Anti-nociceptive and anti-inflammatory effects of the ethanol extract of Arenga pinnata (Wurmb) Merr. fruit.

Arenga pinnata (Wurmb) Merr. is a medicinal and edible plant belonging to family Palmae. The fruits of this plant were used in traditional folk medicine due to its analgesia and anti-inflammatory activities. This study aimed to investigate the analgesic and anti-inflammatory properties and the mechanism of the ethanol extract of A. pinnata (Wurmb) Merr. fruit (EAF) on different experimental models.

High-performance liquid chromatography (HPLC) was used to determine the chromatographic profile and to analyze the composition of EAF. In the acute toxicity test, all mice were orally administered EAF at a maximum dosage of 26 g/kg and were then monitored for 14 days. The potential analgesic activity of EAF was evaluated by using animal pain models, namely the acetic acid-induced writhing test and the hot plate test in mice. The underlying mechanisms of analgesia were determined by pretreating with naloxone, capsaicin and cinnamaldehyde to evaluate the involvement of the opioid system and transient receptor potential channels (TRP channels). The anti-inflammatory activity of EAF was evaluated by using the following inflammatory animal models: xylene-induced ear edema in mice and Complete Freund's adjuvant (CFA)-induced paw swelling in rats. EAF was orally administered at the doses of 1.625, 3.25 and 6.5 g/kg in mice and 1.125, 2.25 and 4.5 g/kg in rats. The underlying mechanism of the anti-inflammatory activity was determined by enzyme-linked immunosorbent assay (ELISA) kits and real time-PCR used to measure the expression levels of interleukin-1β (IL-1β), tumor necrosis factor-α (TNF-α), prostaglandin E2 (PGE₂) and cyclooxygenase-2 (COX-2). Western blot analysis was used to determine the expression levels of proteins related to the nuclear factor-κB (NF-κB) and mitogen-activated protein kinases (MAPK) signaling pathways in paw tissues.

Five compounds, namely (5-(hydroxymethyl) furan-2-yl) methanediol,4'-hydroxy-N-(4-hydroxy-3-methoxybenzoyl)-3',5'-dimethoxybenzamide, (+)-lyonirenisol-3a-O-β-D-glucopyranoside, (-)-lyonirenisol-3a-O-β-glucopyranoside and liquiritin, were firstly identified from A. pinnata (Wurmb) Merr. fruit by HPLC-UV analysis. In the acute toxicity test, no treatment-related toxicological signs or mortality was observed in mice administered doses up to 26 g/kg. Bodyweight was not obviously different among the treatment groups and the vehicle group. EAF significantly inhibited the pain response induced by acetic acid and increased the latency time in the hot plate test in mice. The anti-nociception effect of EAF in the formalin test was not alleviated by pretreatment with naloxone. However, the nociception induced by injection with capsaicin and cinnamaldehyde was significantly reduced by EAF. Compared with vehicle treatment, EAF significantly inhibited the formation of xylene-induced ear edema and CFA adjuvant-induced paw swelling. EAF markedly inhibited the production of IL-1β, TNF-α, PGE₂ and IL-6 induced by CFA in paw tissues. Furthermore, the phosphorylation of IKKα, IKKβ, IκBα, p38, ERK1/2, and JNK and the nuclear translation of NF-κB p65 induced by CFA in paw tissues were significantly inhibited by EAF treatment compared with vehicle treatment.

For the first time, this study provides pharmacological evidence for the analgesic and anti-inflammatory activities of EAF and the underlying mechanism, suggesting that EAF might be a potential candidate for reducing pain and inflammatory disorders.