Watanabe et al. (1975, 1983a,b) evaluated in detail the central depressive effects of magnolol and honokiol. Both intraperitoneally administered magnolol and honokiol disrupted the grip strength in mice for up to 180 min (ED50 131 mg/kg and 217 mg/kg, respectively) (Watanabe et al., 1975), and inhibited the spinal reflexes in chicks for up to 60 min (ED50 10.3 mg/kg and 11.1 mg/kg, respectively) (Watanabe et al., 1983a). Magnolol also inhibited penicillin G potassium-induced convulsions in mice (ED50 31.3 mg/kg) (Watanabe et al, 1983b).
However, it should be considered that considerably higher doses of magnolol and honokiol were administered to develop such behavioral symptoms. Since the concentration of honokiol in Magnolia bark is 0.25—1.7% (Kuribara et al., 1999a), the doses of magnolol and honokiol effective for the development of central depression and muscle relaxation correspond to 15—156 g/kg of Magnolia. In contrast, the elevated plus-maze test in mice revealed that seven daily treatments with honokiol and extracts of Magnolia developed the anxiolytic effect even at 0.2 mg/kg (Kuribara et al., 1998) and 11.6-80 mg/kg (Kuribara et al., 1999a, 2000a), respectively, as did single treatment with 20 mg/kg honokiol (Kuribara et al., 1998). Thus, assessment of pharmacological effects of honokiol at doses equal to the effective doses for anxiolytic effect is proposed.
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