Antinociceptive and anti-inflammatory effects of rosmarinic acid and Thunbergia Laurifolia leaf extract

Abstract

Rang Chuet (Thunbergia laurifolia Lindl., family Acanthaceae) is traditionally used as an antidote for poisons and for treatment of pain, edema, inflammation, and headache. Rosmarinic acid (RA), a main constituent of Rang Chuet, was isolated from an ethanolic extract of T. laurifolia leaves. The antinociceptive activities of RA and T. laurifolia leaf extract (TLE) were assessed in mice using hot-plate, acetic acid-induced writhing, and formalin tests. The anti-inflammatory effect of RA was determined in mouse models of carrageenan-induced paw edema and cotton pellet-induced granuloma formation. The anti-inflammatory effect of TLE was determined in carrageenan-induced paw edema. Orally administered RA (50, 100, and 150 mg/kg) and all doses of TLE (500-2500 mg/kg) showed significant antinociceptive activity in the hot-plate test and these effects were reversed by naloxone suggesting opioid-mediated mechanism. RA at doses of 50 and 100 mg/kg significantly reduced acetic acid-induced writhing by 52.05% and 84.53%, respectively. TLE at doses of 1000, 1500, 2000, and 2500 mg/kg significantly decreased the number of writhes by 32.11%, 45.04%, 47.81%, and 67.89%, respectively. These data indicate that both RA and TLE have antinociceptive effects on inflammatory pain. RA at 100 mg/kg and TLE at doses of 1500 and 2000 mg/kg also caused significant inhibition of formalin-induced pain in the early and late phases. The results from the formalin test were in agreement with those obtained from the hot-plate and writhing tests, thereby indicating that RA and TLE act both centrally and peripherally to achieve pain relief. Most effective doses of RA and TLE showed no detectable relaxant or sedative effects in the rota-rod test. RA at 100 mg/kg significantly suppressed carrageenan-induced paw edema at 3, 4, 5, and 6 h, while all doses of TLE significantly decreased paw edema at 4, 5, and 6 h after carrageenan injection. These results indicate that RA and TLE exerted anti-inflammatory action during the second phase of inflammation. Both RA and TLE showed significant activity against PGE2-induced paw edema. The reduction of PGE2- induced paw edema could explain, at least in part, the inhibitory effects of RA and TLE in the acetic acid-induced writhing and formalin tests. RA at 100 mg/kg effectively inhibited cotton pellet-induced granuloma formation which indicated that RA has anti-inflammatory activity in the chronic phase of inflammation. An acute toxicity assessment confirmed the safety of all doses of RA and TLE used in the study. Taken together, these results show that RA and TLE possess both central and peripheral antinociceptive activity. The analgesic mechanisms of action of RA and TLE are most likely involved with the opioid pathway. RA and TLE have anti-inflammatory effects against acute and chronic inflammation. The anti-inflammatory mechanisms of action of RA and TLE are partly due to direct blockage of prostaglandin receptors. The antinociceptive and anti-inflammatory effects of TLE are partly resulted from its main constituent, rosmarinic acid. These data provide a basis for the rational use of RA and T. laurifolia for treatment of pain and inflammatory disorders.