Physiological profiles of cortical responses to mechanical stimulation of the tooth in the rat: An optical imaging study.

The periodontal ligament (PDL) includes several types of nerve endings, such as Aß-, Aδ-, and C-fibers, which play critical roles in detecting the strength and direction of occlusal force. Previous studies have demonstrated that electrical stimulation of the PDL activates the somatosensory and insular cortices. However, the profile of cortical excitation in response to mechanical PDL stimulation mostly remains unknown. To investigate the differences in cortical responses to electrical and mechanical stimulation of the maxillary first molar, we performed optical imaging to determine the responding cortical regions in combination with a pharmacological approach. The molar was mechanically stimulated by pulling in the rostral direction, and electrical stimulation was applied via bipolar electrodes inserted into the mesial PDL. Mechanical stimulation initially excited the primary somatosensory cortex (S1), whereas electrical stimulation evoked an initial response between the secondary somatosensory cortex (S2) and insular oral region (IOR). The characteristic feature responding to mechanical stimulation was the rebound response evoked at the end of mechanical stimulation. A longer mechanical stimulation evoked a larger amplitude of the rebound response. A paired-pulse protocol of mechanical stimulation revealed that the amplitude of the second response was smaller than the first response, in accordance with the shorter interstimulus interval. Systemic application of morphine, a potent blocker of nociception, reduced the amplitude of the maximum excitation, particularly in S2/IOR compared to S1. These results suggest that S1 and S2/IOR are principally excited by mechanical and electrical stimulation, respectively, and that S2/IOR is involved in nociception processing.

Neural mechanisms of nociception during orthodontic treatment.

Orthodontic tooth movement is accompanied by inflammatory responses in the periodontal ligament. Chemical mediators such as interleukin-1ß have key roles in nociception around teeth. Such nociceptive inputs to the periodontal ligament continue for several days and potentially induce plastic changes in higher brain regions, including the cerebral cortex. This review summarizes research on orthodontic treatment-induced modulation of neural activities in the central nervous system. Furthermore, we describe our recent findings on the spatiotemporal effects of orthodontic treatment in the somatosensory and insular cortices.

Effect of low-intensity pulsed ultrasound (LIPUS) on mandibular condyle growth in rats analyzed with micro-CT.

This study examined the effects of a bite-jumping appliance combined with low-intensity pulsed ultrasound (LIPUS) stimulation on the mandibular condyle of growing rats using micro CT (mCT) and histological examinations. Twelve Wistar rats were divided into three groups of four individuals each: Group 1 was an untreated control group, Group 2 received bite-jumping appliances, and Group 3 received bite-jumping appliances and LIPUS stimulation (15 min/day, 2 weeks) to the temporomandibular region. We measured the length and three-dimensional bone volume of each rat's mandibular condyle using mCT. The condylar cartilage was observed after the rats had been sacrificed. There was no significant difference in condylar sagittal width among the groups. The bite-jumping appliance combined with LIPUS stimulation increased the condylar major axis, mandibular sagittal length and condylar bone volume to a greater degree than use of the bite-jumping appliance alone. Histological examination demonstrated hypertrophy of the condylar cartilage layers, the fibrous layer and hypertrophic cell layer of the rats treated with bite-jumping appliances combined with LIPUS stimulation in comparison to rats treated with bite-jumping appliances alone. (J Oral Sci 58, 415-422, 2016).

Cone-beam computed tomography evaluation of the association of cortical plate proximity and apical root resorption after orthodontic treatment.

We investigated the effects of proximity of the root apex to the maxillary labial cortical plate, palatal cortical plate, and incisive canal cortical plate on apical root resorption. Cone-beam computed tomography was used to measure the amount of root resorption and root apex movement around maxillary right and left central incisors in 30 adults who underwent four-bicuspid extraction followed by treatment with multibracket appliances. The patients were divided into three groups on the basis of the direction of root apex movement, after which the correlation between the amount of root resorption and root apex movement was determined. Mean apical root resorption was 1.80 ± 0.82 mm (range, 0.18-3.96 mm). The amount of root apex movement was positively correlated with the amount of root resorption on the side of pressure. Root apex proximity to the maxillary labial cortical plate, palatal cortical plate, and incisive canal cortical plate was associated with apical root resorption. Orthodontic treatment plans should carefully consider root proximity to the maxillary cortical plate. (J Oral Sci 58, 231-236, 2016).

Opposite effects of mu and delta opioid receptor agonists on excitatory propagation induced in rat somatosensory and insular cortices by dental pulp stimulation.

The insular cortex (IC) contributes to nociceptive information processing. IC neurons express opioid receptors, including the mu (MOR), kappa (KOR), and delta (DOR) subtypes. Opioidergic agonists suppress excitatory synaptic transmission in the cerebral cortex. In addition, morphine injection into the IC reduces responses to noxious thermal stimuli. However, the mechanisms of the opioid-dependent modulation of cortical excitation at the macroscopic level, which bridge the cellular and behavioral findings, have remained unknown. The present in vivo optical imaging study aimed to examine the effects of the agonists of each subtype on cortical excitatory propagation in the IC and the neighboring cortices, the primary (S1) and secondary somatosensory (S2) areas. To assess the opioidergic effects on the cortical circuits, we applied electrical stimulation to the maxillary 1st molar pulp, which induced excitation in the ventral part of S1 and the S2/insular oral region (IOR). The initial excitatory response was observed 10-14ms after stimulation, and then excitation propagated concentrically. DAMGO (10-100µM), an MOR agonist, suppressed the amplitude of cortical excitation and shrank the maximum excitation areas in S1 and S2/IOR. In contrast, 10-100µM DPDPE, a DOR agonist, increased the amplitude of excitation and expanded the area of maximum excitation. U50488 (10-100µM), a KOR agonist, had little effect on cortical excitation. These results suggest that MOR-induced suppression of excitatory propagation in the IC is an underlying mechanism of the powerful analgesic effects of MOR agonists. In contrast, DOR may play a minor role in suppressing acute pain.