The glial-neuronal GRK2 pathway participates in the development of trigeminal neuropathic pain in rats

Connecting nutritional deprivation and pubertal inhibition via GRK2-mediated repression of kisspeptin actions in GnRH neurons

BACKGROUND

Perturbations in the timing of puberty, with potential adverse consequences in later health, are increasingly common. The underlying neurohormonal mechanisms are unfolded, but nutritional alterations are key contributors. Efforts to unveil the basis of normal puberty and its metabolic control have focused on mechanisms controlling expression of Kiss1, the gene encoding the puberty-activating neuropeptide, kisspeptin. However, other regulatory phenomena remain ill-defined. Here, we address the putative role of the G protein-coupled-receptor kinase-2, GRK2, in GnRH neurons, as modulator of pubertal timing via repression of the actions of kisspeptin, in normal maturation and conditions of nutritional deficiency.

METHODS

Hypothalamic RNA and protein expression analyses were conducted in maturing female rats. Pharmacological studies involved central administration of GRK2 inhibitor, βARK1-I, and assessment of gonadotropin responses to kisspeptin or phenotypic and hormonal markers of puberty, under normal nutrition or early subnutrition in female rats. In addition, a mouse line with selective ablation of GRK2 in GnRH neurons, aka G-GRKO, was generated, in which hormonal responses to kisspeptin and puberty onset were monitored, in normal conditions and after nutritional deprivation.

RESULTS

Hypothalamic GRK2 expression increased along postnatal maturation in female rats, especially in the preoptic area, where most GnRH neurons reside, but decreased during the juvenile-to-pubertal transition. Blockade of GRK2 activity enhanced Ca+2 responses to kisspeptin in vitro, while central inhibition of GRK2 in vivo augmented gonadotropin responses to kisspeptin and advanced puberty onset. Postnatal undernutrition increased hypothalamic GRK2 expression and delayed puberty onset, the latter being partially reversed by central GRK2 inhibition. Conditional ablation of GRK2 in GnRH neurons enhanced gonadotropin responses to kisspeptin, accelerated puberty onset, and increased LH pulse frequency, while partially prevented the negative impact of subnutrition on pubertal timing and LH pulsatility in mice.

CONCLUSIONS

Our data disclose a novel pathway whereby GRK2 negatively regulates kisspeptin actions in GnRH neurons, as major regulatory mechanism for tuning pubertal timing in nutritionally-compromised conditions.

Spinal Neuronal GRK2 Contributes to Preventive Effect by Electroacupuncture on Cisplatin-Induced Peripheral Neuropathy in Mice

The main symptoms of chemotherapy-induced peripheral neuropathy (CIPN) include pain and numbness. Neuronal G protein–coupled receptor kinase 2 (GRK2) plays an important role in various pain models. Cisplatin treatment can induce the activation of proinflammatory microglia in spinal cord. The purpose of this study was to investigate the role of spinal neuronal GRK2 in cisplatin-induced CIPN and in the prevention of CIPN by electroacupuncture (EA).

Potential Molecular Targets for Treating Neuropathic Orofacial Pain Based on Current Findings in Animal Models

This review highlights potential molecular targets for treating neuropathic orofacial pain based on current findings in animal models. Preclinical research is currently elucidating the pathophysiology of the disease and identifying the molecular targets for better therapies using animal models that mimic this category of orofacial pain, especially post-traumatic trigeminal neuropathic pain (PTNP) and primary trigeminal neuralgia (PTN). Animal models of PTNP and PTN simulate their etiologies, that is, trauma to the trigeminal nerve branch and compression of the trigeminal root entry zone, respectively. Investigations in these animal models have suggested that biological processes, including inflammation, enhanced neuropeptide-mediated pain signal transmission, axonal ectopic discharges, and enhancement of interactions between neurons and glial cells in the trigeminal pathway, are underlying orofacial pain phenotypes. The molecules associated with biological processes, whose expressions are substantially altered following trigeminal nerve damage or compression of the trigeminal nerve root, are potentially involved in the generation and/or exacerbation of neuropathic orofacial pain and can be potential molecular targets for the discovery of better therapies. Application of therapeutic candidates, which act on the molecular targets and modulate biological processes, attenuates pain-associated behaviors in animal models. Such therapeutic candidates including calcitonin gene-related peptide receptor antagonists that have a reasonable mechanism for ameliorating neuropathic orofacial pain and meet the requirements for safe administration to humans seem worth to be evaluated in clinical trials. Such prospective translation of the efficacy of therapeutic candidates from animal models to human patients would help develop better therapies for neuropathic orofacial pain.

The role of G-protein-coupled receptor kinase 2 in diabetic mechanical hyperalgesia in rats

BACKGROUND

Previous studies have indicated a negative correlation between GRK2 expression and pain development and transmission. Here, we investigated whether G-protein-coupled receptor kinase 2 (GRK2) was involved in regulating diabetic mechanical hyperalgesia (DMH).

METHODS

The adeno-associated viral vectors containing the GRK2 gene (AAV-GRK2) were used to up-regulate GRK2 protein expression. The expression of GRK2 and exchange protein directly activated by cyclic adenosine monophosphate 1 (Epac1) in the dorsal root ganglion (DRG) of lumbar 4-6 was detected via immunoblotting and immunohistochemistry, and the transfection of the GRK2 gene was detected by immunofluorescence.

RESULTS

Low levels of GRK2 were able to sustain STZ-induced pain in DMH rats. Intrathecal injection of AAV-GRK2 vector up-regulated GRK2 expression, providing pain rain to rats with DMH. With an increase in DMH duration, there was a decrease in paw withdrawal threshold (PWT) value, aggravating the pain, resulting in a decreasing pattern in GRK2 protein expression over time, whereas Epac1 protein expression showed an opposite trend.

CONCLUSION

GRK2 expression regulated DMH progression and is expected to play a role in the development of targeted therapy for DMH. GRK2 and Epac1 expressions play a vital role in maintaining pain in DMH rats.

Central VEGF-A pathway plays a key role in the development of trigeminal neuropathic pain in rats

The study reported here investigated the role of the central vascular endothelial growth factor-A (VEGF-A) pathway in the development of trigeminal neuropathic pain following nerve injury. A Sprague-Dawley rat model of trigeminal neuropathic pain was produced using malpositioned dental implants. The left mandibular second molar was extracted under anesthesia and replaced with a miniature dental implant to induce injury to the inferior alveolar nerve. The inferior alveolar nerve injury produced a significant upregulation of astrocytic VEGF-A expression in the medullary dorsal horn. The nerve injury-induced mechanical allodynia was inhibited by an intracisternal infusion of VEGF-A₁₆₄ antibody. Although both VEGF-A Receptor 1 (VEGF-A R1; colocalized with the blood–brain barrier) and VEGF-A Receptor 2 (VEGF-A R2; colocalized with astrocytes) participated in the development of trigeminal neuropathic pain following nerve injury, only the intracisternal infusion of a VEGF-A R1 antibody, and not that of a VEGF-A R2 antibody, inhibited the increased blood–brain barrier permeability produced by nerve injury. Finally, we confirmed the participation of the central VEGF-A pathway in the development of trigeminal neuropathic pain by reducing VEGF-A expression using VEGF-A₁₆₄ siRNA. This suppression of VEGF-A produced significant prolonged anti-allodynic effects. These results suggest that the central VEGF-A pathway plays a key role in the development of trigeminal neuropathic pain following nerve injury through two separate pathways: VEGF-A R1 and VEGF-A R2. Hence, a blockade of the central VEGF-A pathway provides a new therapeutic avenue for the treatment of trigeminal neuropathic pain.

Early Blockade of EphA4 Pathway Reduces Trigeminal Neuropathic Pain

Background

Although the Eph receptor plays an important role in the development of neuropathic pain following nerve injury, there has been no evidence of the participation of the ephrin A4 receptor (EphA4) in the development of trigeminal neuropathic pain. The present study investigated the role of EphA4 in central nociceptive processing in rats with inferior alveolar nerve injury.

Materials and Methods

Male Sprague-Dawley rats were used in all our experiments. A rat model for trigeminal neuropathic pain was produced using malpositioned dental implants. The left mandibular second molar was extracted under anesthesia, followed by the placement of a miniature dental implant to injure the inferior alveolar nerve.

Results

Our current findings show that nerve injury induced by malpositioned dental implants evokes significant mechanical allodynia and up-regulation of EphA4 expression in the ipsilateral trigeminal subnucleus caudalis. Although daily treatment with EphA4-Fc, an EphA4 antagonist, did not produce prolonged anti-allodynic effects after the chronic neuropathic pain had been already established, an early treatment protocol with repeated EphA4-Fc administration significantly attenuated mechanical allodynia before initiation of chronic neuropathic pain. Finally, we confirmed the participation of the central EphA4 pathway in the development of trigeminal neuropathic pain by reducing EphA4 expression using EphA4 siRNA. This suppression of EphA4 produced significantly prolonged anti-allodynic effects.

Conclusion

These results suggest that early blockade of central EphA4 signaling provides a new therapeutic target for the treatment of trigeminal neuropathic pain.

Se@SiO2 nanocomposites suppress microglia-mediated reactive oxygen species during spinal cord injury in rats

Selenium (Se) is an essential trace element with strong antioxidant activity, showing a great prospect in the treatment of spinal cord injury (SCI). However, the narrow gap between the beneficial and toxic effects has limited its further clinical application. In this experiment, we used porous Se@SiO2 nanocomposites (Se@SiO2) modified by nanotechnology as a new means of release control to investigate the anti-oxidative effect in SCI. In vitro Se@SiO2 toxicity, anti-oxidative and anti-inflammatory effects on microglia were assayed. In vivo we investigated the protective effect of Se@SiO2 to SCI rats. Neurological function was evaluated by Basso, Beattie and Bresnahan (BBB). The histopathological analysis, microglia activation, oxidative stress, inflammatory factors (TNF-α, IL-1β and IL-6) and apoptosis were detected at 3 and 14 days after SCI. The favorable biocompatibility of Se@SiO2 suppressed microglia activation, which is known to be associated with oxidative stress and inflammation in vivo and in vitro. In addition, Se@SiO2 improved the rat neurological function and reduced apoptosis via caspase-3, Bax and Bcl-2 pathways in SCI. Se@SiO2 was able to treat SCI and reduce oxidative stress, inflammation and apoptosis induced by microglia activation, which may provide a novel and safe strategy for clinical application.

A role for the purinergic receptor P2X3 in astrocytes in the mechanism of craniofacial neuropathic pain

The purinergic receptor P2X₃, expressed in the central terminals of primary nociceptive neurons in the brainstem, plays an important role in pathological pain. However, little is known about expression of P2X₃ in the brainstem astrocytes and its involvement in craniofacial pathologic pain. To address this issue, we investigated the expression of P2X₃ in astrocytes in the trigeminal caudal nucleus (Vc) in a rat model of craniofacial neuropathic pain, chronic constriction injury of infraorbital nerve (CCI-ION). We found that 1) P2X₃-immunoreactivity is observed in the brainstem astrocytes, preferentially in their fine processes, 2) the number of P2X₃-positive fine astrocytic processes and the density of P2X₃ in these processes were increased significantly in CCI-ION rats, compared to control rats, and 3) administration of MPEP, a specific mGluR5 antagonist, alleviated the mechanical allodynia and abolished the increase in density of P2X₃ in fine astrocytic processes caused by CCI-ION. These findings reveal preferential expression of P2X₃ in the fine astrocytic processes in the brainstem, propose a novel role of P2X₃ in the fine astrocytic process in the mechanism of craniofacial neuropathic pain, and suggest that the expression of astrocytic P2X₃ may be regulated by astrocytic mGluR5.

Constriction of the buccal branch of the facial nerve produces unilateral craniofacial allodynia

Despite pain being a sensory experience, studies of spinal cord ventral root damage have demonstrated that motor neuron injury can induce neuropathic pain. Whether injury of cranial motor nerves can also produce nociceptive hypersensitivity has not been addressed. Herein, we demonstrate that chronic constriction injury (CCI) of the buccal branch of the facial nerve results in long-lasting, unilateral allodynia in the rat. An anterograde and retrograde tracer (3000MW tetramethylrhodamine-conjugated dextran) was not transported to the trigeminal ganglion when applied to the injury site, but was transported to the facial nucleus, indicating that this nerve branch is not composed of trigeminal sensory neurons. Finally, intracisterna magna injection of interleukin-1 (IL-1) receptor antagonist reversed allodynia, implicating the pro-inflammatory cytokine IL-1 in the maintenance of neuropathic pain induced by facial nerve CCI. These data extend the prior evidence that selective injury to motor axons can enhance pain to supraspinal circuits by demonstrating that injury of a facial nerve with predominantly motor axons is sufficient for neuropathic pain, and that the resultant pain has a neuroimmune component.

Methane Suppresses Microglial Activation Related to Oxidative, Inflammatory, and Apoptotic Injury during Spinal Cord Injury in Rats

OBJECTIVE

We investigated the hypothesis that methane-rich saline (MS) can be used to repair spinal cord injury (SCI) in a rat model through suppressing microglial activation related to oxidative, inflammatory, and apoptotic injury.

METHODS

MS was injected intraperitoneally in rats after SCI. Hematoxylin-eosin (HE) staining, oxidative stress, inflammatory parameters, and cell apoptosis were detected 72 h after SCI to determine the optimal dose. Then, we investigated the protective mechanisms and the long-term effects of MS on SCI. HE and microglial activation were observed. Neurological function was evaluated by the Basso, Beattie, and Bresnahan (BBB) scale.

RESULTS

MS can significantly decrease infarct area and inhibit oxidative stress, inflammation, and cell apoptosis 72 h following SCI. The MS protective effect at a dose of 20 ml/kg was better. Moreover, MS can significantly suppress microglial activation related to oxidative and inflammatory injury after SCI and improve hind limb neurological function.

CONCLUSION

MS could repair SCI and reduce the release of oxidative stress, inflammatory cytokines, and cell apoptosis produced by activated microglia. MS provides a novel and promising strategy for the treatment of SCI.

Participation of central GABAA receptors in the trigeminal processing of mechanical allodynia in rats

Here we investigated the central processing mechanisms of mechanical allodynia and found a direct excitatory link with low-threshold input to nociceptive neurons. Experiments were performed on male Sprague-Dawley rats weighing 230-280 g. Subcutaneous injection of interleukin 1 beta (IL-1β) (1 ng/10 µL) was used to produce mechanical allodynia and thermal hyperalgesia. Intracisternal administration of bicuculline, a gamma aminobutyric acid A (GABA_A) receptor antagonist, produced mechanical allodynia in the orofacial area under normal conditions. However, intracisternal administration of bicuculline (50 ng) produced a paradoxical anti-allodynic effect under inflammatory pain conditions. Pretreatment with resiniferatoxin (RTX), which depletes capsaicin receptor protein in primary afferent fibers, did not alter the paradoxical anti-allodynic effects produced by the intracisternal injection of bicuculline. Intracisternal injection of bumetanide, an Na-K-Cl cotransporter (NKCC 1) inhibitor, reversed the IL-1β-induced mechanical allodynia. In the control group, application of GABA (100 µM) or muscimol (3 µM) led to membrane hyperpolarization in gramicidin perforated current clamp mode. However, in some neurons, application of GABA or muscimol led to membrane depolarization in the IL-1β-treated rats. These results suggest that some large myelinated Aβ fibers gain access to the nociceptive system and elicit pain sensation via GABA_A receptors under inflammatory pain conditions.

Antinociceptive Effects of Botulinum Toxin Type A on Trigeminal Neuropathic Pain

Previous studies have demonstrated that botulinum toxin type A (BoNT-A) attenuates orofacial nociception. However, there has been no evidence of the participation of the voltage-gated sodium channels (Navs) in the antinociceptive mechanisms of BoNT-A. This study investigated the cellular mechanisms underlying the antinociceptive effects of BoNT-A in a male Sprague-Dawley rat model of trigeminal neuropathic pain produced by malpositioned dental implants. The left mandibular second molar was extracted under anesthesia, followed by a miniature dental implant placement to induce injury to the inferior alveolar nerve. Mechanical allodynia was monitored after subcutaneous injection of BoNT-A at 3, 7, or 12 d after malpositioned dental implant surgery. Subcutaneous injections of 1 or 3 U/kg of BoNT-A on postoperative day 3 significantly attenuated mechanical allodynia, although 0.3 U/kg of BoNT-A did not affect the air-puff threshold. A single injection of 3 U/kg of BoNT-A produced prolonged antiallodynic effects over the entire experimental period. Treatment with BoNT-A on postoperative days 7 and 12, when pain had already been established, also produced prolonged antiallodynic effects. Double treatments with 1 U/kg of BoNT-A produced prolonged, more antiallodynic effects as compared with single treatments. Subcutaneous administration of 3 U/kg of BoNT-A significantly inhibited the upregulation of Nav isoform 1.7 (Nav1.7) expression in the trigeminal ganglion in the nerve-injured animals. These results suggest that antinociceptive effects of BoNT-A are mediated by an inhibition of upregulated Nav1.7 expression in the trigeminal ganglion. BoNT-A is therefore a potential new therapeutic agent for chronic pain control, including neuropathic pain.

Demyelination/remyelination and expression of interleukin-1β, substance P, nerve growth factor, and glial-derived neurotrophic factor during trigeminal neuropathic pain in rats

The etiology of trigeminal neuropathic pain is not clear, but there is evidence that demyelination, expression of cytokines, neuropeptides, and neurotrophic factors are crucial contributors. In order to elucidate mechanisms underlying trigeminal neuropathic pain, we evaluated the time course of morphological changes in myelinated and unmyelinated trigeminal nerve fibers, expression of cytokine IL-1β, neuropeptide substance P (SP), nerve growth factor (NGF), and glial derived neurotrophic factor (GDNF) in peripheral and ganglion tissues, using a rat model of trigeminal neuropathic pain. Chronic constriction injury (CCI) of the infraorbital nerve (IoN), or a sham surgery, was performed. Mechanical allodynia was evaluated from day 3 to day 15 post-surgery. Trigeminal nerves were divided into 2 sections - distal to CCI and ganglion - for morphological analyses, immunohistochemistry (IL-1β, SP), and protein quantification by ELISA (NGF, GDNF). At early postoperative time points, decreased mechanical responses were observed, which were associated with demyelination, glial cell proliferation, increased immunoexpression of IL-1 β and SP, and impaired GDNF production. In the late postoperative period, mechanical allodynia was present with partial recovery of myelination, glial cell proliferation, and increased immunoreactivity of IL-1β and SP. Our data show that demyelination/remyelination processes are related to the development of pain behavior. IL-1β may have effects both in ganglia and nerves, while SP may be an important mediator at the nerve endings. Additionally, low levels of GDNF may produce impaired signaling, which may be involved in generation of pain.

Activity-triggered tetrapartite neuron-glial interactions following peripheral injury

Recent studies continue to support the proposition that non-neuronal components of the nervous system, mainly glial cells and associated chemical mediators, contribute to the development of neuronal hyperexcitability that underlies persistent pain conditions. In the event of peripheral injury, enhanced or abnormal nerve input is likely the most efficient way to activate simultaneously central neurons and glia. Injury induces phenotypic changes in glia and triggers signaling cascades that engage reciprocal interactions between presynaptic terminals, postsynaptic neurons, microglia and astrocytes. While some responses to peripheral injury may help the nervous system to adapt positively to counter the disastrous effect of injury, the net effect often leads to long-lasting sensitization of pain transmission pathways and chronic pain.

Downregulation of Spinal G Protein-Coupled Kinase 2 Abolished the Antiallodynic Effect of Electroacupuncture

Acupuncture or electroacupuncture (EA) has been demonstrated to have a powerful antihypernociceptive effect on inflammatory pain. The attenuation of G protein-coupled receptor kinase 2 (GRK2) in spinal cord and peripheral nociceptor has been widely acknowledged to promote the transition from acute to chronic pain and to facilitate the nociceptive progress. This study was designed to investigate the possible role of spinal GRK2 in EA antiallodynic in a rat model with complete Freund's adjuvant (CFA) induced inflammatory pain. EA was applied to ST36 (“Zusanli”) and BL60 (“Kunlun”) one day after CFA injection. Single EA treatment at day 1 after CFA injection remarkably alleviated CFA induced mechanical allodynia two hours after EA. Repeated EA displayed significant antiallodynic effect from 2nd EA treatment and a persistent effect was observed during the rest of treatments. However, downregulation of spinal GRK2 by intrathecal exposure of GRK2 antisense 30 mins after EA treatment completely eliminated both the transient and persistent antiallodynic effect by EA treatment. These pieces of data demonstrated that the spinal GRK2 played an important role in EA antiallodynia on inflammatory pain.