Activation of trigeminal ganglion satellite glial cells in CFA-induced tooth pulp pain in rats

The purinergic receptor P2X3 promotes facial pain by activating neurons and cytokines in the trigeminal ganglion

The mechanism underlying allodynia/hyperalgesia caused by dental pulpitis has remained enigmatic. This investigation endeavored to characterize the influence of the purinergic receptor P2X3 on pain caused by experimental pulpitis and the mechanism involved. An experimental model of irreversible pulpitis was produced by the drilling and exposure of the dental pulp of the left upper first and second molars in rats, followed by measuring nociceptive responses in the oral and maxillofacial regions. Subsequently, neuronal activity and the expression of P2X3 and pertinent cytokines in the trigeminal ganglion (TG) were meticulously examined and analyzed. Histological evidence corroborated that significant pulpitis was produced in this model, which led to a distinct escalation in nociceptive responses in rats. The activation of neurons, coupled with the upregulated expression of c-fos, P2X3, p-p38, TNF-α and IL-1β, was identified subsequent to the pulpitis surgery within the TG. The selective inhibition of P2X3 with A-317491 effectively restrained the abnormal allodynia/hyperalgesia following the pulpitis surgery and concurrently inhibited the upregulation of p-p38, TNF-α and IL-1β within the TG. These findings suggest that the P2X3 signaling pathway plays a pivotal role in instigating and perpetuating pain subsequent to the induction of pulpitis in rats, implicating its association with the p38 MAPK signaling pathway and inflammatory factors.

Fructose supplementation shifts rat brain metabolism in experimental migraine

AIMS

We have previously demonstrated that fructose supplementation (FS), given in a scheme used for inducing metabolic syndrome (MS), elicited pain relief in the nitroglycerin (NTG)-elicited rat migraine model. Herein, we evaluated whether FS could reestablish the impaired metabolic pathways in NTG-injected rats.

MAIN METHODS

Male Wistar rats (N=40) were divided into two groups for receiving 10-% FS or tap water. After 45 days, they were subdivided into NTG-injected (10mg/kg; 15 days) or controls. After the fourth NTG injection, ¹⁸F-fluorodeoxyglucose ([¹⁸F] FDG) micro-PET scanning was accomplished. The day after, euthanasia was performed, and blood was collected for glycemia and LDH analysis. The levels of energy molecules, TBARS, PGC-1α, and MCTS1 were evaluated in the brain cortices. The activated satellite glial cells (SGC) were assessed in the trigeminal ganglion (TG).

KEY FINDINGS

There were no variations of glycemia or LDH serum levels. NTG-injected rats showed a significant increase in glucose uptake in the hypothalamus (HT) vs. NTG-free rats. The FS-NTG group showed increased metabolism in the superior colliculus (SC) vs. the NTG group. Moreover, the glucose uptake was amplified in the inferior colliculus (IC) of the FS-NTG vs. FS group. The cortical inosine levels were significantly higher in FS-NTG rats vs. NTG or FS groups, with no changes in TBARS or MCTS1 levels, despite a minor decrease of PGC1-α contents in the FS+NTG group. Finally, there was a significant increase of activated SGC around TG in the FS-NTG rats.

SIGNIFICANCE

We provide novel evidence linking nutrition and metabolism with migraine.

Inhibition of the Nav1.7 Channel in the Trigeminal Ganglion Relieves Pulpitis Inflammatory Pain

Pulpitis causes significant changes in the peripheral nervous system, which induce hyperalgesia. However, the relationship between neuronal activity and Nav1.7 expression following pulpal noxious pain has not yet been investigated in the trigeminal ganglion (TG). The aim of our study was to verify whether experimentally induced pulpitis activates the expression of Nav1.7 peripherally and the neuronal activities of the TGs can be affected by Nav1.7 channel inhibition. Acute pulpitis was induced through allyl isothiocyanate (AITC) application to the rat maxillary molar tooth pulp. Three days after AITC application, abnormal pain behaviors were recorded, and the rats were euthanized to allow for immunohistochemical, optical imaging, and western blot analyses of the Nav1.7 expression in the TG. A significant increase in AITC-induced pain-like behaviors and histological evidence of pulpitis were observed. In addition, histological and western blot data showed that Nav1.7 expressions in the TGs were significantly higher in the AITC group than in the naive and saline group rats. Optical imaging showed that the AITC group showed higher neuronal activity after electrical stimulation of the TGs. Additionally, treatment of ProTxII, selective Nav1.7 blocker, on to the TGs in the AITC group effectively suppressed the hyperpolarized activity after electrical stimulation. These findings indicate that the inhibition of the Nav1.7 channel could modulate nociceptive signal processing in the TG following pulp inflammation.

Polarization of macrophages in the trigeminal ganglion of rats with pulpitis

BACKGROUND

Dental pulp tissues are rich in pain-related afferent nerve fibers, which originate from primary sensory neurons in the trigeminal ganglion (TG). The mechanisms of central nervous system (CNS) underlying ectopic pain following peripheral inflammation have been reported that the macrophages as inflammatory and immunologic mediators in the TG play an important role in the process of pulpitis and hyperalgesia.

OBJECTIVE(S)

To observe the polarization response and dynamic distribution of macrophages in the TG during the development of dental pulp inflammation.

METHODS

A rat model of pulpitis was established using complete Freund's adjuvant (CFA). Hematoxylin-eosin (HE), immunohistochemistry (IHC), immunofluorescence (IF), toluidine blue (TB) staining, and RT-qPCR were performed to observe the expression of macrophage-related factors in the TG.

RESULTS

The results of IHC staining showed that M2 macrophages labeled with CD206 were observed in the TG of both the control and CFA groups. The statistical analysis indicated that the number of CD206-positive macrophages in the TG increased significantly at 24 h after CFA-induced pulpitis, reached a peak at 2 weeks, and then returned to the normal level after 6 weeks. The ratio of M2/M1 in the CFA groups was significantly lower than that in the control group from 24 to 72 h, and this pattern was reversed at 2 weeks after CFA-induced pulpitis; then, the ratio increased significantly and was maintained at a high level for 4 weeks. RT-qPCR results showed that the expression of IL-10 in the TG increased significantly from 1 to 4 weeks after CFA-induced pulpitis.

CONCLUSION

The trend of M2 macrophages was opposite to that of M1 macrophages in the TG during the process of pulpitis induced by CFA, which is consistent with the expression of macrophage-related cytokines. Macrophage polarization in the TG may participate in the neuroinflammation response induced by dental pulpitis.

Glia and Orofacial Pain: Progress and Future Directions

Orofacial pain is a universal predicament, afflicting millions of individuals worldwide. Research on the molecular mechanisms of orofacial pain has predominately focused on the role of neurons underlying nociception. However, aside from neural mechanisms, non-neuronal cells, such as Schwann cells and satellite ganglion cells in the peripheral nervous system, and microglia and astrocytes in the central nervous system, are important players in both peripheral and central processing of pain in the orofacial region. This review highlights recent molecular and cellular findings of the glia involvement and glia–neuron interactions in four common orofacial pain conditions such as headache, dental pulp injury, temporomandibular joint dysfunction/inflammation, and head and neck cancer. We will discuss the remaining questions and future directions on glial involvement in these four orofacial pain conditions.

Establishment of Pulp Damage Repair Models in Miniature Pigs Using Diode Lasers

Objective: To establish a controlled pulp damage repair model in miniature pigs by using a diode laser. Background: Laser is a novel kind of controllable energy, and it is widely used in dentistry. Methods: The premolars of four 24- to 28-month-old miniature pigs were divided into three laser groups, according to the output powers of a diode laser, and the nonirradiated first molars acted as controls. The teeth in laser groups were irradiated under three parameters (output powers 1.5, 2.5, 4 W, continuous wave, frequency 50 Hz for 60 sec). The dental and gingival morphology was observed at 0, 7, 14, and 21 days after laser irradiation. The animals were sacrificed for qualitative and quantitative pulp histopathological analysis. Results: The three laser groups present no seriously irreversible dental and gingival damage. In the 1.5-W group, dental pulp exhibited angiectasis and hyperemia with no inflammation, and did not significantly differ with the control groups at 21 days (p > 0.05). In the 2.5-W group, pulpal inflammation was highest at 7 days and then decreased significantly at 21 days, and the tissue repair appeared at 14 days (p < 0.05). In the 4-W group, pulpal inflammation was significantly highest at 7 days, with an increase in the degree of tissue repair (p < 0.05). Conclusions: The output power of 1.5 W developed a reversible pulpitis model; the output powers of 2.5 and 4 W within 7 days led to the development of irreversible pulpitis models, which proceeded as chronic pulpitis with obvious tissue repair.