Administration of MK-801 decreases c-Fos expression in the trigeminal sensory nuclear complex but increases it in the midbrain during experimental movement of rat molars

The role of the endogenous opioid system in modulating orthodontic-induced neurogenic inflammation of the dental pulp: A comprehensive review of the literature

BACKGROUND

Orthodontic forces may lead to neurogenic inflammation in the dental pulp by triggering the release of somatosensory neuropeptides such as Substance P (SP), Calcitonin gene-related peptide (CGRP) and Neurokinin A (NKA). In the vast majority of patients, acute symptoms are not triggered, probably due to the control of the neurogenic inflammatory process exerted by endogenous opioid systems.

OBJECTIVE

This review aimed to assess the cellular and molecular mechanisms through which the endogenous opioid system modulates the orthodontic-induced neurogenic inflammation of the dental pulp and to identify potential mechanisms for endogenous control of pulp pain.

METHODS

A literature search was conducted in the databases Pubmed, ISI Web of Science and Scopus to find relevant studies using the keywords: "orthodontic movement," "opioids" and "neurogenic inflammation." Following the PRISMA and Amstar recommendations, studies were selected for the literature review.

RESULTS

After removing duplicated and irrelevant articles, and those that does not meet the inclusion criteria, 38 articles were selected and classified according to the opioid peptides analysed in relation to orthodontic movement and dental pulp.

DISCUSSION

Both peripheral and central pathways, via endogenous opioid systems such as somatostatin (SST), dynorphin, β-endorphin, methionine enkephalin, endocannabinoids and anti-inflammatory cytokines, modulate the neurogenic inflammation elicited by orthodontic movements. The bradykinin and monoaminergic systems also appear to display regulatory effects on pain response. These control mechanisms, however, may be insufficient in cases where severe orthodontic forces are applied, thus leading to asymptomatic irreversible pulpitis or necrosis.

CONCLUSION

The opioid system regulates neurogenic pulpal inflammation and pain at the level of the central and peripheral nervous systems by releasing endogenous substances, including SST, opioid peptides, endocannabinoids and anti-inflammatory cytokines.

Glutamate as a Therapeutic Substrate in Migraine

Recurrent and intense headache is a well appreciated cardinal feature of migraine, a common and incapacitating neurological disorder. Often, there are associated canonical sensory abnormalities, such as light and sound sensitivity, as well as associated nausea. Given this phenotype of disordered sensory processing and, in a third of patients, the phenomenon called aura accompanying migraine attacks, it has been suggested that the pathophysiology of migraine is likely to involve glutamate, the main excitatory neurotransmitter in the central nervous system (CNS). Glutamate plays a role in nociception, central sensitization, and cortical spreading depression (CSD), three processes that are deemed important in migraine biology. With an emphasis on the therapeutic potential of targeting various glutamate receptors in migraine, this review will discuss the currently available literature and emerging findings on the role of targeting glutamatergic pathways for the treatment of migraine. A thorough literature review was carried out on the functions of both metabotropic glutamate receptors (mGluRs), and the ionotropic glutamate receptors (NMDA, AMPA, and kainate) in migraine pathogenesis. The ever-present need for new treatments, the role of glutamate in the migraine aura phenomenon, and the consequences of monogenic migraine mutations on mediating prolonged, complex, or permanent aura are all discussed, culminating in a suggestion that glutamatergic targeting may hold particular promise in the management of migraine aura. There are plausible roles for metabotropic receptors in regulating pain processing in important migraine-related brain structures, like the thalamus and trigeminal nucleus. Similarly, ionotropic receptors contribute to excitatory neurotransmission and neuronal hyperexcitability. Recent studies have shown preclinical and early clinical results for treatments targeting these receptors, but there are still significant issues with treatment response, including drug transport, side effects, and efficacy. With ongoing and emerging discoveries in the field, there is increasing promise of new migraine medications targeting glutamate receptors. For bench to bedside translation in this area, continued study of the molecular basis of migraine, receptor subtypes, and exploration of potential drug delivery methods are needed.

Effects of high-frequency near infrared laser irradiation on experimental tooth movement-induced pain in rats

Discomfort and dull pain are known side effects of orthodontic treatment. Pain is expected to be reduced by near-infrared (NIR) lasers; however, the mechanism underlying effects of short-pulse NIR lasers in the oral and maxillofacial area remains unclear. This study aimed to examine the effects of high-frequency NIR diode laser irradiation on pain during experimental tooth movement (ETM) on 120 J. NIR laser with 910 nm wavelength, 45 W maximum output power, 300 mW average output power, and 200 ns pulse width (Lumix 2; (Lumix 2; Fisioline, Verduno CN, Italy) was used for the experiment. A nickel-titanium-closed coil was used to apply a 50-gf force between the maxillary left-side first molar and incisor in 7-week-old Sprague-Dawley rats (280-300 g) to induce ETM. We measured facial-grooming frequency and vacuous chewing movement (VCM) period between laser-irradiation and ETM groups. We performed immunofluorescent histochemistry analysis to quantify levels of Iba-1, astrocytes, and c-fos protein-like immunoreactivity (Fos-IR) in the trigeminal spinal nucleus caudalis (Vc). Compared with the ETM group, the laser irradiation group had significantly decreased facial-grooming frequency (P = 0.0036), VCM period (P = 0.043), Fos-IR (P = 0.0028), Iba-1 levels (P = 0.0069), and glial fibrillary acidic protein (GFAP) levels (P = 0.0071). High-frequency NIR diode laser irradiation appears to have significant analgesic effects on ETM-induced pain, which involve inhibiting neuronal activity, microglia, and astrocytes, and it inhibits c-fos, Iba-1, and GFAP expression, reducing ETM-induced pain in rats. High-frequency NIR diode laser application could be applied to reduce pain during orthodontic tooth movement.

Involvement of astrocytes activation in orofacial hyperalgesia induced by experimental tooth movement

OBJECTIVE

The study aimed to investigate the involvement of astrocytes in the medullary dorsal horn (MDH) in the orofacial hyperalgesia induced by experimental tooth movement (ETM) and related mechanism.

MATERIALS AND METHODS

Experimental tooth movement was produced with nickel-titanium alloy closed-coil spring fixed between the left maxillary first molar and the left upper incisor. Fluorocitrate was administrated through medullary subarachnoid at 3 days after ETM. Pressure pain threshold (PPT) in masseter cutaneous area was measured. The expression of glial fibrillary acidic protein (GFAP) and c-Fos in MDH was measured using immunofluoroscence staining. The expression of interleukin-1β (IL-1β) and phosphorylated N-methyl-D-aspartic acid (NMDA) receptor subunit NR1 (p-NR1) was measured with Western blotting.

RESULTS

Experimental tooth movement-induced orofacial hyperalgesia from 1 to 9 days as the PPT was significantly reduced (P < .05). Immunofluoroscence staining showed that the expression of c-Fos in MDH was dramatically upregulated at 1 day and 3 days after ETM, while GFAP expression with both immunofluoroscence staining and Western blotting was significantly enhanced at 3 days and 7 days after ETM. Western blotting analysis indicated that the expression of IL-1β and p-NR1 in MDH was significantly enhanced at 3 days after ETM. Furthermore, we found that fluorocitrate administration at 3 days after ETM could markedly suppress the expression of c-Fos, GFAP, IL-1β and p-NR1 and attenuate the reduction of PPT induced by ETM.

CONCLUSION

Astrocyte activation in MDH is involved in the mechanical hyperalgesia, and the subsequent upregulated IL-1β and overexpression of p-NR1 may participate in this process.

Glutamate and Its Receptors as Therapeutic Targets for Migraine

There is substantial evidence indicating a role for glutamate in migraine. Levels of glutamate are higher in the brain and possibly also in the peripheral circulation in migraine patients, particularly during attacks. Altered blood levels of kynurenines, endogenous modulators of glutamate receptors, have been reported in migraine patients. Population genetic studies implicate genes that are involved with glutamate signaling in migraine, and gene mutations responsible for familial hemiplegic migraine and other familial migraine syndromes may influence glutamate signaling. Animal studies indicate that glutamate plays a key role in pain transmission, central sensitization, and cortical spreading depression. Multiple therapies that target glutamate receptors including magnesium, topiramate, memantine, and ketamine have been reported to have efficacy in the treatment of migraine, although with the exception of topiramate, the evidence for the efficacy of these therapies is not strong. Also, because all of these therapies have other mechanisms of action, it is not possible to conclude that the efficacy of these drugs is entirely due to their effects on glutamate receptors. Further studies are needed to more clearly delineate the possible roles of glutamate and its specific receptor subtypes in migraine and to identify new ways of targeting glutamate for migraine therapy.

Glutamate receptor antagonists in the management of migraine

Migraine is a neurovascular disorder that is associated with severe headache and neurologic symptoms. The pathogenesis of migraine is believed to involve trigeminovascular system activation with the primary dysfunction located in brainstem. Glutamate, the major excitatory neurotransmitter in the central nervous system, and its receptors have since long been suggested in migraine pathophysiology. Different preclinical studies have confirmed their potential role in migraine. Moreover, several glutamate receptor modulators have been studied in clinical studies, some with promising results. In this review, we will give an overview of what is known about the role of glutamate in the pathogenesis of migraine, which will be followed by an overview of available efficacy, safety and tolerability data for glutamate receptor inhibitors in clinical development for the treatment of migraine.

Novel strategies for the treatment of migraine attacks via the CGRP, serotonin, dopamine, PAC1, and NMDA receptors

INTRODUCTION

Migraine is a common, paroxysmal, and disabling primary headache with a high personal and socioeconomic impact. It involves ∼ 16% of the general population. During the years, a number of hypotheses have been put forward concerning the exact pathomechanism, but the final solution is still undiscovered.

AREAS COVERED

Although the origin is enigmatic, parallel therapeutic efforts have been developed. Current attack therapy does not meet the expectations of the patients or the doctors. This article, based on a PubMed search, reviews the novel pharmacological possibilities that influence the peripheral and central sensitization involved in the disease.

EXPERT OPINION

In order to overcome the therapeutic insufficiency, a calcitonin gene-related peptide receptor antagonist without the side-effect of liver transaminase elevation is required. Another therapeutic option is to develop a neurally acting antimigraine agent, such as a serotonin-1F receptor agonist, with low adverse central nervous system events. Development of a potent dopamine receptor antagonist is necessary to diminish the premonitory symptoms of migraine. A further option is to decrease the headache intensity with a pituitary adenylate cyclase-activating polypeptide type 1 receptor blocker which can cross the blood-brain barrier. Finally, synthetic kynurenine analogues are required to block the pain transmission in the activated trigeminal system.

Neurobehavioral effects, c-Fos/Jun expression and tissue distribution in rat offspring prenatally co-exposed to MeHg and PFOA: PFOA impairs Hg retention

Exposure to methylmercury (MeHg) and perfluorooctanoic acid (PFOA) can occur simultaneously as both contaminants are found in the same food sources, especially fish, seafood, marine mammals and milk. The aim of this study was to assess the effects of exposure to MeHg (10 μg mL(-1) in drinking water) and PFOA (10 μg mL(-1) in drinking water) from gestational day 1 to postnatal day (PND) 21, alone and in combination, on neurobehavioral development and the expression of c-Fos/Jun in different brain regions in the offspring. Our findings showed that exposure to MeHg alone, and exposure to MeHg combined with PFOA significantly induced cliff avoidance reflexes and negative geotaxis reflexes. And these effects appeared to be greater following exposure to MeHg alone. MeHg and/or PFOA exposure did not significantly impair motor coordination functions, or cause significant changes in c-Fos expression in the hippocampus and cerebellum, and spatial learning tests were similar to those in the controls, thus it was impossible to determine whether combined exposure to MeHg and PFOA had any additional effects on both hippocampus and cerebellum regions. However, a significant increase in the frequency of line crossing was observed in rats treated with MeHg or PFOA alone, and there were no significant differences between the MeHg+PFOA-treated group and the controls, suggesting that PFOA was antagonistic to MeHg toxicity in the locomotor activity test. Co-exposure to MeHg and PFOA decreased all tissue Hg concentrations in pups compared to the group exposed to MeHg only, suggesting that PFOA impaired Hg retention in different tissues.

ERK is Involved in Tooth-pressure-induced Fos Expression in Vc Neurons

Discomfort and pain encountered during orthodontic treatment are major problems for patients, but the details of the underlying neural processes and molecular mechanisms are not well-understood. Here we show that noxious tooth mechanical pressure induced by orthodontic elastics resulted in a rapid and transient activation of extracellular signal-regulated protein kinase (ERK) in the trigeminal spinal subnucleus interpolaris and caudalis transition zone (Vi/Vc), trigeminal spinal subnucleus caudalis (Vc), and upper cervical spinal cord (Vc/C2). The phosphorylated ERK (pERK) was observed in neurons but not in astroglia and microglia. Single-plane scanning analysis indicated that the pERK was localized to the nucleus of Vc neurons. In addition, the tooth mechanical pressure led to Fos expression in the pERK-positive Vc neurons that would be suppressed by intrathecal administration of an MEK1/2 inhibitor (PD98059). Taken together, these findings suggest that activation of the ERK signaling cascade following noxious mechanical pressure on the teeth regulates Fos expression in Vc neurons and may thereby contribute to pain associated with orthodontic treatment.

Dopamine D(2) Antagonist-Induced Striatal Nur77 Expression Requires Activation of mGlu5 Receptors by Cortical Afferents

Dopamine D₂ receptor antagonists modulate gene transcription in the striatum. However, the molecular mechanism underlying this effect remains elusive. Here we used the expression of Nur77, a transcription factor of the orphan nuclear receptor family, as readout to explore the role of dopamine, glutamate, and adenosine receptors in the effect of a dopamine D₂ antagonist in the striatum. First, we investigated D₂ antagonist-induced Nur77 mRNA in D_2L receptor knockout mice. Surprisingly, deletion of the D_2L receptor isoform did not reduce eticlopride-induced upregulation of Nur77 mRNA levels in the striatum. Next, we tested if an ibotenic acid-induced cortical lesion could block the effect of eticlopride on Nur77 expression. Cortical lesions strongly reduced eticlopride-induced striatal upregulation of Nur77 mRNA. Then, we investigated if glutamatergic neurotransmission could modulate eticlopride-induced Nur77 expression. A combination of a metabotropic glutamate type 5 (mGlu5) and adenosine A_2A receptor antagonists abolished eticlopride-induced upregulation of Nur77 mRNA levels in the striatum. Direct modulation of Nur77 expression by striatal glutamate and adenosine receptors was confirmed using corticostriatal organotypic cultures. Taken together, these results indicate that blockade of postsynaptic D₂ receptors is not sufficient to trigger striatal transcriptional activity and that interaction with corticostriatal presynaptic D₂ receptors and subsequent activation of postsynaptic glutamate and adenosine receptors in the striatum is required. Thus, these results uncover an unappreciated role of presynaptic D₂ heteroreceptors and support a prominent role of glutamate in the effect of D₂ antagonists.

Glutamate pharmacology and metabolism in peripheral primary afferents: physiological and pathophysiological mechanisms

In addition to using glutamate as a neurotransmitter at central synapses, many primary sensory neurons release glutamate from peripheral terminals. Primary sensory neurons with cell bodies in dorsal root or trigeminal ganglia produce glutaminase, the synthetic enzyme for glutamate, and transport the enzyme in mitochondria to peripheral terminals. Vesicular glutamate transporters fill neurotransmitter vesicles with glutamate and they are shipped to peripheral terminals. Intense noxious stimuli or tissue damage causes glutamate to be released from peripheral afferent nerve terminals and augmented release occurs during acute and chronic inflammation. The site of action for glutamate can be at the autologous or nearby nerve terminals. Peripheral nerve terminals contain both ionotropic and metabotropic excitatory amino acid receptors (EAARs) and activation of these receptors can lower the activation threshold and increase the excitability of primary afferents. Antagonism of EAARs can reduce excitability of activated afferents and produce antinociception in many animal models of acute and chronic pain. Glutamate injected into human skin and muscle causes acute pain. Trauma in humans, such as arthritis, myalgia, and tendonitis, elevates glutamate levels in affected tissues. There is evidence that EAAR antagonism at peripheral sites can provide relief in some chronic pain sufferers.

Tooth movement activates the central amygdala and the lateral hypothalamus by the magnitude of the force applied

OBJECTIVE

To determine if the magnitude of the force used to induce incisor tooth movement promotes distinct activation in cells in the central amygdala (CEA) and lateral hypothalamus (LH) of rats. Also, the effect of morphine on Fos immunoreactivity (Fos-IR) was investigated in these nuclei.

MATERIALS AND METHODS

Adult male rats were anesthetized and divided into six groups: only anesthetized (control), without orthodontic appliance (OA), OA but without force, OA activated with 30g or 70g, OA with 70g in animals pretreated with morphine (2 mg/kg, intraperitoneal). Three hours after the onset of the experiment the rats were reanesthetized and perfused with 4% paraformaldehyde. The brains were removed and fixed, and sections containing CEA and LH were processed for Fos protein immunohistochemistry.

RESULTS

The results show that in the control group, the intramuscular injection of a ketamine/ xylazine mixture did not induce Fos-IR cells in the CEA or in the LH. Again, the without force group showed a little Fos-IR. However, in the 70g group the Fos-IR was the biggest observed (P < .05, Tukey) in the CEA and LH compared with the other groups. In the 30g group, the Fos-IR did not differ from the control group, the without OA group, and the without force group. Furthermore, pretreatment with morphine in the 70g group reduced Fos-IR in these regions.

CONCLUSIONS

Tooth movement promotes Fos-IR in the CEA and LH according to the magnitude of the force applied.

Trigeminal expression of N-methyl-D-aspartate receptor subunit 1 and behavior responses to experimental tooth movement in rats

OBJECTIVE

To test the hypothesis that peripheral N-methyl-D-aspartate (NMDA) receptors play a role in pain induced by experimental tooth movement.

MATERIALS AND METHODS

Male Sprague-Dawley rats weighing between 200 g and 300 g were used in this study. Expression of NMDA receptors subunit 1 (NMDAR1) in the mandibular portion of the trigeminal ganglion (TG) was determined by Western blotting 4 hours and 1, 2, 3, 5, 7, and 14 days after tooth movement. Changes in the time taken by the rats on nocifensive behavior then effects of NMDA receptor antagonist MK-801 and force magnitude on these changes in behavior and NMDAR1 expression were evaluated.

RESULTS

Experimental tooth movement led to a statistically significant increase in NMDAR1 expression at protein level from day 1 to 7 after force application initiating tooth movement. Time spent on nocifensive behavior dramatically increased from day 1 to 7. The rhythm in NMDAR1 expression in the TG and behavioral activities correlated well with the initial orthodontic pain responses. The magnitude of the nocifensive behavior and NMDAR1 expression were both force magnitude dependent and could be reduced by peripheral NMDA receptor antagonist MK-801.

CONCLUSIONS

The hypothesis is accepted. Peripheral NMDA receptors are modulated by experimental tooth movement and involved in the development of tooth movement pain.

Therapeutic potential of novel glutamate receptor antagonists in migraine

BACKGROUND

Migraine is a common and disabling neurological disorder. Although the pharmacotherapy of migraine has advanced in parallel with our understanding of the pathophysiology of the disease, there is still a considerable unmet need to find more effective treatments. Migraine pathophysiology involves activation or the perception of activation of the trigeminovascular system. Glutamate, the major excitatory neurotransmitter in the CNS, is implicated in elements of the pathophysiology of the disorder, including trigeminovascular activation, central sensitization and cortical spreading depression.

OBJECTIVE

The aim of this article is to review the potential use of glutamate receptor antagonists as innovative neuronally targeted treatments of migraine.

METHODS

A systematic search of peer-reviewed publications was performed in PubMed on glutamate and migraine/trigeminovascular activation, and important references providing an insight into migraine pathophysiology are included. The results of unpublished trials were obtained from presentations at national and international meetings.

RESULTS/CONCLUSIONS

The preclinical and clinical data argue strongly for a role of glutamatergic receptor activation in migraine. The pharmacology of glutamatergic trigeminovascular responses in brain areas involved in migraine pathophysiology is relevant to the development of new therapies for this disabling condition. Glutamate receptors represent a promising target for a valuable, non-vasoconstrictor, and perhaps more importantly neuronal-specific therapeutic approach to the treatment of migraine.

Cellular, molecular, and tissue-level reactions to orthodontic force

Remodeling changes in paradental tissues are considered essential in effecting orthodontic tooth movement. The force-induced tissue strain produces local alterations in vascularity, as well as cellular and extracellular matrix reorganization, leading to the synthesis and release of various neurotransmitters, cytokines, growth factors, colony-stimulating factors, and metabolites of arachidonic acid. Recent research in the biological basis of tooth movement has provided detailed insight into molecular, cellular, and tissue-level reactions to orthodontic forces. Although many studies have been reported in the orthodontic and related scientific literature, a concise convergence of all data is still lacking. Such an amalgamation of the rapidly accumulating scientific information should help orthodontic clinicians and educators understand the biological processes that underlie the phenomenon of tooth movement with mechanics (removable, fixed, or functional appliances). This review aims to achieve this goal and is organized to include all major findings from the beginning of research in the biology of tooth movement. It highlights recent developments in cellular, molecular, tissue, and genetic reactions in response to orthodontic force application. It reviews briefly the processes of bone, periodontal ligament, and gingival remodeling in response to orthodontic force. This review also provides insight into the biological background of various deleterious effects of orthodontic forces.