Peripheral group II and III metabotropic glutamate receptors in the knee joint attenuate carrageenan-induced nociceptive behavior in rats

Red nucleus mGluR2 but not mGluR3 mediates inhibitory effect in the development of SNI-induced neuropathological pain by suppressing the expressions of TNF-α and IL-1β

Our previous study has verified that activation of group Ⅰ metabotropic glutamate receptors (mGluRⅠ) in the red nucleus (RN) facilitate the development of neuropathological pain. Here, we further discussed the functions and possible molecular mechanisms of red nucleus mGluR Ⅱ (mGluR2 and mGluR3) in the development of neuropathological pain induced by spared nerve injury (SNI). Our results showed that mGluR2 and mGluR3 both were constitutively expressed in the RN of normal rats. At 2 weeks post-SNI, the protein expression of mGluR2 rather than mGluR3 was significantly reduced in the RN contralateral to the nerve lesion. Injection of mGluR2/3 agonist LY379268 into the RN contralateral to the nerve injury at 2 weeks post-SNI significantly attenuated SNI-induced neuropathological pain, this effect was reversed by mGluR2/3 antagonist EGLU instead of selective mGluR3 antagonist β-NAAG. Intrarubral injection of LY379268 did not alter the PWT of contralateral hindpaw in normal rats, while intrarubral injection of EGLU rather than β-NAAG provoked a significant mechanical allodynia. Further studies indicated that the expressions of nociceptive factors TNF-α and IL-1β in the RN were enhanced at 2 weeks post-SNI. Intrarubral injection of LY379268 at 2 weeks post-SNI significantly suppressed the overexpressions of TNF-α and IL-1β, these effects were reversed by EGLU instead of β-NAAG. Intrarubral injection of LY379268 did not influence the protein expressions of TNF-α and IL-1β in normal rats, while intrarubral injection of EGLU rather than β-NAAG significantly boosted the expressions of TNF-α and IL-1β. These findings suggest that red nucleus mGluR2 but not mGluR3 mediates inhibitory effect in the development of SNI-induced neuropathological pain by suppressing the expressions of TNF-α and IL-1β. mGluR Ⅱ may be potential targets for drug development and clinical treatment of neuropathological pain.

Recent Advances in the Modulation of Pain by the Metabotropic Glutamate Receptors

Metabotropic glutamate receptors (mGluR or mGlu) are G-protein coupled receptors activated by the binding of glutamate, the main classical neurotransmitter of the nervous system. Eight different mGluR subtypes (mGluR1-8) have been cloned and are classified in three groups based on their molecular, pharmacological and signaling properties. mGluRs mediate several physiological functions such as neuronal excitability and synaptic plasticity, but they have also been implicated in numerous pathological conditions including pain. The availability of new and more selective allosteric modulators together with the canonical orthosteric ligands and transgenic technologies has led to significant advances in our knowledge about the role of the specific mGluR subtypes in the pathophysiological mechanisms of various diseases. Although development of successful compounds acting on mGluRs for clinical use has been scarce, the subtype-specific-pharmacological manipulation might be a compelling approach for the treatment of several disorders in humans, including pain; this review aims to summarize and update on preclinical evidence for the roles of different mGluRs in the pain system and discusses knowledge gaps regarding mGluR-related sex differences and neuroimmune signaling in pain.

Development of a novel model of intervertebral disc degeneration by the intradiscal application of monosodium iodoacetate (MIA) in rat

BACKGROUND CONTEXT

Low back pain is one of the most common musculoskeletal disorders. Although, the pathology of intervertebral disc (IVD) degeneration has been modeled using various biological methods, these models are inadequate for simulating similar pathologic states in humans.

PURPOSE

This study investigated whether monosodium iodoacetate (MIA) injection into the IVD of rats could generate a reliable model of IVD degeneration.

STUDY DESIGN/SETTINGS

In vivo animal study.

METHODS

MIA was injected into two-disc spaces (L4-5 and L5-6) of Sprague-Dawley rats. Their behaviors were examined by measuring weight load shifts from hind to forefoot, rearing, and von Frey tests. We examined the inhibition of pain behavior through intraperitoneal morphine injection and measured cyclooxygenase-2 (COX-2) and transcription factor nuclear factor-kappa B (NF-κB) levels in the IVD and dorsal root ganglion (DRG) by Western blot. Bone alterations were assessed by microfocus computed tomography (micro-CT), and IVD and/or cartilage changes were evaluated by hematoxylin and eosin and safranin-O staining and inducible nitric oxide synthase (iNOS) immunohistochemistry. The other authors declare no conflicts of interest. This project funded by the Memorial Fund and the National Research Foundation of Korea (NRF).

RESULTS

We observed increased weight load shifts to the forefoot and decreased rearing. Morphine-injected rats showed reduced pain. NF-κB and COX-2 expression increased in the IVD and left and/or right DRG. Micro-CT analyses suggested progressive bone deformation. Histologic examination showed decreased IVD width and nucleus pulposus area. Cartilaginous changes indicated epiphyseal growth plate loss. Finally, iNOS expression was increased in the subchondral endplate.

CONCLUSIONS

These results suggest that low back pain (LBP) models can be developed by MIA injection into the IVDs of rats and that an animal model is useful for exploring degenerative alterations in the affected discs. Therefore, MIA injection may be a useful model for the study of changes in the IVD to elucidate the mechanisms underlying clinical symptoms, such as LBP, in patients with IVD degeneration.

CLINICAL SIGNIFICANCE

This model in which MIA was injected into the disc better represented the human histologic and behavioral characteristics than the existing puncture model.

Activation of peripheral group III metabotropic glutamate receptors suppressed formalin-induced nociception

Intraplantar injection of formalin produces persistent spontaneous nociception and hyperalgesia. The underlying mechanism, however, remains unclear. The present study was, therefore, designed to determine the roles of peripheral group III metabotropic glutamate receptors (mGluRs) in formalin-evoked spontaneous nociception. Pre-treatment with intraplantar injections of L-serine-O-phosphate (L-SOP), a group III mGluRs agonist, significantly inhibited formalin-induced nociceptive behaviours and decreased Fos production in the spinal dorsal horn. The inhibitory effects of L-SOP were abolished completely by pre-treatment with the group III mGluR antagonist (RS)-a-methylserine-O-phosphate (M-SOP). These data suggest that the activation of group III mGluRs in the periphery may play a differential role in formalin-induced nociception. In addition, L-SOP decreased the formalin-induced upregulation of tumour necrosis factor-α (TNF-α) as well as interleukine-1β (IL-1β) expression in the spinal cord, suggesting that activation of peripheral group III mGluRs reduces formalin-induced nociception through inhibition of the pro-inflammatory cytokines in the spinal cord. Therefore, the agonists acting peripheral group III mGluRs possess therapeutic effectiveness in chronic pain.

Emerging role of glutamate in the pathophysiology and therapeutics of Gulf War illness

Gulf War illness (GWI) is a chronic and multi-symptomatic disorder affecting veterans who served in the Gulf War. The commonly reported symptoms in GWI veterans include mood problems, cognitive impairment, muscle and joint pain, migraine/headache, chronic fatigue, gastrointestinal complaints, skin rashes, and respiratory problems. Neuroimaging studies have revealed significant brain structure alterations in GWI veterans, including subcortical atrophy, decreased volume of the hippocampus, reduced total grey and white matter, and increased brain white matter axial diffusivity. These brain changes may contribute to or increase the severities of the GWI-related symptoms. Epidemiological studies have revealed that neurotoxic exposures and stress may be significant contributors to the development of GWI. However, the mechanism underlying how the exposure and stress could contribute to the multi-symptomatic disorder of GWI remains unclear. We and others have demonstrated that rodent models exposed to GW-related agents and stress exhibited higher extracellular glutamate levels, as well as impaired structure and function of glutamatergic synapses. Restoration of the glutamatergic synapses ameliorated the GWI-related pathological and behavioral deficits. Moreover, recent studies showed that a low-glutamate diet reduced multiple symptoms in GWI veterans, suggesting an important role of the glutamatergic system in GWI. Currently, growing evidence has indicated that abnormal glutamate neurotransmission may contribute to the GWI symptoms. This review summarizes the potential roles of glutamate dyshomeostasis and dysfunction of the glutamatergic system in linking the initial cause to the multi-symptomatic outcomes in GWI and suggests the glutamatergic system as a therapeutic target for GWI.

Disc degeneration induces a mechano-sensitization of disc afferent nerve fibers that associates with low back pain

OBJECTIVE

We aimed to investigate mechano-sensitivity at the afferent nerve fibers projecting to degenerated intervertebral disc (IVD) and nociceptive behaviour in a rat model of low back pain (LBP).

DESIGN

Animal model with LBP was established by lumbar 4/5 IVD puncture and nucleus pulposus aspiration. In vivo single nerve recordings (n = 121) were introduced to measure discharge frequency at the afferent nerve fiber innervating the IVD during mechanical stimulations (von Frey filament or intradiscal pressure). Nerve growth factor (NGF) expression levels in the IVD (n = 20) were assessed by Western blot. LBP-related behaviour (n = 22) was assessed by measuring changes in rearing, mechanical paw-withdrawal threshold, and dynamic weight bearing in a freely walking rat. Inhibitory effect of morphine on the neuronal excitability (n = 19) and painful behaviour (n = 28) was also assessed.

RESULTS

Compared to those with sham or naïve IVD, animal group with degenerated IVD displayed the sensitized neuronal responses and painful behaviour, with hyperexcitability of the afferent nerve fibers in any range of mechanical stimulations (von Frey filament stimulation; 1, 2, and 26 g; intradiscal pressure, 1,500-3,000 mm Hg), strong upregulation of NGF (200-250 % increase), and LBP-like behaviour such as failure of rearing, front limbs-dependent walking pattern, and hypersensitivity in hind-paws. However, the neuronal hyperexcitability and pain behaviour were attenuated after local (30 μM) or systemic (3 mg kg^-1) morphine administration.

CONCLUSIONS

Our study suggests that enhanced mechano-sensitivity at the afferent nerve fiber innervating degenerated IVD is deeply correlated with LBP development, which supports the hypothesis that hyperexcited responses at the nerve fibers represent a decisive source of LBP.

Role of Metabotropic Glutamate Receptors in Neurological Disorders

Glutamate is a fundamental excitatory neurotransmitter in the mammalian central nervous system (CNS), playing key roles in memory, neuronal development, and synaptic plasticity. Moreover, excessive glutamate release has been implicated in neuronal cell death. There are both ionotropic and metabotropic glutamate receptors (mGluRs), the latter of which can be divided into eight subtypes and three subgroups based on homology sequence and their effects on cell signaling. Indeed, mGluRs exert fine control over glutamate activity by stimulating several cell-signaling pathways via the activation of G protein-coupled (GPC) or G protein-independent cell signaling. The involvement of specific mGluRs in different forms of synaptic plasticity suggests that modulation of mGluRs may aid in the treatment of cognitive impairments related to several neurodevelopmental/psychiatric disorders and neurodegenerative diseases, which are associated with a high economic and social burden. Preclinical and clinical data have shown that, in the CNS, mGluRs are able to modulate presynaptic neurotransmission by fine-tuning neuronal firing and neurotransmitter release in a dynamic, activity-dependent manner. Current studies on drugs that target mGluRs have identified promising, innovative pharmacological tools for the treatment of neurodegenerative and neuropsychiatric conditions, including chronic pain.

Emerging Trends in Pain Modulation by Metabotropic Glutamate Receptors

Pain is an essential protective mechanism meant to prevent tissue damages in organisms. On the other hand, chronic or persistent pain caused, for example, by inflammation or nerve injury is long lasting and responsible for long-term disability in patients. Therefore, chronic pain and its management represents a major public health problem. Hence, it is critical to better understand chronic pain molecular mechanisms to develop innovative and efficient drugs. Over the past decades, accumulating evidence has demonstrated a pivotal role of glutamate in pain sensation and transmission, supporting glutamate receptors as promising potential targets for pain relieving drug development. Glutamate is the most abundant excitatory neurotransmitter in the brain. Once released into the synapse, glutamate acts through ionotropic glutamate receptors (iGluRs), which are ligand-gated ion channels triggering fast excitatory neurotransmission, and metabotropic glutamate receptors (mGluRs), which are G protein-coupled receptors modulating synaptic transmission. Eight mGluRs subtypes have been identified and are divided into three classes based on their sequence similarities and their pharmacological and biochemical properties. Of note, all mGluR subtypes (except mGlu6 receptor) are expressed within the nociceptive pathways where they modulate pain transmission. This review will address the role of mGluRs in acute and persistent pain processing and emerging pharmacotherapies for pain management.

Apoptotic changes in a full-lengthened immobilization model of rat soleus muscle

INTRODUCTION

Lengthened immobilization may prevent muscle shortening, and help maintain normal muscle length. However, its apoptotic effects remain unclear. We evaluated the effects of long-term immobilization on apoptotic proteins.

METHODS

Rat soleus muscles were immobilized by casting in a neutral (NEUT) or lengthened (LENG) position for 21 days. We evaluated dynamic weight load and muscle atrophy following the 21-day period using hematoxylin and eosin staining. We measured Bax (pro-apoptotic Bcl-2 family member), MyoD (myogenic differentiation factor D), MYH (myosin heavy chain), and cleaved poly(ADP-ribose)polymerase levels and examined apoptotic nucleus expression.

RESULTS

Decreased dynamic weight load and muscle atrophy changes were observed in LENG. Both NEUT and LENG showed significantly reduced levels of MYH. LENG showed a significant increase in Bax and MyoD expression as well as in the number of apoptotic nuclei.

CONCLUSIONS

Long-term lengthened immobilization may increase apoptotic changes and decrease muscle formation proteins in muscle. Muscle Nerve 59:263-269, 2019.

Synovial membrane receptors as therapeutic targets: A review of receptor localization, structure, and function

Joint pathology and degeneration is a significant cause of pain. The synovial membrane plays an important role in maintenance of the joint, contributes to the pathology of many arthropathies and may be adversely affected in joint disease. Improving knowledge of the receptors present within the synovium will aid in a better understanding of joint pathology and the development of new treatments for diseases such as osteoarthritis and rheumatoid arthritis. Knowledge of the location and function of synovial membrane receptors (both in healthy and diseased synovium) may provide important targets in the treatment of various arthropathies. Classic pain receptors such as opioid receptors in the synovium are a mainstay in local and systemic management of chronic pain in many species. In addition to these, many other receptors such as bradykinin, neurokinin, transient receptor potential vanilloid, and inflammatory receptors, such as prostanoid and interleukin receptors have been discovered within the synovial membrane. These receptors are important in pain, inflammation, and in maintenance of normal joint function and may serve as targets for pharmacologic intervention in pathologic states. The goal of this review is to outline synovial membrane receptor localization and local therapeutic modulation of these receptors, in order to stimulate further research into pharmacological management of arthropathies at the local level. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1589-1605, 2017.

The effects of Chamaecyparis obtusa essential oil on pain-related behavior and expression of pro-inflammatory cytokines in carrageenan-induced arthritis in rats

Chamaecyparis obtusa essential oil (COE) has been widely used to treat allergic diseases and was suggested to exert anti-inflammatory, antioxidant, and antimicrobial effects. This study evaluated the effects of COE on pain-related behavior and pro-inflammatory cytokines in rats with carrageenan (CGN)-induced arthritis. Reduced dynamic weight load on inflamed joint in voluntarily walking rats was used as the behavior test for arthritic pain; 10% COE-treated group was significantly attenuated pain (6–8 h post-CGN injection) compared to VEH (mineral oil)-treated group. In addition, the protein levels of interleukin (IL)-1β, tumor necrosis factor-α, IL-6 (6–8 h), and cyclooxygenase (COX)-2 (8 h) within the synovial membrane, as well as IL-1β, COX-2 (6–8 h), and IL-6 (5–7 h) within the meniscus, of 10% COE-treated group were significantly reduced. The current results implicate that COE has anti-inflammatory and anti-nociceptive effects on arthritis in rats.

Up-regulation of Glutamate in Painful Human Supraspinatus Tendon Tears

BACKGROUND

Pain related to rotator cuff tendinopathy is a common problem, but little is known regarding the origin and cause of pain from the tendon substance. No study to date has looked at the association between tissue changes and patient outcomes.

PURPOSE

To describe the peripheral neuronal phenotype in painful rotator cuff tears and to determine correlations between tissue changes and clinical outcome measures.

STUDY DESIGN

Controlled laboratory study.

METHODS

Tissue samples of the supraspinatus were taken from patients undergoing surgery to repair a rotator cuff tendon tear. Patients were classified as having small/medium or large/massive tears. Control tissue was obtained from patients undergoing surgery for posttraumatic shoulder instability. Immunohistochemical techniques were performed using antibodies to known nociceptive and neuronal markers as well as general tissue structural markers.

RESULTS

There was no correlation between tissue changes and patient-reported outcomes. A significant increase in the expression of glutamate was seen in tendon tears. There were differences in the expression of metabotropic and ionotropic glutamate receptors. Expression changes were also observed for markers of the sensory and autonomic systems; however, no differences were found in neurotrophins.

CONCLUSION

Glutamate and the glutaminergic system play a key role in painful human tendon tears; however, the exact role is still uncertain, as glutamate is highly involved in both pain and metabolic pathways.

CLINICAL RELEVANCE

This study has identified a number of markers that could be potential therapeutic targets.

Progress toward advanced understanding of metabotropic glutamate receptors: structure, signaling and therapeutic indications

The metabotropic glutamate (mGlu) receptors are a group of Class C seven-transmembrane spanning/G protein-coupled receptors (7TMRs/GPCRs). These receptors are activated by glutamate, one of the standard amino acids and the major excitatory neurotransmitter. By activating G protein-dependent and non-G protein-dependent signaling pathways, mGlus modulate glutamatergic transmission both in the periphery and throughout the central nervous system. Since the discovery of the first mGlu receptor, and especially during the last decade, a great deal of progress has been made in understanding the signaling, structure, pharmacological manipulation and therapeutic indications of the 8 mGlu members.