When doses of DZP (0.25 mg/kg) and the EO (2.5 mg/kg) that were ineffective per se were co-administered, they had a synergistic effect on each other. In other words, mice treated with DZP and the EO at their respective ineffective doses showed a significant increase in time spent in the LDB light chamber. In addition, mice treated with effective doses of the EO (5.0 mg/kg) and DZP showed a significantly elevated light-chamber duration, when compared to the EO 5.0 mg/kg alone or with DZP (Fig. 3).