Activation of the N-methyl-D-aspartate receptor contributes to orofacial neuropathic and inflammatory allodynia by facilitating calcium-calmodulin-dependent protein kinase II phosphorylation in mice.

Neuropathic and inflammatory pain are major clinical challenges due to their ambiguous mechanisms and limited treatment approaches. N-methyl-D-aspartate receptor (NMDAR) and calcium-calmodulin-dependent protein kinase II (CaMKII) are responsible for nerve system sensation and are required for the induction and maintenance of pain. However, the roles of NMDAR and CaMKII in regulating orofacial pain are still less well known. Here, we established a neuropathic pain model by transecting a mouse inferior alveolar nerve (IAN) and an inflammatory pain model by injecting complete Freund's adjuvant (CFA) into its whisker pad. The Cre/loxp site-specific recombination system was used to conditionally knock out (KO) NR2B in the trigeminal ganglion (TG). Von Frey filament behavioral tests showed that IANX and CFA-induced mechanical allodynia were altered in NR2B-deficient mice. CFA upregulated CaMKIIα and CaMKIIß in the mouse TG and spinal trigeminal caudate nucleus (SpVc). CaMKIIα first decreased and then increased in the TG after IANX, and CaMKIIß decreased in the TG and SpVc. CFA and IANX both greatly enhanced the expression of phospho (p)-NR2B, p-CaMKII, cyclic adenosine monophosphate (cAMP), p-ERK, and p-cAMP response element binding protein (CREB) in the TG and SpVc. These neurochemical signal pathway alterations were reversed by the conditional KO of NR2B and inhibition of CaMKII. Similarly, IANX- and CFA-related behavioral alterations were reversed by intra-ganglionic (i.g.) -application of inhibitors of CaMKII, cAMP, and ERK. These findings revealed novel molecular signaling pathways (NR2B-CaMKII-cAMP-ERK-CREB) in the TG- and SpVc-derived latent subsequent peripheral and spinal central sensitization under nerve injury and inflammation, which might be beneficial for the treatment of orofacial allodynia.

GABAB receptor activation attenuates inflammatory orofacial pain by modulating interleukin-1ß in satellite glial cells: Role of NF-κB and MAPK signaling pathways.

Orofacial inflammation could activate satellite glial cells (SGCs) in the trigeminal ganglion (TG) to produce interleukin 1ß (IL-1ß) which plays crucial roles in the development of inflammatory pain. Recent studies have shown that gamma-amino butyric acid-B (GABAB) receptor could modulate the expression of inflammatory cytokines in microglia and astrocytes in the spinal cord. The objective of this study was to investigate whether GABAB receptors in TG SGCs attenuate inflammatory facial pain via mediating IL-1ß following inflammation and its mechanisms. Complete Freund's adjuvant (CFA) was injected into the whisker pad of rats to induce inflammation in vivo. Lipopolysaccharide (LPS) was added to culture medium to activate SGCs in vitro. Behavioral measures showed that microinjection of baclofen (a selective GABAB receptor agonist) into the TG ameliorated the mechanical allodynia of CFA-treated rats. Interestingly, baclofen pretreatment inhibited SGC activation and IL-1ß production, however, preserved the decreased expression of GABAB receptors in SGCs activated by CFA in vivo and LPS in vitro. In addition, baclofen suppressed the increased expression of p-NF- κ B p65, p-I κ Bα, and p-p38 MAPK, while reversed the decreased production of I κ Bα, and further enhanced the increased expression of p-ERK(1/2) in LPS-treated SGCs in vitro. Finally, those effects of baclofen were abolished by saclofen (a specific GABAB receptor antagonist) co-administration. Altogether, these results demonstrated for the first time that activation of GABAB receptor might inhibit IL-1ß production by suppressing NF- κ B and p38 MAPK signaling pathway activation and restore GABAB receptor expression in SGCs to attenuate inflammatory facial pain.