Antinociceptive activity of metapramine in mice. Relationship with its pharmacokinetic properties

The antinociceptive effect of acutely and chronically (every brain elimination half-life time) administered metapramine, a tricyclic antidepressant without anticholinergic or cardiotoxic effects, was studied in three different pain tests. In the hot plate test, its action was more potent when jumping was used as a pain parameter (acute ED50 = 19 +/- 3 mg/kg, i.p.) than when pain was assessed by licking of forepaws (only 20 mg/kg, i.p. was weakly active). Five chronic doses of 15 mg/kg were as active in the tail-flick test as an acute dose of 20 mg/kg (only active dose). Metapramine was more effective in the PBQ-induced writhing test after acute (ED50 = 9.9 +/- 0.1 mg/kg, i.p.) and chronic administration. A significant linear correlation was found between the effect in this test and plasma and overall brain levels of metapramine. No correlation was observed with levels of its three desmethylated metabolites. The usefullness of using a well-defined pattern of administration based on pharmacokinetic parameters and the involvement of monoaminergic mechanisms and of some metabolites of metapramine are discussed.