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

Abstract

Infection-induced chronic pain is an under-studied pain condition. One example is apical periodontitis, which evokes considerable mechanical allodynia that persists after treatment in 7% to 12% of patients. Available analgesics often provide incomplete relief. However, a preclinical model to study pain mechanisms associated with apical periodontitis is not available. Here, we report a mouse model of apical periodontitis to facilitate studies determining mechanisms mediating persistent infection-induced pain. Mice were anesthetized and the left first molar was exposed to the oral environment for six weeks. Bone resorption, as an indicator of apical periodontitis, was quantified using microcomputed tomography. Mechanical allodynia was determined using extraoral von-Frey filaments in both male and female mice. The expression of c-fos in the medullary dorsal horn was assessed using immunohistochemistry. Mice with apical periodontitis developed significant mechanical allodynia by day 7 that was maintained for 42 days. Mechanical thresholds were significantly lower in females compared to males. Administration of ibuprofen, morphine, or MK-801 reversed mechanical allodynia. Finally, apical periodontitis triggered an upregulation of c-fos in the medullary dorsal horn. Collectively, this model simulates signs of clinical pain experienced by patients with apical periodontitis, detects sex differences in allodynia, and permits the study of peripheral and central trigeminal pain mechanisms.

Keywords: Apical periodontitis; central sensitization; dental pain; mechanical allodynia; orofacial pain; pain model.

Figure 1.

Images demonstrating pulp exposures and site of mechanical allodynia. (a) Representative image demonstrating pulp exposure of mouse maxillary left first molar tooth. (b) Schematic representing area of von Frey testing for mechanical allodynia.

Figure 2.

Effect of pulp exposures on induction of apical periodontitis using µCT analysis. (a) Representative left (L) and right (R) coronal scans of control animals (upper panel) and apical periodontitis animals (lower panel; arrow). (b) Representative left (L) and right (R) axial scans of control animals (left side) and apical periodontitis animals (right side; arrow). (c) Quantification of % void volume between left and right side of control and apical periodontitis animals. Statistical analysis was performed using two-way ANOVA with Sidak’s multiple comparison test (N = 3 maxillae/group; error bars = standard error of the mean; ****p < 0.001 compared to right side). Note: Data generated using male mice.

Figure 3.

Effect of pulp exposures on development of mechanical allodynia in male mice. (a) EF₅₀ values comparing control and apical periodontitis animals on days 1, 7, 14, 21, 28, 35, and 42 after pulp exposures to the left maxillary left first molar (*BL = Baseline). (b–d) Stimulus-response curves comparing control (c) and apical periodontitis (d) animals on days 1, 7, 14, 21, 28, 35, and 42 after pulp exposures to the left maxillary left first molar. Statistical analysis was performed using two-way ANOVA with Sidak’s multiple comparison test (N = 10 animals/group; error bars = standard error of the mean; ****p < 0.001 compared to control group at all time points).

Figure 4.

Effect of pulp exposures on development of mechanical allodynia in female mice. (a) EF₅₀ values comparing control and apical periodontitis animals on days 1, 7, 14, 21, 28, 35, and 42 after pulp exposures to the left maxillary left first molar (*BL = Baseline). (b–d) Stimulus-response curves comparing control (c) and apical periodontitis (d) animals on days 1, 7, 14, 21, 28, 35, and 42 after pulp exposures to the left maxillary left first molar. Statistical analysis was performed using two-way ANOVA with Sidak’s multiple comparison test (N = 6 animals/group; error bars = standard error of the mean; ****p < 0.001 compared to control group at all time points).

Figure 5.

Comparison of mechanical allodynia between male and female mice. Mean EF₅₀ values between male and female mice at BL and day 21 post-pulp exposure. Statistical analysis was performed using two-way ANOVA with Sidak’s multiple comparison test (N = 6–10 animals/group; error bars = standard error of the mean; ****p < 0.001 within group, **p < 0.01 compared to control group at all time points).

Figure 6.

Effect of known analgesics on reversal of apical periodontitis-induced mechanical allodynia. (a–c) Mean EF₅₀ values at BL and day 21 post-pulp exposure with and without i.p. injection of ibuprofen (a), morphine (b), and MK-801 (c). Statistical analysis was performed using two-way ANOVA with Sidak’s multiple comparison test (N = 4 animals/group; error bars = standard error of the mean; ****p < 0.001 within group, **p < 0.01, p < 0.05 compared to respective control group at all time points). Note: Data generated using male mice.

Figure 7.

Immunohistochemistry of c-fos in trigeminal ganglia and medullar dorsal horn. (a and b) Representative immunohistochemical images evaluating expression of c-fos and CGRP in the V2 region of the trigeminal ganglia between control and apical periodontitis animals. (c and d) Representative immunohistochemical images evaluating expression of c-fos and NeuN in lamina I and II of the left medullary dorsal horn between control and apical periodontitis animals. Arrows in (d) indicates co-localization of c-fos with NeuN. Note: Data generated using male mice.