α1 adrenergic receptor activation has a dynamic effect on masticatory muscle afferent fibers

β2-adrenergic receptor activation decreases the mechanical sensitivity of rat masticatory muscle afferent fibres

BACKGROUND

Activation of β2 adrenergic receptors reduces cutaneous mechanical pain thresholds in rats. While β2 adrenergic receptor activation may contribute to mechanisms that underlie temporomandibular joint pain, its effect on masticatory muscle pain sensitivity is uncertain.

OBJECTIVES

The current study sought to determine the extent to which β adrenergic receptors are expressed by masticatory muscle afferent fibres, and to assess the effect of local activation of these receptors on the mechanical sensitivity of masticatory muscle afferent fibres in rats.

METHODS

Trigeminal ganglion neurons that innervate the rat (n = 12) masseter muscle and lower lip were identified by tissue injection of fluorescent dyes and were then stained with antibodies against β1 or β2 adrenergic receptors. Extracellular recordings from 60 trigeminal ganglion neurons that innervate the masticatory muscle were undertaken in a second group of anaesthetised rats of both sexes (n = 37) to assess afferent mechanical activation thresholds. Thresholds were assessed before and after injection of the β adrenergic receptor agonists into masticatory muscle.

RESULTS

β1 and β2 adrenergic receptor expression was greater in labial skin than in masticatory muscle ganglion neurons (p < .05, one-way ANOVA, Holm-Sidak test). There was a higher expression of β2 adrenergic receptors in masticatory muscle ganglion neurons in males than in females. The mixed β agonist isoproterenol increased afferent mechanical activation threshold in male but not female rats (p < .05, Mann-Whitney test). In male rats, salbutamol, a β2 selective agonist, also increased afferent mechanical activation threshold but hydralazine, a vasodilator, did not (p < .05, Mann-Whitney test).

CONCLUSION

Activation of β2 adrenergic receptors decreases the mechanical sensitivity of masticatory muscle afferent fibres in a sex-related manner.

Sympathetic modulation of tumor necrosis factor alpha-induced nociception in the presence of oral squamous cell carcinoma

ABSTRACT

Head and neck squamous cell carcinoma (HNSCC) causes more severe pain and psychological stress than other types of cancer. Despite clinical evidence linking pain, stress, and cancer progression, the underlying relationship between pain and sympathetic neurotransmission in oral cancer is unknown. We found that human HNSCC tumors and mouse tumor tissue are innervated by peripheral sympathetic and sensory nerves. Moreover, β-adrenergic 1 and 2 receptors (β-ARs) are overexpressed in human oral cancer cell lines, and norepinephrine treatment increased β-AR2 protein expression as well as cancer cell proliferation in vitro. We have recently demonstrated that inhibition of tumor necrosis factor alpha (TNFα) signaling reduces oral cancer-induced nociceptive behavior. Norepinephrine-treated cancer cell lines secrete more TNFα which, when applied to tongue-innervating trigeminal neurons, evoked a larger Ca 2+ transient; TNF-TNFR inhibitor blocked the increase in the evoked Ca 2+ transient. Using an orthotopic xenograft oral cancer model, we found that mice demonstrated significantly less orofacial cancer-induced nociceptive behavior during systemic β-adrenergic inhibitory treatment with propranolol. Furthermore, chemical sympathectomy using guanethidine led to a significant reduction in tumor size and nociceptive behavior. We infer from these results that sympathetic signaling modulates oral cancer pain through TNFα secretion and tumorigenesis. Further investigation of the role of neurocancer communication in cancer progression and pain is warranted.

The contribution of autonomic mechanisms to pain in temporomandibular disorders: A narrative review

BACKGROUND

Temporomandibular disorders (TMD) are diagnosed based on symptom presentation and, like other functional pain disorders, often lack definitive pathology. There is a strong association between elevated stress levels and the severity of TMD-related pain, which suggests that alterations in autonomic tone may contribute to this pain condition.

OBJECTIVES

This narrative review examines the association between altered autonomic function and pain in TMD.

METHODS

Relevant articles were identified by searching PubMed and through the reference list of those studies.

RESULTS

TMD sufferers report an increased incidence of orthostatic hypotension. As in other chronic musculoskeletal pain conditions, TMD is associated with increased sympathetic tone, diminished baroreceptor reflex sensitivity and decreased parasympathetic tone. It remains to be determined whether ongoing pain drives these autonomic changes and/or is exacerbated by them. To examine whether increased sympathetic tone contributes to TMD-related pain through β₂ adrenergic receptor activation, clinical trials with the beta blocker propranolol have been undertaken. Although evidence from small studies suggested propranolol reduced TMD-related pain, a larger clinical trial did not find a significant effect of propranolol treatment. This is consistent with human experimental pain studies that were unable to demonstrate an effect of β₂ adrenergic receptor activation or inhibition on masticatory muscle pain. In preclinical models of temporomandibular joint arthritis, β₂ adrenergic receptor activation appears to contribute to inflammation and nociception, whereas in masticatory muscle, α₁ adrenergic receptor activation has been found to induce mechanical sensitization. Some agents used to treat TMD, such as botulinum neurotoxin A, antidepressants and α₂ adrenergic receptor agonists, may interact with the autonomic nervous system as part of their analgesic mechanism.

CONCLUSION

Even if dysautonomia turns out to be a consequence rather than a causative factor of painful TMD, the study of its role has opened up a greater understanding of the pathogenesis of this condition.

Chronic Pain-Related Jaw Muscle Motor Load and Sensory Processing

Chronic pain associated with temporomandibular disorders (TMDs) may reflect muscle mechanoreceptor afferent barrage and dysregulated sensory processing. This observational study tested for associations between Characteristic Pain Intensity (CPI), physical symptoms (Patient Health Questionnaire-15 [PHQ-15]), and cumulative jaw muscle motor load (mV*s). In accordance with institutional review board oversight and Strengthening the Reporting of Observational Studies in Epidemiology guidelines, adult subjects gave informed consent and were identified via Diagnostic Criteria for TMD (DC-TMD) examination and research protocols. Subjects were assigned to ±Pain groups using DC-TMD criteria for myalgia. CPI scores characterized pain intensity. PHQ-15 scores were surrogate measures of dysregulated sensory processing. Laboratory tests were performed to quantify masseter and temporalis muscle activities (mV) per bite force (N) for each subject. In their natural environments, subjects recorded day- and nighttime electromyography from which cumulative jaw muscle motor loads (mV*s) were determined for activities consistent with bite forces of >1 to ≤2 and >2 to ≤5 N. Data were assessed using univariate analysis of variance, simple effects tests, K-means cluster classification, and 3-dimensional regression analyses. Of 242 individuals screened, 144 enrolled, and 125 with complete data from study protocols, there were 35 females and 15 males for +Pain and 35 females and 40 males for -Pain. Subjects produced 324 daytime and 341 nighttime recordings of average duration 6.9 ± 1.7 and 7.6 ± 1.7 h, respectively. Overall, +Pain compared to -Pain subjects had significantly higher (all P ≤ 0.002) CPI and PHQ-15 scores. Cumulative jaw muscle motor loads showed significant between-subject effects for time, diagnostic group, and sex (all P < 0.003), where motor loads tended to be higher for daytime versus nighttime, +Pain versus -Pain groups, and males versus females. Two clusters were identified, and regression relations showed associations of low-magnitude daytime masseter motor load, PHQ-15, and CPI scores for cluster 1 (n = 105, R2 = 0.44) and cluster 2 (n = 18, R2 = 0.80). Furthermore, these regression relations showed thresholds of motor load and PHQ-15 scores, above which there were nonlinear increases in reported pain.

Transient Receptor Potential (TRP) Ion Channels in Orofacial Pain

Orofacial pain, including temporomandibular joint disorders pain, trigeminal neuralgia, dental pain, and debilitating headaches, affects millions of Americans each year with significant population health impact. Despite the existence of a large body of information on the subject, the molecular underpinnings of orofacial pain remain elusive. Two decades of research has identified that transient receptor potential (TRP) ion channels play a crucial role in pathological pain. A number of TRP ion channels are clearly expressed in the trigeminal sensory system and have critical functions in the transduction and pathogenesis of orofacial pain. Although there are many similarities, the orofacial sensory system shows some distinct peripheral and central pain processing and different sensitivities from the spinal sensory system. Relative to the extensive review on TRPs in spinally-mediated pain, the summary of TRPs in trigeminally-mediated pain has not been well-documented. This review focuses on the current experimental evidence involving TRP ion channels, particularly TRPV1, TRPA1, TRPV4, and TRPM8 in orofacial pain, and discusses their possible cellular and molecular mechanisms.