Synergistic antidepressant-like effect of xylopic acid co-administered with selected antidepressants.

BACKGROUND: Xylopic acid (XA), a kaurene diterpene from the dried fruits of Xylopia aethiopica, has anxiolytic- and antidepressant-like activity in mice and zebrafish. We aimed to assess the potential synergistic antidepressant-like effects of XA when combined with selected antidepressants in the mouse forced-swim test. MATERIALS AND METHODS: The antidepressant-like effect of xylopic acid (XA) (10, 30, 100 mgkg(-1)), fluoxetine (Flx) (3, 10, 30 mgkg(-1)), sertraline (Sert) (3, 10, 30 mgkg(-1)), imipramine (Imi) (10, 30, 100 mgkg-1) and ketamine (Ket) (0.1, 0.3, 1.0 mgkg(-1)), was evaluated in forced swim test. The dose (ED(50)) that achieved a 50% reduction in immobility time was determined from the respective log-dose response curves. XA and the selected antidepressants were co-administered in fixed-dose ratio combinations (1/2:1/2, 1/4:1/4, 1/8:1/8) of the ED(50) to identify the experimental ED(50) (ED(50mix)). The theoretical ED(50)(ED(50add)), of all combinations was determined using isobolograms and compared with the ED(50mix) to identify the nature of the interaction. The effect of dose combinations on general locomotor activity was assessed in the open-field test. RESULTS: The interaction index (γ) for the following XA combinations, XA/Flx, XA/Sert, XA/Imi and XA/Ket were 0.42, 0.41, 0.31 and 0.34. An independent sample t-test revealed that the experimental ED(50) (ED(50mix)) was significantly lower than the theoretical ED(50) (ED(50add)) in all combinations of XA, indicative of a synergistic antidepressant-like effect. However, combinations of XA with ketamine significantly reduced general locomotor activity at all dose combinations. CONCLUSION: The co-administration of xylopic acid and fluoxetine, imipramine, sertraline and ketamine produces a synergistic antidepressant-like effect in mice.