An alpha-adrenoceptor-mediated mechanism of hypoactivity induced by beta-amyrin palmitate

Inhibitory effects of beta-amyrin palmitate in locomotor activity of mice were studied by combining this compound with alpha-adrenergic agonists or antagonists and a dopaminergic agonist. beta-Amyrin palmitate (2.5, 5.0 and 10.0 mg kg-1, i.p.) decreased locomotor activity of mice in a dose-dependent manner. It enhanced hypoactivity of mice treated with clonidine (0.025 mg kg-1, i.p.) and antagonized hyperactivity produced by phenylephrine (40 micrograms, i.c.v.). The inhibitory action of beta-amyrin palmitate was not affected by yohimbine (1.5 mg kg-1, i.p.), but was potentiated by prazosin (0.75 mg kg-1, i.p.). When combined with a dopaminergic agonist, apomorphine (2.0 mg kg-1, i.p.), beta-amyrin palmitate (5.0 and 10.0 mg kg-1, i.p.) did not affect locomotor stimulation produced by apomorphine. These results suggest that beta-amyrin palmitate might inhibit alpha 1-adrenoceptors.