidence for an intrinsic localisation of the receptor protein in the dorsal horn of the spinal cord. These data show that the somatostatin sst2(a) receptor exists on morphologically and neurochemically heterogenous neurones and is closely associated with brain areas involved in analgesia and the modulation of nociception.

Background: The role of the neurotrophin regulated polypeptide, VGF, has been investigated in a rat spared injury model of neuropathic pain. This peptide has been shown to be associated with synaptic strengthening and learning in the hippocampus and while it is known that VGFmRNA is upregulated in dorsal root ganglia following peripheral nerve injury, the role of this VGF peptide in neuropathic pain has yet to be investigated. Results: Prolonged upregulation of VGF mRNA and protein was observed in injured dorsal root ganglion neurons, central terminals and their target dorsal horn neurons. Intrathecal application of TLQP-62, the C-terminal active portion of VGF (5–50 nmol) to naïve rats caused a long-lasting mechanical and cold behavioral allodynia. Direct actions of 50 nM TLQP-62 upon dorsal horn neuron excitability was demonstrated in whole cell patch recordings in spinal cord slices and in receptive field analysis in intact, anesthetized rats where significant actions of VGF were upon spontaneous activity and cold evoked responses. Conclusion: VGF expression is therefore highly modulated in nociceptive pathways following peripheral nerve injury and can cause dorsal horn cell excitation and behavioral hypersensitivity in naïve animals. Together the results point to a novel and powerful role for VGF in neuropathic pain.

We have used in vivo microdialysis in anaesthetised rats to investigate whether somatostatin (SRIF) can play a neuromodulatory role in the striatum. When 100 nM SRIF was retrodialysed for 15 min, it increased concentrations of dopamine (DA) by 28-fold, gamma-aminobutyric acid (GABA) by eightfold, and glutamate (Glu) by sixfold as well as those of aspartate (Asp) and taurine (Tau). These effects were both calcium- and tetrodotoxin-sensitive. Lower (10 or 50 nM) and higher (1 microM) SRIF concentrations were less effective. Rapid sampling showed that whereas Asp and Glu concentrations were raised for 3 min at the start of 15-min SRIF infusions, those of DA were increased for 12 min. A second 15-min application of 100 nM SRIF given 135 min after the first application failed to increase transmitter release. An NMDA receptor antagonist, 2-amino-5-phosphonopentanoic acid (200 microM), blocked SRIF (100 nM)-evoked Asp, Glu, Tau, and GABA release and reduced that of DA. An alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate antagonist, 6,7-dinitroquinoxaline-2,3-dione (100 microM), blocked SRIF-induced DA and Tau release and reduced that of Asp, Glu, and GABA. These results show that SRIF increases DA, Glu, Asp, GABA, and Tau release in the rat striatum and suggest that its actions on DA and GABA release are mainly mediated through increased excitatory amino acid release.

Background Rheumatoid arthritis (RA) patients frequently show weak correlations between the magnitude of pain and inflammation suggesting that mechanisms other than overt peripheral inflammation contribute to pain in RA. We assessed changes in microglial reactivity and spinal excitability and their contribution to pain-like behaviour in the early stages of collagen-induced arthritis (CIA) model. Methods Mechanically evoked hypersensitivity, spinal nociceptive withdrawal reflexes (NWRs) and hind paw swelling were evaluated in female Lewis rats before and until 13 days following collagen immunization. In the spinal dorsal horn, microgliosis was assayed using immunohistochemistry (Iba-1/p-p38) and cyto(chemo)kine levels in the cerebrospinal fluid (CSF). Intrathecal administration of microglia-targeting drugs A-438079 (P2X7 antagonist) and LHVS (cathepsin S inhibitor) were examined upon hypersensitivity, NWRs, microgliosis and cyto(chemo)kine levels in the early phase of CIA. Results The early phase of CIA was associated with mechanical allodynia and exaggerated mechanically evoked spinal NWRs, evident before hind paw swelling, and exacerbated with the development of swelling. Concomitant with the development of hypersensitivity was the presence of reactive spinal microgliosis and an increase of IL-1β levels in CSF (just detectable in plasma). Prolonged intrathecal administration of microglial inhibitors attenuated the development of mechanical allodynia, reduced microgliosis and attenuated IL-1β increments. Acute spinal application of either microglial inhibitor significantly diminished the sensitization of the spinal NWRs. Conclusions Mechanical hypersensitivity in the early phase of CIA is associated with central sensitization that is dependent upon microglial-mediated release of IL-1β in the spinal cord. Blockade of these spinal events may provide pain relief in RA patients.