Gonadal hormones decrease temporomandibular joint kappa-mediated antinociception through a down-regulation in the expression of kappa opioid receptors in the trigeminal ganglia

Sex as a Biological Variable in Oral Diseases: Evidence and Future Prospects

The interest of the scientific community on sex and gender differences in health and disease has increased substantially over the past 25 to 30 y as a result of a long process of events and policies in the biomedical field. This is crucial as compelling evidence from human and animal model studies has demonstrated that sex and gender influence health, molecular and cellular processes, and response and predisposition to disease. The present scoping review aims to provide a synthesis of sex differences in oral diseases, ranging from periodontal disease to orofacial pain conditions, from risk of caries development to apical periodontitis. Overall, findings from this review further support a role for sexual dimorphism influencing disease predisposition and/or progression in oral diseases. Of note, this review also highlights the lack of consideration of additional factors such as gender and other psychosocial and external factors potentially influencing oral health and disease. New conceptual frameworks capable of capturing multiple fundamental domains and measurements should be developed in clinical and preclinical studies to inform sex-based individualized preventive and treatment strategies.

Sex differences in kappa opioid receptor antinociception is influenced by the number of X chromosomes in mouse

Kappa opioid receptor (KOR) agonists produce robust analgesia with minimal abuse liability and are considered promising pharmacological agents to manage chronic pain and itch. The KOR system is also notable for robust differences between the sexes, with females exhibiting lower analgesic response than males. Sexually dimorphic traits can be due to either the influence of gonadal hormones during development or adulthood, or due to the complement of genes expressed on the X or Y chromosome. Previous studies examining sex differences in KOR antinociception have relied on surgical or pharmacological manipulation of the gonads to determine whether sex hormones influence KOR function. While there are conflicting reports whether gonadal hormones influence KOR function, no study has examined these effects in context with sex chromosomes. Here, we use two genetic mouse models, the four core genotypes and XY*, to isolate the chromosomal and hormonal contributions to sex differences in KOR analgesia. Mice were treated with systemic KOR agonist (U50,488H) and thermal analgesia measured in the tail withdrawal assay. We found that KOR antinociception was influenced predominantly by the number of the X chromosomes. These data suggest that the dose and/or parental imprint on X gene(s) contribute significantly to the sexually dimorphism in KOR analgesia.

Peripherally Acting Opioids in Orofacial Pain

The activation of opioid receptors by exogenous or endogenous opioids can produce significant analgesic effects in peripheral tissues. Numerous researchers have demonstrated the expression of peripheral opioid receptors (PORs) and endogenous opioid peptides (EOPs) in the orofacial region. Growing evidence has shown the involvement of PORs and immune cell-derived EOPs in the modulation of orofacial pain. In this review, we discuss the role of PORs and EOPs in orofacial pain and the possible cellular mechanisms involved. Furthermore, the potential development of therapeutic strategies for orofacial pain is also summarized.

Sex differences in the modulation of mouse nest building behavior by kappa opioid receptor signaling

Emerging evidence suggests that females are less sensitive than males to the effects of kappa opioid receptor (KOR) ligands across multiple behavioral measures. The effects of the KOR agonist U50,488 and the KOR antagonist aticaprant were assessed on nest building behavior, an ethologically relevant indicator of overall well-being and affect, in adult male and female C57BL/6J mice. Females required a higher dose of U50,488 to suppress nesting, and a higher dose of aticaprant to restore U50,488-induced impairment of nesting. Females also required a higher dose of aticaprant to decrease immobility scores in the forced swim test. Pretreatment with the estrogen receptor modulator tamoxifen, at a dose which blocked estrogen receptors, augmented the effect of U50,488 on nesting in female mice, suggesting that estrogen receptors play a key role in attenuating the effects of KOR ligands in female mice. Together, these results suggest that females are less sensitive to KOR mediation, requiring a higher dose to achieve comparable results to males. This behavioral sensitivity, as measured by nesting, may be mediated by estrogen receptors. Together these studies highlight the importance of comparing sex differences in response to KOR regulation on behaviors related to affective states.

The role of peripheral opioid receptors in orofacial pain

Opioid receptors are widely distributed in the central and peripheral nervous systems and non-neuronal tissues. Numerous researchers have noted the pivotal role of peripheral opioid receptors (PORs) in analgesia. Accumulating evidence has shown the existence of PORs in the trigeminal nerve system, indicating that PORs may be involved in the modulation of orofacial pain. In this review, we summarise the recent evidence for the role of PORs in orofacial pain and discuss the possible cellular mechanisms.

Analgesic activities of the mixed opioid and NPFF receptors agonist DN-9 in a mouse model of formalin-induced orofacial inflammatory pain

Orofacial pain is one of the most common pain conditions and compromises the quality of life of the sufferer. Several studies have shown that opioid agonists produced significant analgesia in the orofacial pain, and combination of opioids with drugs belonging to other classes induced synergism in the orofacial pain. However, combination therapy of different analgesic drugs improves the risk of drug-drug interactions. Against this background, we sought to investigate the analgesic effects of the multi-functional opioid peptide DN-9, a mixed opioid and NPFF receptors agonist that produced robust analgesia in acute and inflammatory pain models, on formalin-induced orofacial pain. Our results showed that formalin injection caused significant spontaneous pain behaviors and increased the expressions of the mu-opioid receptor, c-Fos and phosphorylated extracellular signal-regulated kinase (p-ERK1/2) in the ipsilateral trigeminal ganglion (TG). In mice pretreated with DN-9, there was a significant reduction in nociceptive behaviors, which was selectively mediated by the mu- and kappa-opioid receptors, independently of the NPFF system. Four hours after formalin injection, the level of c-Fos immunoreactivity in the ipsilateral TG neurons was much lower in mice pretreated with DN-9 or morphine. In addition, DN-9 exhibited a significant inhibition in the expression of p-ERK1/2, which was reversed by the selective antagonists of the mu- and kappa-opioid receptors. In conclusion, our present results demonstrate that central administration of DN-9 produces potential antinociceptive effects via the mu- and kappa-opioid receptors, independently of the NPFF system, and this antinociceptive action is tightly linked with the intracellular ERK activation in TG neurons.

The anti-inflammatory effect of tramadol in the temporomandibular joint of rats

Tramadol is a centrally acting analgesic drug able to prevent nociceptor sensitization when administered into the temporomandibular joint (TMJ) of rats. The mechanism underlying the peripheral anti-inflammatory effect of tramadol remains unknown. This study demonstrated that intra-TMJ injection of tramadol (500µg/TMJ) was able to inhibit the nociceptive response induced by 1.5% formalin or 1.5% capsaicin, suggesting that tramadol has an antinociceptive effect, acting directly on the primary nociceptive neurons activating the nitric oxide/cyclic guanosine monophosphate signaling pathway. Tramadol also inhibited the nociceptive response induced by carrageenan (100µg/TMJ) or 5-hydroxytryptamine (225µg/TMJ) along with inhibition of inflammatory cytokines levels, leukocytes migration and plasma extravasation. In conclusion, the results demonstrate that peripheral administration of tramadol has a potential antinociceptive and anti-inflammatory effect. The antinociceptive effect is mediated by activation of the intracellular nitric oxide/cyclic guanosine monophosphate pathway, at least in part, independently from the opioid system.

Intra-Articular Blockade of P2X7 Receptor Reduces the Articular Hyperalgesia and Inflammation in the Knee Joint Synovitis Especially in Female Rats

Synovitis is a key factor in joint disease pathophysiology, which affects a greater proportion of women than men. P2X7 receptor activation contributes to arthritis, but whether it plays a role in articular inflammatory pain in a sex-dependent manner is unknown. We investigated whether the P2X7 receptor blockade in the knee joint of male and female rats reduces the articular hyperalgesia and inflammation induced by a carrageenan knee joint synovitis model. Articular hyperalgesia was quantified using the rat knee joint incapacitation test and the knee joint inflammation, characterized by the concentration of cytokines tumor necrosis factor-α, interleukin-1β, interleukin-6, and cytokine-induced neutrophil chemoattractant-1, and by neutrophil migration, was quantified using enzyme-linked immunosorbent assay and by myeloperoxidase enzyme activity measurement, respectively. P2X7 receptor blockade by the articular coadministration of selective P2X7 receptor antagonist A740003 with carrageenan significantly reduced articular hyperalgesia, pro-inflammatory cytokine concentrations, and myeloperoxidase activity induced by carrageenan injection into the knee joint of male and estrus female rats. However, a lower dose of P2X7 receptor antagonist was sufficient to significantly induce the antihyperalgesic and anti-inflammatory effects in estrus female but not in male rats. These results suggest that P2X7 receptor activation by endogenous adenosine 5'-triphosphate is essential to articular hyperalgesia and inflammation development in the knee joint of male and female rats. However, female rats are more responsive than male rats to the antihyperalgesic and anti-inflammatory effects induced by P2X7 receptor blockade.

PERSPECTIVE

P2X7 receptors could be promising therapeutic targets in the treatment of knee joint disease symptoms, especially in women, who are more affected than men by these conditions.

P2X3 and P2X2/3 Receptors Play a Crucial Role in Articular Hyperalgesia Development Through Inflammatory Mechanisms in the Knee Joint Experimental Synovitis

Osteoarthritis (OA) is a degenerative and progressive disease characterized by cartilage breakdown and by synovial membrane inflammation, which results in disability, joint swelling, and pain. The purinergic P2X3 and P2X2/3 receptors contribute to development of inflammatory hyperalgesia, participate in arthritis processes in the knee joint, and are expressed in chondrocytes and nociceptive afferent fibers innervating the knee joint. In this study, we hypothesized that P2X3 and P2X2/3 receptors activation by endogenous ATP (adenosine 5'-triphosphate) induces articular hyperalgesia in the knee joint of male and female rats through an indirect sensitization of primary afferent nociceptors dependent on the previous release of pro-inflammatory cytokines and/or on neutrophil migration. We found that the blockade of articular P2X3 and P2X2/3 receptors significantly attenuated carrageenan-induced hyperalgesia in the knee joint of male and estrus female rats in a similar manner. The carrageenan-induced knee joint inflammation increased the expression of P2X3 receptors in chondrocytes of articular cartilage. Further, the blockade of articular P2X3 and P2X2/3 receptors significantly reduced the increased concentration of TNF-α, IL-6, and CINC-1 and the neutrophil migration induced by carrageenan. These findings indicate that P2X3 and P2X2/3 receptors activation by endogenous ATP is essential to hyperalgesia development in the knee joint through an indirect sensitization of primary afferent nociceptors dependent on the previous release of pro-inflammatory cytokines and/or on neutrophil migration.

The role of endogenous opioid peptides in the antinociceptive effect of 15-deoxy(Δ12,14)-prostaglandin J2 in the temporomandibular joint

We have previously demonstrated that peripheral administration of 15d-PGJ2 in the Temporomandibular joint (TMJ) of rats can prevent nociceptor sensitization, mediated by peroxisome proliferator activated receptor-γ (PPAR-γ), and κ- and δ- opioid receptors. However, the mechanism that underlies the signaling of PPAR-γ (upon activation by 15d-PGJ2) to induce antinociception, and how the opioid receptors are activated via 15d-PGJ2 are not fully understood. This study demonstrates that peripheral antinociceptive effect of 15d-PGJ2 is mediated by PPAR-γ expressed in the inflammatory cells of TMJ tissues. Once activated by 15d-PGJ2, PPAR-γ induces the release of β-endorphin and dynorphin, which activates κ- and δ-opioid receptors in primary sensory neurons to induce the antinociceptive effect.

Dorsal root ganglion transcriptome analysis following peripheral nerve injury in mice

BACKGROUND

Peripheral nerve injury leads to changes in gene expression in primary sensory neurons of the injured dorsal root ganglia. These changes are believed to be involved in neuropathic pain genesis. Previously, these changes have been identified using gene microarrays or next generation RNA sequencing with poly-A tail selection, but these approaches cannot provide a more thorough analysis of gene expression alterations after nerve injury.

METHODS

The present study chose to eliminate mRNA poly-A tail selection and perform strand-specific next generation RNA sequencing to analyze whole transcriptomes in the injured dorsal root ganglia following spinal nerve ligation. Quantitative real-time reverse transcriptase polymerase chain reaction assay was carried out to verify the changes of some differentially expressed RNAs in the injured dorsal root ganglia after spinal nerve ligation.

RESULTS

Our results showed that more than 50 million (M) paired mapped sequences with strand information were yielded in each group (51.87 M-56.12 M in sham vs. 51.08 M-57.99 M in spinal nerve ligation). Six days after spinal nerve ligation, expression levels of 11,163 out of a total of 27,463 identified genes in the injured dorsal root ganglia significantly changed, of which 52.14% were upregulated and 47.86% downregulated. The largest transcriptional changes were observed in protein-coding genes (91.5%) followed by noncoding RNAs. Within 944 differentially expressed noncoding RNAs, the most significant changes were seen in long interspersed noncoding RNAs followed by antisense RNAs, processed transcripts, and pseudogenes. We observed a notable proportion of reads aligning to intronic regions in both groups (44.0% in sham vs. 49.6% in spinal nerve ligation). Using quantitative real-time polymerase chain reaction, we confirmed consistent differential expression of selected genes including Kcna2, Oprm1 as well as lncRNAs Gm21781 and 4732491K20Rik following spinal nerve ligation.

CONCLUSION

Our findings suggest that next generation RNA sequencing can be used as a promising approach to analyze the changes of whole transcriptomes in dorsal root ganglia following nerve injury and to possibly identify new targets for prevention and treatment of neuropathic pain.

Gonadal hormones modulate the responsiveness to local β-blocker-induced antinociception in the temporomandibular joint of male and female rats

BACKGROUND

We have previously demonstrated that blockade of β-adrenoreceptors (β-AR) located in the temporomandibular joint (TMJ) of rats suppresses formalin-induced TMJ nociceptive behaviour in both male and female rats, but female rats are more responsive. In this study, we investigated whether gonadal hormones modulate the responsiveness to local β-blocker-induced antinociception in the TMJ of rats.

METHODS

Co-administration of each of the selective β1 (atenolol), β2 (ICI 118.551) and β3 (SR59230A)-AR antagonists with equi-nociceptive concentrations of formalin in the TMJ of intact, gonadectomized and hormone-treated gonadectomized male and female rats.

RESULTS

Atenolol, ICI 118.551 and SR59230A significantly reduced formalin-induced TMJ nociception in a dose response fashion in all groups tested. However, a lower dose of each β-AR antagonist was sufficient to significantly reduce nociceptive responses in gonadectomized but not in intact and testosterone-treated gonadectomized male rats. In the female groups, a lower dose of β1 -AR antagonist was sufficient to significantly reduce nociceptive responses in gonadectomized but not in intact or gonadectomized rats treated with progesterone or a high dose of oestradiol; a lower dose of β2 -AR antagonist was sufficient to significantly reduce nociceptive responses in gonadectomized but not in intact and gonadectomized rats treated with low or high dose of oestradiol.

CONCLUSION

Gonadal hormones may reduce the responsiveness to local β-blocker-induced antinociception in the TMJ of male and female rats. However, their effect depends upon their plasma level, the subtype of β-AR and the dose of β-blockers used.

Local kappa opioid receptor activation decreases temporomandibular joint inflammation

In an attempt to decrease central side effects associated with the use of opioids, some strategies have been developed by targeting peripheral opioid receptors. In this context, kappa receptors are of major interest, since, in contrast to other opioid receptors, their activation is not associated with potent peripheral side effects. We have recently demonstrated that local activation of kappa opioid receptors significantly decreases formalin-induced temporomandibular joint nociception; however, whether it also decreases temporomandibular joint inflammation is not known. To address this issue, we evaluated if a specific kappa opioid receptor agonist, U50,488 (trans-(1S,2S)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide hydrochloride hydrate), administered into the temporomandibular joint decreases formalin-induced plasma extravasation and neutrophil migration. Ipsilateral, but not contralateral, administration of U50,488 into the temporomandibular joint blocked formalin-induced plasma extravasation and neutrophil migration in a dose-dependent manner. This anti-inflammatory effect was reversed by the ipsilateral, but not contralateral, administration of the kappa opioid receptor antagonist nor-BNI (nor-binaltorphimine dihydrochloride). This study demonstrates that local activation of kappa opioid receptors decreases two important parameters of temporomandibular joint inflammation, that is, plasma extravasation and neutrophil migration, in a dose-dependent and antagonist-reversible manner. This anti-inflammatory effect taken together with the potent antinociceptive effect, suggests that drugs targeting peripheral kappa opioid receptors are promising for the treatment of inflammatory temporomandibular joint pain and probably, other articular pain conditions with an inflammatory basis.

Modulation of temporomandibular joint nociception and inflammation in male rats after administering a physiological concentration of 17β-oestradiol

BACKGROUND

Previous studies have shown 17β-estradiol will reduce temporomandibular joint (TMJ) inflammation and hypersensitivity in female rats. Although male rats contain significant amounts of oestradiol, it was unknown whether a physiological concentration of 17β-estradiol would attenuate male TMJ inflammation and nociception.

METHODS

Intact and castrated rats were given a physiological concentration of oestradiol to examine first, if oestradiol will affect male TMJ nociception/inflammation and, second, if administration of oestradiol would act synergistically with endogenous male hormones to attenuate TMJ nociception. The hormonally treated rats were given TMJ injections of complete Freund's adjuvant (CFA) and then nociception was measured using a validated method in which a lengthening in meal duration is directly correlated to the intensity of deep TMJ nociception. Inflammation was assayed by quantitating pro-inflammatory gene expression.

RESULTS

Meal duration was significantly lengthened after TMJ CFA injection and this lengthening was significantly attenuated in the castrated but not intact males after administering a physiological concentration of oestradiol. A physiological concentration of 17β-estradiol also significantly increased IL-6 expression in the inflamed TMJ of castrated males while 17β-estradiol did not alter IL-1β, CXCL2 and CCL20 expression. Castration increased pro-inflammatory mediators IL-6, IL-1β and CXCL2 suggesting male sex hormones were anti-inflammatory. Calcitonin gene-related peptide in the trigeminal ganglia was unchanged.

CONCLUSIONS

Similar to females, male rats with TMJ inflammation showed a reduced nociceptive response after treatment with a physiological concentration of oestradiol suggesting the effects of oestradiol treatment were not constrained by organizational processes in the males.

Blockade of β₁-, β₂- and β₃-adrenoceptors in the temporomandibular joint induces antinociception especially in female rats

BACKGROUND

Temporomandibular joint (TMJ) receives rich sympathetic innervations that may contribute to TMJ pain through the local release of sympathomimetic amines. The aim of this study was to determine whether blockade of β-adrenoceptors in the TMJ of male and female rats reduces formalin-induced TMJ nociceptive behaviour.

METHODS

We co-administrated each one of the selective β(1) -, β(2) - and β(3) -adrenoceptors antagonists, atenolol, ICI 118.551 and SR59230A, respectively, with formalin in the TMJ of male and proestrus and dioestrus female rats. Because intra-temporomandibular joint formalin induces significantly different concentration-dependent responses among the three groups, with dioestrus females showing greater responses than males or proestrus females, equi-nociceptive formalin concentrations were used to test the effects of the β-adrenoceptor antagonists.

RESULTS

We found that atenolol, ICI 118.551 and SR59230A significantly reduced formalin-induced TMJ nociception in a dose response fashion in both males and females. However, a lower dose of each β-adrenoceptor antagonist was sufficient to significantly reduce nociceptive responses in females than in males. Administration of the highest doses of each β-adrenoceptor antagonist in the TMJ contralateral to that receiving formalin did not affect formalin-induced nociception in males and females, confirming the local action of the β-adrenoceptor antagonists.

CONCLUSIONS

We conclude that blockade of β-adrenoceptors in the temporomandibular joint suppresses formalin-induced TMJ nociceptive behaviour in both males and females but females are more responsive. These findings suggest that the use of β-blockers in the treatment of TMJ pain might be of benefit, especially in females.

Are temporomandibular disorder symptoms and diagnoses associated with pubertal development in adolescents? An epidemiological study

BACKGROUND

In addition to low back pain, temporomandibular disorders (TMDs) are the most prevalent pain- and disability-related musculoskeletal conditions. However, the influence of pubertal development on TMD diagnoses remains unknown.

OBJECTIVE

To assess whether the prevalence of TMD diagnoses, in addition to self-reported symptoms (pain, restricted mandibular mobility, clicking), change according to pubertal stage.

METHODS

A random sample of 1,011 children and adolescents was chosen from the general population of 24,129 children and adolescents aged 10-17 years living in the urban area of Halle, Germany. Study participants were examined for TMD symptoms and diagnoses according to the Research Diagnostic Criteria for TMDs (RDC/TMD). Status of pubertal development was assessed using the Pubertal Development Scale (PDS). The χ(2) tests and multivariate logistic regression models were used, and odds ratios (ORs) and 95% confidence intervals calculated.

RESULTS

The observed increase in TMD symptoms during adolescence was mainly due to the higher frequency of self-reports of these symptoms by girls than boys (ORs for girls vs. boys: 1.42-1.53; p ≤ 0.05), whereas clinical TMD diagnoses (any RDC/TMD diagnosis, or RDC/TMD group IIa diagnosis) in adolescence increased mainly due to pubertal development itself (ORs for subjects beyond vs. before puberty: 1.58-2.00; p < 0.05; no significant sex-related effect was found).

CONCLUSION

Pubertal development increases the probability of self-reported TMD symptoms among girls, while the probability thereof decreases among boys. Independent of sex, pubertal growth increases the prevalence of RDC/TMD-related diagnoses-mainly disk displacement-in both sexes.

The role of nitric oxide in orofacial pain

Nitric oxide (NO) is a free radical gas that has been shown to be produced by nitric oxide synthase (NOS) in different cell types and recognized to act as a neurotransmitter or neuromodulator in the nervous system. NOS isoforms are expressed and/or can be induced in the related structures of trigeminal nerve system, in which the regulation of NOS biosynthesis at different levels of gene expression may allow for a fine control of NO production. Several lines of evidence suggest that NO may play a role through multiple mechanisms in orofacial pain processing. This report will review the latest evidence for the role of NO involved in orofacial pain and the potential cellular mechanisms are also discussed.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).