Rapamycin reduces orofacial nociceptive responses and microglial p38 mitogen-activated protein kinase phosphorylation in trigeminal nucleus caudalis in mouse orofacial formalin model

Abstract

The mammalian target of rapamycin (mTOR) plays a role in various cellular phenomena, including autophagy, cell proliferation, and differentiation. Although recent studies have reported its involvement in nociceptive responses in several pain models, whether mTOR is involved in orofacial pain processing is currently unexplored. This study determined whether rapamycin, an mTOR inhibitor, reduces nociceptive responses and the number of Fos-immunoreactive (Fos-ir) cells in the trigeminal nucleus caudalis (TNC) in a mouse orofacial formalin model. We also examined whether the glial cell expression and phosphorylated p38 (p-p38) mitogen-activated protein kinases (MAPKs) in the TNC are affected by rapamycin. Mice were intraperitoneally given rapamycin (0.1, 0.3, or 1.0 mg/kg); then, 30 min after, 5% formalin (10 l) was subcutaneously injected into the right upper lip. The rubbing responses with the ipsilateral forepaw or hindpaw were counted for 45 min. High-dose rapamycin (1.0 mg/kg) produced significant antinociceptive effects in both the first and second phases of formalin test. The number of Fos-ir cells in the ipsilateral TNC was also reduced by high-dose rapamycin compared with vehicle-treated animals. Furthermore, the number of p-p38-ir cells the in ipsilateral TNC was significantly decreased in animals treated with high-dose rapamycin; p-p38 expression was co-localized in microglia, but not neurons and astrocytes. Therefore, the mTOR inhibitor, rapamycin, reduces orofacial nociception and Fos expression in the TNC, and its antinociceptive action on orofacial pain may be associated with the inhibition of p-p38 MAPK in the microglia.

The mTOR inhibitor rapamycin suppresses trigeminal neuropathic pain and p-MKK4/p-p38 mitogen-activated protein kinase-mediated microglial activation in the trigeminal nucleus caudalis of mice with infraorbital nerve injury

“…The sham procedure consisted of exposing the ION, with care taken to avoid stretching the nerve or damaging the epineurium. were selected based on a previous study (Yeo et al, 2021). The final vehicle (VEH) was adjusted to 0.2% DMSO in saline.…”

“…The TNC and TG tissues were scanned using an ECLIPSE 80i (Nikon Corp., Kanagawa, Japan) fluorescent microscope, and individual sections were digitized with 4,096 gray levels using a cooled CCD camera (Cool Snap ES model, Nihon Roper, Tokyo, Japan) connected to a computer-assisted image analysis system (MetaMorph; Universal Imaging, Westchester, PA). As previously described (Yeo et al, 2021), for quantitative analysis in the TNC region, six nonadjacent tissue sections per mouse were randomly selected and analyzed with a computer-assisted image analysis system (MetaMorph version 7.7.2.0). To analyze the p-p38 images, cells that were at least 80% brighter than the average level of each image were counted.…”

“…Previously, we demonstrated that rapamycin reduced nociceptive responses in the first and second phases of the mouse orofacial formalin test. In addition, this antinociceptive effect of rapamycin was associated with the inhibition of p38 mitogen-activated protein kinase (MAPK) in the trigeminal nucleus caudalis (TNC) (Yeo et al, 2021). However, whether rapamycin reduces allodynic behavior in a trigeminal neuropathic pain mouse model and its underlying mechanisms remain to be elucidated.…”

Antimicrobial therapies for chronic pain (part 2): the prevention and treatment of chronic pain

“…Signaling via the mammalian target of rapamycin, which controls mRNA translation and mediates opioid tolerance and hyperalgesia, is a potential target for improving opioid efficacy and analgesia [4]. In preclinical studies, the antifungal agent rapamycin has been shown to reduce inflammatory and neuropathic pain [5,6]. Other animal studies have found that tetracycline derivatives may attenuate neuropathic pain through the inhibition of EphB1 tyrosine kinase, which is a mediator of neuropathic pain and morphine tolerance, and that the antimicrobial peptide piscidin-1 may possess antinociceptive effects in neuropathic pain models [7,8].…”

Editorial: Stress, pain or drug addiction: epigenetics, biological mechanisms and therapeutics

“…Neuropathic pain in the orofacial area is commonly observed in humans and markedly reduces the quality of general life by disrupting food intake, face washing, and tooth brushing (Costigan et al, 2009 ). Yeo's colleagues previously proved that an mTOR inhibitor rapamycin suppressed orofacial formalin-induced pain (Yeo et al, 2021 ). Therefore, to elucidate its underlying mechanism, Yeo and Roh explored the effect of rapamycin on the orofacial pain by nerve injury and the changes of mTOR signaling-related proteins, ERK, JNK, p38 MAPK, and glial cells in the trigeminal nucleus caudalis (TNC).…”