Behavioral Responses and Expression of Nociceptin/Orphanin FQ and Its Receptor (N/OFQ-NOP System) during Experimental Tooth Movement in Rats

Objective. To determine behavioral testing after experimental tooth movement in rats and to explore the role of nociceptin/orphanin FQ and its receptor (the N/OFQ-NOP system) in pain induced by experimental tooth movement. Design. The mouth-wiping behavior of rats was assessed by studying behavioral responses after experimental tooth movement. The distribution of N/OFQ in the periodontal ligament, the trigeminal ganglion (TG), and the caudal one-third of the trigeminal subnucleus caudalis (Vc) was assessed by immunohistochemistry. The variations in N/OFQ expression in the TG and Vc were measured by Western blotting. The ongoing changes in the gene expression of the prepronociceptin gene and opioid receptor-like 1 receptor were assessed in the TG and Vc by real-time polymerase chain reaction (RT-PCR). Results. Overall, the mouth-wiping behavior increased significantly. The behavior first increased and then gradually decreased to a low level, showing cyclical variation. N/OFQ immunoreactivity increased in the periodontal ligament after tooth movement. ppN/OFQ mRNA and protein levels showed a time-dependent increase in the TG and were positively correlated with pain stimulus. NOP gene levels showed large fluctuations. In the Vc, the expression and changes in the N/OFQ-NOP system showed the opposite trend as those noted in TG and the periodontal membrane. Conclusion. The N/OFQ system may have a complex regulatory function in the pain induced by tooth movement and may be related to inflammation caused by orthodontic tooth movement and periodontal damage. The specific mechanism remains to be further studied.

In Vivo Ca2+ Imaging of the Insular Cortex during Experimental Tooth Movement

Pain and discomfort are common problems for patients undergoing orthodontic treatment. We have demonstrated that cortical excitation propagation in the somatosensory and insular cortices (IC) induced by electrical stimulation of the periodontal ligament (PDL) is facilitated 1 d after experimental tooth movement (ETM). However, it is necessary to examine ETM-induced changes in neural responses at a single-cell level to understand the mechanisms of cortical plastic changes, in which excitatory glutamatergic and inhibitory GABAergic neurons are intermingled to form cortical local circuits. We performed in vivo 2-photon Ca2+ imaging by loading the Ca2+ indicator Oregon Green BAPTA with the astrocyte marker sulforhodamine. We focused on the IC region that exhibited the largest neural response to maxillary PDL (mxPDL) stimulation using a VGAT-Venus transgenic rat that expresses venus fluorescent protein in GABAergic neurons and discerned changes in the neural activities of each cortical neuronal subtype before and during ETM treatment of the maxillary incisor and first molar. Notably, 1 d after ETM treatment (1d-ETM), the number of neurons responding to mxPDL stimulation increased from 47.6% to 64.2% in excitatory neurons and from 44.5% to 66.2% in inhibitory neurons. On the other hand, only 3% to 4% of excitatory and inhibitory neurons responded to mandibular molar PDL (mbPDL) stimulation in control rats, and the 1d-ETM group showed significant increases in excitatory (14.0%) and inhibitory neurons (22.5%) responding to mbPDL stimulation. Interestingly, most mbPDL-responding neurons also responded to mxPDL stimulation. The population of excitatory and inhibitory neurons that responded only to mxPDL stimulation was comparable between the control and 1d-ETM groups. The facilitative responses in the 1d-ETM group had almost recovered 7 d after ETM treatment. These results suggest that ETM induces parallel increases in PDL-responding neurons and changes some insensitive neurons to respond to both mxPDL and mbPDL stimulation.