Estrogen exacerbates the nociceptive effects of peripheral serotonin on rat trigeminal sensory neurons

The roles of 5-HT in orofacial pain

OBJECTIVES

Acute and chronic orofacial pain are very common and remain a vexing health problem that has a negative effect on the quality of life. Serotonin (5-HydroxyTryptamine, 5-HT) is a kind of monoamine neurotransmitter that is involved in many physiological and pathological processes. However, its role in orofacial pain remains inconclusive. Therefore, this review aims to summarize the recent advances in understanding the effect exerted by 5-HT on the modulation of orofacial pain.

SUBJECTS AND METHODS

An extensive search was conducted on PubMed and Web of Science for pertinent studies focusing on the effects of 5-HT on the modulation of orofacial pain.

RESULTS

In this review, we concisely review how 5-HT mediates orofacial pain, how 5-HT is regulated and how we can translate these findings into clinical applications for the prevention and/or treatment of orofacial pain.

CONCLUSIONS

5-HT plays a key role in the modulation of orofacial pain, implying that 5-HT modulators may serve as effective treatment for orofacial pain. However, further research on the precise mechanisms underlying the modulation of orofacial pain is still warranted.

Ovariectomy induces hyperalgesia accompanied by upregulated estrogen receptor α and protein kinase B in the rat spinal cord

Hormone supplementation is one of the common therapies for menopause-related disorders. Among different tools, the ovariectomy (OVX) rodents are widely accepted as an appropriate menopausal pain model. Our previous study has showed that OVX produces a significant pain facilitation in both acute pain and tonic pain, however, the underlying mechanisms remain unclear. In this study, we examined the effects of OVX treatment and estradiol (E2) supplementation on formalin-induced nociceptive responses, and explored the associated spinal mechanisms. Female Sprague-Dawley rats underwent bilateral OVX, and E2 supplementation was given subcutaneously from the 5th week after surgery (30 μg/day for 7 days). Our results showed that formalin-induced nociceptive behaviors did not differ between diestrus and proestrus stages of the estrous in intact rats. However, OVX exacerbated formalin-evoked inflammatory pain, especially in the late phase at 4-5 weeks but not 2 weeks post-surgery. E2 supplementation significantly reversed the OVX-triggered hyperalgesia. Double immunofluorescence staining revealed that both ERα and ERβ in the spinal dorsal horn were co-labeled with the neuronal markers, but not with markers of astrocytes or microglia. The spinal ERα (but not ERβ) expression significantly increased in the OVX group, which was reversed by E2 supplementation. Moreover, the OVX individuals showed an increased protein kinase B (AKT) level in lumbar spinal cord, and E2 supplementation diminished the AKT expression in OVX rats. Finally, intrathecal injection Wortmannin, an inhibitor for AKT signaling, effectively reduced the nociceptive behaviors in the late phase and the number of c-fos positive cells. Together, our findings indicate that E2 supplementation alleviates the OVX-induced hyperalgesia, which might be involved in spinal ERα and AKT mechanisms.

Sex-Specific Pain Outcomes Following Microvascular Decompression for Trigeminal Neuralgia

BACKGROUND

Trigeminal neuralgia (TN) is more prevalent among women. However, while microvascular decompression (MVD) is the most effective long-term surgical treatment for TN, it is unclear whether it is equally efficacious for men and women. We sought to characterize the relationship between sex and pain outcomes following MVD for TN.

METHODS

From 2007 to 2020, 938 unilateral TN patients were treated with MVD at our institution. Patient demographics, clinical characteristics, operative features, and pain outcomes were recorded. Differences between men and women were analyzed via t-test and chi-squared analyses. A multivariate ordinal regression was used to establish significant predictors of pain outcome. Differences in time to pain recurrence were assessed via Cox proportional hazards and Kaplan-Meier nonparametric survival analysis.

RESULTS

A majority (67%) of the 938 patients analyzed were female. Men and women presented with similar preoperative pain severity (P = 0.17). Female sex (P = 0.048) and younger age (P = 0.03) were independently associated with worsened Barrow Neurological Institute pain scores at 3-month follow-up on multivariate analysis. Women were also more likely to experience recurrence than men (P = 0.01), and time to recurrence was shorter among women (P = 0.02). Only female sex was independently associated with increased risk of postoperative pain recurrence on multivariate Cox proportional hazards regression (P = 0.01).

CONCLUSIONS

Female TN patients undergoing MVD had worse pain outcomes, more frequent pain recurrence, and shorter time to recurrence. Our results indicate a sex-specific dimorphism in response to MVD among TN patients.

Estrogen modulation of the pronociceptive effects of serotonin on female rat trigeminal sensory neurons is timing dependent and dosage dependent and requires estrogen receptor alpha

ABSTRACT

The role of the major estrogen estradiol (E2) on orofacial pain conditions remains controversial with studies reporting both a pronociceptive and antinociceptive role of E2. E2 modulation of peripheral serotonergic activity may be one mechanism underlying the female prevalence of orofacial pain disorders. We recently reported that female rats in proestrus and estrus exhibit greater serotonin (5HT)-evoked orofacial nocifensive behaviors compared with diestrus and male rats. Further coexpression of 5HT 2A receptor mRNA in nociceptive trigeminal sensory neurons that express transient receptor potential vanilloid 1 ion channels contributes to pain sensitization. E2 may exacerbate orofacial pain through 5HT-sensitive trigeminal nociceptors, but whether low or high E2 contributes to orofacial pain and by what mechanism remains unclear. We hypothesized that steady-state exposure to a proestrus level of E2 exacerbates 5HT-evoked orofacial nocifensive behaviors in female rats, explored the transcriptome of E2-treated female rats, and determined which E2 receptor contributes to sensitization of female trigeminal sensory neurons. We report that a diestrus level of E2 is protective against 5HT-evoked orofacial pain behaviors, which increase with increasing E2 concentrations, and that E2 differentially alters several pain genes in the trigeminal ganglia. Furthermore, E2 receptors coexpressed with 5HT 2A and transient receptor potential vanilloid 1 and enhanced capsaicin-evoked signaling in the trigeminal ganglia through estrogen receptor α. Overall, our data indicate that low, but not high, physiological levels of E2 protect against orofacial pain, and we provide evidence that estrogen receptor α receptor activation, but not others, contributes to sensitization of nociceptive signaling in trigeminal sensory neurons.

Preclinical models of deep craniofacial nociception and temporomandibular disorder pain

Chronic pain in temporomandibular disorder (TMD) is a common health problem. Cumulating evidence indicates that the etiology of TMD pain is complex with multifactorial experience that could hamper the developments of treatments. Preclinical research is a resource to understand the mechanism for TMD pain, whereas limitations are present as a disease-specific model. It is difficult to incorporate multiple risk factors associated with the etiology that could increase pain responses into a single animal. This article introduces several rodent models which are often employed in the preclinical studies and discusses their validities for TMD pain after the elucidations of the neural mechanisms based on the clinical reports. First, rodent models were classified into two groups with or without inflammation in the deep craniofacial tissues. Next, the characteristics of each model and the procedures to identify deep craniofacial pain were discussed. Emphasis was directed on the findings of the effects of chronic psychological stress, a major risk factor for chronic pain, on the deep craniofacial nociception. Preclinical models have provided clinically relevant information, which could contribute to better understand the basis for TMD pain, while efforts are still required to bridge the gap between animal and human studies.