Apical periodontitis-induced mechanical allodynia: A mouse model to study infection-induced chronic pain conditions

Sex-specific nociceptor modulation of the apical periodontitis transcriptome

Introduction: Apical periodontitis (AP) is a painful disease that develops quickly following dental infections and is primarily characterized by robust inflammation surrounding the tissues of the affected tooth, resulting in disruption of bone homeostasis and periradicular bone loss. Moreover, there are distinct clinical presentations, symptoms, and responses to AP treatment between male and female subjects, creating a desperate need to further understand the sex-specific mechanisms of AP. Methods: With the growing evidence that nociceptors modulate AP development, we utilized RNA sequencing in nociceptor-ablated (Nav1.8 cre+/-, diphtheria toxin Alox+/-) transgenic mice to study the nociceptor regulation of the periapical lesion transcriptome using a rodent model of AP in female mice over 14 days. Results: Overall, we found that female mice exhibit unique patterns of differentially expressed genes throughout AP infection compared to male mice and that the expression of these genes is regulated by nociceptors. Additionally, nociceptor ablation results in a more significant enrichment of biological processes related to immune responses earlier compared to cre-control (Nav1.8 cre+/-) females and greater expression of genes involved in inflammatory processes and osteolytic activity. Discussion: Therefore, while nociceptor ablation augments inflammatory and bone resorption responses in both males and females in a mouse model of AP, transcriptomic analyses demonstrate that the mechanisms through which nociceptors modulate AP are distinct between sexes. These studies will provide the foundation needed to study further mechanisms of sex differences in AP, an area with a desperate need for investigation to treat current AP patients. Understanding these mechanisms can ultimately inform treatment options to alleviate suffering for millions of patients suffering from AP.

Sex-dependent differences in the genomic profile of lingual sensory neurons in naïve and tongue-tumor bearing mice

Mechanisms of sex-dependent orofacial pain are widely understudied. A significant gap in knowledge exists about comprehensive regulation of tissue-specific trigeminal sensory neurons in diseased state of both sexes. Using RNA sequencing of FACS sorted retro-labeled sensory neurons innervating tongue tissue, we determined changes in transcriptomic profiles in males and female mice under naïve as well as tongue-tumor bearing conditions Our data revealed the following interesting findings: (1) FACS sorting obtained higher number of neurons from female trigeminal ganglia (TG) compared to males; (2) Naïve female neurons innervating the tongue expressed immune cell markers such as Csf1R, C1qa and others, that weren't expressed in males. This was validated by Immunohistochemistry. (3) Accordingly, immune cell markers such as Csf1 exclusively sensitized TRPV1 responses in female TG neurons. (4) Male neurons were more tightly regulated than female neurons upon tumor growth and very few differentially expressed genes (DEGs) overlapped between the sexes, (5) Male DEGs contained higher number of transcription factors whereas female DEGs contained higher number of enzymes, cytokines and chemokines. Collectively, this is the first study to characterize the effect of sex as well as of tongue-tumor on global gene expression, pathways and molecular function of tongue-innervating sensory neurons.

Early life stress exacerbates bone resorption and inhibits anxiety-like behaviour induced by apical periodontitis in rats

AIM

To evaluate the influence of the early life stress (ELS) on the severity of the apical periodontitis (AP) in Wistar rats.

METHODOLOGY

Forty male Wistar rats were divided into four groups (n = 10): Control rats; AP-rats with AP; ELS-rats subject to ELS; AP + ELS-rats exposed to ELS and subject to AP. ELS was induced by maternal separation (MS) for a period of 3 h for 21 consecutive days. AP was induced via pulp exposure of the first and second right maxillary molars to the oral environment for 40 days. Three days before euthanasia, all rats underwent behavioural analysis to measure anxiety levels by elevated zero maze. Then, the rats were euthanized and the maxillas were removed to assess the occurrence and severity of AP. The periapical region was evaluated for the intensity of the inflammatory infiltrate and the extent of bone loss. The Mann-Whitney test was performed for nonparametric data, and the Tukey's or Student's t-test was performed for parametric data (p < .05).

RESULTS

The intensity of the inflammatory infiltrate was significantly larger in the AP + ELS group when compared with AP group (p < .05). The AP + ELS group exhibited significantly greater alveolar bone loss, with a periapical lesion size of 103.5 ± 29.88, compared with 72.3 ± 22.28 in the AP group (p < .05). Rats with AP displayed higher anxiety-like behaviour in relation to the control group (p < .05). However, exposure to ELS abolished the AP-induced increased anxiety-like 'behaviour' throughout, since that rats from AP + ELS group attended more the open arms than non-stressed rats with AP (p < .05).

CONCLUSION

Early life stress is predictive of the severity of AP exacerbating the inflammatory process and increasing periapical bone resorption.

Increased Anxiety-Like Behavior in the Acute Phase of a Preclinical Model of Periodontal Disease

Periodontal disease (PD) is an infectious-inflammatory oral disease that is highly prevalent among adolescence and adulthood and can lead to chronic orofacial pain and be associated with anxiety, stress and depression. This study aimed to identify anxiety-like behaviors in the ligature-induced murine preclinical model of PD in different phases of the disease (i.e., acute vs. chronic). Also, we investigated orofacial mechanical allodynia thresholds and superficial cortical plasticity along the orofacial motor cortex in both disease phases. To this aim, 25 male Wistar rats were randomly allocated in acute (14 days) or chronic (28 days) ligature-induced-PD groups and further divided into active-PD or sham-PD. Anxiety-like behavior was evaluated using the elevated plus maze, mechanical allodynia assessed using the von Frey filaments test and superficial motor cortex mapping was performed with electrical transdural stimulation. We observed increased anxiety-like behavior in active-PD animals in the acute phase, characterized by decreased number of entries into the open arm extremities [t (1,7) = 2.42, p = 0.04], and reduced time spent in the open arms [t (1,7) = 3.56, p = 0.01] and in the open arm extremities [t (1,7) = 2.75, p = 0.03]. There was also a reduction in the mechanical allodynia threshold in all active-PD animals [Acute: t (1,7) = 8.81, p < 0.001; Chronic: t (1,6) = 60.0, p < 0.001], that was positively correlated with anxiety-like behaviors in the acute group. No differences were observed in motor cortex mapping. Thus, our findings show the presence of anxiety-like behaviors in the acute phase of PD making this a suitable model to study the impact of anxiety in treatment response and treatment efficacy.