The role of adenosine receptor ligands on inflammatory pain: possible modulation of TRPV1 receptor function.

Chronic pain has a debilitating consequences on health and lifestyle. The currently available analgesics are often ineffective and accompanied by undesirable adverse effects. Although adenosine receptors (AR) activation can affect nociceptive, inflammatory, and neuropathic pain states, the specific regulatory functions of its subtypes (A1, A2A, A2B and A3 ARs) are not fully understood. The aim of this study was to investigate the role of different AR ligands on inflammatory pain. The von Frey filament test was used to assess the anti-nociceptive effects of adenosine ligands on Complete Freund's Adjuvant (CFA)-induced mechanical allodynia in (180-220 g) adult male Sprague Dawley rats (expressed as paw withdrawal threshold, PWT). Neither the A2AAR selective agonist CGS 21680 hydrochloride (0.1, 0.32 and 1 mg/kg) nor the A2BAR selective agonist BAY 60-6583 (0.1, 0.32 and 1 mg/kg) produced any significant reversal of the PWT. However, the A1AR selective agonist ( ±)-5'-Chloro-5'-deoxy-ENBA, the A3AR selective agonist 2-Cl-IB-MECA, the A2AAR selective antagonist ZM 241385 and the A2BAR selective antagonist PSB 603 produced a significant reversal of the PWT at the highest dose of 1 mg/kg. Co-administration of the selective antagonists of A1AR and A3AR PSB36 (1 mg/ml) and MRS-3777 (1 mg/ml); respectively, significantly reversed the anti-nociceptive effects of their corresponding agonists. Furthermore, calcium imaging studies reveled that the effective AR ligands in the behavioral assay also significantly inhibit capsaicin-evoked calcium responses in cultured rat dorsal root ganglia (DRG) neurons. In conclusion, modulating the activity of the transient receptor potential vanilloid 1 (TRPV1) receptor by ARs ligands could explain their anti-nociceptive effects observed in vivo. Therefore, the cross talk between ARs and TRPV1 receptor may represent a promising targets for the treatment of inflammatory pain conditions.

Anti-nociceptive effect of Arbutus andrachne L. methanolic leaf extract mediated by CB1, TRPV1 and PPARs in mouse pain models.

Arbutus andrachne L. is a medicinal plant that grows in Jordan and has many valuable effects. In the present study, the anti-nociceptive effect of A. andrachne methanolic leaf extract was determined in mice using thermal and chemical tests. Our findings show that different doses of A. andrachne extract reduced the number of writhings significantly compared to control group. The leaf extract also reduced the time of paw licking in the early and late phases of formalin test. In all the conducted tests, 300 mg/kg body wt. was the best effective dose. A peroxisome proliferator-activated receptor alpha (PPARα) antagonist reversed the action of the plant extract in the early phase of formalin test while antagonists of the PPARα, PPAR gamma (PPARγ) and cannabinoid 1 (CB1) receptors were responsible for abolishing its effect in the late phase of this test. Also, the extract administration increased the latency time in hot plate and tail flick, an effect that was reversed by the antagonists of PPARγ, CB1 and transient receptor potential vanilloid 1 (TRPV1). No effect was noticed for α2-adrenergic receptor antagonist in the action of A. andrachne in any of the conducted tests in this study. Furthermore, analysis of the constituents in the methanolic leaf extract using liquid chromatography mass spectrometry (LCMS) showed that the extract is rich in compounds that have anti-nociceptive and/or anti-inflammatory effects such as arbutin, rutin, linalool, linoleic acid, gallic acid, lauric acid, myristic acid, hydroquinone, ß-sitosterol, ursolic acid, isoquercetin, quercetin, (+)-gallocatechin, kaempferol, α-tocopherol, myricetin 3-O-rhamnoside and catechin gallate. In conclusion, A. andrachne showed promising anti-nociceptive effects in thermal and chemical models of pain. These findings can open an avenue for natural pain relief.

Antinociceptive Action of Moringa peregrina is Mediated by an Interaction with α2-Adrenergic Receptor

Background: Moringa peregrina (M. peregrina) is an edible, drought-resistant tree that is native to semi-arid countries. It is used as a painkiller in folk medicine. Aims: To study the antinociceptive effects of the leaf extract of M. peregrina in mice. Study Design: Animal experimentation. Methods: We employed thermal (hot plate and tail-immersion tests) and chemical (writhing and formalin tests) pain models in male BALB/c mice (eight animals per group) to investigate the mechanisms involved in the antinociceptive actions of M. peregrina. Additionally, we identified the chemical constituents present in the extract of M. peregrina by using liquid chromatography-mass spectrometry analysis, and predicted the possible active constituents that interact with the receptor based on molecular docking simulations. Results: In the writhing test, 200 mg/kg of M. peregrina extract restricted abdominal cramps by up to 55.97% (p<0.001). Further, it reduced the time of paw-licking in the early and late phases of formalin test by up to 56.8% and 65.5%, respectively, as compared to the percentage inhibitions of 50.5% and 48.4% produced by 30 mg/kg diclofenac sodium in the early and late phases, respectively (p<0.05). This effect was abrogated by yohimbine (1 mg/kg, intraperitoneally), but not by methysergide (5 mg/kg, intraperitoneally), in the late phase only, which indicates that the action of M. peregrina in formalin test is not mediated by 5-HT2 serotonin receptors, but rather via α2-adrenergic receptors. In the hot plate test, but not on tail-immersion test, the high dose (400 mg/kg) of the extract increased the latency time after 30 minutes of its administration. Yohimbine antagonized the action of M. peregrina in the hot plate test. Based on LC-MS analysis, the major constituents found in M. peregrina methanolic extract were chrysoeriol 7-O-diglucoside, lupeol acetate, quercetin, and rutin. Depending on the molecular docking results, the activity of M. peregrina extract could be due to the binding of chrysoeriol 7-O-diglucoside, quercetin, and rutin to the α2-adrenergic receptor. Conclusion: Interaction with the α2-adrenergic receptor serves as a possible mechanism of the M. peregrina analgesic effect.

Gastroprotective activity of Loranthus acaciae flower extract in a rodent model of ethanol-induced ulcer.

Loranthus acaciae (Loranthaceae) is a perennial green semi-parasitic plant used in ethnopharmacological medicine for healing wounds. The protective effect of L. acaciae on gastric ulcer induced by ethanol was investigated in a rat model. Ulcer index and total glutathione level were measured and histological and immunohistochemical studies for the expression of cyclooxygenase-2 were performed. Furthermore, chemical constituents of the flower extract were analyzed. Ulcer index was significantly lowered in L. acaciae-treated groups. Protection ratios were 75.9%, 98.9%, and 70.7% for 250 mg/kg and 500 mg/kg of L. acaciae and 40 mg/kg of esomeprazole, respectively. Histological examination revealed fewer hemorrhage in mucosa and less edema in submucosa of L. acaciae-treated groups compared with control. In the esomeprazole-treated group, there was mild disruption in the surface epithelium and mild hemorrhage. However, edema and leucocytes infiltration in the submucosa layer were present. Immunohistochemical staining of stomach sections for cyclooxygenase-2 (COX-2) was negative in the control group as well as in the L. acaciae-treated groups. Total glutathione level in mucosa layer of the stomach was higher in L. acaciae-treated groups compared with control. Liquid chromatography-mass spectrometric analysis revealed the presence of loranthin and rutin as the major constituents. It can be concluded that L. acaciae imparted a gastroprotective action against ethanol-induced ulcer in rats. Novelty 500 mg/kg L. acaciae protected the stomach by 98.9% from ulcerogenic effect of ethanol. L. acaciae increased total glutathione level but not COX-2 expression in gastric mucosa. Loranthin and rutin were the major constituents in L. acaciae flower extract.

Antinociceptive action of Achillea biebersteinii methanolic flower extract is mediated by interaction with cholinergic receptor in mouse pain models.

Achillea biebersteinii is a perennial aromatic herb that grows in the Mediterranean area. The leaves of this plant are used in foods as bittering and appetizing agents. In folk medicine, it is used for the treatment of stomachache and abdominal pain. In this study, the analgesic effect of A. biebersteinii methanolic flower extract was tested in three pain models, namely: writhing, tail-flick and paw-licking (formalin) tests. A. biebersteinii extract inhibited abdominal cramps produced by acetic acid. The effect of A. biebersteinii was better than that of 70 mg/kg indomethacin. In tail flick, A. biebersteinii extract increased latency at 30 min and was as effective as 100 mg/kg diclofenac sodium. In formalin test, A. biebersteinii extracts decreased paw-licking and flinching response in early and late phases. Atropine blocked the action of A. biebersteinii extract (300 mg/kg) in the late phase of formalin test as well as in writhing and tail-flick tests. GC-MS analysis revealed that ascaridole and iso-ascaridole were the main constituents of A. biebersteinii flower extract. In conclusion, this study shows for the first time that the antinociceptive effect of A. biebersteinii is mediated by the cholinergic receptor.