Neurobiology of Pain

Neuropilin-1 (NRP1) is a single pass transmembrane glycoprotein that can form a receptor complex with several tyrosine kinase receptors, including the vascular endothelial growth factor (VEGF) receptor. Previous studies have reported that binding of VEGFA to this receptor complex elicits mechanical allodynia and thermal hyperalgesia through potentiation of voltage-gated sodium and calcium channel activity. We find that Nrp1 mRNA and protein is widely distributed in naIve mouse and rat DRG neurons, including peptidergic afferents. A CGRPcreER: NRP1fl/fl transgenic mice was generated to investigate the role of peptidergic NRP1 in basal nociception. Following in vivo loss of NRP1, mice are hyposensitive to both noxious heat and mechanical stimuli. Under normal conditions, VEGFA elicits mechanical hypersensitivity, an effect that was absent in our NRP1 knockout mouse. Furthermore, VEGFA induced neuronal hyperexcitability was lost in CGRP expressing neurons isolated from this NRP1 knockout mouse. This study validates the NRP1 knockout mouse and confirms previous findings that VEGFA, often released during pathological pain conditions, requires peptidergic NRP1. Interestingly, we find that in the absence of ongoing injury or inflammation, peptidergic NRP1 regulates basal nociception and pain-like behaviors. PerspectiveNRP1 is expressed in sensory neurons including the peptidergic subpopulation. Genetic deletion of NRP1 in healthy adults alters nociception without altering innervation; NRP1 knockout mice are hyposensitive to noxious heat and mechanical stimuli, but lose sensitivity to VEGFA, confirming it is a therapeutic target for growth factor mediated pain conditions.

Nav1.7 voltage-gated sodium channels are strongly expressed in human primary painsensing neurons (nociceptors) and selective Nav1.7 inhibitors have been developed as possible therapeutic agents for treating pain, so far with disappointing clinical results. In contrast, a selective Nav1.8 channel inhibitor (suzetrigine) has had successful clinical trials. Because nociceptors express both Nav1.7 and Nav1.8 channels, it is of interest to compare effects of Nav1.7 and Nav1.8 inhibitors on the excitability of human nociceptors. To compare with previous results with suzetrigine, we characterized the effects of a selective Nav.7 inhibitor, AM-2099, on action potential generation and repetitive firing of dissociated human dorsal root ganglion neurons, studied at 37{degrees}C. Inhibition of Nav1.7 channels by 600 nM AM-2099 generally produced a substantial depolarizing shift of action potential threshold, an increase in rheobase, a decrease in action potential upstroke velocity, decrease in action potential peak, and prolongation of refractory period. Compared to inhibition of Nav1.8 channels, inhibition of Nav1.7 channels had larger effects on threshold and maximal upstroke velocity, while action potential peak was reduced similarly by both. Nav1.8 inhibition produced much more dramatic reduction of repetitive firing than Nav1.7 inhibition. The results show that although the excitability of human DRG neurons is affected by inhibition of Nav1.7 channels, most notably by an increase in threshold and increase in refractory period, repetitive firing of the neurons in response to strong stimuli is little affected. Significance statementNav1.7 sodium channels are highly expressed in primary pain-sensing neurons and humans with null mutations in Nav1.7 channels have loss of pain sensation. However, unlike the Nav1.8 inhibitor suzetrigine, Nav1.7 inhibitors have so far not reached clinical use. We compared effects of Nav1.7 on electrical excitability of human dorsal root ganglion neurons with those of suzetrigine and found that while Nav1.7 inhibition affects spike threshold more than suzetrigine, there is little effect on repetitive firing with strong stimuli.

BackgroundPain is one of the primary symptoms of endometriosis, a chronic inflammatory condition characterised by the presence of endometrial tissue outside the uterus. Endometriosis-associated pain is commonly considered as nociceptive in nature but its clinical presentation suggests that it might have neuropathic-like properties in a subgroup of patients. MethodsThis is a cross sectional study using an online survey. The survey was distributed by patient support websites. The survey was composed of validated questionnaires assessing pain symptoms, psychological measures and questions about number of surgeries. Main results and the role of chanceWe had 1417 responses which met the inclusion criteria. Using standard painDETECT cut-off scores, we found that pain was classified as neuropathic in 40% of patients and as mixed neuropathic/nociceptive in a further 35%. In line with observations in other neuropathic conditions, the neuropathic subgroup reported higher pain intensities, greater psychological distress and cognitive impairment. Neuropathic pain was also more likely in those with more surgeries to the abdomen and a longer history of pain. As revealed by a cluster analysis, those with a neuropathic pain component could further be divided into two subgroups based on their sensory profile. ConclusionsThe data presented here indicate that endometriosis-associated pain includes a neuropathic-like component in a substantial proportion of women. Although further investigation is required, our finding challenges the current conceptualisation of endometriosis-associated pain as nociceptive and advocates for a new perspective on this type of pain, which is so debilitating to a large number of women.

The central nucleus of amygdala (CeA) comprises diverse populations of neurons, forming a complex network responsible for regulating various behavioral responses. Among these, neurons expressing calcitonin gene-related peptide receptors (CGRPR) have emerged as key players in CGRP neuropeptide-mediated pain modulation. While previous studies emphasize CGRPs key role in synaptic plasticity and its connection with pain behavior in the CeA, the precise functional attributes and contributions of CeA-CGRPR-expressing neurons in pain processing remain elusive. This study reveals the co-localization of CGRPR-expressing neurons in the CeA with phosphorylated extracellular signal-regulated kinase (pERK), a marker indicating pain plasticity, in a neuropathic pain model. Electrophysiological assessments of these neurons in slice preparations unveiled heightened intrinsic excitability after sciatic nerve cuff implantation, contingent upon their rostro-caudal positioning within the CeA. Furthermore, our behavioral experiments using chemogenetic inhibition of CeA-CGRPR neurons demonstrated the ability to reverse nerve injury-induced hypersensitivity. Conversely, activating these neurons induced pain-related hypersensitivity even in the absence of injury. Our findings also highlight a sex-specific role of CeA-CGRPR neurons in formalin-induced spontaneous pain response. Collectively, these data reinforce the involvement of CeA-CGRPR neurons in pain processing, contributing to a better understanding of how neural circuits are affected in persistent pain conditions. Significance StatementOur study shows the role of CGRPR-expressing neurons within the CeA during pain processing. Using a cuff-implanted neuropathic mouse model, we discovered that CGRPR-expressing neurons co-localize with phosphorylated extracellular signal-regulated kinase (pERK), a hallmark of pain plasticity, in both male and female mice. Furthermore, our electrophysiological investigations reveal that posterior CeA-CGRPR neurons exhibit increased excitability following sciatic nerve cuff implantation. Importantly, we demonstrate that CeA-CGRPR neurons exert bidirectional effects on pain behavior in mice, irrespective of sex differences in nerve injury-induced pain responses while showing sex-specific spontaneous pain responses in the formalin-induced model. These findings show the role of CeA-CGRPR neurons in pain modulation, underscoring their potential significance in understanding and addressing persistent pain conditions.

Inflammatory bowel disease (IBD) is characterised by chronic pain, a debilitating symptom for which effective treatments are few and far between. IBD pathogenesis includes the prevalence of a variety of pro-inflammatory cytokines, including the Interleukin-6 (IL-6) family members Il-6 and Oncostatin M (OSM). Previous research has shown disruption of OSM signaling can modulate nociceptor sensitization and activation, however the downstream signalling pathway is unknown. When an in silico analysis of murine colonic sensory neuronal populations was undertaken for receptor expression for OSM and other factors necessary for intracellular signaling, we can find diverse expression indicative of functional signaling. We were able to observe that hyper Il-6 (Il-6 bound to the soluble Il-6 receptor) and OSM can elicit activation of a subset of murine sensory neurons by finding an increase in calcium mobilization following superfusion. This could then be attenuated by the pharmacologic inhibition of all janus kinases or interestingly, TYK2 alone. Furthermore, inhibition of transient receptor potential vanilloid 1 or transient receptor potential ankyrin 1 ion channels, which are known to be sensitized by OSM in other sensory neurons also reduced the proportion of OSM-responsive neurons. This further understanding of OSM signaling in sensory neurons creates avenues for more extensive research into the molecular mechanisms occurring as well as the potential to exploit these therapeutically to induce analgesia in a subset of neurons.

Nociceptive pain is the most common pain condition, and moderate-to-severe nociceptive pain substantially impacts daily functioning, constituting a significant public health burden. Despite this, most studies investigating the neural mechanisms underlying somatosensory processing and inhibition have focused on other pain conditions (e.g., neuropathic, nociplastic, or mixed pain). Thus, the extent to which neural aberrancies detected in these other populations extend to or differentiate from nociceptive pain conditions remains largely unknown. In this study, 29 individuals with moderate-to-severe nociceptive pain (MSNP) and 47 pain-free (PF) controls underwent magnetoencephalography (MEG) alongside a paired-pulse somatosensory stimulation paradigm to examine somatosensory cortical processing and functional inhibition. Pain status and intensity were determined using validated pain questionnaires, painDETECT and PROMIS-29, respectively. MEG oscillatory responses were source localized via a beamformer to the primary somatosensory cortex (S1) and time series data were extracted from the peak voxel to quantify the dynamics of somatosensory gating (SG; index of cortical inhibitory processing), oscillatory response power, and spontaneous power. We found that adults with MSNP exhibit aberrant theta SG in contralateral S1 compared to PF controls, reflecting reduced functional inhibition of innocuous stimulus processing in this region. Additionally, individuals with MSNP demonstrated exaggerated gamma responses but blunted alpha responses in contralateral S1 to innocuous stimulation. Finally, individuals with MSNP were characterized by weaker spontaneous alpha in contralateral S1 that scaled with self-reported pain intensity. Together, these findings suggest that experiencing MSNP is associated with disrupted somatosensory and cortical inhibitory processing.

Nerve injury causes an imbalance of glutamatergic excitation over GABAergic inhibition, contributing thereby to lasting neuropathic pain. Transgenic GAD67-GFP knock-in reporter mice were developed to visualize GABAergic interneurons. The knock-in into glutamate decarboxylase (GAD67) causes haploinsufficiency that manifest in low GABA levels. In this model, we studied if diminished GABA exacerbates neuropathic pain after nerve injury. Adolescent male and female GAD67-GFP knock-in mice were subjected to Spared Sciatic Nerve Injury (SNI). At baseline, nociception and thermal preferences were equal but after SNI, GAD67-GFP mice developed thermal allodynia which was not detected in wildtype littermates. At the electrophysiology level, SNI caused a partial decrease in the excitability in layer 2/3 pyramidal neurons in the projection-hemisphere in wildtype mice. This effect was exacerbated in GAD67-GFP, affecting both sides, and was accompanied with imbalance of field-potential (FP) amplitudes between projection and non-projection hemisphere, which did not occur in wildtype mice. The results suggest that GABA deficiency can be compensated to protect from hyperexcitability at baseline, but it cannot be further upscaled, ultimately leading to network hyperactivity after injury. Metabolomic studies confirmed the moderate loss of GABA in ipsi- and contralateral cortex and lumbar spinal cord of GAD67-GFP mice and failure to raise GABA in the ipsilateral dorsal horn after injury. Carnosine, cystathionine, and glutathione, three important anti-oxidative metabolites, were co-reduced with GABA suggesting that GABAergic activity and anti-oxidative capacity are interconnected and failure of this axis contributes to neuropathic "pain".

Pain management has been challenging and a major obstacle lies in the limited translational success between preclinical studies, often based on rodent models and evoked nociception behavioral assays, whose validity is often questioned. The dorsal root ganglia (DRG) contains diverse nociceptor subtypes that serve as the primary afferent pathways for detecting painful stimuli and analgesics often target proteins expressed in nociceptors. This makes the distinct protein repertoires and molecular interactors within nociceptor subtypes a key focus for understanding which molecular players drive pain processing and how they may be therapeutically targeted. The confirmation of cross-species conservation of pain-related signaling pathways, mediated by nociceptors, could help to elucidate the molecular mechanisms by which the drugs act across species. In this context, we constructed and compared experimentally-validated protein-protein interaction (PPI) networks based on drug targets and their direct binding partners for nociceptor subtypes supported by single-nuclei transcriptome data from mouse and human DRGs. We found that overall gene expression is more conserved across mice than in human nociceptor subtypes, indicating a higher degree of molecular specialization of human nociceptors. Overall signaling network analyses revealed subtype- and species-specific conservation related to pain signaling, with some particularities, in which key drug targets mediate broader cellular processes beyond pain signaling and neuronal depolarization. Altogether, this resource may help to further understand the molecular mechanisms of specific drug targeting, and the proposed workflow can be used to identify and prioritize pain-related pathways in the DRG, advancing target identification and translational medicine.

Trigeminal neuralgia (TN) is a severe and disabling facial pain condition and is characterized by intermittent, severe, electric shock-like pain in one (or more) trigeminal subdivisions. This pain can be triggered by an innocuous stimulus or can be spontaneous. Presently available therapies for TN include both surgical and pharmacological management; however, the lack of a known etiology for TN contributes to the unpredictable response to treatment and the variability in long-term clinical outcomes. Given this, a range of peripheral and central mechanisms underlying TN pain remain to be understood. We acquired functional magnetic resonance imaging (fMRI) data from TN patients who (1) rested comfortably in the scanner during a resting state session and (2) rated their pain levels in real time using a calibrated tracking ball-controlled scale in a pain tracking session. Following data acquisition, the data was analyzed using the conventional correlation analysis and two artificial intelligence (AI)-inspired deep learning methods: convolutional neural network (CNN) and graph convolutional neural network (GCNN). Each of the three methods yielded a set of brain regions related to the generation and perception of pain in TN. There were six regions that were identified by all three methods, including the superior temporal cortex, the insula, the fusiform, the precentral gyrus, the superior frontal gyrus, and the supramarginal gyrus. Additionally, 17 regions, including dorsal anterior cingulate cortex(dACC) and the thalamus, were identified by at least two of the three methods. Collectively, these 23 regions represent signature centers of TN pain and provide target areas for future studies relating to central mechanisms of TN.

Spinal cord injury (SCI) results in dysfunction of both motor and sensory systems, which can be characterized by neuropathic pain, hypersensitivity, muscular spasticity and rigidity. Most SCIs result from incidents such as vehicle accidents or falls, resulting in polytraumatic SCI that includes peripheral injuries in addition to direct CNS damage. Recent findings suggest that spinal cord synaptic plasticity plays a crucial role in neuropathic pain pathophysiology, specifically in association with spinal sensitization and the consequent onset of AMPA-related maladaptive plasticity. Further findings have demonstrated that nociceptive peripheral stimulation in the acute phase of SCI results in maladaptive spinal synaptic plasticity by overdriving GluA2-lacking calcium-permeable AMPARs (CP-AMPARs). Here, we investigated the effect of a spared nerve injury (SNI) in conjunction with SCI to determine the effect of polytraumatic SCI on maladaptive plasticity in the spinal cord. Near-IR quantitative Western blot analysis demonstrated that SCI+SNI increases spinal GluA1 expression, but not GluA2. Patch-clamp confirmed that AMPAR currents in spinal motorneurons increase after SCI with SNI, and decrease after the administration of NASPM, a CP-AMPAR antagonist. Data-driven analysis using non-linear principal components analysis (NL-PCA) also demonstrated that SCI with SNI produces a multivariate signature of AMPAR plasticity that is observed in other forms of nociceptive peripheral input, indicating a general mechanism for maladaptive plasticity in spinal motor systems in response to polytraumatic SCI.

IntroductionThe role of pain as a warning system necessitates a rapid transmission of information from the periphery for the execution of appropriate motor responses. The nociceptive withdrawal reflex (NWR) is a physiological response to protect the limb from a painful stimulus and is often considered an objective measure of spinal nociceptive excitability. The NWR is commonly defined by its latency in the presumed Ad-fiber range consistent with the canonical view that "fast pain" is signaled by Ad nociceptors. We recently demonstrated that human skin is equipped with ultrafast (A{beta} range) nociceptors. Here, we investigated the short-latency component of the reflex and explored the relationship between reflex latency and pain perception. MethodsWe revisited our earlier work on NWR measurements in which, following convention, only those reflex responses were selected that were in the presumed Ad range (taken to be latencies [≥]90 ms in that study). In our current analysis, we expanded the time window to search for shorter latency responses and compared those with pain ratings. ResultsIn both cohorts, we found an abundance of recordings with short-latency reflex responses. In nearly 90% of successful recordings, only single reflex responses (not dual) were seen which allowed us to compare pain ratings to reflex latencies. We found that shorter latency reflexes were just as painful as those in the conventional latency range. DiscussionWe found a preponderance of short-latency painful reflex responses. Based on this finding, we suggest that short-latency responses must be considered in future studies. We predict these might be signaled by the ultrafast nociceptors, warranting further investigation.

The vagus nerve conveys interoceptive information, yet how specific vagal sensory afferents regulate pain remains unclear. Here, we tested whether vagus nerve stimulation (VNS) modulates temporomandibular disorder (TMD)-related pain. In a mouse model of TMD, auricular VNS (aVNS) attenuated temporomandibular joint (TMJ) pain behaviors and suppressed sensitization of trigeminal nociceptors. We identified a subset of vagal sensory afferents with dopaminergic features that was sufficient to mediate these effects, as selective activation of these afferents recapitulated the analgesic actions of aVNS. These findings highlight an underappreciated peripheral interoceptive pathway and provide a mechanistic framework for targeted neuromodulation in chronic craniofacial pain.

Pain perception is initiated upon activation of nociceptors of the dorsal root ganglia (DRG) and trigeminal ganglia. We identified G protein-coupled receptors (GPCRs) expressed in CGRP+ mouse and human nociceptors and found that agonists of several identified Gi/o-coupled and orphan GPCRs attenuated neuronal excitability. Experiments focusing on the Gi/o-coupled serotonin receptor Htr1b, which is expressed in mouse and human CGRP+ DRG neurons, revealed that Htr1b/1d agonists, the triptans sumatriptan and zolmitriptan, attenuated CGRP+ neuron excitability in vitro and exhibited analgesia across several pain models, including neuropathic pain. Conditional genetic deletion experiments showed that triptan-induced analgesia is mediated by Htr1b expressed in A-fiber mechanonociceptors. Also, triptan-associated adverse effects are partially mediated by Htr1b-independent targets. Further testing identified the GPCR Gpr19 as an additional promising target for treating pain. These findings establish a preclinical screening platform for identifying novel analgesics and reveal nociceptor GPCRs that may be targeted to treat pain.

Chronic pain, a complex multidimensional disorder, remains a major health care issue and a therapeutic challenge. Neuropathic pain is a chronic pain condition that results from damage or dysfunction in the nervous system. While mechanisms of neuropathic pain at the peripheral and spinal cord level have been extensively studied, pain mechanisms in the brain remain underexplored. The amygdala, a limbic brain region, has emerged as a critical brain area for the emotional-affective dimension of pain and pain modulation. Amygdala neuroplasticity has been associated with pain states, but exact molecular and cellular mechanisms underlying these states and the transition from acute to chronic pain are not well understood. Here, we used the spinal nerve ligation (SNL) model of neuropathic pain in male rats to investigate changes in gene expression in the amygdala at the chronic pain stage using RNA sequencing (RNA-Seq). Two amygdala nuclei, basolateral (BLA) and central (CeA), were investigated in a hemisphere-dependent manner. We used an integrative approach that focuses on functional significance and cell type specificity of differentially expressed genes (DEGs) to nominate mechanistic targets for central regulation of chronic pain. Our integrative transcriptomic and bioinformatic analyses identified individual genes (e.g., Cxcl10, Cxcl12, Mbp, Plp1, Mag, Mog, Slc17a6, Gad1, Sst), molecular pathways (e.g., cytokine-mediated signaling pathway), biological processes (e.g., myelination, synaptic transmission), and specific cell types (e.g., oligodendrocytes, glutamatergic and GABA-ergic neurons) affected by chronic pain. Our results also provide evidence for the emerging concept of hemispheric lateralization of pain processing in the amygdala. Overall, our study proposes oligodendrocyte dysfunction in the amygdala, neuroimmune signaling in CeA, and glutamatergic neurotransmission in BLA as mechanistic determinants of and potential therapeutic targets for the management of chronic neuropathic pain.

Neuropathic pain is a leading cause of high impact pain, is often disabling and is poorly managed by current therapeutics. Here we focused on a unique group of neuropathic pain patients undergoing thoracic vertebrectomy where the DRG is removed as part of the surgery allowing for molecular characterization and identification of mechanistic drivers of neuropathic pain independently of preclinical models. Our goal was to quantify whole transcriptome RNA abundances using RNA-seq in pain-associated human DRGs from these patients, allowing comprehensive identification of molecular changes in these samples by contrasting them with non-pain associated DRGs. We sequenced 70 human DRGs, including over 50 having mRNA libraries with neuronal mRNA. Our expression analysis revealed profound sex differences in differentially expressed genes including increase of IL1B, TNF, CXCL14, and OSM in male and including CCL1, CCL21, PENK and TLR3 in female DRGs associated with neuropathic pain. Co-expression modules revealed enrichment in members of JUN-FOS signaling in males, and centromere protein coding genes in females. Neuro-immune signaling pathways revealed distinct cytokine signaling pathways associated with neuropathic pain in males (OSM, LIF, SOCS1) and females (CCL1, CCL19, CCL21). We validated cellular expression profiles of a subset of these findings using RNAscope in situ hybridization. Our findings give direct support for sex differences in underlying mechanisms of neuropathic pain in patient populations.

Neonatal pain is a significant clinical issue but the mechanisms by which pain is produced early in life are poorly understood. Our recent work has linked the transcription factor serum response factor downstream of local growth hormone (GH) signaling to incision-related hypersensitivity in neonates. However, it remains unclear if similar mechanisms contribute to inflammatory pain in neonates. We found that local GH treatment inhibited neonatal inflammatory myalgia but appeared to do so through a unique signal transducer and activator of transcription (STAT) dependent pathway within sensory neurons. The STAT1 transcription factor appeared to regulate peripheral inflammation itself by modulation of monocyte chemoattractant protein 1 (MCP1) release from sensory neurons. Data suggests that STAT1 upregulation, downstream of GH signaling, contributes to neonatal nociception during muscle inflammation through a novel neuroimmune loop involving cytokine release from primary afferents. Results could uncover new ways to treat muscle pain and inflammation in neonates. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=132 SRC="FIGDIR/small/605393v2_ufig1.gif" ALT="Figure 1"> View larger version (28K): org.highwire.dtl.DTLVardef@18b0bdorg.highwire.dtl.DTLVardef@fb5ee9org.highwire.dtl.DTLVardef@114179aorg.highwire.dtl.DTLVardef@19979b_HPS_FORMAT_FIGEXP M_FIG C_FIG

Patients with schwannomatosis (SWN) develop multiple tumors along major peripheral nerves, with most experiencing significant pain, though each patients symptoms are unique. Neuropathic, nociceptive, and inflammatory pain types have been reported, but many patients describe severe pain when a schwannoma is palpated or even lightly touched. Currently, the only effective treatment for pain relief is surgical removal. We are investigating the root causes of tumor-induced pain. In some cases, tumor growth increases pressure on nearby nerves, resulting in pain. Additionally, schwannoma cells in culture secrete proinflammatory cytokines into the surrounding medium. This conditioned medium (CM) sensitizes sensory neurons to painful stimuli both in vitro and in vivo. When injected into the glabrous skin of a mouse hindpaw, CM from painful schwannomas increases neuron sensitivity to light touch, as demonstrated by a fourfold reduction in paw withdrawal threshold (measured using the Von Frey assay) one hour post-injection (p = 0.006), with effects persisting for 24 hours (p = 0.002).We hypothesize that this increase in sensitivity is linked to mechanosensitive ion channels (MSCs), which detect pressure and stretch. These channels can be blocked by the peptide GsMTx-4. This peptide penetrates deeper into cell membranes under mechanical pressure to block MSCs from opening without affecting other ion channels. When co-injected with CM into the mouse hindpaw, 10 {micro}M GsMTx-4 prevents heightened sensitivity to light touch. Moreover, GsMTx-4 can reverse hyperalgesia, restoring withdrawal thresholds to baseline levels. Thus, local injection of GsMTx-4 near painful tumors presents a promising, minimally invasive therapeutic approach for SWN patients. SignificancePain is a confounding comorbidity in the multiple tumor syndrome schwannomatosis. Patients harbor benign peripheral nerve sheath tumors that rarely become malignant or cause neurological deficits. Yet, patients undergo numerous surgeries for the removal of painful tumors. A non-invasive treatment for tumor-related pain is in dire need. We are examining the small peptide GsMTx-4, a blocker of mechanosensitive ion channels, as a potential therapy for painful tumors in the context of schwannomatosis.

IntroductionOffset analgesia (OA) is defined as a disproportionate reduction in pain perception following a small decrease in noxious stimulation. However, the mechanisms underlying this phenomenon remain unclear, with ongoing debate on peripheral versus central contributions. ObjectivesThis experimental study aimed to differentiate first and second pain perception during the OA paradigm, thereby assessing fiber-specific influences on OA. MethodsThirty-two healthy participants were asked to distinguish a double pain sensation (first and second pain), to assess pain quality descriptors related to A-{delta} and C-fibers, and to indicate response times to brief noxious heat stimuli. This procedure was repeated while implementing heat pulses in an OA paradigm. ResultsNo significant differences were found between offset and constant trials in the reported double pain sensation or the fiber specific pain descriptors (p > 0.05). Nevertheless, significant differences in response times were observed depending on the type of trial and the timing of the stimulus. Response time to noxious stimuli was delayed after prolonged stimulation in both offset and constant trials (p < 0.05). ConclusionsThe findings suggest that A-{delta} and C-fiber response characteristics were unaffected during the OA paradigm; however, higher stimulation intensities or prolonged pain induce a notable response delay. This indicates a negligible role of specific peripheral nerve fibers in OA, emphasizing the predominance of central mechanisms, particularly those related to attention and cognitive resources, which merit further investigation.

BackgroundThe experience of pain has been dissociated into two interwoven aspects: a sensory-discriminative aspect and an affective-motivational aspect. We aimed to explore which of the pain descriptors is more deeply rooted in the human brain. FindingsParticipants were asked to evaluate applied cold pain. The majority of the trials showed distinct ratings: some were rated higher for unpleasantness and others for intensity. We compared the relationship between functional data recorded from 7 tesla MRI with unpleasantness and intensity ratings and revealed a stronger relationship between cortical data and unpleasantness ratings. ConclusionsThe present study underlines the importance of the emotional-affective aspects of pain-related cortical processes in the brain. The findings corroborate previous studies showing a higher sensitivity to pain unpleasantness compared to ratings of pain intensity. For the processing of pain in healthy subjects, this effect may reflect the more direct and intuitive evaluation of emotional aspects of the pain system, which is to prevent harm and to preserve the physical integrity of the body.

BackgroundThe Delta-opioid receptor (DOR) has gained attention as a promising target for the treatment of migraine and headache disorders. This is largely attributed to its unique pharmacological profile, which suggests that DOR-targeting treatment offers effective therapeutic benefit with a lower risk of medication overuse headache (MOH), reduced abuse liability, and minimal potential for physical dependence. These advantages have driven the development of a novel DOR agonist PN6047 (3-[[4-(dimethylcarbamoyl) phenyl]-[1-(thiazol-5-ylmethyl)-4-piperidylidene] methyl]benzamide), which has completed Phase I clinical trial and showed a favorable safety and tolerability profile. Although PN6047 has shown promising effects in neuropathic pain models, its efficacy in preclinical models of headache-associated pain remains to be evaluated. Here, we investigated the effects of PN6047 in models of migraine-associated pain and aura as well as post-traumatic headache (PTH) and MOH. MethodsC57BL6/J mice were used to examine the effects of PN6047 in the following migraine models: chronic intermittent nitroglycerin (NTG)-induced migraine-associated pain, PTH, KCl-induced cortical spreading depression (CSD), and optogenetic evoked CSD in a freely behaving transgenic mice expressing ChR2-eYFP. In addition, we tested whether chronic PN6047 induced MOH and whether it could prevent the development of MOH induced by sumatriptan. ResultsA single injection of PN6047 blocked chronic cephalic allodynia established by chronic intermittent NTG and PTH. Moreover, chronic PN6047 treatment prevented the development of MOH induced by sumatriptan, without causing MOH itself. In addition, PN6047 significantly reduced the number of CSD events in the KCl-induced CSD model, and delayed CSD onset triggered in freely behaving mice along with subsequent CSD-evoked allodynia. ConclusionPN6047, a novel DOR agonist, strikingly blocks headache-associated mechanism and symptoms in preclinical models of chronic migraine, migraine aura, PTH, and MOH. Importantly, prolonged PN6047 treatment did not induce MOH or analgesic tolerance. Together, these data demonstrate that despite the distinct mechanisms underlying migraine and headache disorder, PN6047 exhibits robust efficacy without inducing MOH, and displays a favorable safety and tolerability profile.