Neurobiology of estrogen status in deep craniofacial pain

The association between chronotype profile and temporomandibular disorders among college students

BACKGROUND

Temporomandibular joint disorders (TMDs) are common in young adults, and the link between chronotype profile and TMDs is unclear.

OBJECTIVE

This study examined TMD prevalence and chronotype distribution and explored the relationship between chronotype and TMDs in young adults.

MATERIALS AND METHODS

A total of 663 students from Sichuan University completed questionnaires. Chronotype profiles were assessed using the Morningness-Eveningness Questionnaire, and TMDs were screened using the Fonseca Memory Index. To validate the findings, 68 TMD patients and 136 controls were enrolled.

RESULTS

The prevalence of TMDs was 69.7%, with significant differences among chronotype profiles. The intermediate profile was the most common chronotype. Eveningness profile was associated with higher TMDs prevalence and severity. Muscle pain and side movement difficulty scores were higher in eveningness and intermediate profiles. Female gender (OR 2.345; 95% CI 1.668-3.297) was a TMD risk factor, while morningness profile (OR 0.537; 95% CI 0.297-0.970) was protective. Validation with TMD patients and controls supported these findings, showing higher eveningness profile prevalence in the TMD groups.

CONCLUSIONS

TMDs have a high prevalence in college students, chronotype profiles shown to be associated with TMDs. Morningness is the protection factor in TMDs and PT, eveningness is a risk factor for IT.

Temporomandibular disorder symptoms in the university context

OBJECTIVE

To assess the impact of the university context of different academic fields and class times and the sociodemographic context on the symptoms of temporomandibular disorder (TMD).

METHODS

A total of 2912 university students, divided into 26 undergraduate courses distributed in the fields of biology and exact and human sciences in the day and night participated in this study. The symptoms of orofacial pain and TMD were assessed using a self-explanatory questionnaire for screening, as recommended by the American Academy of Orofacial Pain. The data were analyzed using frequency distribution tables and multiple logistic regression at a 5% significance level.

RESULTS

There was a significant association between the female sex and the attended class time. Women and undergraduates of night courses were 1.60 times more likely to present symptoms related to TMD compared to women studying during the day.

CONCLUSION

It was concluded that, within the university context, women and undergraduate students enrolled in night courses were more likely to present with TMD symptoms.

The degeneration-pain relationship in the temporomandibular joint: Current understandings and rodent models

Temporomandibular disorders (TMD) represent a group of musculoskeletal conditions involving the temporomandibular joints (TMJ), the masticatory muscles and associated structures. Painful TMD are highly prevalent and conditions afflict 4% of US adults annually. TMD include heterogenous musculoskeletal pain conditions, such as myalgia, arthralgia, and myofascial pain. A subpopulations of TMD patients show structural changes in TMJ, including disc displacement or degenerative joint diseases (DJD). DJD is a slowly progressing, degenerative disease of the TMJ characterized by cartilage degradation and subchondral bone remodeling. Patients with DJD often develop pain (TMJ osteoarthritis; TMJ OA), but do not always have pain (TMJ osteoarthrosis). Therefore, pain symptoms are not always associated with altered TMJ structures, which suggests that a causal relationship between TMJ degeneration and pain is unclear. Multiple animal models have been developed for determining altered joint structure and pain phenotypes in response to various TMJ injuries. Rodent models of TMJOA and pain include injections to induce inflammation or cartilage destruction, sustained opening of the oral cavity, surgical resection of the articular disc, transgenic approaches to knockout or overexpress key genes, and an integrative approach with superimposed emotional stress or comorbidities. In rodents, TMJ pain and degeneration occur during partially overlapping time periods in these models, which suggests that common biological factors may mediate TMJ pain and degeneration over different time courses. While substances such as intra-articular pro-inflammatory cytokines commonly cause pain and joint degeneration, it remains unclear whether pain or nociceptive activities are causally associated with structural degeneration of TMJ and whether structural degeneration of TMJ is necessary for producing persistent pain. A thorough understanding of the determining factors of pain-structure relationships of TMJ during the onset, progression, and chronification by adopting novel approaches and models should improve the ability to simultaneously treat TMJ pain and TMJ degeneration.

Estrogen Status and Trigeminal Ganglion Responses to Jaw Movement

Chronic temporomandibular joint disorders (TMDs) present with pain in the temporomandibular joint (TMJ) and muscles of mastication. Risk factors for TMD include localized joint/muscle inflammation and estrogen status. This study determined whether mild tissue inflammation and estrogen status influenced the responses of trigeminal ganglion neurons to jaw palpation or jaw movement, 2 key diagnostic features of clinical TMD, in adult rats. Neuronal activity was recorded from male rats, ovariectomized (OvX) female rats, and OvX female rats injected with 17β-estradiol 24 h prior to testing (OvXE). Neurons were tested for responses to deep press over the TMJ region and jaw movement in 3 directions (open, protrusion, lateral) 10 d after intra-TMJ injection of a low dose of complete Freund's adjuvant (CFA) or vehicle (sham). Deep press evoked similar responses in all treatment groups. The response magnitude to jaw opening and protrusion was significantly greater for neurons recorded from OvXE CFA-treated rats than from OvX CFA-treated or OvXE sham rats. The responses to lateral movement of the jaw were similar across all treatment groups. Most neurons (70% to 90%) displayed a static response pattern to jaw movement independent of direction. Estradiol treatment also increased the proportion of neurons that were excited by jaw movement in >1 direction as compared with untreated OvX females or males. These results suggest that mild localized inflammation in the TMJ region during periods of elevated estrogen were sufficient to increase the peripheral driving force for jaw movement-evoked hyperalgesia.

Chronic Orofacial Pain: Models, Mechanisms, and Genetic and Related Environmental Influences

Chronic orofacial pain conditions can be particularly difficult to diagnose and treat because of their complexity and limited understanding of the mechanisms underlying their aetiology and pathogenesis. Furthermore, there is considerable variability between individuals in their susceptibility to risk factors predisposing them to the development and maintenance of chronic pain as well as in their expression of chronic pain features such as allodynia, hyperalgesia and extraterritorial sensory spread. The variability suggests that genetic as well as environmental factors may contribute to the development and maintenance of chronic orofacial pain. This article reviews these features of chronic orofacial pain, and outlines findings from studies in animal models of the behavioural characteristics and underlying mechanisms related to the development and maintenance of chronic orofacial pain and trigeminal neuropathic pain in particular. The review also considers the role of environmental and especially genetic factors in these models, focussing on findings of differences between animal strains in the features and underlying mechanisms of chronic pain. These findings are not only relevant to understanding underlying mechanisms and the variability between patients in the development, expression and maintenance of chronic orofacial pain, but also underscore the importance for considering the strain of the animal to model and explore chronic orofacial pain processes.

P2X3 receptor expression in dorsal horn of spinal cord and pain threshold after estrogen therapy for prevention therapy in neuropathic pain

INTRODUCTION

Neuropathic pain may arise from conditions that affecting the central or peripheral nervous system. This study was held to determine the difference P2X3 receptor expression in the dorsal horn of the spinal cord and pain threshold after estrogen therapy in neuropathic pain.

METHODS

This study design was an experimental research laboratory. The 24 mice samples divided into negative control group, positive control, and treatment groups. The treatment groups were given subcutaneous injections of estrogen 0.4 ml and also examined for the onset of thermal hyperalgesia in every rat. On day 15, an autopsy was performed on rats, and the spine was taken. The spinal cord was stained by hematoxylin-eosin, and the expression of P2X3 receptors was investigated. P2X3 receptor expression was examined in the dorsal horn on each sample.

RESULTS

From 24 subjects of the study revealed an increase in the onset of thermal hyperalgesia on the estrogen group compared with the placebo group, a higher start. This study also obtained a decrease in the expression of P2X3 on the therapy group compared to the positive control group with significant differences. Statistical test results revealed the appearance of the P2X3 estrogen group had a substantial difference with the placebo group (p = 0.000) and the mean of the negative control group (p = 0.030). The placebo group had a significant difference from the negative control group (p = 0.035).

CONCLUSION

Estrogen could decrease the expression of P2X3 receptors and prolonged the onset of thermal hyperalgesia. So, both of these explained that estrogen has a role in preventing the occurrence of neuropathic pain after peripheral nerve lesions.

Women’s oral health-related quality of life as a risk factor for TMD symptoms. A case-control study

OBJECTIVE

To investigate the association between temporomandibular disorder (TMD) symptoms and women's oral health-related quality of life (OHRQoL).Methods: An observational case-control study was performed with 765 adult women aged 21.5 years on average. Oral Health Impact Profile (OHIP-14) assessed OHRQoL and determined the selection of cases and controls matched by age and marital status at a ratio of 1:4 (153 cases: 612 controls). American Academy of Orofacial Pain's self-explanatory questionnaire assessed the TMD symptoms. The TMD symptoms' impact on OHRQoL was analyzed, estimating the odds ratio with a 95% confidence interval.

RESULTS

Of the women, 86.9% in the case group, and 69.6% in the control group presented some TMD symptoms (p< 0.05). Women with an impacted OHRQoL are 2.95 (95% CI:1.79-4.86) times more likely to report some TMD symptoms (p< 0.05).

DISCUSSION

Women with a negative impact on OHRQoL are more likely to report TMD symptoms.

Peripheral and Central Mechanisms of Persistent Orofacial Pain

Neuroplastic changes in the neuronal networks involving the trigeminal ganglion (TG), trigeminal spinal subnucleus caudalis (Vc), and upper cervical spinal cord (C1/C2) are considered the mechanisms underlying the ectopic orofacial hypersensitivity associated with trigeminal nerve injury or orofacial inflammation. It has been reported that peripheral nerve injury causes injury discharges in the TG neurons, and a barrage of action potentials is generated in TG neurons and conveyed to the Vc and C1/C2 after trigeminal nerve injury. Long after trigeminal nerve injury, various molecules are produced in the TG neurons, and these molecules are released from the soma of TG neurons and are transported to the central and peripheral terminals of TG neurons. These changes within the TG cause neuroplastic changes in TG neurons and they become sensitized. The neuronal activity of TG neurons is further accelerated, and Vc and C1/C2 neurons are also sensitized. In addition to this cascade, non-neuronal glial cells are also involved in the enhancement of the neuronal activity of TG, Vc, and C1/C2 neurons. Satellite glial cells and macrophages are activated in the TG after trigeminal nerve injury and orofacial inflammation. Microglial cells and astrocytes are also activated in the Vc and C1/C2 regions. It is considered that functional interaction between non-neuronal cells and neurons in the TG, Vc, and C1/C2 regions is a key mechanism involved in the enhancement of neuronal excitability after nerve injury or inflammation. In this article, the detailed mechanisms underlying ectopic orofacial hyperalgesia associated with trigeminal nerve injury and orofacial inflammation are addressed.

Sex Differences in Estradiol Secretion by Trigeminal Brainstem Neurons

Estrogen status is a significant risk factor in the development of temporomandibular joint disorders (TMD). Classically, estrogen status is thought to derive mainly from ovarian sources; however, it is well known that estradiol (E2) also is synthesized by neurons in the brain. This study tested the hypothesis that E2 is produced by neurons in trigeminal subnucleus caudalis (Vc), the principal site of termination for sensory afferents that supply the temporomandibular joint (TMJ), to modify evoked responses in a model of TMJ nociception in male and female rats. Intra-TMJ injection of the small fiber excitant, allyl isothiocyanate (AIC), increased the levels of E2 collected from microdialysis probes sites at Vc of ovariectomized (OvX) female rats, ipsilateral to the stimulus, whereas males displayed no change. Dialysate levels of E2 collected from probe sites in the contralateral Vc or cerebellum in OvX rats were not affected by TMJ stimulation. Reverse dialysis of anastrozole, an aromatase (ARO) inhibitor, via the probe reduced perfusate levels of E2 in Vc. Systemic administration of letrozole, a non-steroid ARO inhibitor, for 4 days prevented TMJ-evoked increases in masseter muscle electromyography (MMemg) activity. ARO-positive neurons were distributed mainly in superficial laminae (I-III) at Vc and cell counts revealed no significant difference between OvX and male rats. Intra-TMJ injection of AIC revealed similar numbers of ARO/Fos dual-labeled neurons in OvX and male rats. By contrast, the percentage of ARO neurons co-labeled for glutamic acid decarboxylase (GAD), the biosynthetic enzyme for GABA, was greater in OvX (35%) than male rats (14%). Few ARO-positive neurons were co-labeled for estrogen receptor alpha. These data indicate that E2 is secreted continuously by Vc neurons and that acute stimulation of TMJ nociceptors evokes further secretion in a sex-dependent manner. Reduced TMJ-evoked MMemg activity after ARO inhibition suggests that locally produced E2 by Vc neurons acts via paracrine mechanisms to modify TMJ nociception in female rats.

Inhibitory effects of fluoxetine, an antidepressant drug, on masseter muscle nociception at the trigeminal subnucleus caudalis and upper cervical spinal cord regions in a rat model of psychophysical stress

This study aimed to determine whether psychophysical stress conditionings had facilitatory effects on masseter muscle nociception in the central nervous system via serotonergic mechanisms in rats. Two experiments were conducted to assess: (1) whether repeated forced swim stress for 3 days increased the number of Fos-positive neurons evoked by masseter muscle injury due to formalin injection; and (2) whether serotonin-reuptake inhibitor, fluoxetine, administered daily after each stress conditioning, had modulatory roles on Fos expression. The number of Fos-positive cells was quantified in several areas within the trigeminal subnucleus caudalis (Vc) and upper cervical spinal cord regions (Vc areas), including the ventrolateral area of the trigeminal subnucleus interpolaris/Vc transition, and the middle or caudal portion of the Vc regions, since nociceptive neural activity in the Vc region could play critical roles in deep craniofacial nociception. We found that forced swim stress conditionings increased depression-like behaviors, which was prevented by fluoxetine. Repeated forced swim stress significantly increased Fos expression in all Vc areas compared with those of non-stressed rats, while systemic administration of fluoxetine significantly decreased Fos expression in all areas, but mainly in the caudal Vc region, in stressed rats. Fluoxetine had no effect on Fos expression in non-stressed rats. These results indicate that repeated forced swim stress conditionings increase Fos expression in the Vc areas, and the contribution of serotonergic mechanisms to masseter muscle nociception could be greater in stressed rats than in sham rats. These results support the hypothesis that changes in brain function, including serotonergic mechanisms, in the Vc areas play critical roles in enhanced masseter muscle nociceptive responses under psychophysical stress conditions.

Tratamento para disfunções temporomandibulares: uma revisão sistemática

RESUMO Introdução: A disfunção temporomandibular (DTM) é complexa e multifatorial. São encontrados, na literatura, estudos que comparam diferentes métodos de tratamento. Objetivo: Investigar estudos sobre o tratamento das disfunções temporomandibulares (DTMs) nas diversas áreas da saúde, avaliando a eficácia das técnicas empregadas, principalmente no que se refere ao uso da terapia miofuncional orofacial. Estratégia de pesquisa: Os artigos compilados neste estudo foram selecionados por meio da base de dados PubMed, utilizando os descritores “temporomandibular disorders and oral motor therapy”, “orofacial myofunctional therapy and temporomandibular disorders” e “temporomandibular disorders and myofunctional rehabilitation”. O levantamento realizado limitou-se aos artigos publicado nos idiomas Inglês e Português, entre janeiro de 2006 e dezembro de 2016. Critérios de seleção: Foram incluídos artigos sobre os tratamentos das DTMs associados aos exercícios musculares e/ou terapias manuais. Publicações sem acesso completo, repetidas por sobreposição das palavras-chave, revisões de literatura, cartas ao editor e não relacionadas diretamente ao tema foram excluídas. Resultados: Dos 102 estudos selecionados, 22 atenderam aos critérios estabelecidos. Em geral, a maioria dos tratamentos descritos apresentou efeitos benéficos para pacientes com DTMs. Foi observada grande variabilidade da metodologia adotada para a aplicação e verificação dos efeitos dos tratamentos e somente poucos estudos fizeram uso de grupo controle. Conclusão: Apesar do crescimento no número de pesquisas sobre DTMs, ainda não é possível estabelecer qual a melhor técnica de tratamento. Após análise dos artigos selecionados, observou-se que as técnicas combinadas de terapia (ex.: exercício associado ao uso de equipamento para redução da dor) produzem melhores resultados, com maior redução da dor e melhora da mobilidade mandibular.

Prevalence of temporomandibular symptoms and parafunctional habits in a university dental clinic and association with gender, age, and missing teeth

Objective To assess the prevalence of symptoms of temporomandibular disorder (TMD) and parafunctional habits as well as to investigate their association with age, gender, and number of missing teeth. Methods This retrospective study is based on 4204 randomly selected patients who were examined to determine their symptoms of TMD, such as clicking, difficulty chewing, difficulty opening/closing, as well self-reported joint pain and parafunctional habits. Results Clenching/grinding was reported by 26.5% of the examined population, clicking by 14.8%, and difficulty chewing and closing by 3.6%. Significant associations between the number of missing teeth with clenching/grinding (p = 0.05) and difficulty chewing (p < 0.001) were detected. Age and gender showed significant effects on the examined parameters (p < 0.05). Discussion Understanding the TMD subjective symptoms in relation to age, gender, and tooth loss would be of great value for treatment planning and could provide other perspectives to establish preventive measures.

Signs and symptoms of temporomandibular disorders in women and men

INTRODUCTION

Women are more likely to present temporomandibular disorders (TMD); however, studies comparing genders in Brazilian samples are rare.

PURPOSE

To analyze the proportion of men and women, as well as the association between gender and age, problem duration, and TMD symptoms in patients admitted to an university clinic for treatment.

METHODS

Interview and assessment data of protocols from 1,000 patients diagnosed with TMD were collected and analyzed and then divided into two groups, male (n = 177) and female (n = 823). The exploratory analysis was based on contingency tables and χ2 test was carried out. Subsequently, the logistic regression model was used and the odds ratios (OR) concerning the evaluated comparisons were calculated.

RESULTS

Females were more prevalent in the sample, and mean ages and TMD duration were similar between the groups, with higher occurrence in young adults (19 to 40 years old). The OR values showed an association between the female gender and the signs/symptoms of pain in the temporomandibular joint, pain in the facial muscles, neck and shoulders, headache, fatigue in the muscles of mastication, otologic symptoms, and dysphonia. Women had two times higher chances of presenting these symptoms than men.

CONCLUSION

In the sample of Brazilian patients with TMD, the number of women who presented a higher prevalence of painful symptoms was greater, followed by otologic symptoms and complaints of dysphonia. The prevalence of joint noise was similar in both studied groups.

Local group I mGluR antagonists reduce TMJ-evoked activity of trigeminal subnucleus caudalis neurons in female rats

Group I metabotropic glutamate receptors (mGluR1 and mGluR5) are functionally linked to estrogen receptors and play a key role in the plasticity of central neurons. Estrogen status strongly influences sensory input from the temporomandibular joint (TMJ) to neurons at the spinomedullary (Vc/C1-2) region. This study tested the hypothesis that TMJ input to trigeminal subnucleus caudalis/upper cervical cord (Vc/C1-2) neurons involved group I mGluR activation and depended on estrogen status. TMJ-responsive neurons were recorded in superficial laminae at the Vc/C1-2 region in ovariectomized (OvX) female rats treated with low-dose estradiol (2 μg/day, LE) or high-dose estradiol (20 μg/day, HE) for 2 days. TMJ-responsive units were activated by adenosine triphosphate (ATP, 1mM) injected into the joint space. Receptor antagonists selective for mGluR1 (CPCCOEt) or mGluR5 (MPEP) were applied topically to the Vc/C1-2 surface at the site of recording 10 min prior to the intra-TMJ ATP stimulus. In HE rats, CPCCOEt (50 and 500 μM) markedly reduced ATP-evoked unit activity. By contrast, in LE rats, a small but significant increase in neural activity was seen after 50 μM CPCCOEt, while 500 μM caused a large reduction in activity that was similar in magnitude as that seen in HE rats. Local application of MPEP produced a significant inhibition of TMJ-evoked unit activity independent of estrogen status. Neither mGluR1 nor mGluR5 antagonism altered the spontaneous activity of TMJ units in HE or LE rats. High-dose MPEP caused a small reduction in the size of the convergent cutaneous receptive field in HE rats, while CPCCOEt had no effect. These data suggest that group I mGluRs play a key role in sensory integration of TMJ nociceptive input to the Vc/C1-2 region and are largely independent of estrogen status.

Inhibition of temporomandibular joint input to medullary dorsal horn neurons by 5HT3 receptor antagonist in female rats

Repeated forced swim (FS) conditioning enhances nociceptive responses to temporomandibular joint (TMJ) stimulation in female rats. The basis for FS-induced TMJ hyperalgesia remains unclear. To test the hypothesis that serotonin 3 receptor (5HT3R) mechanisms contribute to enhanced TMJ nociception after FS, ovariectomized female rats were treated with estradiol and subjected to FS for three days. On day 4, rats were anesthetized with isoflurane and TMJ-responsive neurons were recorded from superficial and deep laminae at the trigeminal subnucleus caudalis/upper cervical (Vc/C1-2) region and electromyographic (EMG) activity was recorded from the masseter muscle. Only Vc/C1-2 neurons activated by intra-TMJ injections of ATP were included for further analysis. Although neurons in both superficial and deep laminae were activated by ATP, only neurons in deep laminae displayed enhanced responses after FS. Local application of the 5HT3R antagonist, ondansetron (OND), at the Vc/C1-2 region reduced the ATP-evoked responses of neurons in superficial and deep laminae and reduced the EMG response in both sham and FS rats. OND also decreased the spontaneous firing rate of neurons in deep laminae and reduced the high-threshold convergent cutaneous receptive field area of neurons in superficial and deep laminae in both sham and FS rats. These results revealed that central application of a 5HT3R antagonist, had widespread effects on the properties of TMJ-responsive neurons at the Vc/C1-2 region and on jaw muscle reflexes under sham and FS conditions. It is concluded that 5HT3R does not play a unique role in mediating stress-induced hyperalgesia related to TMJ nociception.

Characteristics of upper quadrant posture of young women with temporomandibular disorders

[Purpose] This study aimed to investigate the characteristics of upper quadrant posture of young women with temporomandibular disorders. [Subjects] The participants were 19 female patients with temporomandibular disorders (patient group: mean age, 30.1 years) and 14 controls (control group: mean age, 24.6 years). [Methods] Outcome measures were the neck inclination angle (formed by a line connecting C7 and the ear tragus with a horizontal line), the angle of the shoulder (formed by a line connecting C7 and the acromial angle with a horizontal line), the cranial rotation angle (formed by a line connecting the ear tragus and the corner of the eye with a horizontal line), and the neck-length/shoulder-width ratio [the ratio of the neck length (from C7 to the tragus) to the width of the shoulder between the acromial angle]. The maximum range of mouth opening was measured using a scale. [Results] The neck inclination angle and maximum range of mouth opening were significantly smaller in the patient group than in the control group. No significant differences were observed in the other outcome measures between the two groups. [Conclusion] Temporomandibular disorders with limited mouth opening in young females are associated with the head position relative to the trunk.

Expression of estrogen receptors α and β in the trigeminal mesencephalic nucleus of adult women and men

Temporomandibular disorders are more prevalent in women than in men and phases of pain relate to the estrous cycle. Several studies described the location of estrogen receptors (ER) in the temporomandibular joint (TMJ), the masseteric muscles and cartilage, but it was unknown whether they are also expressed within the pseudounipolar neurons of the trigeminal mesencephalic nucleus, which receives direct sensory inputs from these structures. Therefore, we studied expression of ERα and ERβ protein in the trigeminal mesencephalic nucleus of ten human brains (five female/five male). Both receptors were uniformly expressed on neurons, but not other cell types within the target structure. Thus, sensory inputs from the TMJ and adjacent structures are likely to be modulated by estrogen at the level of the first sensory neuron which may underlie the well-known correlation of pain incidence and phases of the estrous cycle.

GABAergic influence on temporomandibular joint-responsive spinomedullary neurons depends on estrogen status

Sensory input from the temporomandibular joint (TMJ) to neurons in superficial laminae at the spinomedullary (Vc/C1-2) region is strongly influenced by estrogen status. This study determined if GABAergic mechanisms play a role in estrogen modulation of TMJ nociceptive processing in ovariectomized female rats treated with high- (HE) or low-dose (LE) estradiol (E2) for 2days. Superficial laminae neurons were activated by ATP (1mM) injections into the joint space. The selective GABAA receptor antagonist, bicuculline methiodide (BMI, 5 or 50μM, 30μl), applied at the site of recording greatly enhanced the magnitude and duration of ATP-evoked responses in LE rats, but not in units from HE rats. The convergent cutaneous receptive field (RF) area of TMJ neurons was enlarged after BMI in LE but not HE rats, while resting discharge rates were increased after BMI independent of estrogen status. By contrast, the selective GABAA receptor agonist, muscimol (50μM, 30μl), significantly reduced the magnitude and duration of ATP-evoked activity, resting discharge rate, and cutaneous RF area of TMJ neurons in LE and HE rats, whereas lower doses (5μM) affected only units from LE rats. Protein levels of GABAA receptor β3 isoform at the Vc/C1-2 region were similar for HE and LE rats. These results suggest that GABAergic mechanisms contribute significantly to background discharge rates and TMJ-evoked input to superficial laminae neurons at the Vc/C1-2 region. Estrogen status may gate the magnitude of GABAergic influence on TMJ neurons at the earliest stages of nociceptive processing at the spinomedullary region.

Estrogenic influences in pain processing

Gonadal hormones not only play a pivotal role in reproductive behavior and sexual differentiation, they also contribute to thermoregulation, feeding, memory, neuronal survival, and the perception of somatosensory stimuli. Numerous studies on both animals and human subjects have also demonstrated the potential effects of gonadal hormones, such as estrogens, on pain transmission. These effects most likely involve multiple neuroanatomical circuits as well as diverse neurochemical systems and they therefore need to be evaluated specifically to determine the localization and intrinsic characteristics of the neurons engaged. The aim of this review is to summarize the morphological as well as biochemical evidence in support for gonadal hormone modulation of nociceptive processing, with particular focus on estrogens and spinal cord mechanisms.

Estrogen status and psychophysical stress modify temporomandibular joint input to medullary dorsal horn neurons in a lamina-specific manner in female rats

Estrogen status and psychological stress contribute to the expression of several chronic pain conditions including temporomandibular muscle and joint disorders (TMJD). Sensory neurons that supply the temporomandibular joint (TMJ) region terminate in laminae I and V of the spinal trigeminal nucleus (Vc/C1-2 region); however, little is known about lamina-specificity and environmental influences on the encoding properties of TMJ brainstem neurons. To test the hypothesis that Vc/C1-2 neurons integrate both interoceptive and exteroceptive signals relevant for TMJ nociception, we recorded TMJ-evoked activity in superficial and deep laminae of ovariectomized rats under high and low estradiol (E2) and stress conditions. Rats received daily injections of low (LE) or high (HE) dose E2 and were subjected to forced swim (FS) or sham swim conditioning for 3days. The results revealed marked lamina-specificity in that HE rats displayed enhanced TMJ-evoked activity in superficial, but not deep, laminae independent of stress conditioning. By contrast, FS conditioned rats displayed increased background firing and TMJ-evoked activity of neurons in deep, but not superficial, laminae independent of E2 status. FS also enhanced TMJ-evoked masseter muscle activity and suggested the importance of deep dorsal horn neurons in mediating evoked jaw muscle activity. In conclusion, E2 status and psychophysical stress play a significant role in modifying the encoding properties of TMJ-responsive medullary dorsal horn neurons with a marked lamina-specificity.

Temporomandibular joint-evoked responses by spinomedullary neurons and masseter muscle are enhanced after repeated psychophysical stress

Psychological stress is a risk factor for the development of musculoskeletal pain of the head and neck; however, the basis for this relationship remains uncertain. This study tested the hypothesis that psychophysical stress alone was sufficient to alter the encoding properties of spinomedullary dorsal horn neurons and masseter muscle activity in male rats. Repeated forced swim conditioning increased markedly both the background firing rate and temporomandibular joint (TMJ)-evoked activity of neurons in deep dorsal horn, while neurons in superficial laminae were less affected. Stress also increased the responses to stimulation of facial skin overlying the TMJ of neurons in deep and superficial dorsal horn. TMJ-evoked masseter muscle activity was enhanced significantly in stressed rats, an effect that was reduced by prior blockade of the spinomedullary junction region. These data indicated that repeated psychophysical stress induced widespread effects on the properties of medullary dorsal horn neurons and masseter muscle activity. The effects of stress were seen preferentially on neurons in deep dorsal horn and included enhanced responses to chemosensory input from the TMJ and mechanical input from overlying facial skin. The stress-induced elevation in TMJ-evoked masseter muscle activity matched well with the changes seen in dorsal horn neurons. It is concluded that the spinomedullary junction region plays a critical role in the integration of psychophysical stress and sensory information relevant for nociception involving deep craniofacial tissues.

Differential ascending projections of temporomandibular joint-responsive brainstem neurons to periaqueductal gray and posterior thalamus of male and female rats

Several craniofacial pain conditions, including temporomandibular joint disorders (TMJDs), are more prevalent in women than men. The basis for sex differences in deep craniofacial pain is not known. The present study compared the magnitude of ascending projections from temporomandibular joint (TMJ)-responsive neurons in trigeminal brainstem with the ventrolateral periaqueductal gray (vlPAG) or posterior nucleus of the thalamus (Po) in males and female rats. Fluorogold (FG) was injected into vlPAG or Po, and TMJ-responsive neurons were identified by Fos-like immunoreactivity (Fos-LI) after mustard oil injection. TMJ-evoked Fos-LI was similar in males and females; however, significant differences in cell counts were seen for FG single-labeled and Fos/FG double-labeled neurons in trigeminal brainstem. After vlPAG injections, the number of FG-labeled neurons in trigeminal subnucleus interpolaris (Vi), ventral interpolaris/caudalis transition (vl-Vi/Vc), and dorsal paratrigeminal region (dPa5) was greater in females than males. The percentage of Fos/FG double-labeled neurons in vl-Vi/Vc and dPa5 after vlPAG injection also was greater in females than males. In contrast, after Po injections, males displayed a greater number of FG-labeled neurons in superficial laminae (Lam I/II) of trigeminal subnucleus caudalis (Vc) and upper cervical spinal cord (C(1-2)) and deeper laminae (Lam III/V) at C(1-2) than females. The percentage of Fos/FG double-labeled neurons in Lam I/II of Vc after Po injection also was greater in males than females. These data revealed significant sex differences in ascending projections from TMJ-responsive neurons in trigeminal brainstem. Such differences may influence the ability of males and females to recruit autonomic reflexes and endogenous pain control circuits relevant for TMJ nociception.

Rapid estrogenic effects on TMJ-responsive brainstem neurons

Estrogen status is a risk factor for temporomandibular muscle and joint disorders (TMJD) and other craniofacial pain conditions. The basis for estrogen modulation of pain is poorly understood and has often been attributed to long-term genomic effects. However, estrogens also act rapidly through membrane-initiated mechanisms to alter neural activity. To assess if estrogens act rapidly to affect TMJ-responsive neurons, we applied 17β-estradiol (E2) directly at the spinomedullary (Vc/C(1-2)) region, the initial brainstem site for synaptic integration of TMJ sensory signals, while recording single neuron activity. In ovariectomized female rats, E2 rapidly (within 10 minutes) and reversibly reduced TMJ-evoked neural activity at the Vc/C(1-2) region. The effect was estrogen receptor (ER) subtype-specific, since ERβ agonists inhibited, while an ERβ agonist enhanced, evoked activity. A membrane-mediated mechanism was indicated, since the membrane-impermeable analogue, E(2)-BSA, mimicked the inhibitory effect of E2 and was prevented by an ER antagonist. This study demonstrated that E2 acted rapidly, through membrane-mediated pathways, and locally at the Vc/C(1-2) region, to modulate sensory signals from the TMJ region. These results were consistent with the hypothesis that estrogens can act rapidly at the level of the trigeminal brainstem complex to influence sensory integration of TMJ-related information.

Cortical NR2B NMDA subunit antagonism reduces inflammatory pain in male and female rats

Background

Studies have shown that N-methyl-D-aspartate (NMDA) receptors play a critical role in pain processing at different levels of the central nervous system.

Methods

In this study, we used adult Wistar rats to examine gender differences in the effects of NR2B NMDA antagonism at the level of the anterior cingulate cortex in phasic pain, and in the first and second phases of a formalin test. Rats underwent stereotactic surgery for cannula implantation in the anterior cingulate cortex. After recovery, paw withdrawal latency to a noxious thermal stimulus was assessed. Rats were also subjected to a formalin pain test whereby 60 μL of 5% formalin was injected into the right hind paw.

Results

Female and male rats that received Ro 25-6981, an NR2B antagonist, before formalin injection showed significantly reduced pain responses to the formalin test compared with saline-injected control rats (P < 0.05). No gender differences in phasic pain responses were found in rats treated with Ro 25-6981.

Conclusion

These results suggest that cortical antagonism of the NR2B subunit reduces inflammatory pain levels in both genders of rat.