Spinal AMPA receptors: Amenable players in central sensitization for chronic pain therapy?

Spinal nerve transection-induced upregulation of SAP97 via promoting membrane trafficking of GluA1-containing AMPA receptors in the dorsal horn contributes to the pathogenesis of neuropathic pain

Emerging evidence has implicated an important role of synapse-associated protein-97 (SAP97)-regulated GluA1-containing AMPARs membrane trafficking in cocaine restate and in contextual episodic memory of schizophrenia. Herein, we investigated the role of SAP97 in neuropathic pain following lumbar 5 spinal nerve transection (SNT) in rats. Our results showed that SNT led to upregulation of SAP97, enhanced the interaction between SAP97 and GluA1, and increased GluA1-containing AMPARs membrane trafficking in the dorsal horn. Microinjection of AAV-EGFP-SAP97 shRNA in lumbar 5 spinal dorsal horn inhibited SAP97 production, decreased SAP97-GluA1 interaction, reduced the membrane trafficking of GluA1-containing AMPARs, and partially attenuated neuropathic pain following SNT. Intrathecal injections of SAP97 siRNA or NASPM, an antagonist of GluA1-containing AMPARs, also partially reversed neuropathic pain on day 7, but not on day 14, after SNT. Spinal overexpression of SAP97 by AAV-EGFP-SAP97 enhanced SAP97-GluA1 interaction, increased the membrane insertion of GluA1-containing AMPARs, and induced abnormal pain in naïve rats. In addition, treatment with SAP97 siRNA or NASPM i.t. injection alleviated SNT-induced allodynia and hyperalgesia and exhibited a longer effect in female rats. Together, our results indicate that the SNT-induced upregulation of SAP97 via promoting GluA1-containing AMPARs membrane trafficking in the dorsal horn contributes to the pathogenesis of neuropathic pain. Targeting spinal SAP97 might be a promising therapeutic strategy to treatment of chronic pain.

Nerve Preparation and Recordings for Pharmacological Tests of Sensory and Nociceptive Fiber Conduction Ex Vivo

Measuring signal propagation through nerves is a classical electrophysiological technique established decades ago to evaluate sensory and motor functions in the nervous system. The whole-nerve preparation provides a valuable model to investigate nerve function ex vivo; however, it requires specific knowledge to ensure successful and stable measurements. Although the methodology for sciatic nerve recordings has long existed, a method for reliable and long-lasting recordings from myelinated and non-myelinated (nociceptive) fibers still needs to be adapted for pharmacological testing. This protocol takes benefits from epineurium sheath removal for pharmacological tests and provides a detailed description of how to make accurate nerve preparations, from the dissection and handling of nerves to epineurium cleaning, fabrication of adaptable suction electrodes for appropriate fiber stimulation and recordings, setting of electrophysiological protocols for compound action potential (CAP) recordings to distinguish between myelinated and non-myelinated (nociceptive) fibers, and finally to the analysis of the datasets of CAP components. We also demonstrate the feasibility of CAP recordings from individual branches in epineurium-free nerve preparations and provide clues to help retain nerve viability and maintain stable recordings over time. Although a sciatic nerve preparation was used here, the methodology can be applied to other nerve-type preparations. Key features • Detailed and simplified protocol for peripheral nerve preparation for recording sensory inputs ex vivo. • Recordings from myelinated and non-myelinated (nociceptive) fibers can be performed hours after nerve preparation. • The protocol involves the epineurium removal to facilitate drug permeability into nerve tissue for pharmacological tests. • The protocol allows physiological and pathological studies (pain/chronic pain conditions).

Whole Body Cryostimulation: A New Adjuvant Treatment in Central Sensitization Syndromes? An Expert Opinion

Central sensitisation is defined as a multifactorial etiopathogenetic condition involving an increase in the reactivity of nociceptive neurons and alterations in pain transmission and perception in the central nervous system. Patients may present with widespread chronic pain, fatigue, sleep disturbance, dizziness, psychological (e.g., depression, anxiety, and anger) and social impairment. Pain can be spontaneous in onset and persistence, characterised by an exaggerated response and spread beyond the site of origin, and sometimes triggered by a non-painful stimulus. Whole-body cryostimulation (WBC) could be an adjuvant therapy in the management of this type of pain because of its global anti-inflammatory effect, changes in cytokines and hormone secretion, reduction in nerve conduction velocity, autonomic modulation, and release of neurotransmitters involved in the pain pathway. In several conditions (e.g., fibromyalgia, rheumatoid arthritis, and chronic musculoskeletal pain), WBC affects physical performance, pain perception, and psychological aspects. Given its multiple targets and effects at different organs and levels, WBC appears to be a versatile adjuvant treatment for a wide range of conditions of rehabilitation interest. Further research is needed to fully understand the mechanisms of analgesic effect and potential actions on pain pathways, as well as to study long-term effects and potential uses in other chronic pain conditions.

[Tacrolimus Induces Pain by Upregulating the Synaptic Expression of AMPA Receptors in the Spinal Cord Dorsal Horn]

Objective

To investigate the effects of long-term administration of tacrolimus (also known as FK506) on the pain-related behaviors in mice and to study the underlying mechanism of pain induced by FK506 via measuring the effect of FK506 on the synaptic expression and phosphorylation of alpha-amino-3-hyroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor in the spinal cord dorsal horn of mice.

Methods

1) A total of 24 mice were evenly and randomly assigned to two groups, a FK506 group and a Saline group. The FK506 group was given daily intraperitoneal injection of FK506 and the Saline group received normal saline. Both groups received injection once a day for 7 days in a row. Some of the mice ( n=6 in each group) were monitored for the changes in the paw withdrawal threshold (PWT), the paw withdrawal latency (PWL), and the spontaneous pain behaviors to establish the pain model. The other mice ( n=6 in each group) of each group underwent isolation of the dorsal horn when obvious pain symptoms were induced on day 7 of injection. Then, immunoblotting was performed to determine the synaptic expression and phosphorylation levels of GluA1 and GluA2 subunits of AMPA receptors. 2) The mice were randomly divided into two groups, FK506+calcineurin (CaN) group and FK506+Saline group ( n=6 in each group). After the pain model was constructed, the mice were given intrathecal injection of recombinant CaN (also know as 33 U) or normal saline. Then, 60 minutes later, the PWT and the PWL of the mice were measured to investigate the role of CaN in FK506-induced pain. 3) Another18 mice were selected. The mice were randomly and evenly assigned to three groups, a control group (receiving intraperitoneal injection of normal saline followed by intrathecal injection of normal saline), FK506+Saline group (receiving intraperitoneal injection of FK506 followed by intrathecal injection of normal saline) and FK506+CaN group (receiving intraperitoneal injection of FK506 followed by intrathecal injection of CaN). Then, 60 minutes later, the spinal cords were isolated and subjected to immunoblotting assay to determine the role of CaN in FK506-induced AMPA receptor modification.

Results

1) After 7 consecutive days of intraperitoneal injection of FK506, the PWT and PWL of mice dropped significantly, reaching on day 7 as low as 22.3%±0.05% and 66.6%±0.05% of the control group, respectively ( P<0.01). The FK506-treated mice displayed evident spontaneous pain behavior, presenting significantly increased licking activities ( P<0.01). These results indicated that FK506-induced pain model was successfully established. Immunoblotting assay showed that the total expressions of GluA1 and GluA2 subunits in the spinal dorsal horn of the FK506 group remained unchanged in comparison with those of the Saline group. However, FK506 specifically induced an increase in the synaptic expression of GluA1. In addition, the phosphorylation levels of GluA1 at Ser845 and Ser831 in FK506-treated mice were significantly increased in comparison with those of the control group ( P<0.05). 2) Compared with those of the mice in the FK506+Saline group, the PWT and the PWL of mice in the FK506+CaN group were significantly increased ( P<0.05). 3) Compared with those of the FK506+Saline group, the synaptic expression of GluA1 were decreased in FK506+CaN group ( P<0.01) and the phosphorylation levels of GluA1 at Ser845 and Ser831 were significantly downregulated ( P<0.001).

Conclusion

The hyper-expression and hyperphosphorylation of GluA1 subunit in the spinal cord dorsal horn resulting from CaN inhibition contributes to the FK506-induced pain syndrome. FK506 induces the synaptic hyper-expression and hyperphosphorylation of GluA1 in the dorsal horn of the spinal cord through CaN inhibition, thereby inducing pain.

Presynaptic glutamate receptors in nociception

Chronic pain is a frequent, distressing and poorly understood health problem. Plasticity of synaptic transmission in the nociceptive pathways after inflammation or injury is assumed to be an important cellular basis for chronic, pathological pain. Glutamate serves as the main excitatory neurotransmitter at key synapses in the somatosensory nociceptive pathways, in which it acts on both ionotropic and metabotropic glutamate receptors. Although conventionally postsynaptic, compelling anatomical and physiological evidence demonstrates the presence of presynaptic glutamate receptors in the nociceptive pathways. Presynaptic glutamate receptors play crucial roles in nociceptive synaptic transmission and plasticity. They modulate presynaptic neurotransmitter release and synaptic plasticity, which in turn regulates pain sensitization. In this review, we summarize the latest understanding of the expression of presynaptic glutamate receptors in the nociceptive pathways, and how they contribute to nociceptive information processing and pain hypersensitivity associated with inflammation / injury. We uncover the cellular and molecular mechanisms of presynaptic glutamate receptors in shaping synaptic transmission and plasticity to mediate pain chronicity, which may provide therapeutic approaches for treatment of chronic pain.

NETO2-GluK2 interaction contributes to postoperative pain hypersensitivity through inducing PKCγ activation and synaptic incorporation of AMPA receptor GluR1 subunits in rat dorsal horn

Important roles in the initiation and maintenance of postoperative pain are played by the functional control of kainate (KA) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors in the rat dorsal horn (DH). However, the mechanisms underpinning the cross-talk between spinal KA and AMPA receptors in postoperative pain are poorly understood. We hypothesized that after the rat's plantar incision, the synaptic incorporation of AMPA receptor GluR1 subunits in the DH ipsilateral to the incision would increase due to the interaction between GluK2 and neuropilin tolloid-like 2 (NETO2). Our findings showed that incision stimuli caused severe pain responses, as measured by cumulative pain scores. GluK2-NETO2 but not GluK2-NETO1interaction was upregulated in ipsilateral dorsal horn neurons (DHNs) at 6 h post-incision. At 6 h post-incision, NETO2 small interfering ribonucleic acid (siRNA) intrathecal pretreatment increased mechanical withdrawal thresholds to von Freys and decreased ipsilateral paw cumulative pain scores. Further, PKCγactivation and synaptic abundance of GluK2 and GluR1 subunits in the ipsilateral DH were decreased by intrathecal pretreatment with NETO2 siRNA at 6 h post-incision. In conclusion, our findings imply that GluK2-NETO2 interaction could trigger PKCγactivation and the synaptic incorporation of AMPA receptor GluR1 subunits in rat DHs, which in turn led to the enhanced pain hypersensitivity after surgery. It sheds light on the interplay between KA and AMPA receptors in DHNs, which is thought to contribute to postoperative pain.

Electrophysiological Assessment of Newly Synthesized 2,3-Benzodiazepine Derivatives for Inhibiting the AMPA Receptor Channel

Three major subtypes of ionotropic receptors regulate glutamatergic synaptic transmission, one of which is α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPARs). They are tetrameric, cation-permeable ionotropic glutamate receptors found across the brain. Abnormalities in AMPA receptor trafficking and synaptic assembly are linked to cognitive decline and neurological diseases such as Alzheimer's, Parkinson's, and Huntington's. The present study will investigate the effects of four novel 2,3-benzodiazepine derivatives on AMPA receptor subunits by comparing their effects on synaptic responses, desensitization, and deactivation rate in human embryonic kidney cells (HEK293T) recombinant AMPAR subunits using whole-cell patch-clamp electrophysiology. All four 2,3-BDZ compounds showed inhibitory activity against all the homomeric and heteromeric subunits tested. While the desensitization and deactivation rates in 2,3-BDZ-1 and 2,3-BDZ-2 decreased and increased, respectively, in the other two compounds (i.e., 2,3-BDZ-3 and 2,3-BDZ-4), there was no change in the desensitization or deactivation rates. These results contribute to a better understanding of AMPARs by identifying potential 2,3-BDZ drugs that demonstrate inhibitory effects on the AMPAR subunits.

Pain in Huntington's disease and its potential mechanisms

Pain is common and frequent in many neurodegenerative diseases, although it has not received much attention. In Huntington's disease (HD), pain is often ignored and under-researched because attention is more focused on motor and cognitive decline than psychiatric symptoms. In HD progression, pain symptoms are complex and involved in multiple etiologies, particularly mental issues such as apathy, anxiety and irritability. Because of psychiatric issues, HD patients rarely complain of pain, although their bodies show severe pain symptoms, ultimately resulting in insufficient awareness and lack of research. In HD, few studies have focused on pain and pain-related features. A detailed and systemic pain history is crucial to assess and explore pain pathophysiology in HD. This review provides an overview concentrating on pain-related factors in HD, including neuropathology, frequency, features, affecting factors and mechanisms. More attention and studies are still needed in this interesting field in the future.

Diagnosis and Management of Neuropathic Pain in Spine Diseases

Neuropathic pain is generally defined as a non-physiological pain experience caused by damage to the nervous system. It can occur spontaneously, as a reaction to a given stimulus, or independently of its action, leading to unusual pain sensations usually referred to as firing, burning or throbbing. In the course of spine disorders, pain symptoms commonly occur. According to available epidemiological studies, a neuropathic component of pain is often present in patients with spinal diseases, with a frequency ranging from 36% to 55% of patients. Distinguishing between chronic nociceptive pain and neuropathic pain very often remains a challenge. Consequently, neuropathic pain is often underdiagnosed in patients with spinal diseases. In reference to current guidelines for the treatment of neuropathic pain, gabapentin, serotonin and norepinephrine reuptake inhibitors and tricyclic antidepressants constitute first-line therapeutic agents. However, long-term pharmacologic treatment often leads to developing tolerance and resistance to used medications. Therefore, in recent years, a plethora of therapeutic methods for neuropathic pain have been developed and investigated to improve clinical outcomes. In this review, we briefly summarized current knowledge about the pathophysiology and diagnosis of neuropathic pain. Moreover, we described the most effective treatment approaches for neuropathic pain and discussed their relevance in the treatment of spinal pain.

Ca2+-Permeable AMPA Receptors Contribute to Changed Dorsal Horn Neuronal Firing and Inflammatory Pain

The dorsal horn (DH) neurons of the spinal cord play a critical role in nociceptive input integration and processing in the central nervous system. Engaged neuronal classes and cell-specific excitability shape nociceptive computation within the DH. The DH hyperexcitability (central sensitisation) has been considered a fundamental mechanism in mediating nociceptive hypersensitivity, with the proven role of Ca²⁺-permeable AMPA receptors (AMPARs). However, whether and how the DH hyperexcitability relates to changes in action potential (AP) parameters in DH neurons and if Ca²⁺-permeable AMPARs contribute to these changes remain unknown. We examined the cell-class heterogeneity of APs generated by DH neurons in inflammatory pain conditions to address these. Inflammatory-induced peripheral hypersensitivity increased DH neuronal excitability. We found changes in the AP threshold and amplitude but not kinetics (spike waveform) in DH neurons generating sustained or initial bursts of firing patterns. In contrast, there were no changes in AP parameters in the DH neurons displaying a single spike firing pattern. Genetic knockdown of the molecular mechanism responsible for the upregulation of Ca²⁺-permeable AMPARs allowed the recovery of cell-specific AP changes in peripheral inflammation. Selective inhibition of Ca²⁺-permeable AMPARs in the spinal cord alleviated nociceptive hypersensitivity, both thermal and mechanical modalities, in animals with peripheral inflammation. Thus, Ca²⁺-permeable AMPARs contribute to shaping APs in DH neurons and nociceptive hypersensitivity. This may represent a neuropathological mechanism in the DH circuits, leading to aberrant signal transfer to other nociceptive pathways.

The Scaffold Protein PICK1 as a Target in Chronic Pain

Well-tolerated and effective drugs for treating chronic pain conditions are urgently needed. Most chronic pain patients are not effectively relieved from their pain and suffer from debilitating drug side effects. This has not only drastic negative consequences for the patients’ quality of life, but also constitute an enormous burden on society. It is therefore of great interest to explore new potent targets for effective pain treatment with fewer side effects and without addiction liability. A critical component of chronic pain conditions is central sensitization, which involves the reorganization and strengthening of synaptic transmission within nociceptive pathways. Such changes are considered as maladaptive and depend on changes in the surface expression and signaling of AMPA-type glutamate receptors (AMPARs). The PDZ-domain scaffold protein PICK1 binds the AMPARs and has been suggested to play a key role in these maladaptive changes. In the present paper, we review the regulation of AMPARs by PICK1 and its relation to pain pathology. Moreover, we highlight other pain-relevant PICK1 interactions, and we evaluate various compounds that target PICK1 and have been successfully tested in pain models. Finally, we evaluate the potential on-target side effects of interfering with the action of PICK1 action in CNS and beyond. We conclude that PICK1 constitutes a valid drug target for the treatment of inflammatory and neuropathic pain conditions without the side effects and abuse liability associated with current pain medication.

Conditional knockout of CRMP2 in neurons, but not astrocytes, disrupts spinal nociceptive neurotransmission to control the initiation and maintenance of chronic neuropathic pain

ABSTRACT

Mechanistic studies principally focusing on primary afferent nociceptive neurons uncovered the upregulation of collapsin response mediator protein 2 (CRMP2)-a dual trafficking regulator of N-type voltage-gated calcium (Cav2.2) as well as Nav1.7 voltage-gated sodium channels-as a potential determinant of neuropathic pain. Whether CRMP2 contributes to aberrant excitatory synaptic transmission underlying neuropathic pain processing after peripheral nerve injury is unknown. Here, we interrogated CRMP2's role in synaptic transmission and in the initiation or maintenance of chronic pain. In rats, short-interfering RNA-mediated knockdown of CRMP2 in the spinal cord reduced the frequency and amplitude of spontaneous excitatory postsynaptic currents, but not spontaneous inhibitory postsynaptic currents, recorded from superficial dorsal horn neurons in acute spinal cord slices. No effect was observed on miniature excitatory postsynaptic currents and inhibitory postsynaptic currents. In a complementary targeted approach, conditional knockout of CRMP2 from mouse neurons using a calcium/calmodulin-dependent protein kinase II alpha promoter to drive Cre recombinase expression reduced the frequency and amplitude of spontaneous excitatory postsynaptic currents, but not miniature excitatory SCss. Conditional knockout of CRMP2 from mouse astrocytes using a glial fibrillary acidic protein promoter had no effect on synaptic transmission. Conditional knockout of CRMP2 in neurons reversed established mechanical allodynia induced by a spared nerve injury in both male and female mice. In addition, the development of spared nerve injury-induced allodynia was also prevented in these mice. Our data strongly suggest that CRMP2 is a key regulator of glutamatergic neurotransmission driving pain signaling and that it contributes to the transition of physiological pain into pathological pain.

Presynaptic AMPA Receptors in Health and Disease

AMPA receptors (AMPARs) are ionotropic glutamate receptors that play a major role in excitatory neurotransmission. AMPARs are located at both presynaptic and postsynaptic plasma membranes. A huge number of studies investigated the role of postsynaptic AMPARs in the normal and abnormal functioning of the mammalian central nervous system (CNS). These studies highlighted that changes in the functional properties or abundance of postsynaptic AMPARs are major mechanisms underlying synaptic plasticity phenomena, providing molecular explanations for the processes of learning and memory. Conversely, the role of AMPARs at presynaptic terminals is as yet poorly clarified. Accruing evidence demonstrates that presynaptic AMPARs can modulate the release of various neurotransmitters. Recent studies also suggest that presynaptic AMPARs may possess double ionotropic-metabotropic features and that they are involved in the local regulation of actin dynamics in both dendritic and axonal compartments. In addition, evidence suggests a key role of presynaptic AMPARs in axonal pathology, in regulation of pain transmission and in the physiology of the auditory system. Thus, it appears that presynaptic AMPARs play an important modulatory role in nerve terminal activity, making them attractive as novel pharmacological targets for a variety of pathological conditions.

Deciphering the mechanisms of regulation of an excitatory synapse via cyclooxygenase-2. A review

Cyclooxygenase (COX) is a heme-containing enzyme that produces prostaglandins (PGs) via a pathway known as the arachidonic acid (AA) cascade. Two isoforms of COX enzyme (COX-1 and COX-2) and splice variant (COX-3) have been described so far. COX-2 is a neuronal enzyme that is intensively produced during activation of the synapse and glutamate (Glu) release. The end product of COX-2 action, prostaglandin E2 (PGE2), regulates Glu level in a retrograde manner. At the same time, the level of Glu, the primary excitatory neurotransmitter, is regulated in the excitatory synapse via Glu receptors, both ionotropic and metabotropic ones. Glu receptors are known modulators of behavior, engaged in cognition and mood. So far, the interaction between ionotropic N-methyl-D-aspartate (NMDA) receptors or metabotropic glutamate (mGluRs) receptors and COX-2 was found. Here, based on literature data and own research, a new mechanism of action of COX-2 in an excitatory synapse will be presented.