The results of our present study demonstrate that mal-positioned dental implants significantly increase the early phosphorylation of p38 MAPK in microglia and late phosphorylation of NF-κB in astrocyte. On POD 3, the intracisternal administration of SB203580 significantly inhibited both mechanical allodynia and the expression of p-p38 MAPK. On POD 21, the intracisternal administration of SN50 significantly inhibited both mechanical allodynia and the expression of p-NF-κB. Moreover, treatment with dexamethasone, which produces potent anti-nociceptive effects, significantly attenuated the expression of p-p38 MAPK and p-NF-κB in rats with inferior nerve injury produced by mal-positioned dental implants. These results suggest that the activation of p38 MAPK and NF-κB play a critical role in the development and maintenance of trigeminal neuropathic pain.
Previous studies have reported that chronic constriction injury of the infraorbital nerve or inferior alveolar nerve damage produces abnormal nociceptive behavior in the trigeminal region [11–15]. These nociceptive behaviors, including allodynia, are similar to the symptoms of patients with neuropathic pain caused by nerve injury. Dental implantation, one of the most common prosthetic treatments, often results in injuries to the nerve including the inferior alveolar or mental nerve. Han et al. have previously described a novel animal model for trigeminal neuropathic pain, which was produced by mal-position dental implants . In our present study, we used this animal model and found that it produced severe long-lasting nociceptive behavior in the ipsilateral and contralateral sides compare to the naïve or sham groups. Accordingly, these results suggest that this is an appropriate animal model for the clinical study of trigeminal neuropathic pain. However, the underlying cellular mechanisms involved in the development and maintenance of trigeminal neuropathic pain remain unclear.
P38 MAPK, one of the families of intracellular signaling molecules that transduce extracellular responses, plays an important role in the development and maintenance of nerve injury-induced pain hypersensitivity . Emerging evidence now indicates that activated p38 MAPK is involved in the development of neuropathic pain after nerve injury. The activation of p38 MAPK has been previously observed in the spinal cord following spinal nerve injury [17–19], spared nerve injury , and spinal cord injury . Moreover, the intrathecal administration of a p38 MAPK inhibitor prevents the generation of neuropathic pain in rats with nerve injury [17, 18, 20, 22]. In our present study, the intracisternal administration of SB203580 on POD 3 significantly inhibited mechanical allodynia induced by mal-positioned dental implants. On POD 21 however, SB203580 did not affect mechanical allodynia. This early role of p38 MAPK is consistent with the findings of a previous study showing that the p38 MAPK levels rapidly increase and reached a peak at three days after nerve injury . The results of our present study also show that an inferior alveolar nerve injury markedly induces an increase in p-p38 MAPK expression on POD 3 and 7. Moreover, p38 MAPK immunoreactive cells were found to co-localize with OX42 but not with NeuN or GFAP. These results, taken together with our behavioral analysis, suggest that a blockade of early p38 MAPK expression in microglia may be a potential therapeutic strategy for the treatment of trigeminal neuropathic pain.
Similarly to p38 MAPK, NF-κB is also involved in environmental stress-responsive pathways, including those that are activated by inflammatory insults . Recent studies also indicate that NF-κB is involved in the pathogenesis of neuropathic and inflammatory pain [24–26]. NF-κB is activated in dorsal root ganglia following partial sciatic nerve injury  and in the dorsal horn following spinal nerve ligation . The results of our present study demonstrate that the expression of p-NF-κB is significantly increased following nerve injury on POD 7 and 21. The intracisternal injection of SN50, an NF-κB inhibitor, was found to attenuate the expression of p-NF-κB as well as mechanical allodynia on POD 21, but not on POD 3. Moreover the p-NF-κB immunoreactive cells were found to co-localize with GFAP but not with NeuN or OX42. These results suggest that the activation of NF-κB in astrocyte, produced by mal-positioned dental implants, play an important role in the maintenance of trigeminal neuropathic pain. This suggested role of NF-κB in astrocyte in the late stages of neuropathic pain is consistent with the results of previous studies. NF-κB has been shown in an earlier report to mainly co-localize with GFAP [28, 29] and the activation of NF-κB in nerve injury models has previously been demonstrated to occur in the later stages of neuropathic pain [7, 30]. These results, taken together with our current behavioral analysis, suggest that a blockade of NF-κB in astrocyte may be a potential future therapy for already established trigeminal neuropathic pain.
Spinal glial cells, mainly microglia and astrocyte, play an important role in innate immunity and after peripheral nerve injury are converted to an activated state through a series of cellular and molecular changes [2, 3]. Recent studies have also indicated that activated microglia and astrocyte play important roles in the development and maintenance of neuropathic pain caused by nerve injury [7, 13, 17, 31]. Following nerve injury, activated microglia and astrocyte release a variety of pro-inflammatory cytokines, such as IL-1β, IL-6, or TNF-α, which may augment the nociceptive signals in the spinal cord . Moreover, treatment with specific inhibitors of microglia or astrocyte significantly attenuates nociceptive responses following nerve injury . In addition, microglial activation following nerve injury increases in the spinal dorsal horn within several hours  whereas astrocyte activation seems to initiate several days after nerve injury [7, 31]. However, there is no direct behavioral evidence for the differential regulation of glial cells in rats with trigeminal neuropathic pain. Our current analyses demonstrate that an early blockade of p38MAPK in microglia and a late blockade of NF-κB in astrocyte significantly attenuate mechanical allodynia following inferior alveolar nerve injury. These results suggest that nerve injuries produce a differential activation of spinal glial cells which may contribute to the development or maintenance of trigeminal neuropathic pain.
Previous reports have demonstrated that glucocorticoids attenuate nociceptive behavior in neuropathic pain animal models [33, 34] and inhibit the development and maintenance of neuropathic pain . Recently, Han et al. reported that early treatment with dexamethasone produces a prolonged inhibition of mechanical allodynia in a rat model of trigeminal neuropathic pain produced by mal-positioned dental implants . Daily treatment with dexamethasone significantly reduced the expression of p-p38 MAPK and p-NF-κB on POD 7 in our present experiments. Taken together, these results indicate that dexamethasone produces anti-allodynic effects through the inhibition of p38 MAPK and NF-κB activation.