An isobolographic analysis of the anti-nociceptive effect of geraniin in combination with morphine or diclofenac.

BACKGROUND: Geraniin, a dehydroellagitannin, is a major component of the aqueous extract of the aerial parts of Phyllanthus muellerianus (Kuntze) Exell. (Euphorbiaceae). Several Phyllanthus species are traditionally used for painful disorders. The anti-nociceptive effects of the aqueous extract of the aerial parts of P. muellerianus and of geraniin have been scientifically established. The aim of the paper is to determine whether a combination of geraniin and diclofenac or geraniin and morphine leads to better anti-nociceptive effects.

METHODS: The nature of the interactions of morphine and diclofenac with geraniin was evaluated by undertaking the isobolographic analysis. Mice were treated with geraniin (3-30 mg/kg), morphine (1-10 mg/kg), and diclofenac (10-100 mg/kg) to obtain the ED50 values of the agents in the formalin test. Dose-response curves were then obtained and analyzed after the co-administration of geraniin with morphine or diclofenac in fixed ratio (1:1) combinations based on specific fractions (1/2, 1/4, and 1/8) of their respective ED50 values for the formalin test.

RESULTS: Geraniin was less potent than morphine but more potent than diclofenac in the formalin-induced nociception. The isobolographic analysis of geraniin/morphine (G/M) and geraniin/diclofenac combinations (G/D) at different fractions revealed the potentiation of their anti-nociceptive effects. The degrees of potentiation, which were calculated as interaction indices, showed synergism for both combinations in both phase I (G/M: 0.040, G/D: 0.017) and phase II (G/M: 0.004, G/D: 0.002) of the formalin test.

CONCLUSIONS: The present study demonstrates synergism for the co-administration of geraniin with both morphine and diclofenac.