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Abstract 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, p < 0.001, 3-way ANOVA, n = 6 per group). Importantly, no effect was observed on thermal thresholds. This hypersensivity was accompanied by macrophage, microglial and astrocyte activation in the DRG and dorsal horn, but no significant change in dorsal horn p38 or JNK expression. Preemptive minocycline (daily 40 mg/kg, s.c) did not prevent the effect. Ketamine (20 mg/kg, s.c), on the other hand, produced a dose-dependent reversal of mechanical nociceptive thresholds ipsilateral to the nerve injury such that thresholds return to control levels at the highest doses of 20 mg/Kg. Conclusions 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.

Abstract 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.

Abstract 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 investigated how the biodistribution of cannabidiol (CBD) within the central nervous system (CNS) is influenced by two different formulations, an oil-in-water (O/W) nanoemulsion and polymer-coated nanoparticles (PCNPs). We observed that both CBD formulations administered were preferentially retained in the spinal cord, with high concentrations reaching the brain within 10 min of administration. The CBD nanoemulsion reached C max in the brain at 210 ng/g within 120 min (T max ), whereas the CBD PCNPs had a C max of 94 ng/g at 30 min (T max ), indicating that rapid brain delivery can be achieved through the use of PCNPs. Moreover, the AUC 0 - 4 h of CBD in the brain was increased 3.7-fold through the delivery of the nanoemulsion as opposed to the PCNPs, indicating higher retention of CBD at this site. Both formulations exhibited immediate anti-nociceptive effects in comparison to the respective blank formulations., (Copyright © 2023 The Authors. Published by Elsevier Inc. All rights reserved.)

Osteoarthritis (OA) is a chronic joint disease that represents an increasingly substantial global burden. Joint pain is the most significant symptom of OA. Unfortunately, current pharmacological treatments for OA pain are often not wholly efficacious, or are associated with serious adverse effects. This lack of effective pain relief has seen the prescription of opioids for OA pain increase over the past decades. The long-term adverse effects of prescribed opioids alongside the increasing prevalence of OA pain highlights the need for alternative analgesics. Understanding the mechanisms that drive this chronic joint pain is crucial for the development of novel analgesics. OA is a heterogeneous disease, and this is reflected by the diversity of pain phenotypes in people with the disease. Herein, we review current understanding of the biological changes at the joint and within the central nervous system that drive this chronic pain. We particularly focus on the most recent advances in our understanding of the peripheral nociceptive mechanisms that underlie chronic OA pain and highlight how targeting peripheral OA inflammation may open up opportunities for novel analgesics., (Copyright © 2022 The Authors. Published by Elsevier Ltd.. All rights reserved.)

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.

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., Competing Interests: SG, LB, FM, ED, GH, CH, IT, RS No competing interests declared, (© 2018, Goksan et al.)

Advances in pediatric cancer treatment have led to a ten year survival rate greater than 75%. Platinum-based chemotherapies (e.g. cisplatin) induce peripheral sensory neuropathy in adult and pediatric cancer patients. The period from birth through to adulthood represents a period of maturation within nociceptive systems. Here we investigated how cisplatin impacts upon postnatal maturation of nociceptive systems. Neonatal Wistar rats (Postnatal day (P) 7) were injected (i.p.) daily with either vehicle (PBS) or cisplatin (1mg/kg) for five consecutive days. Neither group developed mechanical or thermal hypersensitivity immediately during or after treatment. At P22 the cisplatin group developed mechanical (P < 0.05) and thermal (P < 0.0001) hypersensitivity versus vehicle group. Total DRG or dorsal horn neuronal number did not differ at P45, however there was an increase in intraepidermal nerve fiber density in cisplatin-treated animals at this age. The percentage of IB 4 +ve, CGRP+ve and NF200+ve DRG neurons was not different between groups at P45. There was an increase in TrkA+ve DRG neurons in the cisplatin group at P45, in addition to increased TrkA, NF200 and vGLUT2 immunoreactivity in the lumbar dorsal horn versus controls. These data highlight the impact pediatric cancer chemotherapy has upon the maturation of pain pathways and later life pain experience., (Copyright © 2018. Published by Elsevier Ltd.)

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., Competing Interests: The authors declare no conflict of interest.

Background: Harnessing the actions of the resolvin pathways has the potential for the treatment of a wide range of conditions associated with overt inflammatory signalling. Aspirin-triggered resolvin D1 (AT-RvD1) has robust analgesic effects in behavioural models of pain; however, the potential underlying spinal neurophysiological mechanisms contributing to these inhibitory effects in vivo are yet to be determined. This study investigated the acute effects of spinal AT-RvD1 on evoked responses of spinal neurones in vivo in a model of acute inflammatory pain and chronic osteoarthritic (OA) pain and the relevance of alterations in spinal gene expression to these neurophysiological effects., Methods: Pain behaviour was assessed in rats with established carrageenan-induced inflammatory or monosodium iodoacetate (MIA)-induced OA pain, and changes in spinal gene expression of resolvin receptors and relevant enzymatic pathways were examined. At timepoints of established pain behaviour, responses of deep dorsal horn wide dynamic range (WDR) neurones to transcutaneous electrical stimulation of the hind paw were recorded pre- and post direct spinal administration of AT-RvD1 (15 and 150 ng/50 μl)., Results: AT-RvD1 (15 ng/50 μl) significantly inhibited WDR neurone responses to electrical stimuli at C- (29 % inhibition) and Aδ-fibre (27 % inhibition) intensities. Both wind-up (53 %) and post-discharge (46 %) responses of WDR neurones in carrageenan-treated animals were significantly inhibited by AT-RvD1, compared to pre-drug response (p < 0.05). These effects were abolished by spinal pre-administration of a formyl peptide receptor 2 (FPR2/ALX) antagonist, butoxy carbonyl-Phe-Leu-Phe-Leu-Phe (BOC-2) (50 μg/50 μl). AT-RvD1 did not alter evoked WDR neurone responses in non-inflamed or MIA-treated rats. Electrophysiological effects in carrageenan-inflamed rats were accompanied by a significant increase in messenger RNA (mRNA) for chemerin (ChemR23) receptor and 5-lipoxygenase-activating protein (FLAP) and a decrease in 15-lipoxygenase (15-LOX) mRNA in the ipsilateral spinal cord of the carrageenan group, compared to controls., Conclusions: Our data suggest that peripheral inflammation-mediated changes in spinal FLAP expression may contribute to the novel inhibitory effects of spinal AT-RvD1 on WDR neuronal excitability, which are mediated by FPR2/ALX receptors. Inflammatory-driven changes in this pathway may offer novel targets for inflammatory pain treatment.

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 S1 in juvenile rats are different to those in the adult. We recorded electroencephalogram (EEG) responses from S1 of lightly-anesthetized Sprague-Dawley rats at either postnatal day 21 or postnatal day 40 during the presentation of noxious (55 °C) or innocuous (30 °C) 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 affected by age. Noxious heat evoked a significant increase in theta band (4-8 Hz) activity in adults only (P<0.0001 compared to baseline; P<0.0001 compared to juveniles). There were no significant differences in EEG responses to innocuous thermal stimuli. These data show that there are significant alterations in the processing of nociceptive inputs within the maturing cortex and that cortical theta activity is involved only in the adult cortical response to noxious stimulation., (Copyright © 2015 IBRO. Published by Elsevier Ltd. All rights reserved.)

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.

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.

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.