Modulation of Inflammatory and Pain Pathways in Trigeminal Ganglion Neurons and Glial Cells by Plant Extracts

Aims: To evaluate the modulatory effect of plant extracts on the expression of mitogen-activated protein kinase phosphatase-1 (MKP-1) and the protein kinases P-ERK and P-p38 in primary cultures of trigeminal ganglion from Sprague Dawley rats.

Study Design: In vitro analysis of methanol extracts on proteins implicated in inflammation and pain signaling.

Place and Duration of Study: Center for Biomedical and Life Sciences, Missouri State University, between September 2017 and December 2018.

Methodology: Methanol extracts were prepared from L. barbarum, S. canadensis, R. copallinum, and V. missurica, while an aqueous extract was prepared from V. californica. Primary cultures of rat trigeminal ganglion cells were utilized to investigate cellular changes mediated by the extracts on basal levels of MKP-1 and sorbitol-stimulated expression of the signaling proteins P-ERK and P-p38 using immunocytochemistry and fluorescent microscopy. Toxicity of each extract was evaluated by trypan blue exclusion and polyphenol levels were determined using the Folin-Ciocalteu reagent and reported as gallic acid equivalents.

Results: Each extract tested caused a significant increase (P<.05) in basal levels of MKP-1 in trigeminal neurons and satellite glial cells when compared to untreated cells. Sorbitol treatment of cultured cells stimulated expression of the inflammatory signaling proteins P-ERK and P-p38 in neurons and glial cells. This stimulatory effect on ERK and p38 was significantly inhibited (P<.05) to near basal levels in both neurons and glia by overnight incubation with the extracts. None of the extracts caused cell toxicity and all extracts were determined to contain polyphenols, with V. californica and V. missurica exhibiting the highest levels.

Conclusions: The findings of this study provide evidence of a cellular mechanism by which plant extracts modulate trigeminal ganglion neurons and glial cells to inhibit inflammatory and pain signaling, and thus may be beneficial in managing orofacial pain conditions such as migraine and temporomandibular disorder.