Decreased face primary motor cortex (face-M1) excitability induced by noxious stimulation of the rat molar tooth pulp is dependent on the functional integrity of medullary astrocytes

Toll-Like Receptor 4 in the Rat Caudal Medulla Mediates Tooth Pulp Inflammatory Pain

The aims of this study were to investigate if Toll-like receptor 4 (TLR4) is expressed in the medullary dorsal horn (MDH) and if medullary application of a TLR4 antagonist (lipopolysaccharides from Rhodobacter sphaeroides, LPS-RS) can attenuate changes in nociceptive sensorimotor responses or TLR4 expression that might be evoked by mustard oil (MO) application to the right maxillary first molar tooth pulp. Of 41 adult male Sprague-Dawley rats used in the study, 23 received intrathecal application of the TLR4 antagonist LPS-RS (25 μg/10 μl; LPS-RS group) or isotonic saline (10 μl; vehicle control group) 10 min before pulpal application of MO (95%; 0.2 μl). Bilateral electromyographic (EMG) activities of the anterior digastric and masseter muscles were recorded continuously before and until 15 min after the MO application to the pulp. In 6 of these 23 rats and an additional 18 rats, the caudal medulla containing the ipsilateral and contralateral MDH was removed after euthanasia for subsequent Western Blot analysis of TLR4 expression in LPS-RS (n = 8) and vehicle (n = 8) groups and a naïve group (n = 8). The % change from baseline in the MO-evoked EMG activities within the anterior digastric muscles were significantly smaller in the LPS-RS group than the control group (two-way ANOVA, post hoc Bonferroni, P < 0.0001). Western Blot analysis revealed similar levels of TLR4 expression in the caudal medulla of the naïve, vehicle and LPS-RS groups. These novel findings suggest that TLR4 signaling in the caudal medulla may mediate MO-induced acute dental inflammatory pain in rats.

Alteration of occlusal vertical dimension induces signs of neuroplastic changes in corticomotor control of masseter muscles: Preliminary findings

PURPOSE

To investigate the effect of altering occlusal vertical dimension (OVD) in patients with severe attrition on corticomotor control of the masseter muscles as assessed by navigated transcranial magnetic stimulation (nTMS).

METHODS

Seven patients (58.6 ± 8.4 years) with decreased OVD due to severe attrition were given mandibular occlusal splints to alter the OVD with the instruction to wear during the whole awake time for a period of four weeks. Motor-evoked potentials (MEPs) and the motor cortex maps of the masseter muscles and first dorsal interosseous (FDI) muscles as control were recorded by nTMS at baseline and at least 4 weeks after the alteration of OVD. The stimulus-response curves of MEPs were analysed with two-way repeated-measures ANOVA, and the numerical rating scale scores, motor thresholds, onset latencies, motor cortex maps and centre of gravity (COG) were analysed with paired t tests.

RESULTS

There was a significant increase in the amplitude of the masseter muscle MEPs (P = 0.036), but no change in the motor cortex map areas (P = 0.111) four weeks after the alteration of OVD. Furthermore, there was no significant difference in either the amplitude of the FDI muscle MEPs (P = 0.466) or the motor cortex map areas (P = 0.230) before and after OVD alteration.

CONCLUSION

The results suggest that alteration of OVD in patients with severe attrition was associated with signs of neuroplastic changes in the corticomotor control of the masseter muscles. The results of the study may add to our understanding of the putative mechanisms related to cortical changes in response to OVD alterations.

Widespread Volumetric Brain Changes following Tooth Loss in Female Mice

Tooth loss is associated with altered sensory, motor, cognitive and emotional functions. These changes vary highly in the population and are accompanied by structural and functional changes in brain regions mediating these functions. It is unclear to what extent this variability in behavior and function is caused by genetic and/or environmental determinants and which brain regions undergo structural plasticity that mediates these changes. Thus, the overall goal of our research program is to identify genetic variants that control structural and functional plasticity following tooth loss. As a step toward this goal, here our aim was to determine whether structural magnetic resonance imaging (sMRI) is sensitive to detect quantifiable volumetric differences in the brains of mice of different genetic background receiving tooth extraction or sham operation. We used 67 adult female mice of 7 strains, comprising the A/J (A) and C57BL/6J (B) strains and a randomly selected sample of 5 of the 23 AXB-BXA strains (AXB1, AXB4, AXB24, BXA14, BXA24) that were produced from the A and B parental mice by recombinations and inbreeding. This panel of 25 inbred strains of genetically diverse inbred strains of mice is used for mapping chromosomal intervals throughout the genome that harbor candidate genes controlling the phenotypic variance of any trait under study. Under general anesthesia, 39 mice received extraction of 3 right maxillary molar teeth and 28 mice received sham operation. On post-extraction day 21, post-mortem whole-brain high-resolution sMRI was used to quantify the volume of 160 brain regions. Compared to sham operation, tooth extraction was associated with a significantly reduced regional and voxel-wise volumes of cortical brain regions involved in processing somatosensory, motor, cognitive and emotional functions, and increased volumes in subcortical sensorimotor and temporal limbic forebrain regions including the amygdala. Additionally, comparison of the 10 BXA14 and 21 BXA24 mice revealed significant volumetric differences between the two strains in several brain regions. These findings highlight the utility of high-resolution sMRI for studying tooth loss-induced structural brain plasticity in mice, and provide a foundation for further phenotyping structural brain changes following tooth loss in the full AXB-BXA panel to facilitate mapping genes that control brain plasticity following orofacial injury.