Hyperalgesia in response to traumatic occlusion and GFAP expression in rat parabranchial nucleus: modulation with fluorocitrate

Up-regulation of IL-1β and sPLA2-III in the medial prefrontal cortex contributes to orofacial and somatic hyperalgesia induced by malocclusion via glial-neuron crosstalk

The medial prefrontal cortex (mPFC) has been identified as a key brain region involved in the modulation of chronic pain. Our recent study demonstrated that unilateral anterior crossbite (UAC) developed the comorbidity model of temporomandibular disorders (TMD) and fibromyalgia syndrome (FMS), which was characterized by both orofacial and somatic hyperalgesia. In the present study, UAC rats exhibited significant changes in gene expression in the mPFC. Enrichment analysis revealed that the significantly involved pathways were cytokines-cytokine receptor interaction and immune response. The expression of group III secretory phospholipase A2 (sPLA2-III) was significantly increased in the mPFC of UAC rats. Silencing sPLA2-III expression in the mPFC blocked the orofacial and somatic hyperalgesia. Immunofluorescence showed that sPLA2-III was mainly localized in neurons. The expression of interleukin-1β (IL-1β) in the mPFC significantly increased after UAC. Injection of IL-1β antibody into the mPFC blocked orofacial and somatic hyperalgesia. IL-1β was mainly localized in microglia cells. Furthermore, injection of IL-1β antibody significantly reduced the expression of sPLA2-III. These results indicate that neuroinflammatory cascade responses induced by glial-neuron crosstalk in the mPFC may contribute to the development of TMD and FMS comorbidity, and IL-1β and sPLA2-III are identified as novel potential therapeutic targets for the treatment of chronic pain in the comorbidity of TMD and FMS.

Repeat mild traumatic brain injuries (RmTBI) modify nociception and disrupt orexinergic connectivity within the descending pain pathway

Repeat mild traumatic brain injuries (RmTBI) result in substantial burden to the public health system given their association with chronic post-injury pathologies, such as chronic pain and post-traumatic headache. Although this may relate to dysfunctional descending pain modulation (DPM), it is uncertain what mechanisms drive changes within this pathway. One possibility is altered orexinergic system functioning, as orexin is a potent anti-nociceptive neuromodulator. Orexin is exclusively produced by the lateral hypothalamus (LH) and receives excitatory innervation from the lateral parabrachial nucleus (lPBN). Therefore, we used neuronal tract-tracing to investigate the relationship between RmTBI and connectivity between lPBN and the LH, as well as orexinergic projections to a key site within the DPM, the periaqueductal gray (PAG). Prior to injury induction, retrograde and anterograde tract-tracing surgery was performed on 70 young-adult male Sprague Dawley rats, targeting the lPBN and PAG. Rodents were then randomly assigned to receive RmTBIs or sham injuries before undergoing testing for anxiety-like behaviour and nociceptive sensitivity. Immunohistochemical analysis identified distinct and co-localized orexin and tract-tracing cell bodies and projections within the LH. The RmTBI group exhibited altered nociception and reduced anxiety as well as a loss of orexin cell bodies and a reduction of hypothalamic projections to the ventrolateral nucleus of the PAG. However, there was no significant effect of injury on neuronal connectivity between the lPBN and orexinergic cell bodies within the LH. Our identification of structural losses and the resulting physiological changes in the orexinergic system following RmTBI begins to clarify acute post-injury mechanistic changes that drive may drive the development of post-traumatic headache and the chronification of pain.

Spinal CCK1 Receptors Contribute to Somatic Pain Hypersensitivity Induced by Malocclusion via a Reciprocal Neuron-Glial Signaling Cascade

Recent studies have shown that the incidence of chronic primary pain including temporomandibular disorders (TMD) and fibromyalgia syndrome (FMS) often exhibit comorbidities. We recently reported that central sensitization and descending facilitation system contributed to the development of somatic pain hypersensitivity induced by orofacial inflammation combined with stress. The purpose of this study was to explore whether TMD caused by unilateral anterior crossbite (UAC) can induce somatic pain hypersensitivity, and whether the cholecystokinin (CCK) receptor-mediated descending facilitation system promotes hypersensitivity through neuron-glia cell signaling cascade. UAC evoked thermal and mechanical pain hypersensitivity of the hind paws from day 5 to 70 that peaked at week 4 post UAC. The expression levels of CCK1 receptors, interleukin-18 (IL-18) and IL-18 receptors (IL-18R) were significantly up-regulated in the L4 to L5 spinal dorsal horn at 4 weeks post UAC. Intrathecal injection of CCK1 and IL-18 receptor antagonists blocked somatic pain hypersensitivity. IL-18 mainly co-localized with microglia, while IL-18R mainly co-localized with astrocytes and to a lesser extent with neurons. These findings indicate that the signaling transduction between neurons and glia at the spinal cord level contributes to the descending pain facilitation through CCK1 receptors during the development of the comorbidity of TMD and FMS. PERSPECTIVE: CCK1 receptor-dependent descending facilitation may mediate central mechanisms underlying the development of widespread somatic pain via a reciprocal neuron-glial signaling cascade, providing novel therapeutic targets for the clinical treatment of TMD and FMS comorbidities.

Occlusal disharmony and chronic oro-facial pain: from clinical observation to animal study

BACKGROUND

Occlusion can be viewed as the most sensitive susceptor of the central nervous system in the oro-facial region. Its inalienable relationships to the temporomandibular joint, the muscles, the stomatognathic system and even the central nervous system are self-evident. Almost all the dental treatments inevitably change the occlusion, potentially or actually, locally or extensively, and immediately or gradually.

OBJECTIVE

The objective of this study was to present a narrative literature on occlusal disharmony and chronic oro-facial pain.

METHODS

Literature reviews focusing on clinical studies about the relationship between occlusal disharmony and myofascial oro-facial pain, and related preclinical studies about the animal models of, as well as the peripheral and central mechanisms underlying this condition related to, occlusal disharmony were used as starting point and guidelines to describe the topics mentioned. A search of the PubMed database was performed mainly with the following search terms: "occlusion," "occlusal interference," "occlusal disharmony," "occlusal change," "oro-facial pain" and "myofascial pain."

RESULTS

Relevant literature from the past 70 years until the present day was meticulously studied. The literature review together with three related characteristic clinical cases revealed an intimate association between occlusal disharmony and chronic oro-facial pain, involving pathological changes, extending from the peripheral tissues to the central nervous system. The patients suffered from psychological distress, sleep disturbance and poor life quality.

CONCLUSION

Occlusal disharmony-related oro-facial pain is a clinical problem that deserves attention, although there are no universally accepted clinical protocols. The existing literature provides some constructive suggestions, but further research is needed.

Glial role in oxaliplatin-induced neuropathic pain

Oxaliplatin, a platinum-based chemotherapeutic agent, has become a standard treatment for advanced colorectal cancer. The dose-limiting toxicity of this compound is the development of peripheral neuropathy. A tangled panel of symptoms, sensory loss, paresthesia, dysesthesia and pain, may be disabling for patients and adversely affect their quality of life. Recently, we described a characteristic glial activation profile in a rat model of oxaliplatin-induced neuropathy. Glial cells are considered a new pharmacological target for neuropathic pain relief but its relevance in chemotherapy-dependent neuropathies is debated. Aimed to evaluate the significance of glial activation in pain generated by oxaliplatin, the microglial inhibitor minocycline or the astrocyte inhibitor fluorocitrate were continuously infused by intrathecal route in oxaliplatin-treated rats. Both compounds significantly reduced oxaliplatin-evoked pain though the efficacy of fluorocitrate was higher revealing a prominent role of astrocytes. Immunohistochemical analysis of the dorsal horn confirmed the specific Iba1-positive cell inhibition caused by minocycline as well as the selectivity of fluorocitrate on GFAP-positive cells. The activation of astrocytes in minocycline-treated rats suggests a microglia-independent modulation of astrocytes by oxaliplatin neurotoxicity. Neither the selective activation of astrocyte after minocycline treatment nor the exclusive microglial response after fluorocitrate is able to evoke pain. Morphometric and morphological determinations performed on dorsal root ganglia evidenced that the glial inhibitors did not prevent the oxaliplatin-dependent increase of eccentric nucleoli and multinucleolated neurons. The decrease of soma area was also unaltered. In summary, these data highlight the role of central glial cells in oxaliplatin-dependent neuropathic pain. On the other hand, glial inhibition is not associated with neuroprotective effects suggesting the need for careful modulation of glial signaling to prevent the pathophysiology that leads to persistent neuropathic pain.

Importance of glial activation in neuropathic pain

Glia plays a crucial role in the maintenance of neuronal homeostasis in the central nervous system. The microglial production of immune factors is believed to play an important role in nociceptive transmission. Pain may now be considered a neuro-immune disorder, since it is known that the activation of immune and immune-like glial cells in the dorsal root ganglia and spinal cord results in the release of both pro- and anti-inflammatory cytokines, as well as algesic and analgesic mediators. In this review we presented an important role of cytokines (IL-1alfa, IL-1beta, IL-2, IL-4, IL-6, IL-10, IL-15, IL-18, TNFalpha, IFNgamma, TGF-beta 1, fractalkine and CCL2); complement components (C1q, C3, C5); metaloproteinases (MMP-2,-9) and many other factors, which become activated on spinal cord and DRG level under neuropathic pain. We discussed the role of the immune system in modulating chronic pain. At present, unsatisfactory treatment of neuropathic pain will seek alternative targets for new drugs and it is possible that anti-inflammatory factors like IL-10, IL-4, IL-1alpha, TGF-beta 1 would fulfill this role. Another novel approach for controlling neuropathic pain can be pharmacological attenuation of glial and immune cell activation. It has been found that propentofylline, pentoxifylline, minocycline and fluorocitrate suppress the development of neuropathic pain. The other way of pain control can be the decrease of pro-nociceptive agents like transcription factor synthesis (NF-kappaB, AP-1); kinase synthesis (MEK, p38MAPK, JNK) and protease activation (cathepsin S, MMP9, MMP2). Additionally, since it is known that the opioid-induced glial activation opposes opioid analgesia, some glial inhibitors, which are safe and clinically well tolerated, are proposed as potential useful ko-analgesic agents for opioid treatment of neuropathic pain. This review pointed to some important mechanisms underlying the development of neuropathic pain, which led to identify some possible new approaches to the treatment of neuropathic pain, based on the more comprehensive knowledge of the interaction between the nervous system and glial and immune cells.

Differential activation of spinal microglial and astroglial cells in a mouse model of peripheral neuropathic pain

The pharmacological attenuation of glial activation represents a novel approach for controlling neuropathic pain, but the role of microglial and astroglial cells is not well established. To better understand the potential role of two types of glial cells, microglia and astrocytes, in the pathogenesis of neuropathic pain, we examined markers associated with them by quantitative RT-PCR, western blot and immunohistochemical analyses in the dorsal horn of the lumbar spinal cord 7days after chronic constriction injury (CCI) to the sciatic nerve in mice. The mRNA and protein of microglial cells were labeled with C1q and OX42(CD11b/c), respectively. The mRNA and protein of astrocytes were labeled with GFAP. The RT-PCR results indicated an increase in C1q mRNA that was more pronounced than the increased expression of GFAP mRNA ipsilateral to the injury in the dorsal spinal cord. Similarly, western blot and immunohistochemical analyses demonstrated an ipsilateral upregulation of OX42-positive cells (72 and 20%, respectively) and no or little (8% upregulation) change in GFAP-positive cells in the ipsilateral dorsal lumbar spinal cord. We also found that chronic intraperitoneal injection of the minocycline (microglial inhibitor) and pentoxifylline (cytokine inhibitor) attenuated CCI-induced activation of microglia, and both, but not fluorocitrate (astroglial inhibitor), diminished neuropathic pain symptoms and tactile and cold sensitivity. Our findings indicate that spinal microglia are more activated than astrocytes in peripheral injury-induced neuropathic pain. These findings implicate a glial regulation of the pain response and suggest that pharmacologically targeting microglia could effectively prevent clinical pain syndromes in programmed and/or anticipated injury.

Experimental occlusal interference induces long-term masticatory muscle hyperalgesia in rats

Temporomandibular joint or related masticatory muscle pain represents the most common chronic orofacial pain condition. Patients frequently report this kind of pain after dental alterations in occlusion. However, lack of understanding of the mechanisms of occlusion-related temporomandibular joint and muscle pain prevents treating this problem successfully. To explore the relationship between improper occlusion (occlusal interference) and masticatory muscle pain, we created an occlusal interference animal model by directly bonding a crown to a maxillary molar to raise the masticating surface of the tooth in rats. We raised the occlusal surface to three different heights (0.2, 0.4, and 0.6mm), and for one month we quantitatively measured mechanical nociceptive thresholds of the temporal and masseter muscles on both sides. Results showed a stimulus-response relationship between the height of occlusal interference and muscle hyperalgesia. Removal of the crown 6 days after occlusal interference showed that the removal at this time could not terminate the 1 month duration of mechanical hyperalgesia in the masticatory muscles. Lastly, we systemically administered NMDA antagonist MK801 (0.2, 0.1, and 0.05 mg/kg) to the treated rats and found that MK801 dose dependently attenuated the occlusal interference-induced hyperalgesia. These findings suggest that occlusal interference is directly related to masticatory muscle pain, and that central sensitization mechanisms are involved in the maintenance of the occlusal interference-induced mechanical hyperalgesia.