Dopamine D2 blocking activity and plasma concentrations of remoxipride and its main metabolites in the rat

Remoxipride and its active metabolites, the phenolic compounds FLA797(-) and FLA908(-) and the catecholic NCQ436(-) and haloperidol, were examined for their ability to block hypothermia in the rat induced by dopamine (DA) D2 receptor stimulation. In addition, plasma levels of remoxipride and its active metabolites were measured using HPLC methods. Remoxipride (1 mumol/kg), given 30 or 15 min prior to, or 5 and 15 min after, the DA agonists, blocked the hypothermia induced by the DA D2 receptor agonists quinpirole (0.25 mg/kg s.c.) and pergolide (0.1 mg/kg s.c.). Administration of remoxipride by the i.v. or s.c. routes was more effective than by the i.p. route. FLA797(-), FLA908(-), and haloperidol were more effective than remoxipride in preventing the hypothermia caused by quinpirole, while NCQ436(-) was less effective than remoxipride. The variation in time of remoxipride's action and effectiveness in blocking the induced hypothermia followed the variations in plasma concentrations. The plasma concentrations of the active metabolites were below the limit of determination (< 2 nmol/l). Based on estimation of free brain concentrations at effective dose levels together with in vitro affinities for the DA D2 receptor it was concluded that the metabolites FLA797(-), FLA908(-), and NCQ436(-) do not appear to contribute to the antagonism of DA D2 mediated neurotransmission following a low remoxipride dose (1 mumol/kg).