Chronic pain recruits hypothalamic dynorphin/kappa opioid receptor signalling to promote wakefulness and vigilance

Descending facilitation from rostral ventromedial medulla mu opioid receptor-expressing neurons is necessary for maintenance of sensory and affective dimensions of chronic neuropathic pain

ABSTRACT

Pharmacological ablation of rostral ventromedial medulla (RVM) mu opioid receptor-expressing cells before peripheral nerve injury prevents the development of neuropathic pain. However, whether these neurons are required for the expression of established neuropathic pain is not known. Male Oprm1Cre heterozygous (MOR Cre ) or wild-type (MOR WT ) mice received AAV8-hSyn-DIO-hM4D(Gi)-mCherry in the RVM. After partial sciatic nerve ligation (PSNL), we evaluated pain behaviors and descending control of nociception in response to acute or sustained chemogenetic inhibition of RVM-MOR cells expressing hM4D(Gi). A single systemic administration of hM4D(Gi) agonist clozapine-N-oxide (CNO) reversibly inhibited hind paw tactile allodynia and produced conditioned place preference only in MOR Cre mice with PSNL. Intrathecal CNO also reversibly inhibited PSNL-induced hind paw allodynia, suggesting that the spinal projections from these RVM-MOR cells are critical for manifestation of pain behaviors. Consistent with enhanced descending facilitation from RVM-MOR cells, MOR Cre -hM4D(Gi) mice with PSNL showed diminished descending control of nociception that was restored by systemic CNO. Sustained CNO in drinking water before PSNL prevented expression of chronic pain without affecting acute surgical pain; however, relief of chronic pain required sustained CNO treatment. Thus, in male mice, activity of spinally projecting RVM-MOR cells is required (1) for expression and manifestation of both sensory and affective dimensions of established neuropathic pain and (2) to promote descending facilitation that overcomes apparently intact descending inhibition to maintain chronic pain. Enhanced descending facilitation likely regulates the output signal from the spinal cord to the brain to shape the pain experience and may provide a mechanism for nonopioid management of pain.

Cortical kappa opioid receptors integrate negative affect and sleep disturbance

Sleep disruption and negative affect are attendant features of many psychiatric and neurological conditions that are often co-morbid including major depressive disorder, generalized anxiety disorder and chronic pain. Whether there is a causal relationship between negative affect and sleep disruption remains unclear. We therefore asked if mechanisms promoting negative affect can disrupt sleep and whether inhibition of pathological negative affect can normalize disrupted sleep. Signaling at the kappa opioid receptor (KOR) elicits dysphoria in humans and aversive conditioning in animals. We tested the possibility that (a) increased KOR signaling in the anterior cingulate cortex (ACC), a brain region associated with negative emotions, would be sufficient to promote both aversiveness and sleep disruption and (b) inhibition of KOR signaling would normalize pathological negative affect and sleep disruption induced by chronic pain. Chemogenetic Gi-mediated inhibition of KOR-expressing ACC neurons produced conditioned place aversion (CPA) as well as sleep fragmentation in naïve mice. CRISPR/Cas9 editing of ACC KOR normalized both the negative affect and sleep disruption elicited by pathological chronic pain while maintaining the physiologically critical sensory features of pain. These findings suggest therapeutic utility of KOR antagonists for treatment of disease conditions that are associated with both negative affect and sleep disturbances.

Endogenous opiates and behavior: 2023

This paper is the forty-sixth consecutive installment of the annual anthological review of research concerning the endogenous opioid system, summarizing articles published during 2023 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 (1), the roles of these opioid peptides and receptors in pain and analgesia in animals (2) and humans (3), opioid-sensitive and opioid-insensitive effects of nonopioid analgesics (4), opioid peptide and receptor involvement in tolerance and dependence (5), stress and social status (6), learning and memory (7), eating and drinking (8), drug and alcohol abuse (9), sexual activity and hormones, pregnancy, development and endocrinology (10), mental illness and mood (11), seizures and neurologic disorders (12), electrical-related activity and neurophysiology (13), general activity and locomotion (14), gastrointestinal, renal and hepatic functions (15), cardiovascular responses (16), respiration and thermoregulation (17), and immunological responses (18).

Investigating the brain functional abnormalities underlying pain hypervigilance in chronic neck and shoulder pain: a resting-state fMRI study

PURPOSE

To investigate pain hypervigilance in individuals suffering from chronic neck and shoulder pain (CNSP) and its underlying brain mechanism.

METHODS

The evaluation of pain vigilance was conducted through the utilization of pain vigilance and awareness questionnaires. Voxel-wise regional homogeneity (ReHo) from 60 CNSP patients and 60 healthy controls (HCs) using resting-state fMRI data. Voxel-wise two-sample T-test was conducted to reveal the ReHo variations between CNSP and HC. Correlation analyses were utilized to reveal the connection between brain abnormalities and medical measurements. Furthermore, a mediation analysis was conducted to elucidate the pathway-linking changes in brain function with medical measurements.

RESULTS

Our present study revealed three main findings. Firstly, patients with CSNP demonstrated a heightened vigilance of pain in comparison to healthy adults, a common occurrence among individuals with chronic pain conditions. Secondly, we observed brain abnormalities in various brain regions in CSNP patients, and these alterations were associated with the extent of pain vigilance. Lastly, the pain hypervigilance impact on the severity of pain was found to be controlled by regional neural activity in the anterior cingulate cortex (ACC) in subjects with CSNP.

CONCLUSION

Our findings suggested that long-term repetitive nociceptive input caused by chronic pain further aggravates the pain intensity by impairing the vigilance-related pain processing within the anterior cingulate cortex in CNSP patients.

Targeting of KOR by famotidine promotes OPC maturation differentiation and CNS remyelination via STAT3 signaling pathway

This study aims to identify FDA-approved drugs that can target the kappa-opioid receptor (KOR) for the treatment of demyelinating diseases. Demyelinating diseases are characterized by myelin sheath destruction or formation that results in severe neurological dysfunction. Remission of this disease is largely dependent on the differentiation of oligodendrocyte precursor cells (OPCs) into mature oligodendrocytes (OLGs) in demyelinating lesions. KOR is an important regulatory protein and drug target for the treatment of demyelinating diseases. However, no drug targeting KOR has been developed due to the long clinical trials for drug discovery. Here, a structure-based virtual screening was applied to identify drugs targeting KOR among 1843 drugs of FDA-approved drug libraries, and famotidine was screen out by its high affinity cooperation with KOR as well as the clinical safety. We discovered that famotidine directly promoted OPC maturation and remyelination using the complementary in vitro and in vivo models. Administration of famotidine was not only effectively enhanced CNS myelinogenesis, but also promoted remyelination. Mechanically speaking, famotidine promoted myelinogenesis or remyelination through KOR/STAT3 signaling pathway. In general, our study provided evidence of new clinical applicability of famotidine for the treatment of demyelinating diseases for which there is currently no effective therapy.

Visualization analysis of research frontiers and trends in the treatment of sciatic nerve injury

Objective

To visualize and analyze the literature related to sciatic nerve injury treatment from January 2019 to December 2023, and summarize the current status, hotspots, and development trends of research in this field.

Methods

Using CiteSpace and VOSviewer software, we searched the Web of Science database for literature related to the treatment of sciatic nerve injury. Then we analyzed and plotted visualization maps to show the number of publications, countries, institutions, authors, keywords, references, and journals.

Results

A total of 2,653 articles were included in the English database. The annual number of publications exceeded 230, and the citation frequency increased yearly. The United States and China were identified as high-influence nations in this field. Nantong University was the leading institution in terms of close cooperation among institutions. The authors Wang Yu had the highest number of publications and were highly influential in this field. Keyword analysis and reference Burst revealed a research focus on nerve regeneration and neuropathic pain, which involve regenerative medicine and neural tissue engineering. Chronic pain resulting from sciatic nerve injury often manifests alongside anxiety, depression, cognitive-behavioral disorders, and other issues. Interventions such as stem cells, electrical stimulation, electroacupuncture, total joint replacement, pharmacological interventions, gene therapy, nerve conduits, chitosan scaffolds, and exercise promote nerve repair and alleviate pain. Schwann cells have been the focus of much attention in nerve repair and regeneration. Improving the outcome of sciatic nerve injury is a current research challenge and focus in this field. Based on keyword Burst, nerve conduits and grafts may become a potential research hotspot in the treatment of sciatic nerve injury.

Conclusion

This visual analysis summarizes research trends and developments of sciatic nerve injury treatment and predicts potential research frontiers and hot directions.

Nociceptor spontaneous activity is responsible for fragmenting non-rapid eye movement sleep in mouse models of neuropathic pain

Spontaneous pain, a major complaint of patients with neuropathic pain, has eluded study because there is no reliable marker in either preclinical models or clinical studies. Here, we performed a comprehensive electroencephalogram/electromyogram analysis of sleep in several mouse models of chronic pain: neuropathic (spared nerve injury and chronic constriction injury), inflammatory (Freund's complete adjuvant and carrageenan, plantar incision) and chemical pain (capsaicin). We find that peripheral axonal injury drives fragmentation of sleep by increasing brief arousals from non-rapid eye movement sleep (NREMS) without changing total sleep amount. In contrast to neuropathic pain, inflammatory or chemical pain did not increase brief arousals. NREMS fragmentation was reduced by the analgesics gabapentin and carbamazepine, and it resolved when pain sensitivity returned to normal in a transient neuropathic pain model (sciatic nerve crush). Genetic silencing of peripheral sensory neurons or ablation of CGRP+ neurons in the parabrachial nucleus prevented sleep fragmentation, whereas pharmacological blockade of skin sensory fibers was ineffective, indicating that the neural activity driving the arousals originates ectopically in primary nociceptor neurons and is relayed through the lateral parabrachial nucleus. These findings identify NREMS fragmentation by brief arousals as an effective proxy to measure spontaneous neuropathic pain in mice.

Olfactory ensheathing cells as candidate cells for chronic pain treatment

Chronic pain is often accompanied by tissue damage and pain hypersensitivity. It easily relapses and is challenging to cure, which seriously affects the patients' quality of life and is an urgent problem to be solved. Current treatment methods primarily rely on morphine drugs, which do not address the underlying nerve injury and may cause adverse reactions. Therefore, in recent years, scientists have shifted their focus from chronic pain treatment to cell transplantation. This review describes the classification and mechanism of chronic pain through the introduction of the characteristics of olfactory ensheathing cells (OECs), an in-depth discussion of special glial cells through the phagocytosis of nerve debris, receptor-ligand interactions, providing nutrition, and other inhibition of neuroinflammation, and ultimately supporting axon regeneration and mitigation of chronic pain. This review summarizes the potential and limitations of OECs for treating chronic pain by objectively analyzing relevant clinical trials and methods to enhance efficacy and future development prospects.

Sex differences in descending control of nociception (DCN) responses after chronic orofacial pain induction in rats and the contribution of kappa opioid receptors

Descending control of nociception (DCN), a measure of efficiency of descending pain inhibition, can be assessed in animals by the combined application of test and conditioning noxious stimuli. Evidence from pre-clinical and clinical studies indicates that this mechanism of pain control may differ between sexes and might be impaired in many chronic pain states. However, little is known about sex differences in DCN efficiency in models of acute and chronic orofacial pain. Herein, we first evaluated DCN responses in male and female rats by the applying formalin into the upper lip or capsaicin into the forepaw as the conditioning stimulus, followed by mechanical stimulation (Randall-Selitto) of the hind paw as the test stimulus. The same protocol (i.e., capsaicin in the forepaw followed by mechanical stimulation of the hind paw) was evaluated in male and female rats on day 3 after intraoral incision and on day 15 and 30 after chronic constriction injury of the infraorbital nerve (CCI-ION). Additionally, we assessed the effect of the kappa opioid receptor (KOR) antagonist Norbinaltorphimine (nor-BNI) on DCN responses of female nerve-injured rats. This study shows that naïve female rats exhibit less efficient DCN compared to males. Postoperative pain did not alter DCN responses in female and male rats, but CCI-ION induced loss of DCN responses in females but not in males. Systemic pretreatment with nor-BNI prevented the loss of DCN induced by CCI-ION in female rats. The results reveal sex differences in DCN responses and female-specific impairment of DCN following chronic orofacial pain. Moreover, the findings suggest that, at least for females, blocking KOR could be a promising therapeutic approach to prevent maladaptive changes in chronic orofacial pain.

Unraveling the directional relationship of sleep and migraine-like pain

Migraine and sleep disorders are common co-morbidities. Patients frequently link their sleep to migraine attacks suggesting a potential causal relationship between these conditions. However, whether migraine pain promotes or disrupts sleep or whether sleep disruption can increase the risk of migraine remains unknown. We assessed the potential impact of periorbital allodynia, a measure consistent with migraine-like pain, from multiple preclinical models on sleep quantity and quality. Additionally, we evaluated the possible consequences of sleep deprivation in promoting susceptibility to migraine-like pain. Following the implantation of electroencephalogram/electromyography electrodes to record sleep, mice were treated with either single or repeated systemic injections of nitroglycerin at the onset of their active phase (i.e. nocturnal awake period). Neither single nor repeated nitroglycerin affected the total sleep time, non-rapid eye movement sleep, rapid eye movement sleep, sleep depth or other measures of sleep architecture. To account for the possible disruptive effects of the surgical implantation of electroencephalogram/electromyography electrodes, we used immobility recordings as a non-invasive method for assessing sleep-wake behaviour. Neither single nor repeated nitroglycerin administration during either the mouse sleep (i.e. daylight) or active (i.e. night) periods influenced immobility-defined sleep time. Administration of an inflammatory mediator mixture onto the dura mater at either sleep or active phases also did not affect immobility-defined sleep time. Additionally, inhalational umbellulone-induced migraine-like pain in restraint-stressed primed mice did not alter immobility-defined sleep time. The possible influence of sleep disruption on susceptibility to migraine-like pain was evaluated by depriving female mice of sleep over 6 h with novel objects, a method that does not increase circulating stress hormones. Migraine-like pain was not observed following acute sleep deprivation. However, in sleep-deprived mice, subthreshold doses of systemic nitroglycerin or dural calcitonin gene-related peptide induced periorbital cutaneous allodynia consistent with migraine-like pain. Our data reveal that while migraine-like pain does not significantly disrupt sleep, sleep disruption increases vulnerability to migraine-like pain suggesting that a therapeutic strategy focused on improving sleep may diminish migraine attacks.

Kappa opioid receptor agonists produce sexually dimorphic and prolactin-dependent hyperalgesic priming

ABSTRACT

Repeated stress produces hyperalgesic priming in preclinical models, but underlying mechanisms remain uncertain. As stress engages kappa opioid receptors (KORs), we hypothesized that repeated administration of KOR agonists might mimic, in part, stress-induced hyperalgesic priming. The potential contribution of circulating prolactin (PRL) and dysregulation of the expression of PRL receptor (PRLR) isoforms in sensory neurons after KOR agonist administration was also investigated. Mice received 3 daily doses of U-69593 or nalfurafine as a "first-hit" stimulus followed by assessment of periorbital tactile allodynia. Sixteen days after the first KOR agonist administration, animals received a subthreshold dose of inhalational umbellulone, a TRPA1 agonist, as the second-hit stimulus and periorbital allodynia was assessed. Cabergoline, a dopamine D2 receptor agonist, was used to inhibit circulating PRL in additional cohorts. Prolactin receptor isoforms were quantified in the V1 region of the trigeminal ganglion after repeated doses of U-69593. In both sexes, KOR agonists increased circulating PRL and produced allodynia that resolved within 14 days. Hyperalgesic priming, revealed by umbellulone-induced allodynia in animals previously treated with the KOR agonists, also occurred in both sexes. However, repeated U-69593 downregulated the PRLR long isoform in trigeminal neurons only in female mice. Umbellulone-induced allodynia was prevented by cabergoline co-treatment during priming with KOR agonists in female, but not male, mice. Hyperalgesic priming therefore occurs in both sexes after either biased or nonbiased KOR agonists. However, a PRL/PRLR-dependence is observed only in female nociceptors possibly contributing to pain in stress-related pain disorders in females.

Post-injury pain and behaviour: a control theory perspective

Injuries of various types occur commonly in the lives of humans and other animals and lead to a pattern of persistent pain and recuperative behaviour that allows safe and effective recovery. In this Perspective, we propose a control-theoretic framework to explain the adaptive processes in the brain that drive physiological post-injury behaviour. We set out an evolutionary and ethological view on how animals respond to injury, illustrating how the behavioural state associated with persistent pain and recuperation may be just as important as phasic pain in ensuring survival. Adopting a normative approach, we suggest that the brain implements a continuous optimal inference of the current state of injury from diverse sensory and physiological signals. This drives the various effector control mechanisms of behavioural homeostasis, which span the modulation of ongoing motivation and perception to drive rest and hyper-protective behaviours. However, an inherent problem with this is that these protective behaviours may partially obscure information about whether injury has resolved. Such information restriction may seed a tendency to aberrantly or persistently infer injury, and may thus promote the transition to pathological chronic pain states.

Exploring the neurobiology of the premonitory phase of migraine preclinically - a role for hypothalamic kappa opioid receptors?

Background

The migraine premonitory phase is characterized in part by increased thirst, urination and yawning. Imaging studies show that the hypothalamus is activated in the premonitory phase. Stress is a well know migraine initiation factor which was demonstrated to engage dynorphin/kappa opioid receptors (KOR) signaling in several brain regions, including the hypothalamus. This study proposes the exploration of the possible link between hypothalamic KOR and migraine premonitory symptoms in rodent models.

Methods

Rats were treated systemically with the KOR agonist U-69,593 followed by yawning and urination monitoring. Apomorphine, a dopamine D1/2 agonist, was used as a positive control for yawning behaviors. Urination and water consumption following systemic administration of U-69,593 was also assessed. To examine if KOR activation specifically in the hypothalamus can promote premonitory symptoms, AAV8-hSyn-DIO-hM4Di (Gi-DREADD)-mCherry viral vector was microinjected into the right arcuate nucleus (ARC) of female and male KOR^CRE or KOR^WT mice. Four weeks after the injection, clozapine N-oxide (CNO) was administered systemically followed by the assessment of urination, water consumption and tactile sensory response.

Results

Systemic administration of U-69,593 increased urination but did not produce yawning in rats. Systemic KOR agonist also increased urination in mice as well as water consumption. Cell specific Gi-DREADD activation (i.e., inhibition through Gi-coupled signaling) of KOR^CRE neurons in the ARC also increased water consumption and the total volume of urine in mice but did not affect tactile sensory responses.

Conclusion

Our studies in rodents identified the KOR in a hypothalamic region as a mechanism that promotes behaviors consistent with clinically-observed premonitory symptoms of migraine, including increased thirst and urination but not yawning. Importantly, these behaviors occurred in the absence of pain responses, consistent with the emergence of the premonitory phase before the headache phase. Early intervention for preventive treatment even before the headache phase may be achievable by targeting the hypothalamic KOR.

Supplementary Information

The online version contains supplementary material available at 10.1186/s10194-022-01497-7.