Intrathecal injection of the neurosteroid, DHEAS, produces mechanical allodynia in mice: involvement of spinal sigma-1 and GABA receptors

The Role of Cortisol and Dehydroepiandrosterone in Obesity, Pain, and Aging

Obesity, chronic pain, and aging are prevalent global challenges with profound implications for health and well-being. Central to these processes are adrenal hormones, particularly cortisol and dehydroepiandrosterone (DHEA), along with its sulfated form (DHEAS). Cortisol, essential for stress adaptation, can have adverse effects on pain perception and aging when dysregulated, while DHEA/S possess properties that may mitigate these effects. This review explores the roles of cortisol and DHEA/S in the contexts of obesity, acute and chronic pain, aging, and age-related diseases. We examine the hormonal balance, specifically the cortisol-to-DHEA ratio (CDR), as a key marker of stress system functionality and its impact on pain sensitivity, neurodegeneration, and physical decline. Elevated CDR and decreased DHEA/S levels are associated with worsened outcomes, including increased frailty, immune dysfunction, and the progression of age-related conditions such as osteoporosis and Alzheimer's disease. This review synthesizes the current literature to highlight the complex interplay between these hormones and their broader implications for health. It aims to provide insights into potential future therapies to improve pain management and promote healthy weight and aging. By investigating these mechanisms, this work contributes to a deeper understanding of the physiological intersections between pain, aging, and the endocrine system.

Sigma-1 Receptor Modulates CFA-Induced Inflammatory Pain via Sodium Channels in Small DRG Neurons

The sigma-1 receptor (Sig-1R) has emerged as a significant target in the realm of pain management and has been the subject of extensive research. Nonetheless, its specific function in inflammatory pain within dorsal root ganglion (DRG) neurons remains inadequately elucidated. This study utilized whole-cell patch clamp techniques, single-cell real-time PCR, and immunohistochemistry to examine the influence of Sig-1R on inflammatory pain induced by complete Freund's adjuvant (CFA) in a rat model. Our results revealed several key findings: (1) The expression of Sig-1R was found to be upregulated during the progression of inflammatory pain, with a notable translocation from the cytoplasm to the membrane; (2) Inhibition of peripheral Sig-1R using S1RA resulted in a reduction of CFA-induced allodynia; (3) Activation of Sig-1R through PRE-084 led to a decrease in the fast sodium current in isolated DRG neurons from CFA-treated rats, which was associated with a diminished action potential (AP) peak and maximum depolarizing rate (MDR), as well as an increased rheobase; (4) Furthermore, PRE-084 was observed to enhance the slow component of the sodium current, resulting in hyperpolarization of the threshold potential and an increase in AP firing frequency, alongside an elevation in the mRNA expression of the slow sodium channel Nav1.9 in CFA-treated rats. In conclusion, our findings suggest that the modulation of sodium channels by Sig-1R in DRG neurons plays a significant role in the mechanisms underlying inflammatory pain.

Neuropeptidergic control of neurosteroids biosynthesis

Neurosteroids are steroids synthesized within the central nervous system either from cholesterol or by metabolic reactions of circulating steroid hormone precursors. It has been suggested that neurosteroids exert pleiotropic activities within the central nervous system, such as organization and activation of the central nervous system and behavioral regulation. It is also increasingly becoming clear that neuropeptides exert pleiotropic activities within the central nervous system, such as modulation of neuronal functions and regulation of behavior, besides traditional neuroendocrinological functions. It was hypothesized that some of the physiological functions of neuropeptides acting within the central nervous system may be through the regulation of neurosteroids biosynthesis. Various neuropeptides reviewed in this study possibly regulate neurosteroids biosynthesis by controlling the activities of enzymes that catalyze the production of neurosteroids. It is now required to thoroughly investigate the neuropeptidergic control mechanisms of neurosteroids biosynthesis to characterize the physiological significance of this new neuroendocrinological phenomenon.

Pharmacokinetics and brain σ1 receptor occupancy of MR309, a selective σ1 receptor antagonist

AIMS

Preclinical studies of MR309, a selective sigma-1 receptor (σ1R) antagonist, support a potential role in treating neuropathic pain. We report 2 studies that provide insight into the pharmacokinetics (PK) and brain σ1R binding of MR309.

METHODS

Steady-state PK of MR309 (400 mg once daily and 200 mg twice-daily [BID] for 10 days; EudraCT 2015-001818-99 [PK study]) and the relationship between MR309 plasma exposure and brain σ1R occupancy (EudraCT 2017-000670-11 [positron emission tomography study]) were investigated in healthy volunteers. Positron emission tomography using the σ1R ligand [11 C]SA4503 was conducted at baseline, and 2 and 8 hours after a single dose of MR309 (200-800 mg). The relationship between brain σ1R occupancy and MR309 exposure was explored using data-driven model fitting.

RESULTS

MR309 was well tolerated, brain σ1R occupancy ranged between 30.5 and 74.9% following single-dose MR309 (n = 7). MR309 BID provided a plasma PK profile with less fluctuation than once daily dosing (n = 16). MR309 200 mg BID yielded average steady state plasma concentrations between 2000 and 4000 ng/mL in the PK study, which corresponded to an estimated brain σ1R occupancy of 59-74%.

CONCLUSION

MR309 200 mg BID dose was below the 75% σ1R occupancy threshold expected to elicit maximal antinociceptive effect as observed in neuropathic pain models. Further investigations of MR309 for neuropathic pain will require higher brain σ1R occupancy, and establish the optimal dose by elucidating the clinical impact of a broad range of brain σ1R occupancy across different neuropathic pain indications.

Gut feelings: the microbiota-gut-brain axis on steroids

The intricate connection between central and enteric nervous systems is well established with emerging evidence linking gut microbiota function as a significant new contributor to gut-brain axis signaling. Several microbial signals contribute to altered gut-brain communications, with steroids representing an important biological class that impacts central and enteric nervous system function. Neuroactive steroids contribute pathologically to neurological disorders, including dementia and depression, by modulating the activity of neuroreceptors. However, limited information is available on the influence of neuroactive steroids on the enteric nervous system and gastrointestinal function. In this review, we outline how steroids can modulate enteric nervous system function by focusing on their influence on different receptors that are present in the intestine in health and disease. We also highlight the potential role of the gut microbiota in modulating neuroactive steroid signaling along the gut-brain axis.

Partial Agonistic Actions of Sex Hormone Steroids on TRPM3 Function

Sex hormone steroidal drugs were reported to have modulating actions on the ion channel TRPM3. Pregnenolone sulphate (PS) presents the most potent known endogenous chemical agonist of TRPM3 and affects several gating modes of the channel. These includes a synergistic action of PS and high temperatures on channel opening and the PS-induced opening of a noncanonical pore in the presence of other TRPM3 modulators. Moreover, human TRPM3 variants associated with neurodevelopmental disease exhibit an increased sensitivity for PS. However, other steroidal sex hormones were reported to influence TRPM3 functions with activating or inhibiting capacity. Here, we aimed to answer how DHEAS, estradiol, progesterone and testosterone act on the various modes of TRPM3 function in the wild-type channel and two-channel variants associated with human disease. By means of calcium imaging and whole-cell patch clamp experiments, we revealed that all four drugs are weak TRPM3 agonists that share a common steroidal interaction site. Furthermore, they exhibit increased activity on TRPM3 at physiological temperatures and in channels that carry disease-associated mutations. Finally, all steroids are able to open the noncanonical pore in wild-type and DHEAS also in mutant TRPM3. Collectively, our data provide new valuable insights in TRPM3 gating, structure-function relationships and ligand sensitivity.

Synergistic interaction between haloperidol and gabapentin in a model of neuropathic nociception in rat

Preclinical studies have reported that sigma-1 receptor antagonists may have efficacy in neuropathic pain states. The sigma-1 receptor is a unique ligand-operated chaperone present in crucial areas for pain control, in both the peripheral and central nervous system. This study assesses the synergistic antihyperalgesic and antiallodynic effect of haloperidol, a sigma-1 antagonist, combined with gabapentin in rats with peripheral neuropathy. Wistar rats male were subjected to chronic constriction injury (CCI) of the sciatic nerve. The effects of systemic administration of gabapentin and the sigma-1 receptor antagonist, haloperidol, were examined at 11 days post-CCI surgery. An analysis of Surface of Synergistic Interaction was used to determine whether the combination's effects were synergistic. Twelve combinations showed various degrees of interaction in the antihyperalgesic and antiallodynic effects. In hyperalgesia, three combinations showed additive effects, four combinations showed supra-additive effects, and three combinations produced an effect limited by the maximum effect. In allodynia, five combinations showed additive effects, two combinations showed supra-additive effects, and five combinations produced antihyperalgesic effects limited by the maximum effect. These findings indicate that the administration of some specific combination of gabapentin and haloperidol can synergistically reduce nerve injury-induced allodynia and hyperalgesia. This suggests that the haloperidol-gabapentin combination can improve the antiallodynic and antihyperalgesic effects in a neuropathic pain model.

Dehydroepiandrosterone (DHEA) and its Sulphate (DHEAS) in Alzheimer's Disease

BACKGROUND

Neurosteroids Dehydroepiandrosterone (DHEA) and Dehydroepiandrosterone Sulphate (DHEAS) are involved in many important brain functions, including neuronal plasticity and survival, cognition and behavior, demonstrating preventive and therapeutic potential in different neuropsychiatric and neurodegenerative disorders, including Alzheimer's disease.

OBJECTIVE

The aim of the article was to provide a comprehensive overview of the literature on the involvement of DHEA and DHEAS in Alzheimer's disease.

METHODS

PubMed and MEDLINE databases were searched for relevant literature. The articles were selected considering their titles and abstracts. In the selected full texts, lists of references were searched manually for additional articles.

RESULTS

We performed a systematic review of the studies investigating the role of DHEA and DHEAS in various in vitro and animal models, as well as in patients with Alzheimer's disease, and provided a comprehensive discussion on their potential preventive and therapeutic applications.

CONCLUSION

Despite mixed results, the findings of various preclinical studies are generally supportive of the involvement of DHEA and DHEAS in the pathophysiology of Alzheimer's disease, showing some promise for potential benefits of these neurosteroids in the prevention and treatment. However, so far small clinical trials brought little evidence to support their therapy in AD. Therefore, large-scale human studies are needed to elucidate the specific effects of DHEA and DHEAS and their mechanisms of action, prior to their applications in clinical practice.

Neurosteroid dehydroepiandrosterone sulphate enhances pain transmission in rat spinal cord dorsal horn

BACKGROUND

The neurosteroid dehydroepiandrosterone sulphate (DHEAS) activates the sigma-1 receptor, inhibits gamma-aminobutyric acid A (GABA_A) and glycine receptors, and induces hyperalgesic effects. Although its effects have been studied in various tissues of the nervous system, its synaptic mechanisms in nociceptive pathways remain to be elucidated.

METHODS

The threshold of mechanical hypersensitivity and spontaneous pain behaviour was assessed using the von Frey test in adult male Wistar rats after intrathecal administration of DHEAS. We also investigated the effects of DHEAS on synaptic transmission in the spinal dorsal horn using slice patch-clamp electrophysiology.

RESULTS

Intrathecally administered DHEAS elicited dose-dependent mechanical hyperalgesia and spontaneous pain behaviours (withdrawal threshold: saline; 51.0 [20.1] g, 3 μg DHEAS; 14.0 [7.8] g, P<0.01, 10 μg DHEAS; 6.9 [5.2] g, 15 min after administration, P<0.001). DHEAS at 100 μM increased the frequency of miniature postsynaptic currents in the rat dorsal spinal horn; this increase was extracellular Ca²⁺-dependent but not sigma-1 and N-methyl-d-aspartate receptor-dependent. DHEAS suppressed the frequency of miniature inhibitory postsynaptic currents in a GABA_A receptor- and sigma-1 receptor-dependent manner.

CONCLUSIONS

These results suggest that DHEAS participates in the pathophysiology of nociceptive synaptic transmission in the spinal cord by potentiation of glutamate release and inhibition of the GABA_A receptor.

An exploratory pilot investigation of neurosteroids and self-reported pain in female Iraq/Afghanistan-era Veterans

Female Veterans are the most rapidly growing segment of new users of the Veterans Health Administration (VHA), and a significant proportion of female Veterans receiving treatment from VHA primary care providers report persistent pain symptoms. Currently, available data characterizing the neurobiological underpinnings of pain disorders are limited. Preclinical data suggest that neurosteroids may be involved in the modulation of pain symptoms, potentially via actions at gamma-aminobutyric acid (GABA) and N-methyl-D-aspartate (NMDA) receptors. Dehydroepiandrosterone (DHEA) and dehydroepiandrosterone sulfate (DHEAS) are neurosteroids that modulate inhibitory GABA receptors and excitatory NMDA receptors, producing complex neuronal effects. Emerging evidence from male Iraq/Afghanistan-era Veterans suggests that reductions in neurosteroid levels are associated with increased pain symptoms and that neurosteroids may be promising biomarker candidates. The current exploratory study thus examined associations between self-reported pain symptoms in 403 female Iraq/Afghanistan-era Veterans and serum DHEAS and DHEA levels. Serum DHEAS levels were inversely correlated with low back pain in female Veterans (Spearman r = -0.103; p = 0.04). Nonparametric analyses indicate that female Veterans reporting moderate/extreme low back pain demonstrated significantly lower DHEAS levels than those reporting no/little low back pain (|Z| = 2.60; p = 0.009). These preliminary findings support a role for DHEAS in pain physiology of low back pain and the rationale for neurosteroid therapeutics in pain analgesia.

Sigma 1 receptor mediated HMGB1 expression in spinal cord is involved in the development of diabetic neuropathic pain

No study has been conducted to examine the interactions of sigma-1 receptor (Sigma-1R) and high mobility group box 1 protein (HMGB1) in the development of diabetic peripheral neuropathy. Thus, we examined the effects of streptozotocin (STZ) treatment on expression of HMGB1 in subcellular levels in the dorsal root ganglion (DRG) in both wild-type and Sigma-1R-/- mice and evaluated the effects of repeated intrathecal administrations of selective Sigma-1R antagonists BD1047, agonist PRE-084, or HMGB1 inhibitor glycyrrhizin on peripheral neuropathy in wild-type mice. We found that STZ-induced tactile allodynia and thermal hyperalgesia was associated with increased total HMGB1 expression in DRG. STZ treatment promoted the distribution of HMGB1 into cytoplasm. Furthermore, STZ induced modest peripheral neuropathy and did not alter HMGB1 levels in DRG or the distribution of either cytoplasmic or nuclear HMGB1 in Sigma-1R-/- mice compared to sham control mice. Additionally, repeated stimulation of Sigma-1R in the spinal cord induced tactile allodynia and thermal hyperalgesia at 1 week. This phenomenon was associated with increased cytoplasmic HMGB1 translocation and HMGB1 expression in DRG. Finally, we found that repeated blockade of either Sigma-1R or HMGB1 in the spinal cord after STZ treatment prevent the development of tactile allodynia and thermal hyperalgesia at 1 week. These effects were associated with decreased cytoplasmic HMGB1 translocation and HMGB1 expression in DRG. Taken together, our results suggest that Sigma-1R-mediated enhancement of HMGB1 expression in the DRG is critical for the development of peripheral neuropathy in type 1 diabetes.

Acetaminophen (Paracetamol) Use Modifies the Sulfation of Sex Hormones

Background

Acetaminophen (paracetamol) is one of the most common medications used for management of pain in the world. There is lack of consensus about the mechanism of action, and concern about the possibility of adverse effects on reproductive health.

Methods

We first established the metabolome profile that characterizes use of acetaminophen, and we subsequently trained and tested a model that identified metabolomic differences across samples from 455 individuals with and without acetaminophen use. We validated the findings in a European ancestry adult twin cohort of 1880 individuals (TwinsUK), and in a study of 1235 individuals of African American and Hispanic ancestry. We used genomics to elucidate the mechanisms targeted by acetaminophen.

Findings

We identified a distinctive pattern of depletion of sulfated sex hormones with use of acetaminophen across all populations. We used a Mendelian randomization approach to characterize the role of Sulfotransferase Family 2A Member 1 (SULT2A1) as the site of the interaction. Although CYP3A7-CYP3A51P variants also modified levels of some sulfated sex hormones, only acetaminophen use phenocopied the effect of genetic variants of SULT2A1. Overall, acetaminophen use, age, gender and SULT2A1 and CYP3A7-CYP3A51P genetic variants are key determinants of variation in levels of sulfated sex hormones in blood. The effect of taking acetaminophen on sulfated sex hormones was roughly equivalent to the effect of 35 years of aging.

Interpretation

These findings raise concerns of the impact of acetaminophen use on hormonal homeostasis. In addition, it modifies views on the mechanism of action of acetaminophen in pain management as sulfated sex hormones can function as neurosteroids and modify nociceptive thresholds.

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We use metabolome analysis of 3570 individuals to identify the effect of acetaminophen on metabolic processes.

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Acetaminophen use is associated with decrease sulfation of sexual hormones.

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These findings are relevant in the context of current debate on the use of acetaminophen during pregnancy

Despite decades-long use of acetaminophen, there is an incomplete understanding of the mechanism of action, and of the potential for adverse metabolic effects. Recent epidemiological and animal work supports an effect of acetaminophen on reproductive processes and hormonal homeostasis. We observe a consistent and reproducible effect of acetaminophen use on the levels of sulfated sex hormones. This is relevant to the investigation of hormonal homeostasis during pregnancy – acetaminophen is the most commonly used analgesic by pregnant women. It also opens the door to investigating the role of sulfated hormones in pain management.

Neuroactive steroids, nociception and neuropathic pain: A flashback to go forward

The present review discusses the potential role of neurosteroids/neuroactive steroids in the regulation of nociceptive and neuropathic pain, and recapitulates the current knowledge on the main mechanisms involved in the reduction of pain, especially those occurring at the dorsal horn of the spinal cord, a crucial site for nociceptive processing. We will make special focus on progesterone and its derivative allopregnanolone, which have been shown to exert remarkable actions in order to prevent or reverse the maladaptive changes and pain behaviors that arise after nervous system damage in various experimental neuropathic conditions.

Peripheral neurosteroids enhance P2X receptor-induced mechanical allodynia via a sigma-1 receptor-mediated mechanism

The role of peripheral neurosteroids and their related mechanisms on nociception have not been thoroughly investigated. Based on emerging evidence in the literature indicating that neurosteroids and their main target receptors, i.e., sigma-1, GABAA and NMDA, affect P2X-induced changes in neuronal activity, this study was designed to investigate the effect of peripherally injected dehydroepiandrosterone sulphate (DHEAS) and pregnenolone sulfate (PREGS) on P2X receptor-mediated mechanical allodynia in rats. Intraplantar injection of either neurosteroids alone did not produced any detectable changes in paw withdrawal frequency to the innocuous mechanical stimulation in naïve rats. However, When DHEAS or PREGS were co-injected with a sub-effective dose of αβmeATP, mechanical allodynia was developed and this was dose dependently blocked by pre-injection of the P2X antagonist, TNP-ATP. These results demonstrates that DHEAS and PREGS potentiate the activity of P2X receptors which results in the enhancement of αβmeATP-induced mechanical allodynia. In order to investigate the potential role of peripheral sigma-1, GABAA and NMDA receptors in this facilitatory action, we pretreated animals with BD-1047 (a sigma-1 antagonist), muscimol (a GABAA agonist) or MK-801 (a NMDA antagonist) prior to DHEAS or PREGS+αβmeATP injection. Only BD-1047 effectively prevented the facilitatory effects induced by neurosteroids on αβmeATP-induced mechanical allodynia. Collectively, we have shown that peripheral neurosteroids potentiate P2X-induced mechanical allodynia and that this action is mediated by sigma-1, but not by GABAA nor NMDA, receptors.

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.

Sigma-1 Receptor Antagonist BD1047 Reduces Mechanical Allodynia in a Rat Model of Bone Cancer Pain through the Inhibition of Spinal NR1 Phosphorylation and Microglia Activation

Previous studies have demonstrated that sigma-1 receptor plays important roles in the induction phase of rodent neuropathic pain; however, whether it is involved in bone cancer pain (BCP) and the underlying mechanisms remain elusive. The aim of this study was to examine the potential role of the spinal sigma-1 receptor in the development of bone cancer pain. Walker 256 mammary gland carcinoma cells were implanted into the intramedullary space of the right tibia of Sprague-Dawley rats to induce ongoing bone cancer-related pain behaviors; our findings indicated that, on days 7, 10, 14, and 21 after operation, the expression of sigma-1 receptor in the spinal cord was higher in BCP rats compared to the sham rats. Furthermore, intrathecal injection of 120 nmol of sigma-1 receptor antagonist BD1047 on days 5, 6, and 7 after operation attenuated mechanical allodynia as well as the associated induction of c-Fos and activation of microglial cells, NR1, and the subsequent Ca(2+)-dependent signals of BCP rats. These results suggest that sigma-1 receptor is involved in the development of bone cancer pain and that targeting sigma-1 receptor may be a new strategy for the treatment of bone cancer pain.

Antinociceptive effects of fisetin against diabetic neuropathic pain in mice: Engagement of antioxidant mechanisms and spinal GABAA receptors

Peripheral painful neuropathy is one of the most common complications in diabetes and necessitates improved treatment. Fisetin, a naturally occurring flavonoid, has been reported to exert antidepressant-like effect in previous studies. As antidepressant drugs are employed clinically to treat neuropathic pain, this work aimed to investigate whether fisetin possess beneficial effect on diabetic neuropathic pain and explore the mechanism(s). We subjected mice to diabetes by a single intraperitoneal (i.p.) injection of streptozotocin (200mg/kg), and von Frey test or Hargreaves test was used to assess mechanical allodynia or thermal hyperalgesia, respectively. Chronic treatment of diabetic mice with fisetin not only ameliorated the established symptoms of thermal hyperalgesia and mechanical allodynia, but also arrested the development of neuropathic pain when given at low doses. Although chronic fisetin administration did not impact on the symptom of hyperglycemia in diabetic mice, it reduced exacerbated oxidative stress in tissues of spinal cord, dorsal root ganglion (DRG) and sciatic verve. Furthermore, the analgesic actions of fisetin were abolished by repetitive co-treatment with the reactive oxygen species (ROS) donor tert-butyl hydroperoxide (t-BOOH), but potentiated by the ROS scavenger phenyl-N-tert-butylnitrone (PBN). Finally, acute blockade of spinal GABAA receptors by bicuculline totally counteracted such fisetin analgesia. These findings indicate that chronic fisetin treatment can delay or correct neuropathic hyperalgesia and allodynia in mice with type 1 diabetes. Mechanistically, the present fisetin analgesia may be associated with its antioxidant activity, and spinal GABAA receptors are likely rendered as downstream targets.

Sigma-1 receptors and animal studies centered on pain and analgesia

INTRODUCTION

Neuropathic pain is difficult to relieve with standard analgesics and tends to be resistant to opioid therapy. Sigma-1 receptors activated during neuropathic injury may sustain pain. Neuropathic injury activates sigma-1 receptors, which results in activation of various kinases, modulates the activity of multiple ion channels, ligand activated ion channels and voltage-gated ion channels; alters monoamine neurotransmission and dampens opioid receptors G-protein activation. Activation of sigma-1 receptors tonically inhibits opioid receptor G-protein activation and thus dampens analgesic responses. Therefore, sigma-1 receptor antagonists are potential analgesics for neuropathic and adjuvants to opioid therapy.

AREAS COVERED

This article reviews the importance of sigma-1 receptors as pain generators in multiple animal models in order to illustrate both the importance of these unique receptors in pathologic pain and the potential benefits to sigma-1 receptor antagonists as analgesics.

EXPERT OPINION

Sigma-1 receptor antagonists have a great potential as analgesics for acute neuropathic injury (herpes zoster, acute postoperative pain and chemotherapy induced neuropathy) and may, as an additional benefit, prevent the development of chronic neuropathic pain. Antagonists are potentially effective as adjuvants to opioid therapy when used early to prevent analgesic tolerance. Drug development is complicated by the complexity of sigma-1 receptor pharmacodynamics and its multiple targets, the lack of a specific sigma-1 receptor antagonist, and potential side effects due to on-target toxicities (cognitive impairment, depression).

Chronic activation of sigma-1 receptor evokes nociceptive activation of trigeminal nucleus caudalis in rats

Primary headache disorders, including migraine, are thought to be mediated by prolonged nociceptive activation of the trigeminal nucleus caudalis (TNC), but the precise mechanisms are poorly understood. Our past studies demonstrated that sigma-1 receptors (Sig-1R) facilitate spinal nociceptive transmission in several pain models. Based on these findings, this study asked if chronic activation of Sig-1R by intracisternal administration of the selective Sig-1R agonist, PRE084, produced TNC neuronal activation as a migraine trigger in rats. A single infusion of PRE084 (10, 50, 100, 500 nmol) significantly increased the number of Fos immunoreactive neurons (Fos-IR) in TNC, which BD1047 (a Sig-1R antagonist) reversed. Chronic infusion of PRE084 (100 nmol for 1, 3, 7 and 14 days) time-dependently elevated Fos-IR in TNC. The number of Fos-IR elevation from day 7 of infusion was comparable with a single capsaicin infusion as a headache model. Increase in face grooming/scratching behavior was evident from day 7, and peaked at day 14 of chronic PRE084 infusion, which was correlated with ΔFosB elevation and phosphorylation of extracellular signal-regulated kinase, and the NMDA receptor NR1 subunit in TNC. Following 14 days of PRE084 infusion, the number of Fos-IR increased until day 7 after final infusion. Moreover, by day 14, Fos-IR associated with PRE084 infusion was significantly reversed by NMDA receptor antagonist MK801, rather than BD1047. These findings indicated that chronic activation of Sig-1R could evoke prolonged neuronal activation in the trigeminovascular system.

Sigma 1 receptor: a new therapeutic target for pain

Sigma 1 receptor (σ₁ receptor) is a unique ligand-regulated molecular chaperone located mainly in the endoplasmic reticulum and the plasma membrane. σ₁ receptor is activated under stress or pathological conditions and interacts with several neurotransmitter receptors and ion channels to modulate their function. The effects reported preclinically with σ₁ receptor ligands are consistent with a role for σ₁ receptor in central sensitization and pain hypersensitivity and suggest a potential therapeutic use of σ₁ receptor antagonists for the management of neuropathic pain as monotherapy. Moreover, data support their use in opioid adjuvant therapy: combination of σ₁ receptor antagonists and opioids results in potentiation of opioid analgesia, without significant increases in opioid-related unwanted effects. Results from clinical trials using selective σ₁ receptor antagonists in several pain conditions are eagerly awaited to ascertain the potential of σ₁ receptor modulation in pain therapy.

Sigma-1 receptor expression in sensory neurons and the effect of painful peripheral nerve injury

BACKGROUND

The sigma-1 receptor (σ1R), an endoplasmic reticulum chaperone protein, is widely distributed and regulates numerous intracellular processes in neurons. Nerve injury alters the structure and function of axotomized dorsal root ganglion (DRG) neurons, contributing to the development of pain. The σ1R is enriched in the spinal cord and modulates pain after peripheral nerve injury. However, σ1R expression in the DRG has not been studied. We therefore characterized σ1R expression in DRGs at baseline and following spinal nerve ligation (SNL) in rats.

RESULTS

Immunohistochemical (IHC) studies in DRG sections show σ1R in both neuronal somata and satellite glial cells. The punctate distribution of σ1R in the neuronal cytoplasm suggests expression in the endoplasmic reticulum. When classified by neuronal size, large neurons (>1300 μm) showed higher levels of σ1R staining than other groups (700-1300 μm, <700 μm). Comparing σ1R expression in neuronal groups characterized by expression of calcitonin gene-related peptide (CGRP), isolectin-B4 (IB4) and neurofilament-200 (NF-200), we found σ1R expression in all three neuronal subpopulations, with highest levels of σ1R expression in the NF-200 group. After SNL, lysates from L5 DRGs that contains axotomized neurons showed decreased σ1R protein but unaffected transcript level, compared with Control DRGs. IHC images also showed decreased σ1R protein expression, in SNL L5 DRGs, and to a lesser extent in the neighboring SNL L4 DRGs. Neurons labeled by CGRP and NF-200 showed decreased σ1R expression in L5 and, to a lesser extent, L4 DRGs. In IB4-labeled neurons, σ1R expression decreased only in axotomized L5 DRGs. Satellite cells also showed decreased σ1R expression in L5 DRGs after SNL.

CONCLUSIONS

Our data show that σ1R is present in both sensory neurons and satellite cells in rat DRGs. Expression of σ1R is down-regulated in axotomized neurons as well as in their accompanying satellite glial cells, while neighboring uninjured neurons show a lesser down-regulation. Therefore, elevated σ1R expression in neuropathic pain is not an explanation for pain relief after σ1R blockade. This implies that increased levels of endogenous σ1R agonists may play a role, and diminished neuroprotection from loss of glial σ1R may be a contributing factor.

Gonadotropin-releasing hormone stimulates the biosynthesis of pregnenolone sulfate and dehydroepiandrosterone sulfate in the hypothalamus

The sulfated neurosteroids pregnenolone sulfate (Δ(5)PS) and dehydroepiandrosterone sulfate (DHEAS) are known to play a role in the control of reproductive behavior. In the frog Pelophylax ridibundus, the enzyme hydroxysteroid sulfotransferase (HST), responsible for the biosynthesis of Δ(5)PS and DHEAS, is expressed in the magnocellular nucleus and the anterior preoptic area, two hypothalamic regions that are richly innervated by GnRH1-containing fibers. This observation suggests that GnRH1 may regulate the formation of sulfated neurosteroids to control sexual activity. Double labeling of frog brain slices with HST and GnRH1 antibodies revealed that GnRH1-immunoreactive fibers are located in close vicinity of HST-positive neurons. The cDNAs encoding 3 GnRH receptors (designated riGnRHR-1, -2, and -3) were cloned from the frog brain. RT-PCR analyses revealed that riGnRHR-1 is strongly expressed in the hypothalamus and the pituitary whereas riGnRHR-2 and -3 are primarily expressed in the brain. In situ hybridization histochemistry indicated that GnRHR-1 and GnRHR-3 mRNAs are particularly abundant in preoptic area and magnocellular nucleus whereas the concentration of GnRHR-2 mRNA in these 2 nuclei is much lower. Pulse-chase experiments using tritiated Δ(5)P and DHEA as steroid precursors, and 3'-phosphoadenosine 5'-phosphosulfate as a sulfonate moiety donor, showed that GnRH1 stimulates, in a dose-dependent manner, the biosynthesis of Δ(5)PS and DHEAS in frog diencephalic explants. Because Δ(5)PS and DHEAS, like GnRH, stimulate sexual activity, our data strongly suggest that some of the behavioral effects of GnRH could be mediated via the modulation of sulfated neurosteroid production.

Sigma-1 Receptor Antagonist BD1047 Reduces Allodynia and Spinal ERK Phosphorylation Following Chronic Compression of Dorsal Root Ganglion in Rats

Many therapeutic roles have been proposed for sigma-1 receptor (Sig-1R), but the involvement of Sig-1R in neuropathic pain has currently not been well explored. The present study aimed to evaluate the anti-nociceptive effect of Sig-1R antagonist (BD1047) in a rat model of chronic compression of the dorsal root ganglion (CCD), which is a model of human foraminal stenosis and radicular pain. When stainless steel rods were inserted into the intervertebral foramen of lumbar vertebrae 4 and 5, the CCD developed reliable mechanical (from 3 day) and cold allodynia (from 1 day) as compared with the sham operation group. The spinal expressions of Sig-1R and phosphorylation of extracellular signal-regulated kinase (pERK) were significantly increased from day 3 to day 14 after CCD surgery, as is consistent with the manifestation of allodynia. The BD 1047 (10, 30, 100 mg/kg) administered on postoperative days 0~5 dose-dependently suppressed both the induction of allodynia and the elevation of the spinal pERK expression in a manner comparable with that of gabapentin (100 mg/kg). At 7 days post-CCD surgery, BD1047 (10, 30, 100 mg/kg) administration also produced anti-nociceptive effects on the mechanical and cold allodynia similar with those of gabapentin (100 mg/kg). Therefore, this data suggested that Sig-1R may play an important role in both the development and maintenance of CCD-induced neuropathy.

An increase in spinal dehydroepiandrosterone sulfate (DHEAS) enhances NMDA-induced pain via phosphorylation of the NR1 subunit in mice: involvement of the sigma-1 receptor

Our laboratory has recently demonstrated that an increase in the spinal neurosteroid, dehydroepiandrosterone sulfate (DHEAS) facilitates nociception via the activation of sigma-1 receptors and/or the allosteric inhibition GABA(A) receptors. Several lines of evidence have suggested that DHEAS positively modulates N-methyl-d-aspartate (NMDA) receptor activity within the central nervous system. Moreover, we have demonstrated that the activation of sigma-1 receptors increases NMDA receptor activity. Since NMDA receptors play a key role in the enhancement of pain perception, the present study was designed to determine whether spinally administered DHEAS modulates NMDA receptor-mediated nociceptive activity and whether this effect is mediated by sigma-1 or GABA(A) receptors. Intrathecal (i.t.) DHEAS was found to significantly potentiate i.t. NMDA-induced spontaneous pain behaviors. Subsequent immunohistochemical analysis demonstrated that i.t. DHEAS also increased protein kinase C (PKC)- and protein kinase A (PKA)-dependent phosphorylation of the NMDA receptor subunit NR1 (pNR1), which was used as a marker of NMDA receptor sensitization. The sigma-1 receptor antagonist, BD-1047, but not the GABA(A) receptor agonist, muscimol, dose-dependently suppressed DHEAS's facilitatory effect on NMDA-induced nociception and pNR1 expression. In addition, pretreatment with either a PKC or PKA blocker significantly reduced the facilitatory effect of DHEAS on NMDA-induced nociception. Conversely the GABA(A) receptor antagonist, bicuculline did not affect NMDA-induced pain behavior or pNR1 expression. The results of this study suggest that the DHEAS-induced enhancement of NMDA-mediated nociception is dependent on an increase in PKC- and PKA-dependent pNR1. Moreover, this effect of DHEAS on NMDA receptor activity is mediated by the activation of spinal sigma-1 receptors and not through the inhibition of GABA(A) receptors.