Modality-specific peripheral antinociceptive effects of μ-opioid agonists on heat and mechanical stimuli: Contribution of sigma-1 receptors

Pharmacological modulation of Sigma-1 receptor ameliorates pathological neuroinflammation in rats with diabetic neuropathic pain via the AKT/GSK-3β/NF-κB pathway

Diabetic neuropathic pain (DNP) is a common complication of diabetes mellitus (DM) and is characterized by spontaneous pain and neuroinflammation. The Sigma-1 receptor (Sig-1R) has been proposed as a target for analgesic development. It is an important receptor with anti-inflammatory properties and has been found to regulate DNP. However, it is not known whether Sig-1R can ameliorate pathological neuroinflammation in DNP. The present study used a rat model of DNP and a highly selective agonist of Sig-1R to assess the effects of the protein on neuropathic pain in rats with type 2 diabetes mellitus. The rats were divided into Control, Model, Sig-1R agonist PRE-084 (0.3, 0.6, 1 mg/kg), and metformin (Met, 20 mg/kg) groups, with seven rats per group, and their body weight, fasting blood glucose, mechanical withdrawal threshold and thermal withdrawal latency were tested weekly for two weeks. After treatment with PRE-084, the pain thresholds in the DNP rats were significantly improved, together with pathological changes in the dorsal root ganglion, reductions in the serum levels of TNF-α, IL-1β, IL-6, MOD, and prostaglandin E2 (PGE2), and the activity of superoxide dismutase was increased. The mRNA levels of TNF-α, IL-1β, and cyclooxygenase 2 (COX-2) were reduced. Pharmacological inhibition of Sig-1R with BD1047 (10 μM) abolished Sig-1R-mediated activation of lipopolysaccharide-treated BV-2 microglial cells. It was also found that PRE-084 increased phosphorylation of serine/threonine protein kinase B (AKT) and glycogen synthase kinase 3β (GSK-3β) at Ser9, inhibiting nuclear factor kappa B (NF-κB)-mediated neuroinflammation in the dorsal root ganglion, thus reducing DNP. The findings suggest that the effect of Sig-1R agonist PRE-084 on DNP may reduce the level of inflammation through the up-regulation of AKT/GSK-3β and down-regulation of the NF-κB signaling, thereby contributing to the treatment of the disease.

The role of neutrophils in pain: systematic review and meta-analysis of animal studies

ABSTRACT

The peripheral inflammatory response is an attractive therapeutic target for pain treatment. Neutrophils are the first circulating inflammatory cells recruited to sites of injury, but their contribution to pain outcomes is unclear. We performed a systematic review and meta-analysis of original preclinical studies, which evaluated the effect of preemptive neutrophil depletion on pain outcomes (PROSPERO registration number: CRD42022364004). Literature search (PubMed, January 19, 2023) identified 49 articles, which were meta-analyzed using a random-effects model. The risk of bias was evaluated using SYRCLE's tool. The pooled effect considering all studies showed that neutrophil depletion induced a consistent pain reduction. Inflammatory, joint, neuropathic, and visceral pain showed significant pain alleviation by neutrophil depletion with medium-large effect sizes. However, muscle and postoperative pain were not significantly alleviated by neutrophil depletion. Further analysis showed a differential contribution of neutrophils to pain outcomes. Neutrophils had a higher impact on mechanical hyperalgesia, followed by nociceptive behaviors and mechanical allodynia, with a smaller contribution to thermal hyperalgesia. Interspecies (mice or rats) differences were not appreciated. Analyses regarding intervention unveiled a lower pain reduction for some commonly used methods for neutrophil depletion, such as injection of antineutrophil serum or an anti-Gr-1 antibody, than for other agents such as administration of an anti-Ly6G antibody, fucoidan, vinblastine, CXCR1/2 inhibitors, and etanercept. In conclusion, the contribution of neutrophils to pain depends on pain etiology (experimental model), pain outcome, and the neutrophil depletion strategy. Further research is needed to improve our understanding on the mechanisms of these differences.

Sigma-1 Receptors Control Neuropathic Pain and Peripheral Neuroinflammation After Nerve Injury in Female Mice: A Transcriptomic Study

The mechanisms for neuropathic pain amelioration by sigma-1 receptor inhibition are not fully understood. We studied genome-wide transcriptomic changes (RNAseq) in the dorsal root ganglia (DRG) from wild-type and sigma-1 receptor knockout mice prior to and following Spared Nerve Injury (SNI). In wildtype mice, most of the transcriptomic changes following SNI are related to the immune function or neurotransmission. Immune function transcripts contain cytokines and markers for immune cells, including macrophages/monocytes and CD4 + T cells. Many of these immune transcripts were attenuated by sigma-1 knockout in response to SNI. Consistent with this we found, using flow cytometry, that sigma-1 knockout mice showed a reduction in macrophage/monocyte recruitment as well as an absence of CD4 + T cell recruitment in the DRG after nerve injury. Sigma-1 knockout mice showed a reduction of neuropathic (mechanical and cold) allodynia and spontaneous pain-like responses (licking of the injured paw) which accompany the decreased peripheral neuroinflammatory response after nerve injury. Treatment with maraviroc (a CCR5 antagonist which preferentially inhibits CD4 + T cells in the periphery) of neuropathic wild-type mice only partially replicated the sigma-1 knockout phenotype, as it did not alter cold allodynia but attenuated spontaneous pain-like responses and mechanical hypersensitivity. Therefore, modulation of peripheral CD4 + T cell activity might contribute to the amelioration of spontaneous pain and neuropathic tactile allodynia seen in the sigma-1 receptor knockout mice, but not to the effect on cold allodynia. We conclude that sigma-1 receptor inhibition decreases DRG neuroinflammation which might partially explain its anti-neuropathic effect.

Towards Multitargeted Ligands as Pain Therapeutics: Dual Ligands of the Cavα2δ-1 Subunit of Voltage-Gated Calcium Channel and the μ-Opioid Receptor

The synthesis and pharmacological activity of a new series of dual ligands combining activities towards the α2δ-1 subunit of voltage-gated calcium channels (Cavα2δ-1) and the μ-opioid receptor (MOR) as novel pain therapeutics are reported. A careful exploration of the pharmacophores related to both targets, which in principle had few common characteristics, led to the design of novel compounds exhibiting both activities. The construction of the dual ligands started from published Cavα2δ-1 ligands, onto which MOR ligand pharmacophoric elements were added. This exercise led to new amino-acidic substances with good affinities on both targets as well as good metabolic and physicochemical profiles and low potential for drug-drug interactions. A representative compound, (2S,4S)-4-(4-chloro-3-(((cis)-4-(dimethylamino)-4-phenylcyclohexyl)methyl)-5-fluorophenoxy)pyrrolidine-2-carboxylic acid, displayed promising analgesic activities in several in vivo pain models as well as a reduced side-effect profile in relation to morphine.

Sigma-1 receptor agonism exacerbates immune-driven nociception: Role of TRPV1 + nociceptors

The analgesic effects of sigma-1 antagonists are undisputed, but the effects of sigma-1 agonists on pain are not well studied. Here, we used a mouse model to show that the administration of the sigma-1 agonists dextromethorphan (a widely used antitussive drug), PRE-084 (a standard sigma-1 ligand), and pridopidine (a selective drug being investigated in clinical trials for the treatment of neurodegenerative diseases) enhances PGE2-induced mechanical hyperalgesia. Superficial plantar incision induced transient weight-bearing asymmetry at early time points, but the mice appeared to recover at 24 h, despite noticeable edema and infiltration of neutrophils (a well-known cellular source of PGE2) at the injured site. Sigma-1 agonists induced a relapse of weight bearing asymmetry in a manner dependent on the presence of neutrophils. The effects of sigma-1 agonists were all reversed by administration of the sigma-1 antagonist BD-1063 in wild-type mice, and were absent in sigma-1 knockout mice, supporting the selectivity of the effects observed. The proalgesic effects of sigma-1 agonism were also abolished by the TRP antagonist ruthenium red and by in vivo resiniferatoxin ablation of TRPV1 + peripheral sensory neurons. Therefore, sigma-1 agonism exacerbates pain-like responses in mice with a mild inflammatory state through the action of TRPV1 + nociceptors. We also show that sigma-1 receptors are present in most (if not all) mouse and human DRG neurons. If our findings translate to humans, further studies will be needed to investigate potential proalgesic effects induced by sigma-1 agonism in patients treated with sigma-1 agonists.

The sigma-1 receptor curtails endogenous opioid analgesia during sensitization of TRPV1 nociceptors

BACKGROUND AND PURPOSE

Peripheral sensitization contributes to pathological pain. While prostaglandin E2 (PGE2) and nerve growth factor (NGF) sensitize peptidergic C-nociceptors (TRPV1+), glial cell line-derived neurotrophic factor (GDNF) sensitizes non-peptidergic C-neurons (IB4+). The sigma-1 receptor (sigma-1R) is a Ca²⁺ -sensing chaperone known to modulate opoid analgesia. This receptor binds both to TRPV1 and the μ opioid receptor, although the functional repercussions of these physical interactions in peripheral sensitization are unknown.

EXPERIMENTAL APPROACH

We tested the effects of sigma-1 antagonism on PGE2-, NGF-, and GDNF-induced mechanical and heat hyperalgesia in mice. We used immunohistochemistry to determine the presence of endomorphin-2, an endogenous μ receptor agonist, on dorsal root ganglion (DRG) neurons. Recombinant proteins were used to study the interactions between sigma-1R, μ- receptor, and TRPV1. We used calcium imaging to study the effects of sigma-1 antagonism on PGE2-induced sensitization of TRPV1+ nociceptors.

KEY RESULTS

Sigma1 antagonists reversed PGE2- and NGF-induced hyperalgesia but not GDNF-induced hyperalgesia. Endomorphin-2 was detected on TRPV1+ but not on IB4+ neurons. Peripheral opioid receptor antagonism by naloxone methiodide or administration of an anti-endomorphin-2 antibody to a sensitized paw reversed the antihyperalgesia induced by sigma-1 antagonists. Sigma-1 antagonism transfers sigma-1R from TRPV1 to μ receptors, suggesting that sigma-1R participate in TRPV1-μ receptor crosstalk. Moreover, sigma-1 antagonism reversed, in a naloxone-sensitive manner, PGE2-induced sensitization of DRG neurons to the calcium flux elicited by capsaicin, the prototypic TRPV1 agonist.

CONCLUSION AND IMPLICATIONS

Sigma-1 antagonism harnesses endogenous opioids produced by TRPV1+ neurons to reduce hyperalgesia by increasing μ receptor activity.

Nucleoporin POM121 signals TFEB-mediated autophagy via activation of SIGMAR1/sigma-1 receptor chaperone by pridopidine

Macroautophagy/autophagy is an essential process for cellular survival and is implicated in many diseases. A critical step in autophagy is the transport of the transcription factor TFEB from the cytosol into the nucleus, through the nuclear pore (NP) by KPNB1/importinβ1. In the C9orf72 subtype of amyotrophic lateral sclerosis-frontotemporal lobar degeneration (ALS-FTD), the hexanucleotide (G4C2)RNA expansion (HRE) disrupts the nucleocytoplasmic transport of TFEB, compromising autophagy. Here we show that a molecular chaperone, the SIGMAR1/Sigma-1 receptor (sigma non-opioid intracellular receptor 1), facilitates TFEB transport into the nucleus by chaperoning the NP protein (i.e., nucleoporin) POM121 which recruits KPNB1. In NSC34 cells, HRE reduces TFEB transport by interfering with the association between SIGMAR1 and POM121, resulting in reduced nuclear levels of TFEB, KPNB1, and the autophagy marker LC3-II. Overexpression of SIGMAR1 or POM121, or treatment with the highly selective and potent SIGMAR1 agonist pridopidine, currently in phase 2/3 clinical trials for ALS and Huntington disease, rescues all of these deficits. Our results implicate nucleoporin POM121 not merely as a structural nucleoporin, but also as a chaperone-operated signaling molecule enabling TFEB-mediated autophagy. Our data suggest the use of SIGMAR1 agonists, such as pridopidine, for therapeutic development of diseases in which autophagy is impaired.Abbreviations: ALS-FTD, amyotrophic lateral sclerosis-frontotemporal dementiaC9ALS-FTD, C9orf72 subtype of amyotrophic lateral sclerosis-frontotemporal dementiaCS, citrate synthaseER, endoplasmic reticulumGSS, glutathione synthetaseHRE, hexanucleotide repeat expansionHSPA5/BiP, heat shock protein 5LAMP1, lysosomal-associated membrane protein 1MAM, mitochondria-associated endoplasmic reticulum membraneMAP1LC3/LC3, microtubule-associated protein 1 light chain 3NP, nuclear poreNSC34, mouse motor neuron-like hybrid cell lineNUPs, nucleoporinsPOM121, nuclear pore membrane protein 121SIGMAR1/Sigma-1R, sigma non-opioid intracellular receptor 1TFEB, transcription factor EBTMEM97/Sigma-2R, transmembrane protein 97.

Knocking Out Sigma-1 Receptors Reveals Diverse Health Problems

Sigma-1 receptor (Sig-1R) is a protein present in several organs such as brain, lung, and heart. In a cell, Sig-1R is mainly located across the membranes of the endoplasmic reticulum and more specifically at the mitochondria-associated membranes. Despite numerous studies showing that Sig-1R could be targeted to rescue several cellular mechanisms in different pathological conditions, less is known about its fundamental relevance. In this review, we report results from various studies and focus on the importance of Sig-1R in physiological conditions by comparing Sig-1R KO mice to wild-type mice in order to investigate the fundamental functions of Sig-1R. We note that the Sig-1R deletion induces cognitive, psychiatric, and motor dysfunctions, but also alters metabolism of heart. Finally, taken together, observations from different experiments demonstrate that those dysfunctions are correlated to poor regulation of ER and mitochondria metabolism altered by stress, which could occur with aging.

Structural and Molecular Insight into Piperazine and Piperidine Derivatives as Histamine H3 and Sigma-1 Receptor Antagonists with Promising Antinociceptive Properties

In an attempt to extend recent studies showing that some clinically evaluated histamine H3 receptor (H3R) antagonists possess nanomolar affinity at sigma-1 receptors (σ1R), we selected 20 representative structures among our previously reported H3R ligands to investigate their affinity at σRs. Most of the tested compounds interact with both sigma receptors to different degrees. However, only six of them showed higher affinity toward σ1R than σ2R with the highest binding preference to σ1R for compounds 5, 11, and 12. Moreover, all these ligands share a common structural feature: the piperidine moiety as the fundamental part of the molecule. It is most likely a critical structural element for dual H3/σ1 receptor activity as can be seen by comparing the data for compounds 4 and 5 (hH3R Ki = 3.17 and 7.70 nM, σ1R Ki = 1531 and 3.64 nM, respectively), where piperidine is replaced by piperazine. We identified the putative protein-ligand interactions responsible for their high affinity using molecular modeling techniques and selected compounds 5 and 11 as lead structures for further evaluation. Interestingly, both ligands turned out to be high-affinity histamine H3 and σ1 receptor antagonists with negligible affinity at the other histamine receptor subtypes and promising antinociceptive activity in vivo. Considering that many literature data clearly indicate high preclinical efficacy of individual selective σ1 or H3R ligands in various pain models, our research might be a breakthrough in the search for novel, dual-acting compounds that can improve existing pain therapies. Determining whether such ligands are more effective than single-selective drugs will be the subject of our future studies.

Bifunctional μ opioid and σ1 receptor ligands as novel analgesics with reduced side effects

Opioid analgesics are highly effective painkillers for the treatment of moderate or severe pain, but they are associated with a number of undesirable adverse effects, including the development of tolerance, addiction, constipation and life-threatening respiratory depression. The development of new and safer analgesics with innovative mechanisms of action, which can enhance the efficacy in comparison to available treatments and reduce their side effects, is urgently needed. The sigma-1 receptor (σ1R), a unique Ca2+-sensing chaperone protein, is expressed throughout pain-modulating tissues and affects neurotransmission by interacting with different protein partners, including molecular targets that participate in nociceptive signalling, such as the μ-opioid receptor (MOR), N-methyl-d-aspartate receptor (NMDAR) and cannabinoid 1 receptor (CB1R). Overwhelming pharmacological and genetic evidence indicates that σ1R antagonists induce anti-hypersensitive effects in sensitising pain conditions (e.g. chemically induced, inflammatory and neuropathic pain) and enhance opioid analgesia but not opioid-mediated detrimental effects. It has been suggested that balanced modulation of MORs and σ1Rs may improve both the therapeutic efficacy and safety of opioids. This review summarises the functional profiles of ligands with mixed MOR agonist and σ1R antagonist activities and highlights their therapeutic potentials for pain management. Dual MOR agonism/σ1R antagonism represents a promising avenue for the development of potent and safer analgesics.

Emerging Benefits: Pathophysiological Functions and Target Drugs of the Sigma-1 Receptor in Neurodegenerative Diseases

The sigma-1 receptor (Sig-1R) is encoded by the SIGMAR1 gene and is a nonopioid transmembrane receptor located in the mitochondrial-associated endoplasmic reticulum membrane (MAM). It helps to locate endoplasmic reticulum calcium channels, regulates calcium homeostasis, and acts as a molecular chaperone to control cell fate and participate in signal transduction. It plays an important role in protecting neurons through a variety of signaling pathways and participates in the regulation of cognition and motor behavior closely related to neurodegenerative diseases. Based on its neuroprotective effects, Sig-1R has now become a breakthrough target for alleviating Alzheimer's disease and other neurodegenerative diseases. This article reviews the most cutting-edge research on the function of Sig-1R under normal or pathologic conditions and target drugs of the sigma-1 receptor in neurodegenerative diseases.

Genomic Action of Sigma-1 Receptor Chaperone Relates to Neuropathic Pain

Sigma-1 receptors (Sig-1Rs) are endoplasmic reticulum (ER) chaperones implicated in neuropathic pain. Here we examine if the Sig-1R may relate to neuropathic pain at the level of dorsal root ganglia (DRG). We focus on the neuronal excitability of DRG in a "spare nerve injury" (SNI) model of neuropathic pain in rats and find that Sig-1Rs likely contribute to the genesis of DRG neuronal excitability by decreasing the protein level of voltage-gated Cav2.2 as a translational inhibitor of mRNA. Specifically, during SNI, Sig-1Rs translocate from ER to the nuclear envelope via a trafficking protein Sec61β. At the nucleus, the Sig-1R interacts with cFos and binds to the promoter of 4E-BP1, leading to an upregulation of 4E-BP1 that binds and prevents eIF4E from initiating the mRNA translation for Cav2.2. Interestingly, in Sig-1R knockout HEK cells, Cav2.2 is upregulated. In accordance with those findings, we find that intra-DRG injection of Sig-1R agonist (+)pentazocine increases frequency of action potentials via regulation of voltage-gated Ca2+ channels. Conversely, intra-DRG injection of Sig-1R antagonist BD1047 attenuates neuropathic pain. Hence, we discover that the Sig-1R chaperone causes neuropathic pain indirectly as a translational inhibitor.

Sigma-1 receptor: A drug target for the modulation of neuroimmune and neuroglial interactions during chronic pain

Immune and glial cells play a pivotal role in chronic pain. Therefore, it is possible that the pharmacological modulation of neurotransmission from an exclusively neuronal perspective may not be enough for adequate pain management, and the modulation of complex interactions between neurons and other cell types might be needed for successful pain relief. In this article, we review the current scientific evidence for the modulatory effects of sigma-1 receptors on communication between the immune and nervous systems during inflammation, as well as the influence of this receptor on peripheral and central neuroinflammation. Several experimental models of pathological pain are considered, including peripheral and central neuropathic pain, osteoarthritic, and cancer pain. Sigma-1 receptor inhibition prevents peripheral (macrophage infiltration into the dorsal root ganglion) and central (activation of microglia and astrocytes) neuroinflammation in several pain models, and enhances immune-driven peripheral opioid analgesia during painful inflammation, maximizing the analgesic potential of peripheral immune cells. Therefore, sigma-1 antagonists may constitute a new class of analgesics with an unprecedented mechanism of action and potential utility in several painful disorders.

Endogenous Opiates and Behavior: 2018

This paper is the forty-first consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2018 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides and receptors as well as effects of opioid/opiate agonists and antagonists. The review is subdivided into the following specific topics: molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (2), the roles of these opioid peptides and receptors in pain and analgesia in animals (3) and humans (4), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (5), opioid peptide and receptor involvement in tolerance and dependence (6), stress and social status (7), learning and memory (8), eating and drinking (9), drug abuse and alcohol (10), sexual activity and hormones, pregnancy, development and endocrinology (11), mental illness and mood (12), seizures and neurologic disorders (13), electrical-related activity and neurophysiology (14), general activity and locomotion (15), gastrointestinal, renal and hepatic functions (16), cardiovascular responses (17), respiration and thermoregulation (18), and immunological responses (19).

Sigma-1 Receptor Positron Emission Tomography: A New Molecular Imaging Approach Using (S)-(-)-[18F]Fluspidine in Glioblastoma

Glioblastoma multiforme (GBM) is the most devastating primary brain tumour characterised by infiltrative growth and resistance to therapies. According to recent research, the sigma-1 receptor (sig1R), an endoplasmic reticulum chaperone protein, is involved in signaling pathways assumed to control the proliferation of cancer cells and thus could serve as candidate for molecular characterisation of GBM. To test this hypothesis, we used the clinically applied sig1R-ligand (S)-(-)-[18F]fluspidine in imaging studies in an orthotopic mouse model of GBM (U87-MG) as well as in human GBM tissue. A tumour-specific overexpression of sig1R in the U87-MG model was revealed in vitro by autoradiography. The binding parameters demonstrated target-selective binding according to identical KD values in the tumour area and the contralateral side, but a higher density of sig1R in the tumour. Different kinetic profiles were observed in both areas, with a slower washout in the tumour tissue compared to the contralateral side. The translational relevance of sig1R imaging in oncology is reflected by the autoradiographic detection of tumour-specific expression of sig1R in samples obtained from patients with glioblastoma. Thus, the herein presented data support further research on sig1R in neuro-oncology.

Urinary bladder sigma-1 receptors: A new target for cystitis treatment

No adequate treatment is available for painful urinary bladder disorders such as interstitial cystitis/bladder pain syndrome, and the identification of new urological therapeutic targets is an unmet need. The sigma-1 receptor (σ1-R) modulates somatic pain, but its role in painful urological disorders is unexplored. The urothelium expresses many receptors typical of primary sensory neurons (e.g. TRPV1, TRPA1 and P2X3) and high levels of σ1-R have been found in these neurons; we therefore hypothesized that σ1-R may also be expressed in the urothelium and may have functional relevance in this tissue. With western blotting and immunohistochemical methods, we detected σ1-R in the urinary bladder in wild-type (WT) but not in σ1-R-knockout (σ1-KO) mice. Interestingly, σ1-R was located in the bladder urothelium not only in mouse, but also in human bladder sections. The severity of histopathological (edema, hemorrhage and urothelial desquamation) and biochemical alterations (enhanced myeloperoxidase activity and phosphorylation of extracellular regulated kinases 1/2 [pERK1/2]) that characterize cyclophosphamide-induced cystitis was lower in σ1-KO than in WT mice. Moreover, cyclophosphamide-induced pain behaviors and referred mechanical hyperalgesia were dose-dependently reduced by σ1-R antagonists (BD-1063, NE-100 and S1RA) in WT but not in σ1-KO mice. In contrast, the analgesic effect of morphine was greater in σ1-KO than in WT mice. Together these findings suggest that σ1-R plays a functional role in the mechanisms underlying cyclophosphamide-induced cystitis, and modulates morphine analgesia against urological pain. Therefore, σ1-R may represent a new drug target for urinary bladder disorders.

Sigma-1 receptors control neuropathic pain and macrophage infiltration into the dorsal root ganglion after peripheral nerve injury

Neuron-immune interaction in the dorsal root ganglia (DRG) plays a pivotal role in the neuropathic pain development after nerve injury. Sigma-1 receptor (Sig-1R) is expressed by DRG neurons but its role in neuropathic pain is not fully understood. We investigated the effect of peripheral Sig-1R on neuroinflammation in the DRG after spared (sciatic) nerve injury (SNI) in mice. Nerve injury induced a decrease in NeuN staining along with the nuclear eccentricity and ATF3 expression in the injured DRG. Sig-1R was present in all DRG neurons examined, and after SNI this receptor translocated to the periphery of the soma and the vicinity of the nucleus, especially in injured ATF3 + neurons. In WT mice, injured DRG produced the chemokine CCL2, and this was followed by massive infiltration of macrophages/monocytes, which clustered mainly around sensory neurons with translocated Sig-1R, accompanied by robust IL-6 increase and mechanical allodynia. In contrast, Sig-1R knockout (Sig-1R-KO) mice showed reduced levels of CCL2, decreased macrophage/monocyte infiltration into DRG, and less IL-6 and neuropathic mechanical allodynia after SNI. Our findings point to an important role of peripheral Sig-1R in sensory neuron-macrophage/monocyte communication in the DRG after peripheral nerve injury; thus, these receptors may contribute to the neuropathic pain phenotype.

4-Aryl-1-oxa-4,9-diazaspiro[5.5]undecane Derivatives as Dual μ-Opioid Receptor Agonists and σ1 Receptor Antagonists for the Treatment of Pain

The synthesis and pharmacological activity of a new series of 1-oxa-4,9-diazaspiro[5.5]undecane derivatives as potent dual ligands for the sigma-1 receptor (σ1R) and the μ-opioid receptor (MOR) are reported. The different positions of the central scaffold, designed using a merging strategy of both target pharmacophores, were explored using a versatile synthetic approach. Phenethyl derivatives in position 9, substituted pyridyl moieties in position 4 and small alkyl groups in position 2 provided the best profiles. One of the best compounds, 15au, showed a balanced dual profile (i.e., MOR agonism and sigma antagonism) and a potent analgesic activity, comparable to the MOR agonist oxycodone in the paw pressure test in mice. Contrary to oxycodone, as expected from the addition of σ1R antagonism, 15au showed local, peripheral activity in this test, which was reversed by the σ1R agonist PRE-084. At equianalgesic doses, 15au showed less constipation than oxycodone, providing evidence that dual MOR agonism and σ1R antagonism may be a useful strategy for obtaining potent and safer analgesics.

The search for translational pain outcomes to refine analgesic development: Where did we come from and where are we going?

Pain measures traditionally used in rodents record mere reflexes evoked by sensory stimuli; the results thus may not fully reflect the human pain phenotype. Alterations in physical and emotional functioning, pain-depressed behaviors and facial pain expressions were recently proposed as additional pain outcomes to provide a more accurate measure of clinical pain in rodents, and hence to potentially enhance analgesic drug development. We aimed to review how preclinical pain assessment has evolved since the development of the tail flick test in 1941, with a particular focus on a critical analysis of some nonstandard pain outcomes, and a consideration of how sex differences may affect the performance of these pain surrogates. We tracked original research articles in Medline for the following periods: 1973-1977, 1983-1987, 1993-1997, 2003-2007, and 2014-2018. We identified 606 research articles about alternative surrogate pain measures, 473 of which were published between 2014 and 2018. This indicates that preclinical pain assessment is moving toward the use of these measures, which may soon become standard procedures in preclinical pain laboratories.

Intracolonic Mustard Oil Induces Visceral Pain in Mice by TRPA1-Dependent and -Independent Mechanisms: Role of Tissue Injury and P2X Receptors

Both TRPA1 and purinergic P2X receptors have been proposed as potential targets for the treatment of visceral pain. We found that the intracolonic administration of a low dose mustard oil (0.5%), a well-known TRPA1 agonist, produced nociceptive responses and abdominal wall referred mechanical hyperalgesia, without inducing apparent tissue damage. Both nociceptive responses and referred hyperalgesia were abolished by the ablation of TRPV1-expressing neurons (and the consequent ablation of TRPA1+ nociceptors) by resiniferatoxin (RTX) treatment, and by the TRPA1 antagonist AP18. However, a higher dose of mustard oil (2.5%) damaged the colonic epithelium and induced pERK activation in the spinal cord, and these processes were clearly independent of TRPV1-expressing neurons ablated by RTX. This higher dose of mustard oil induced nociceptive responses and referred mechanical hyperalgesia which were insensitive or only slightly sensitive to resiniferatoxin or AP18, but were markedly reduced by the P2X antagonist TNP-ATP, which is known to inhibit nociceptive actions induced by ATP released from injured tissues. In conclusion, whereas a low dose of intracolonic mustard oil induces visceral pain in a manner fully dependent on TRPA1 actions, when a high dose of this chemical irritant is used, visceral pain becomes mostly independent of TRPA1 activation but clearly enhanced by ATP purportedly released by the damaged colonic epithelium. Therefore, TRPA1 inhibition is not sufficient to substantially decrease visceral pain during tissue injury, whereas purinergic antagonism appears to be a more effective strategy.

Sigma-1 Receptor Inhibition Reduces Neuropathic Pain Induced by Partial Sciatic Nerve Transection in Mice by Opioid-Dependent and -Independent Mechanisms

Sigma-1 (σ₁) receptor antagonists are promising tools for neuropathic pain treatment, but it is unknown whether σ₁ receptor inhibition ameliorates the neuropathic signs induced by nerve transection, in which the pathophysiological mechanisms and response to drug treatment differ from other neuropathic pain models. In addition, σ₁ antagonism ameliorates inflammatory pain through modulation of the endogenous opioid system, but it is unknown whether this occurs during neuropathic pain. We investigated the effect of σ₁ inhibition on the painful hypersensitivity associated with the spared nerve injury (SNI) model in mice. Wild-type (WT) mice developed prominent cold (acetone test), mechanical (von Frey test), and heat hypersensitivity (Hargreaves test) after SNI. σ₁ receptor knockout (ခσ₁-KO) mice did not develop cold allodynia and showed significantly less mechanical allodynia, although they developed heat hyperalgesia after SNI. The systemic acute administration of the selective σ₁ receptor antagonist S1RA attenuated all three types of SNI-induced hypersensitivity in WT mice. These ameliorative effects of S1RA were reversed by the administration of the σ₁ agonist PRE-084, and were absent in σ₁-KO mice, indicating the selectivity of S1RA-induced effects. The opioid antagonist naloxone and its peripherally restricted analog naloxone methiodide prevented S1RA-induced effects in mechanical and heat hypersensitivity, but not in cold allodynia, indicating that opioid-dependent and -independent mechanisms are involved in the effects of this σ₁ antagonist. The repeated administration of S1RA twice a day during 10 days reduced SNI-induced cold, mechanical, and heat hypersensitivity without inducing analgesic tolerance during treatment. These effects were observed up to 12 h after the last administration, when S1RA was undetectable in plasma or brain, indicating long-lasting pharmacodynamic effects. These data suggest that σ₁ antagonism may have therapeutic value for the treatment of neuropathic pain induced by the transection of peripheral nerves.

Spinal cytochrome P450c17 plays a key role in the development of neuropathic mechanical allodynia: Involvement of astrocyte sigma-1 receptors

While evidence indicates that sigma-1 receptors (Sig-1Rs) play an important role in the induction of peripheral neuropathic pain, there is limited understanding of the role that the neurosteroidogenic enzymes, which produce Sig-1R endogenous ligands, play during the development of neuropathic pain. We examined whether sciatic nerve injury upregulates the neurosteroidogenic enzymes, cytochrome P450c17 and 3β-hydroxysteroid dehydrogenase (3β-HSD), which modulate the expression and/or activation of Sig-1Rs leading to the development of peripheral neuropathic pain. Chronic constriction injury (CCI) of the sciatic nerve induced a significant increase in the expression of P450c17, but not 3β-HSD, in the ipsilateral lumbar spinal cord dorsal horn at postoperative day 3. Intrathecal administration of the P450c17 inhibitor, ketoconazole during the induction phase of neuropathic pain (day 0 to day 3 post-surgery) significantly reduced the development of mechanical allodynia and thermal hyperalgesia in the ipsilateral hind paw. However, administration of the 3β-HSD inhibitor, trilostane had no effect on the development of neuropathic pain. Sciatic nerve injury increased astrocyte Sig-1R expression as well as dissociation of Sig-1Rs from BiP in the spinal cord. These increases were suppressed by administration of ketoconazole, but not by administration of trilostane. Co-administration of the Sig-1R agonist, PRE084 restored the development of mechanical allodynia originally suppressed by the ketoconazole administration. However, ketoconazole-induced inhibition of thermal hyperalgesia was not affected by co-administration of PRE084. Collectively these results demonstrate that early activation of P450c17 modulates the expression and activation of astrocyte Sig-1Rs, ultimately contributing to the development of mechanical allodynia induced by peripheral nerve injury.

Supraspinal and Peripheral, but Not Intrathecal, σ1R Blockade by S1RA Enhances Morphine Antinociception

Sigma-1 receptor (σ₁R) antagonism increases the effects of morphine on acute nociceptive pain. S1RA (E-52862) is a selective σ₁R antagonist widely used to study the role of σ₁Rs. S1RA alone exerted antinociceptive effect in the formalin test in rats and increased noradrenaline levels in the spinal cord, thus accounting for its antinociceptive effect. Conversely, while systemic S1RA failed to elicit antinociceptive effect by itself in the tail-flick test in mice, it did potentiate the antinociceptive effect of opioids in this acute pain model. The present study aimed to investigate the site of action and the involvement of spinal noradrenaline on the potentiation of opioid antinociception by S1RA on acute thermal nociception using the tail-flick test in rats. Local administration was performed after intrathecal catheterization or intracerebroventricular and rostroventral medullar (RVM) cannulae implantation. Noradrenaline levels in the spinal cord were evaluated using the concentric microdialysis technique in awake, freely-moving rats. Systemic or supraspinal administration of S1RA alone, while having no effect on antinociception, enhanced the effect of morphine in rats. However, spinal S1RA administration did not potentiate the antinociceptive effect of morphine. Additionally, the peripherally restricted opioid agonist loperamide was devoid of antinociceptive effect but produced antinociception when combined with S1RA. Neurochemical studies revealed that noradrenaline levels in the dorsal horn of the spinal cord were not increased at doses exerting potentiation of the antinociceptive effect of the opioid. In conclusion, the site of action of σ₁R for opioid modulation on acute thermal nociception is located at the peripheral and supraspinal levels, and the opioid-potentiating effect is independent of the spinal noradrenaline increase produced by S1RA.

Modulation by Sigma-1 Receptor of Morphine Analgesia and Tolerance: Nociceptive Pain, Tactile Allodynia and Grip Strength Deficits During Joint Inflammation

Sigma-1 receptor antagonism increases the effects of morphine on nociceptive pain, even in morphine-tolerant animals. However, it is unknown whether these receptors are able to modulate morphine antinociception and tolerance during inflammatory pain. Here we used a mouse model to test the modulation of morphine effects by the selective sigma-1 antagonist S1RA (MR309), by determining its effect on inflammatory tactile allodynia (von Frey filaments) and on grip strength deficits induced by joint inflammation (a measure of pain-induced functional disability), and compared the results with those for nociceptive heat pain recorded with the unilateral hot plate (55°C) test. The subcutaneous (s.c.) administration of morphine induced antinociceptive effects to heat stimuli, and restored mechanical threshold and grip strength in mice with periarticular inflammation induced by Complete Freund’s Adjuvant. S1RA (80 mg/kg, s.c.) administered alone did not induce any effect on nociceptive heat pain or inflammatory allodynia, but was able to partially reverse grip strength deficits. The association of S1RA with morphine, at doses inducing little or no analgesic-like effects when administered alone, resulted in a marked antinociceptive effect to heat stimuli and complete reversion of inflammatory tactile allodynia. However, S1RA administration did not increase the effect of morphine on grip strength deficits induced by joint inflammation. When S1RA (80 mg/kg, s.c.) was administered to morphine-tolerant animals, it rescued the analgesic-like effects of this opioid in all three pain measures. However, when S1RA was repeatedly given during the induction of morphine tolerance (and not on the day of behavioral evaluation) it failed to affect tolerance to the effects of morphine on nociceptive heat pain or inflammatory allodynia, but completely preserved the effects of this opioid on grip strength deficits. These effects of S1RA on morphine tolerance cannot be explained by pharmacokinetic interactions, given that the administration of S1RA did not modify concentrations of morphine or morphine-3-glucuronide (a major morphine metabolite) in morphine-tolerant animals in plasma or brain tissue. We conclude that sigma-1 receptors play a pivotal role in the control of morphine analgesia and tolerance in nociceptive and inflammatory pain, although in a manner dependent on the type of painful stimulus explored.