Sigma-1 receptor and inflammatory pain

Genetic and Pharmacological Blockade of Sigma-1 Receptors Attenuates Inflammation-Associated Hypersensitivity during Acute Colitis in CD1 Mice

Sigma-1 receptors (σ1Rs) are implicated in nociception, including pain sensitization, and inflammation. We assessed the role of σ1Rs on acute colitis-associated hypersensitivity using both genetic (constitutive knockout) and pharmacological blockade of the receptor. Colitis was induced in CD1 wild-type (WT) and σ1R KO mice (exposure to dextran sodium sulfate, 3%). A von Frey test was used to assess referred mechanosensitivity (abdominal and plantar withdrawal responses). The effects of the selective σ1R antagonists BD1063 and E-52862 were also assessed in WT animals. The expression of immune and sensory-related markers (RT-qPCR, Western blot) was assessed in the colon and lumbosacral spinal cord. The genetic ablation or pharmacological blockade of σ1Rs attenuated acute colonic inflammation in a similar manner. Mechanosensitivity was similar in WT and σ1R KO mice before colitis. In WT mice, but not in σ1R KO, colitis was associated with the development of referred mechanical hypersensitivity, manifested as a reduction in the withdrawal thresholds to mechanical probing (paw and abdominal wall). In WT mice, BD1063 and E-52862 blocked colitis-associated hypersensitivity. A genotype- and treatment-related differential regulation of sensory-related markers was detected locally (colon) and within the spinal cord. σ1Rs are involved in the development of acute intestinal inflammation and its associated referred mechanical hypersensitivity. The selective modulation of sensory-related pathways within the colon and spinal cord might be part of the underlying mechanisms. These observations support the pharmacological use of σ1R antagonists for the treatment of intestinal inflammation-induced hypersensitivity.

Resilience to Pain-Related Depression in σ1 Receptor Knockout Mice Is Associated with the Reversal of Pain-Induced Brain Changes in Affect-Related Genes

Mice lacking the σ1 receptor chaperone (σ1R-/-) are resilient to depressive-like behaviors secondary to neuropathic pain. Examining the resilience's brain mechanisms could help develop conceptually novel therapeutic strategies. We explored the diminished motivation for a natural reinforcer (white chocolate) in the partial sciatic nerve ligation (PSNL) model in wild-type (WT) and σ1R-/- mice. In the same mice, we performed a comprehensive reverse transcription quantitative PCR (qPCR) analysis across ten brain regions of seven genes implicated in pain regulation and associated affective disorders, such as anxiety and depression. PSNL induced anhedonic-like behavior in WT but not in σ1R-/- mice. In WT mice, PSNL up-regulated dopamine transporter (DAT) and its rate-limiting enzyme, tyrosine hydroxylase (Th), in the ventral tegmental area (VTA) and periaqueductal gray (PAG) as well as the serotonin transporters (SERT) and its rate-limiting enzyme tryptophan hydroxylase 2 (Tph2) in VTA. In addition, μ-opioid receptor (MOR) and σ1R were up-regulated in PAG, and MOR was also elevated in the somatosensory cortex (SS) but down-regulated in the striatum (STR). Finally, increased BDNF was found in the medial prefrontal cortex (mPFC) and hypothalamus (HPT). Sham surgery also produced PSNL-like expression changes in VTA, HPT, and STR. Genetic deletion of the σ1R in mice submitted to PSNL or sham surgery prevented changes in the expression of most of these genes. σ1R is critically involved in the supraspinal gene expression changes produced by PSNL and sham surgery. The changes in gene expression observed in WT mice may be related to pain-related depression, and the absence of these changes observed in σ1R-/- mice may be related to resilience.

Discovery of RC-752, a Novel Sigma-1 Receptor Antagonist with Antinociceptive Activity: A Promising Tool for Fighting Neuropathic Pain

Neuropathic pain (NP) is a chronic condition resulting from damaged pain-signaling pathways. It is a debilitating disorder that affects up to 10% of the world's population. Although opioid analgesics are effective in reducing pain, they present severe risks; so, there is a pressing need for non-opioid pain-relieving drugs. One potential alternative is represented by sigma-1 receptor (S1R) antagonists due to their promising analgesic effects. Here, we report the synthesis and biological evaluation of a series of S1R antagonists based on a 2-aryl-4-aminobutanol scaffold. After assessing affinity toward the S1R and selectivity over the sigma-2 receptor (S2R), we evaluated the agonist/antagonist profile of the compounds by investigating their effects on nerve growth factor-induced neurite outgrowth and aquaporin-mediated water permeability in the presence and absence of oxidative stress. (R/S)-RC-752 emerged as the most interesting compound for S1R affinity (K_i S1R = 6.2 ± 0.9) and functional antagonist activity. Furthermore, it showed no cytotoxic effect in two normal human cell lines or in an in vivo zebrafish model and was stable after incubation in mouse plasma. (R/S)-RC-752 was then evaluated in two animal models of NP: the formalin test and the spinal nerve ligation model. The results clearly demonstrated that compound (R/S)-RC-752 effectively alleviated pain in both animal models, thus providing the proof of concept of its efficacy as an antinociceptive agent.

Protective effects of sigma 1 receptor agonist PRE084 on 2,4,6-trinitrobenzene sulfonic acid-induced experimental colitis in mice

Purpose

We aimed to investigate the protective effect of sigma 1 receptor agonist and antagonist, PRE084 and BD1047, respectively, on 2,4,6-trinitrobenzene sulfonic acid (TNBS)-induced colitis in mice.

Methods

Thirty male ICR mice were randomly divided into 5 groups: control, 50% ethanol, colitis, PRE084 + colitis, and combined (PRE084 + BD1047 + colitis). Colitis was induced by intrarectal administration of TNBS. PRE084 and BD1047 were injected daily, starting 3 days before colitis induction. Distal colon tissue was excised for histopathological evaluation, and levels of glutathione (GSH), superoxide dismutase (SOD), myeloperoxidase (MPO), and lipid peroxidation were determined.

Results

Colitis caused weight loss, mucosal damage, upregulation of tumor necrosis factor-α, interleukin (IL)-1β, IL-6, MPO, and thiobarbituric acid reactive substance activities, and downregulation of GSH and SOD activities. These changes caused by TNBS-induced colitis were significantly ameliorated by PRE084 pretreatment. However, the combined pretreatment with BD1047 significantly attenuated the protective effect of PRE084, thereby reverting to the colitis-induced state.

Conclusion

We conclude that the sigma 1 receptor agonist PRE084 exhibits significant protective effects against TNBS-induced colitis, which appears to be at least partly mediated by the inhibition of inflammation and oxidative stress, and enhancement of antioxidant properties. Collectively, these results suggest that PRE084 might be an effective drug for the treatment of ulcerative colitis.

Long COVID, neuropsychiatric disorders, psychotropics, present and future

Long COVID refers to the lingering symptoms which persist or appear after the acute illness. The dominant long COVID symptoms in the two years since the pandemic began (2020-2021) have been depression, anxiety, fatigue, concentration and cognitive impairments with few reports of psychosis. Whether other symptoms will appear later on is not yet known. For example, dopamine-dependent movement disorders generally take many years before first symptoms are seen. Post-stroke depression and anxiety may explain many of the early long COVID cases. Hemorrhagic, hypoxic and inflammatory damages of the central nervous system, unresolved systematic inflammation, metabolic impairment, cerebral vascular accidents such as stroke, hypoxia from pulmonary damages and fibrotic changes are among the major causes of long COVID. Glucose metabolic and hypoxic brain issues likely predispose subjects with pre-existing diabetes, cardiovascular or lung problems to long COVID as well. Preliminary data suggest that psychotropic medications may not be a danger but could instead be beneficial in combating COVID-19 infection. The same is true for diabetes medications such as metformin. Thus, a focus on sigma-1 receptor ligands and glucose metabolism is expected to be useful for new drug development as well as the repurposing of current drugs. The reported protective effects of psychotropics and antihistamines against COVID-19, the earlier reports of reduced number of sigma-1 receptors in post-mortem schizophrenic brains, with many antidepressant and antipsychotic drugs being antihistamines with significant affinity for the sigma-1 receptor, support the role of sigma and histamine receptors in neuroinflammation and viral infections. Literature and data in all these areas are accumulating at a fast rate. We reviewed and discussed the relevant and important literature.

Sigma-1 receptor activity in primary sensory neurons is a critical driver of neuropathic pain

The Sigma-1 receptor (σ1R) is highly expressed in the primary sensory neurons (PSNs) that are the critical site of initiation and maintenance of pain following peripheral nerve injury. By immunoblot and immunohistochemistry, we observed increased expression of both σ1R and σ1R-binding immunoglobulin protein (BiP) in the lumbar (L) dorsal root ganglia (DRG) ipsilateral to painful neuropathy induced by spared nerve injury (SNI). To evaluate the therapeutic potential of PSN-targeted σ1R inhibition at a selected segmental level, we designed a recombinant adeno-associated viral (AAV) vector expressing a small hairpin RNA (shRNA) against rat σ1R. Injection of this vector into the L4/L5 DRGs induced downregulation of σ1R in DRG neurons of all size groups, while expression of BiP was not affected. This was accompanied by attenuation of SNI-induced cutaneous mechanical and thermal hypersensitivity. Whole-cell current-clamp recordings of dissociated neurons showed that knockdown of σ1R suppressed neuronal excitability, suggesting that σ1R silencing attenuates pain by reversal of injury-induced neuronal hyperexcitability. These findings support a critical role of σ1R in modulating PSN nociceptive functions, and that the nerve injury-induced elevated σ1R activity in the PSNs can be a significant driver of neuropathic pain. Further understanding the role of PSN-σ1R in pain pathology may open routes to exploit this system for DRG-targeted pain therapy.

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.

Intestinal inflammation-associated hypersensitivity is attenuated in a DSS model of colitis in Sigma-1 knockout C57BL/6 mice

Sigma-1 receptors (σ1R) have been implicated in several pain pathways. We assessed the implication of σ1Rs in the development of intestinal inflammation and inflammation-associated referred hypersensitivity in a model of colitis in σ1R knockout (KO) mice. Colitis was induced with dextran sulfate sodium (DSS) in wild type (WT) and σ1R KO mice. The development of referred mechanical hypersensitivity (von Frey test) was assessed. Colonic and spinal changes in expression of immune- and sensory-related markers were also investigated (RT-qPCR/Western blot). Absence of σ1Rs had little impact in colitis generation and progression, although during the chronic phase a reduction in edema and a down-regulation of iNOS gene expression was observed. In σ1R KO mice, inflammation-associated hypersensitivity was significantly attenuated (paw) or completely prevented (abdomen). During colitis, in WT mice, changes in the colonic expression of nociceptive markers were observed during the acute and chronic phases of inflammation. Although σ1R KO mice showed similar regulation in the acute phase, an attenuated response was observed during the chronic phase of colitis. These differences were especially relevant for CB2 and TRPV1 receptors, which could play an important role in σ1-mediated regulation of sensitivity. No changes were detected on ERK phosphorylation at the level of the lumbosacral spinal cord. In summary, intestinal inflammation-associated referred hyperalgesia was reduced (paw) or absent (abdomen) in σ1R KO mice, thus confirming an important role for σ1R in the development of colitis-associated hypersensitivity. These results identify σ1Rs as a possible therapeutic target for the treatment of hypersensitivity associated to intestinal inflammation.

The Peripheral Role of CCL2 in the Anti-Nociceptive Effect of Sigma-1 Receptor Antagonist BD1047 on Inflammatory Hyperalgesia in Rats

Our recent study demonstrated that the CC-chemokine ligand 2 (CCL2) present in primary afferent fibers (PAFs) plays an important role in the microglia-dependent neuronal activation associated with zymosan-induced inflammatory pain. The present study was aimed to evaluate whether BD1047 (a prototypical sigma-1 receptor (Sig-1R) antagonist) is capable of modifying elevated levels of inflammation-evoked CCL2 as a peripheral antinociceptive mechanism. In DRG primary culture, zymosan dose-dependently increased CCL2 release from isolectin B4 (IB4)-positive DRG neurons, a process that was inhibited by co-culture with BD1047. Single treatment of BD1047 before intraplantar injection of zymosan in rats significantly reduced thermal hyperalgesia and mechanical hyperalgesia, as well as CCL2 expression in DRG neurons and microglia activation in the spinal dorsal horn. In the Complete Freund's adjuvant (CFA)-induced inflammation model, repeated administration of BD1047 dramatically attenuated thermal hyperalgesia and mechanical hyperalgesia, and significantly diminished CCL2 immunoreactivity and microglia activation. Notably, CFA-induced inflammation significantly increased Sig-1R immunoreactivity in DRG neurons, which was co-localized with CCL2 and IB4, respectively. Taken together, our results suggest that BD1047's anti-nociceptive property was substantially mediated by the inhibition of CCL2 release in unmyelinated PAFs and that this may, in turn, have attenuated the spinal microglia activation that is associated with inflammatory pain.

Sigma-1 Receptor Changes Observed in Chronic Pelvic Pain Patients: A Pilot PET/MRI Study

Introduction: Chronic pelvic pain is a highly prevalent pain condition among women, but identifying the exact cause of pelvic pain remains a significant diagnostic challenge. In this study, we explored a new diagnostic approach with PET/MRI of the sigma-1 receptor, a chaperone protein modulating ion channels for activating nociceptive processes.

Methods: Our approach is implemented by a simultaneous PET/MRI scan with a novel radioligand [18F]FTC-146, which is highly specific to the sigma-1 receptor. We recruited 5 chronic pelvic pain patients and 5 healthy volunteers and compared our PET/MRI findings between these two groups.

Results: All five patients showed abnormally increased radioligand uptake on PET compared to healthy controls at various organs, including the uterus, vagina, pelvic bowel, gluteus maximus muscle, and liver. However, on MRI, only 2 patients showed abnormalities that could be potentially associated with the pain symptoms. For a subset of patients, the association of pain and the abnormally increased radioligand uptake was further validated by successful pain relief outcomes following surgery or trigger point injections to the identified abnormalities.

Conclusion: In this preliminary study, sigma-1 receptor PET/MRI demonstrated potential for identifying abnormalities associated with chronic pelvic pain. Future studies will need to correlate samples with imaging findings to further validate the correlation between S1R distribution and pathologies of chronic pelvic pain.

Trial Registration: The clinical trial registration date is June 2, 2018, and the registration number of the study is NCT03195270 (https://clinicaltrials.gov/ct2/show/NCT03556137).

Preliminary in vitro approach to evaluate the drug-drug interaction potential of EST73502, a dual µ-opioid receptor partial agonist and σ1 receptor antagonist

The potential for drug-drug interactions (DDI) of EST73502 was preliminary explored in vitro. EST73502 is a new chemical entity intended for oral pain treatment with dual sigma-1 receptor (σ1R) antagonism and μ-opioid receptor (MOR) partial agonism, that presents a promising potent analgesic activity.Several enzymes were involved in EST73502 metabolism catalysing the formation of different metabolites, CYP3A4 and CYP2D6 being the main ones.Fraction unbound was determined due to its impact in interactions, a considerable proportion of EST73502 being available.EST73502 showed a low potential for CYP inhibition, except for CYP2D6 that showed time-dependent inhibition.No induction potential was found for CYP1A2 and 3A4, while CYP2B6 was induced at high concentration.EST73502 seemed to be a potential efflux transporter substrate (efflux ratio ≥ 2) but a negligible in vivo impact would be expected due to its high solubility and permeability in Caco-2 cells. P-gp inhibition was observed while no BCRP inhibition was detected.Preliminary in vitro interaction studies suggested that neither CYPs nor efflux transporters interactions would preclude further development of EST73502 to thoroughly assess the clinical relevance of these findings.

Dual-targeting Approach on Histamine H3 and Sigma-1 Receptor Ligands as Promising Pharmacological Tools in the Treatment of CNS-linked Disorders

With the recent market approval of Pitolisant (Wakix®), the interest in clinical application for novel multifunctional histamine H3 receptor antagonists has clearly increased. Several combinations of different H3R pharmacophores with pharmacophoric elements of other G-protein coupled receptors, transporters, or enzymes have been synthesized by numerous pharmaceutical companies and academic institutions. Since central nervous system disorders are characterized by diverse physiological dysfunctions and deregulations of a complex network of signaling pathways, optimal multipotent drugs should simultaneously and peculiarly modulate selected groups of biological targets. Interestingly, very recent studies have shown that some clinically evaluated histamine H3 receptor antagonists possess a nanomolar affinity for sigma-1 receptor binding sites, suggesting that this property might play a role in their overall efficacy. The sigma-1 receptor, unusual and yet obscure protein, is supposed to be involved in numerous CNS pathologies through neuroprotection and neuroplasticity. These two different biological structures, histamine H3 and sigma-1 receptors, combined, can represent a potential fruitful target for therapeutic developments in tackling numerous human diseases.

Bioanalytical method development and pharmacokinetics of MCI-92, a sigma-1 receptor ligand

Sigma-1 receptors are found throughout the nervous system and play a role in regulating nociception. They are highly expressed in nerve injury, making them a potential target for the treatment of neuropathic pain. Although sigma-1 receptor antagonists have been shown to have anti-nociceptive and anti-allodynic effects, improved selectivity of these ligands is needed to further investigate their potential to treat neuropathic pain. MCI-92 is a novel, selective sigma-1 receptor ligand developed to address this need. A sensitive and rapid ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) method was developed and validated for the quantification of MCI-92 in mouse plasma and brain homogenate. A structural analog of the analyte, MCI-147, was used as the internal standard (IS). The chromatographic separation was achieved on an Acquity UPLC BEH C18 column using a mobile phase consisting of water acidified with 0.1 % v/v formic acid and acetonitrile with gradient elution over 3.2 min. The method was linear over a concentration range of 1-200 ng/mL. Multiple reaction monitoring in the positive ionization mode was used for the mass spectrometric quantitation using m/z transitions 369.2 > 126.0 for MCI-92 and 448.9 > 350.1 for the IS. The method was successfully applied to the analysis of plasma and brain samples obtained in the course of oral and intravenous pharmacokinetic studies in CD-1 mice. MCI-92 showed a high volume of distribution (11.3 ± 0.6 L/kg) and rapid clearance (6.1 ± 0.8 L/h/kg) from systemic circulation. The concentration of the MCI-92 was higher in the brain than in plasma throughout all terminal time points, indicating high blood-to-brain partitioning and slow brain clearance.

Pharmacological characterization of high-affinity σ1 receptor ligands with spirocyclic thienopyran and thienofuran scaffold

OBJECTIVES

In this study, the pharmacological properties of six spirocyclic piperidines 1-6 showing very high σ1 receptor affinity (Ki  = 0.2-16 nm) were investigated.

METHODS

In vitro receptor binding studies, retinal ganglion assay and in vivo capsaicin assay were used to determine the affinity, selectivity and activity. Influence on human tumour cell growth (cell lines A427, LCLC-103H, 5637 and DAN-G) was determined in different assays. The effect on the ergosterol and cholesterol biosynthesis was determined by GLC/MS analysis.

KEY FINDINGS

Receptor binding studies demonstrated high selectivity for the σ1 receptor. The increased Ca2+ influx mediated by 2 and the analgesic activity of 1, 4, 5 and 6 confirm σ1 receptor antagonistic activity. Inhibition of human tumour cell growth further supports the σ1 antagonistic effects. Treatment of A427 tumour cells with 2 led to cell detachment and cell degradation. Whereas the ergosterol biosynthesis was not affected, the sterol C14-reductase, a key enzyme in the cholesterol biosynthesis, was weakly inhibited.

CONCLUSIONS

Due to the high selectivity, off-target effects are not expected. The antiallodynic activity underlines the clinical potential of the spirocyclic piperidines for the treatment of neuropathic pain. Due to the antiproliferative activity, the spirocyclic σ1 antagonists represent promising antitumour agents.

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.

Repeated Sigma-1 Receptor Antagonist MR309 Administration Modulates Central Neuropathic Pain Development After Spinal Cord Injury in Mice

Up to two-thirds of patients affected by spinal cord injury (SCI) develop central neuropathic pain (CNP), which has a high impact on their quality of life. Most of the patients are largely refractory to current treatments, and new pharmacological strategies are needed. Recently, it has been shown that the acute administration of the σ1R antagonist MR309 (previously developed as E-52862) at 28 days after spinal cord contusion results in a dose-dependent suppression of both mechanical allodynia and thermal hyperalgesia in wild-type CD-1 Swiss female mice. The present work was addressed to determine whether MR309 might exert preventive effects on CNP development by repeated administration during the first week after SCI in mice. To this end, the MR309 (16 or 32 mg/kg i.p.) modulation on both thermal hyperalgesia and mechanical allodynia development were evaluated weekly up to 28 days post-injury. In addition, changes in pro-inflammatory cytokine (TNF-α, IL-1β) expression and both the expression and activation (phosphorylation) of the N-methyl-D-aspartate receptor subunit 2B (NR2B-NMDA) and extracellular signal-regulated kinases (ERK1/2) were analyzed. The repeated treatment of SCI-mice with MR309 resulted in significant pain behavior attenuation beyond the end of the administration period, accompanied by reduced expression of central sensitization-related mechanistic correlates, including extracellular mediators (TNF-α and IL-1β), membrane receptors/channels (NR2B-NMDA) and intracellular signaling cascades (ERK/pERK). These findings suggest that repeated MR309 treatment after SCI may be a suitable pharmacologic strategy to modulate SCI-induced CNP development.

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.

Critical role of sigma-1 receptors in central neuropathic pain-related behaviours after mild spinal cord injury in mice

Sigma-1 receptor (σ1R) knockout (KO) CD1 mice, generated by homologous recombination, and separate pharmacological studies in wild type (WT) mice were done to investigate the role of this receptor in the development of pain-related behaviours (thermal hyperalgesia and mechanical allodynia) in mice after spinal cord contusion injury (SCI) - a model of central neuropathic pain. The modulatory effect of σ1R KO on extracellular mediators and signalling pathways in the spinal cord was also investigated. In particular, changes in the expression of inflammatory cytokines (tumour necrosis factor TNF-α, interleukin IL-1β) and both the expression and activation (phosphorylation) of the N-methyl-D-aspartate receptor subunit 2B (NR2B-NMDA) and extracellular signal-regulated kinases (ERK1/2) were analysed. Compared with WT mice, both mechanical and thermal hypersensitivity were attenuated in σ1R KO mice following SCI. Accordingly, treatment of WT mice with the σ1R antagonist MR309 (previously developed as E-52862; S1RA) after SCI exerted antinociceptive effects (i.e. reduced mechanical allodynia and thermal hyperalgesia). Attenuated nociceptive responses in σ1R KO were accompanied by reduced expression of TNF- α and IL-1β as well as decreased activation/phosphorylation of NR2B-NMDA receptors and ERK1/2. These findings suggest that σ1R may modulate central neuropathic pain and point to regulation of sensitization-related phenomena as a possible mechanism.

Biodistribution and Radiation Dosimetry of 18F-FTC-146 in Humans

The purpose of this study was to assess safety, biodistribution, and radiation dosimetry in humans for the highly selective σ-1 receptor PET agent 18F-6-(3-fluoropropyl)-3-(2-(azepan-1-yl)ethyl)benzo[d]thiazol-2(3H)-one (18F-FTC-146). Methods: Ten healthy volunteers (5 women, 5 men; age ± SD, 34.3 ± 6.5 y) were recruited, and written informed consent was obtained from all participants. Series of whole-body PET/MRI examinations were acquired for up to 3 h after injection (357.2 ± 48.8 MBq). Blood samples were collected, and standard vital signs (heart rate, pulse oximetry, and body temperature) were monitored at regular intervals. Regions of interest were delineated, time-activity curves were calculated, and organ uptake and dosimetry were estimated. Results: All subjects tolerated the PET/MRI examination well, and no adverse reactions to 18F-FTC-146 were reported. High accumulation of 18F-FTC-146 was observed in σ-1 receptor-dense organs such as the pancreas and spleen, moderate uptake in the brain and myocardium, and low uptake in bone and muscle. High uptake was also observed in the kidneys and bladder, indicating renal tracer clearance. The effective dose of 18F-FTC-146 was 0.0259 ± 0.0034 mSv/MBq (range, 0.0215-0.0301 mSv/MBq). Conclusion: First-in-human studies with clinical-grade 18F-FTC-146 were successful. Injection of 18F-FTC-146 is safe, and absorbed doses are acceptable. The potential of 18F-FTC-146 as an imaging agent for a variety of neuroinflammatory diseases is currently under investigation.

Pharmacological Modulation of the Sigma 1 Receptor and the Treatment of Pain

There is a critical need for new analgesics acting through new mechanisms of action, which could increase the efficacy with respect to existing therapies and reduce their unwanted effects. Current preclinical evidence supports the modulatory role of sigma-1 receptors (σ1R) in nociception, mainly based on the pain-attenuated phenotype of σ1R knockout mice and on the antinociceptive effect exerted by σ1R antagonists on pains of different etiologies. σ1R is highly expressed in different pain areas of the CNS and the periphery (particularly dorsal root ganglia), and interacts and modulates the functionality of different receptors and ion channels . The antagonism of σ1R leads to decreased amplification of pain signaling within the spinal cord (central sensitization), but recent data also support a role at the periphery. σ1R antagonists have consistently demonstrated efficacy in neuropathic pain , but also in other types of pain including inflammatory, orofacial, visceral, and post-operative pain. Apart from acting alone, when combined with opioids, σ1R antagonists enhance opioid analgesia but not opioid-induced unwanted effects. Interestingly, unlike opioids, σ1R antagonists do not modify normal sensory mechanical and thermal sensitivity thresholds but they exert antihypersensitive effects in sensitizing conditions, enabling the reversal of nociceptive thresholds back to normal values. Accordingly, σ1R antagonists are not strictly analgesics; they are antiallodynic and antihyperalgesic drugs acting when the system is sensitized following prolonged noxious stimulation or persistent abnormal afferent input (e.g., secondary to nerve injury). These are distinctive features allowing σ1R antagonists to exert a modulatory effect specifically in pathophysiological conditions such as chronic pain .

Sigma-1 Receptor and Pain

There is a critical need for new analgesics acting through new mechanisms of action, which could increase the efficacy respect to existing therapies and/or reduce their unwanted effects. Current preclinical evidence supports the modulatory role of the sigma-1 receptor (σ1R) in nociception, mainly based on the pain-attenuated phenotype of σ1R knockout mice and on the antinociceptive effect exerted by σ1R antagonists on pain of different etiology, very consistently in neuropathic pain, but also in nociceptive, inflammatory, and visceral pain. σ1R is highly expressed in different pain areas of the CNS and the periphery, particularly dorsal root ganglia (DRG), and interacts and modulates the functionality of different receptors and ion channels. Accordingly, antinociceptive effects of σ1R antagonists both acting alone and in combination with other analgesics have been reported at both central and peripheral sites. At the central level, behavioral, electrophysiological, neurochemical, and molecular findings support a role for σ1R antagonists in inhibiting augmented excitability secondary to sustained afferent input. Moreover, the involvement of σ1R in mechanisms regulating pain at the periphery has been recently confirmed. Unlike opioids, σ1R antagonists do not modify normal sensory mechanical and thermal sensitivity thresholds but they exert antihypersensitivity effects (antihyperalgesic and antiallodynic) in sensitizing conditions, enabling the reversal of nociceptive thresholds back to normal values. These are distinctive features allowing σ1R antagonists to exert a modulatory effect specifically in pathophysiological conditions such as chronic pain.

Sigma-1 Receptor Antagonists: A New Class of Neuromodulatory Analgesics

The sigma-1 receptor is a unique ligand-operated chaperone present in key areas for pain control, in both the peripheral and central nervous system. Sigma-1 receptors interact with a variety of protein targets to modify their function. These targets include several G-protein-coupled receptors such as the μ-opioid receptor, and ion channels such as the N-methyl-D-aspartate receptor (NMDAR). Sigma-1 antagonists modify the chaperoning activity of sigma-1 receptor by increasing opioid signaling and decreasing NMDAR responses, consequently enhancing opioid antinociception and decreasing the sensory hypersensitivity that characterizes pathological pain conditions. However, the participation in pain relief of other protein partners of sigma-1 receptors in addition to opioid receptors and NMDARs cannot be ruled out. The enhanced opioid antinociception by sigma-1 antagonism is not accompanied by an increase in opioid side effects , including tolerance, dependence or constipation, so the use of sigma-1 antagonists may increase the therapeutic index of opioids. Furthermore, sigma-1 antagonists (in the absence of opioids) have been shown to exert antinociceptive effects in preclinical models of neuropathic pain induced by nerve trauma or chemical injury (the antineoplastic paclitaxel), and more recently in inflammatory and ischemic pain. Although most studies attributed the analgesic properties of sigma-1 antagonists to their central actions, it is now known that peripheral sigma-1 receptors also participate in their effects. Overwhelming preclinical evidence of the role of sigma-1 receptors in pain has led to the development of the first selective sigma-1 antagonist with an intended indication for pain treatment, which is currently in Phase II clinical trials.

Sigma-1 receptor expression in the dorsal root ganglion: Reexamination using a highly specific antibody

Sigma-1 receptor (S1R) is a unique pluripotent modulator of living systems and has been reported to be associated with a number of neurological diseases including pathological pain. Intrathecal administration of S1R antagonists attenuates the pain behavior of rodents in both inflammatory and neuropathic pain models. However, the S1R localization in the spinal cord shows a selective ventral horn motor neuron distribution, suggesting the high likelihood of S1R in the dorsal root ganglion (DRG) mediating the pain relief by intrathecally administered drugs. Since primary afferents are the major component in the pain pathway, we examined the mouse and rat DRGs for the presence of the S1R. At both mRNA and protein levels, quantitative RT-PCR (qRT-PCR) and Western confirmed that the DRG contains greater S1R expression in comparison to spinal cord, cortex, or lung but less than liver. Using a custom-made highly specific antibody, we demonstrated the presence of a strong S1R immuno-fluorescence in all rat and mouse DRG neurons co-localizing with the Neuron-Specific Enolase (NSE) marker, but not in neural processes or GFAP-positive glial satellite cells. In addition, S1R was absent in afferent terminals in the skin and in the dorsal horn of the spinal cord. Using immuno-electron microscopy, we showed that S1R is detected in the nuclear envelope and endoplasmic reticulum (ER) of DRG cells. In contrast to other cells, S1R is also located directly at the plasma membrane of the DRG neurons. The presence of S1R in the nuclear envelope of all DRG neurons suggests an exciting potential role of S1R as a regulator of neuronal nuclear activities and/or gene expression, which may provide insight toward new molecular targets for modulating nociception at the level of primary afferent neurons.

Antinociception by Sigma-1 Receptor Antagonists: Central and Peripheral Effects

There is plenty of evidence supporting the modulatory role of sigma-1 receptors (σ1Rs) in nociception, mainly based on the pain-attenuated phenotype of σ1R knockout mice and on the antinociceptive effect exerted by σ1R antagonists, particularly in nonacute sensitizing conditions involving sustained afferent drive, activity-dependent plasticity/sensitization, and ultimately pain hypersensitivity, as it is the case in chronic pains of different etiology. Antinociceptive effects of σ1R antagonists both when acting alone and in combination with opioids (to enhance opioid analgesia) have been reported at both central and peripheral sites. At the central level, findings at the behavioral (animal pain models), electrophysiological (spinal wind-up recordings), neurochemical (spinal release of neurotransmitters) and molecular (NMDAR function) level supports a role for σ1R antagonists in inhibiting augmented excitability secondary to sustained afferent input. Attenuation of activity-induced plastic changes (central sensitization) following tissue injury/inflammation or nerve damage could thus underlie the central inhibitory effect of σ1R antagonists. Moreover, recent pieces of information confirm the involvement of σ1R in mechanisms regulating pain at the periphery, where σ1Rs are highly expressed, particularly in dorsal root ganglia. Indeed, local peripheral administration of σ1R antagonists reduces inflammatory hyperalgesia. Potentiation of opioid analgesia is also supported, particularly at supraspinal sites and at the periphery, where locally administered σ1R antagonists unmask opioid analgesia. Altogether, whereas σ1R activation is coupled to pain facilitation and inhibition of opioid antinociception, σ1R antagonism inhibits pain hypersensitivity and "releases the brake" enabling opioids to exert enhanced antinociceptive effects, both at the central nervous system and at the periphery.

Antinociceptive effects of a new sigma-1 receptor antagonist (N-(2-morpholin-4-yl-ethyl)-2-(1-naphthyloxy)acetamide) in two types of nociception

Pain has become an active clinical challenge due its etiological heterogeneity, symptoms and mechanisms of action. In the search for new pharmacological therapeutic alternatives, sigma receptors have been proposed as drug targets. This family consists of sigma-1 and sigma-2 receptors. The sigma-1 system is involved in nociception through its chaperone activity. Additionally, it has been shown that agonist to these receptors promote related sensitisation and pain hypersensitisation, suggesting the possible use of antagonists for sigma-1 receptors as an alternative therapy. The aim of this study was to evaluate the antinociceptive effect of a new sigma-1 receptor antagonist N-(2-morpholin-4-yl-ethyl)-2-(1-naphthyloxy)acetamida (NMIN) in two types of pain (arthritic and neuropathic) and to compare its efficacy and potency with reference drugs. The antinociceptive effects of NMIN were quantitatively evaluated using the pain-induced functional impairment model in the rat and the acetone test in a rat model of neuropathic pain. NMIN (sigma-1 receptor affinity of 324nM) did not show any antinociceptive activity in the arthritic pain model but showed a dose-dependent anti-allodynic effect in neuropathic pain. NMIN showed a similar efficacy compared to the effects obtained with morphine and the sigma-1 antagonist BD-1063. However, these reference drugs showed increased potency compared with NMIN. Our results suggest that sigma-1 receptors may play an important direct role in neuropathic pain but not in arthritic pain, supporting the hypothesis that NMIN may be useful for the treatment of neuropathic pain.