Antinociceptive activity of the aqueous leaves of Pongamia pinnata in rats

In Sri Lankan traditional medicine, the decoction of leaves of the plant Pongamia pinnata (L.) is claimed to possess antinociceptive activity. However, as yet, its antinociceptive potential has not been scientifically investigated. The aim of this study was to investigate the antinociceptive potential of an aqueous leaf extraction (ALE) of P. pinnata in rats, using three algesiometric methods (hot-plate, tail-flick and formalin tests) and different doses (500, 1000, 1500 and 3000 mg/kg, given orally). The result showed that the ALE possesses marked and significant (p<0.05) antinociceptive action when evaluated in hot-plate test and formalin tests (only the highest dose tested) but not in the tail-flick test. The antinociceptive action of ALE had a quick onset (within 1h) and a moderately long duration of action (up to 6h). The antinociceptive action of ALE was blocked by metoclopramide (D2 type dopamine receptor antagonist) and atropine (a muscarinic receptor antagonist) but not by nalaxone (an opioid receptor antagonist). Moreover, the ALE has no sedative (as judged by hole board test) activity. ALE contained a wide range of chemical constituents of which alkaloids, flavonoids, steroids, and polyphenols which may have contributed to antinociceptive action of ALE. Collectively, these observations suggest that ALE-induced antinociception was mediated centrally, at supraspinal level, and peripherally. The antinociceptive action is likely to be mediated via dopaminergic and cholinoganic muscarinic mechanisms. The results also showed that ALE is effective against neurogenic and inflammatory pains. In conclusion, this study show, for the first time, that ALE of P. pinnata possesses moderately strong antinociceptive activity, justifying its therapetic claim in traditional medicine as a pain killer. Key word: Antinociceptive activity, dopaminergic mechanisms, muscarinic mechanisms, pain impairment, Pongamiapinnata, toxicology