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

Functional roles of descending projections from the cerebral cortex to the trigeminal spinal subnucleus caudalis in orofacial nociceptive information processing

BACKGROUND

The trigeminal spinal subnucleus caudalis (Sp5C), also known as the medullary dorsal horn, receives orofacial somatosensory inputs, particularly nociceptive inputs, from the trigeminal nerve. In the Sp5C, excitatory and inhibitory neurons, glutamatergic and GABAergic/glycinergic neurons, respectively, form the local circuits. The axons of the glutamatergic neurons in lamina I ascend toward the thalamic and parabrachial nuclei, and this projection is the main pathway of orofacial nociception. Additionally, the axons of the higher brain regions, including the locus coeruleus, dorsal raphe, and cerebral cortex, are sent to the Sp5C.

HIGHLIGHT

Among these descending projections, this review focuses on the functional profiles of the corticotrigeminal projections to the Sp5C, along with their anatomical aspects. The primary and secondary somatosensory and insular cortices are of particular interest.

CONCLUSION

Corticotrigeminal projections from the somatosensory cortex to the Sp5C play a suppressive role in nociceptive information processing, whereas recent studies have demonstrated a facilitative role of the insular cortex in nociceptive information processing at the Sp5C level.

Periodontitis causally affects the brain cortical structure: A Mendelian randomization study

OBJECTIVE

To estimate whether genetically proxied periodontitis causally impacts the brain cortical structure using Mendelian randomization (MR).

BACKGROUND

Periodontitis is one of the most prevalent inflammatory conditions globally, and emerging evidence has indicated its influences on distal organs, including the brain, whose disorders are always accompanied by magnetic resonance imaging (MRI)-identified brain cortical changes. However, to date, no available evidence has revealed the association between periodontitis and brain cortical structures.

METHODS

The instrumental variables (IVs) were adopted from previous genome-wide association study (GWAS) studies and meta-analyses of GWAS studies of periodontitis from 1844 to 5266 cases and 8255 to 12 515 controls. IVs were linked to GWAS summary data of 51 665 patients from the ENIGMA Consortium, assessing the impacts of genetically proxied periodontitis on the surficial area (SA) or the cortical thickness (TH) of the global and 34 MRI-identified functional regions of the brain. Inverse-variance weighted was used as the primary estimate; the MR pleiotropy residual sum and outlier (MR-PRESSO), the MR-Egger intercept test, and leave-one-out analyses were used to examine the potential horizontal pleiotropy.

RESULTS

Genetically proxied periodontitis affects the SA of the medial orbitofrontal cortex, the lateral orbitofrontal cortex, the inferior temporal cortex, the entorhinal cortex, and the temporal pole, as well as the TH of the entorhinal. No pleiotropy was detected.

CONCLUSIONS

Periodontitis causally influences the brain cortical structures, implying the existence of a periodontal tissue-brain axis.

From teeth to brain: dental caries causally affects the cortical thickness of the banks of the superior temporal sulcus

OBJECTIVES

Dental caries is one of the most prevalent oral diseases and causes of tooth loss. Cross-sectional studies observed epidemiological associations between dental caries and brain degeneration disorders, while it is unknown whether dental caries causally affect the cerebral structures. This study tested whether genetically proxied DMFS (the sum of Decayed, Missing, and Filled tooth Surfaces) causally impacts the brain cortical structure using Mendelian randomization (MR).

METHODS

The summary-level GWAS meta-analysis data from the GLIDE consortium were used for DMFS, including 26,792 participants. ENIGMA (Enhancing NeuroImaging Genetics through Meta Analysis) consortium GWAS summary data of 51,665 patients were used for brain structure. This study estimated the causal effects of DMFS on the surface area (SA) and thickness (TH) of the global cortex and functional cortical regions accessed by magnetic resonance imaging (MRI). Inverse-variance weighted (IVW) was used as the primary estimate, the MR pleiotropy residual sum and outlier (MR-PRESSO), the MR-Egger intercept test, and leave-one-out analyses were used to examine the potential horizontal pleiotropy.

RESULTS

Genetically proxied DMFS decreases the TH of the banks of the superior temporal sulcus (BANSSTS) with or without global weighted (weighted, β = - 0.0277 mm, 95% CI: - 0.0470 mm to - 0.0085 mm, P = 0.0047; unweighted, β = - 0.0311 mm, 95% CI: - 0.0609 mm to - 0.0012 mm, P = 0.0412). The causal associations were robust in various sensitivity analyses.

CONCLUSIONS

Dental caries causally decrease the cerebral cortical thickness of the BANKSSTS, a cerebral cortical region crucial for language-related functions, and is the most affected brain region in Alzheimer's disease. This investigation provides the first evidence that dental caries causally affects brain structure, proving the existence of teeth-brain axes. This study also suggested that clinicians should highlight the causal effects of dental caries on brain disorders during the diagnosis and treatments, the cortical thickness of BANKSSTS is a promising diagnostic measurement for dental caries-related brain degeneration.

Insular cortical descending projections facilitate neuronal responses to noxious but not innoxious stimulation in rat trigeminal spinal subnucleus caudalis

The insular cortex (IC) receives orofacial nociceptive information. Pyramidal neurons in IC layer V send their axons to various brain regions, such as the trigeminal spinal subnucleus caudalis (Sp5C), parabrachial nucleus, and periaqueductal gray. However, little information has been available about the functions of these descending projections from the IC. This study aimed to elucidate the effect of IC → Sp5C on neuronal spike firings responding to noxious and innoxious stimuli to the face of the rat receiving an injection of adeno-associated virus encoding modified channelrhodopsin-2 (ChR2) fused to mCherry under the control of the human synapsin promotor. We classified Sp5C neurons responding to mechanical stimuli into three groups: low-threshold (LT), nociceptive specific (NS), and wide dynamic range (WDR) neurons, which respond to innoxious stimuli (brushing) only, noxious mechanical stimuli (pinching) only, and both noxious and innoxious stimuli, respectively. Neuronal activities of IC neurons were activated by photostimulation (repetitive pulses at 20 Hz for 5 Hz) to the IC that consistently induced action potentials in IC layer V pyramidal neurons. LT neurons showed comparable spike firing rates to brushing the facial skin before and during ChR2 activation induced by photostimulation. In contrast, NS neurons showed an increase in their firing frequency to pinching during ChR2 activation. On the other hand, WDR neurons increased their Sp5C neuronal firing to pinching during ChR2 activation without changing their firing rates to innoxious mechanical stimuli. These results suggest that the IC descending projections facilitate nociception by increasing Sp5C neuronal activities responding to noxious mechanical stimuli.

Descending projections from the insular cortex to the trigeminal spinal subnucleus caudalis facilitate excitatory outputs to the parabrachial nucleus in rats

ABSTRACT

Nociceptive information from the orofacial area projects to the trigeminal spinal subnucleus caudalis (Sp5C) and is then conveyed to several nuclei, including the parabrachial nucleus (PBN). The insular cortex (IC) receives orofacial nociceptive information and sends corticofugal projections to the Sp5C. The Sp5C consists of glutamatergic and GABAergic/glycinergic interneurons that induce excitatory postsynaptic currents and inhibitory postsynaptic currents, respectively, in projection neurons. Therefore, quantification of glutamatergic IC inputs in combination with identifying postsynaptic neuronal subtypes is critical to elucidate IC roles in the regulation of Sp5C activities. We investigated features of synaptic transmission from the IC to glutamatergic and GABAergic/glycinergic Sp5C neurons of laminae I/II using vesicular GABA transporter-Venus transgenic rats that received an injection of adeno-associated virus-channelrhodopsin-2-mCherry into the IC. Selective stimulation of IC axon terminals in Sp5C slice preparations induced monosynaptic excitatory postsynaptic currents in both excitatory glutamatergic and inhibitory GABAergic/glycinergic Sp5C neurons with a comparable amplitude. Paired whole-cell patch-clamp recordings showed that unitary inhibitory postsynaptic currents from inhibitory neurons influencing excitatory neurons, including neurons projecting to the PBN, exhibited a high failure rate and were suppressed by both bicuculline and strychnine, suggesting that excitatory neurons in the Sp5C receive both GABAergic and glycinergic inhibition with low impact. Moreover, selective stimulation of IC axons increased the firing rate at the threshold responses. Finally, we demonstrated that selective stimulation of IC axons in the Sp5C by a chemogenetic approach decreased the thresholds of both mechanical and thermal nociception. Thus, IC projection to the Sp5C is likely to facilitate rather than suppress excitatory outputs from the Sp5C.

Repetitive nociceptive stimulation increases spontaneous neural activation similar to nociception-induced activity in mouse insular cortex

Recent noninvasive neuroimaging technology has revealed that spatiotemporal patterns of cortical spontaneous activity observed in chronic pain patients are different from those in healthy subjects, suggesting that the spontaneous cortical activity plays a key role in the induction and/or maintenance of chronic pain. However, the mechanisms of the spontaneously emerging activities supposed to be induced by nociceptive inputs remain to be established. In the present study, we investigated spontaneous cortical activities in sessions before and after electrical stimulation of the periodontal ligament (PDL) by applying wide-field and two-photon calcium imaging to anesthetized GCaMP6s transgenic mice. First, we identified the sequential cortical activation patterns from the primary somatosensory and secondary somatosensory cortices to the insular cortex (IC) by PDL stimulation. We, then found that spontaneous IC activities that exhibited a similar spatiotemporal cortical pattern to evoked activities by PDL stimulation increased in the session after repetitive PDL stimulation. At the single-cell level, repetitive PDL stimulation augmented the synchronous neuronal activity. These results suggest that cortical plasticity induced by the repetitive stimulation leads to the frequent PDL stimulation-evoked-like spontaneous IC activation. This nociception-induced spontaneous activity in IC may be a part of mechanisms that induces chronic pain.

A new phenotype identification method with the fluorescent expression in cross-sectioned tails in Thy1-GCaMP6s transgenic mice

PURPOSE

Unless the phenotype of the transgenic mice is distinguishable, genotyping in each mouse is required prior to experiments. This study aimed to establish a new identification method for the phenotype in Thy1-GCaMP6s transgenic mice to reduce the cost and time.

METHODS

Tail biopsies (2 mm) were performed under general anesthesia with isoflurane in 3 to 4-week-old mice. Then, the resected tail was cut again with a sharp razor, and the cross-sections were observed with two-photon microscopy (excitation wavelength = 940 nm). The emitted light was split into green and red light by a dichroic mirror (570 nm) with bandpass filters (495-540 nm for green, 575-645 nm for red).

RESULTS

Two types of expressed fluorescent pattern were found in the tail tissue: the presence of green fluorescent structures (type 1) and the absence of the structures (type 2). Cortical imaging confirmed that type 1 expressed the cortical GCaMP6s, while type 2 did not.

CONCLUSION

These results suggest that observation of the cross-sectioned tail in Thy1-GCaMP6s mice enabled to identify the phenotype within approximately 10 min/mouse, which reduces the cost and time for genotyping.

N/OFQ modulates orofacial pain induced by tooth movement through CGRP-dependent pathways

BACKGROUND

Nociceptin/orphanin FQ (N/OFQ) has been revealed to play bidirectional roles in orofacial pain modulation. Calcitonin gene-related peptide (CGRP) is a well-known pro-nociceptive molecule that participates in the modulation of orofacial pain. We aimed to determine the effects of N/OFQ on the modulation of orofacial pain and on the release of CGRP.

METHODS

Orofacial pain model was established by ligating springs between incisors and molars in rats for the simulation of tooth movement. The expression level of N/OFQ was determined and pain level was scored in response to orofacial pain. Both agonist and antagonist of N/OFQ receptor were administered to examine their effects on pain and the expression of CGRP in trigeminal ganglia (TG). Moreover, gene therapy based on the overexpression of N/OFQ was delivered to validate the modulatory role of N/OFQ on pain and CGRP expression.

RESULTS

Tooth movement elicited orofacial pain and an elevation in N/OFQ expression. N/OFQ exacerbated orofacial pain and upregulated CGRP expression in TG, while UFP-101 alleviated pain and downregulated CGRP expression. N/OFQ-based gene therapy was successful in overexpressing N/OFQ in TG, which resulted in pain exacerbation and elevation of CGRP expression in TG.

CONCLUSIONS

N/OFQ exacerbated orofacial pain possibly through upregulating CGRP.