Involvement of interferon gamma signaling in spinal trigeminal caudal subnucleus astrocyte in orofacial neuropathic pain in rats with infraorbital nerve injury.

Background: Trigeminal nerve injury causes orofacial pain that can interfere with activities of daily life. However, the underlying mechanism remains unknown, and the appropriate treatment has not been established yet. This study aimed to examine the involvement of interferon gamma (IFN-γ) signaling in the spinal trigeminal caudal subnucleus (Vc) in orofacial neuropathic pain. Methods: Infraorbital nerve (ION) injury (IONI) was performed in rats by partial ION ligation. The head-withdrawal reflex threshold (HWT) to mechanical stimulation of the whisker pad skin was measured in IONI or sham rats, as well as following a continuous intracisterna magna administration of IFN-γ and a mixture of IFN-γ and fluorocitrate (inhibitor of astrocytes activation) in naïve rats, or an IFN-γ antagonist in IONI rats. The IFN-γ receptor immunohistochemistry and IFN-γ Western blotting were analyzed in the Vc after IONI or sham treatment. The glial fibrillary acid protein (GFAP) immunohistochemistry and Western blotting were also analyzed after administration of IFN-γ and the mixture of IFN-γ and fluorocitrate. Moreover, the change in single neuronal activity in the Vc was examined in the IONI, sham, and IONI group administered IFN-γ antagonist. Results: The HWT decreased after IONI. The IFN-γ and IFN-γ receptor were upregulated after IONI, and the IFN-γ receptor was expressed in Vc astrocytes. IFN-γ administration decreased the HWT, whereas the mixture of IFN-γ and fluorocitrate recovered the decrement of HWT. IFN-γ administration upregulated GFAP expression, while the mixture of IFN-γ and fluorocitrate recovered the upregulation of GFAP expression. IONI significantly enhanced the neuronal activity of the mechanical-evoked responses, and administration of an IFN-γ antagonist significantly inhibited these enhancements. Conclusions: IFN-γ signaling through the receptor in astrocytes is a key mechanism underlying orofacial neuropathic pain associated with trigeminal nerve injury. These findings will aid in the development of therapeutics for orofacial neuropathic pain.

PAR2-dependent phosphorylation of TRPV4 at the trigeminal nerve terminals contributes to tongue cancer pain.

OBJECTIVE: This study aimed to clarify the interactions between the tongue and primary afferent fibers in tongue cancer pain. METHODS: A pharmacological analysis was conducted to evaluate mechanical hypersensitivity of the tongues of rats with squamous cell carcinoma (SCC). Changes in trigeminal ganglion (TG) neurons projecting to the tongue were analyzed using immunohistochemistry and western blotting. RESULTS: SCC inoculation of the tongue caused persistent mechanical sensitization and tumor formation. Trypsin expression was significantly upregulated in cancer lesions. Continuous trypsin inhibition or protease-activated receptor 2 (PAR2) antagonism in the tongue significantly inhibited SCC-induced mechanical sensitization. No changes were observed in PAR2 and transient receptor potential vanilloid 4 (TRPV4) levels in the TG or the number of PAR2-and TRPV4-expressing TG neurons after SCC inoculation. In contrast, the relative amount of phosphorylated TRPV4 in the TG was significantly increased after SCC inoculation and abrogated by PAR2 antagonism in the tongue. TRPV4 antagonism in the tongue significantly ameliorated the mechanical sensitization caused by SCC inoculation. CONCLUSIONS: Our findings indicate that tumor-derived trypsin sensitizes primary afferent fibers by PAR2 stimulation and subsequent TRPV4 phosphorylation, resulting in severe tongue pain.

Neonatal Injury Modulates Incisional Pain Sensitivity in Adulthood: An Animal Study.

Neonatal pain experiences including traumatic injury influence negatively on development of nociceptive circuits, resulting in persistent pain hypersensitivity in adults. However, the detailed mechanism is not yet well understood. In the present study, to clarify the pathogenesis of orofacial pain hypersensitivity associated with neonatal injury, the involvement of the voltage-gated sodium channel (Nav) 1.8 and the C-C chemokine ligand 2 (CCL2)/C-C chemokine receptor 2 (CCR2) signaling in the trigeminal ganglion (TG) in facial skin incisional pain hypersensitivity was examined in 190 neonatal facial-injured and sham male rats. The whisker pad skin was incised on postnatal day 4 and week 7 (Incision-Incision group). Compared to the group without neonatal incision (Sham-Incision group), mechanical hypersensitivity in the whisker pad skin was enhanced in Incision-Incision group. The number of Nav1.8-immunoreactive TG neurons and the amount of CCL2 expressed in the macrophages and satellite glial cells in the TG were increased on day 14 after re-incision in the Incision-Incision group, compared with Sham-Incision group. Blockages of Nav1.8 in the incised region and CCR2 in the TG suppressed the enhancement of mechanical hypersensitivity in the Incision-Incision group. Administration of CCL2 into the TG enhanced mechanical hypersensitivity in the Sham-Sham, Incision-Sham and Sham-Incision group. Our results suggest that neonatal facial injury accelerates the TG neuronal hyperexcitability following orofacial skin injury in adult in association with Nav1.8 overexpression via CCL2 signaling, resulting in the enhancement of orofacial incisional pain hypersensitivity in the adulthood.