Aim: To determine whether Porphyromonas gingivalis lipopolysaccharide (LPS) can directly activate trigeminal neurons, to identify which receptors are involved and to establish whether activation leads to secretion of the neuropeptide calcitonin gene-related peptide (CGRP) and/or the translocation of NF-κB.
Methodology: Mouse trigeminal ganglion (TG) cells were cultured in vitro for 2 days. The effect of P. gingivalis LPS (20 μg mL-1 ) on calcium signalling was assessed (by calcium imaging using Cal-520 AM) in comparison with the transient receptor potential channel A1 (TRPA1) agonist cinnamaldehyde (CA; 100 μmol L-1 ), the TRP channel V1 (TRPV1) agonist capsaicin (CAP; 1 μmol L-1 ) and high potassium (60 mmol L-1 KCl). TG cultures were pre-treated with either 1 μmol L-1 CLI-095 to block Toll-like receptor 4 (TLR4) signalling or with 3 μmol L-1 HC-030031 to block TRPA1 signalling. CGRP release was determined using ELISA, and nuclear translocation of NF-κB was investigated using immunocytochemistry. Data were analysed by one-way analysis of variance, followed by Bonferroni's post hoc test as appropriate.
Results: Porphyromonas gingivalis LPS directly exerted a rapid excitatory response on sensory neurons and non-neuronal cells (P < 0.001 to P < 0.05). The effects on neurons appear to be mediated via TLR4- and TRPA1-dependent pathways. The responses were accompanied by an increased release of CGRP (P < 0.001) and by NF-κB nuclear translocation (P < 0.01).
Conclusions: Porphyromonas gingivalis LPS directly activated trigeminal sensory neurons (via TLR4 and TRPA1 receptors) and non-neuronal cells, resulting in CGRP release and NF-κB nuclear translocation. This indicates that P. gingivalis can directly influence activity in trigeminal sensory neurons and this may contribute to acute and chronic inflammatory pain.
Keywords: TLR4; TRPA1; mouse; pain; symptomatic pulpitis; trigeminal.
© 2020 The Authors. International Endodontic Journal published by John Wiley & Sons Ltd on behalf of British Endodontic Society.