Non-amphetaminic mechanism of stimulant locomotor effect of modafinil in mice

Previously established dose-response curves indicated that modafinil 20-40 mg/kg i.p. elicited in mice an obvious stimulation of locomotor activity roughly similar to that induced by (+)amphetamine 2-4 mg/kg. The effects of various agents modifying dopamine transmission were compared on the locomotor response to both drugs. The preferential D2 dopamine receptor antagonist haloperidol 37.5-150 micrograms/kg i.p. suppressed the stimulant effect of (+)amphetamine in a dose dependent manner, but not that of modafinil. The D1 dopamine receptor antagonist SCH 23390 (7.5-30 micrograms/kg s.c.) reversed the (+)amphetamine but not the modafinil induced hyperactivity. The tyrosine hydroxylase inhibitor alpha-methyl-para-tyrosine (200 mg/kg) suppressed the hyperactivity induced by 4 mg/kg dexamphetamine but not that induced by 20 mg/kg modafinil. Associating L-DOPA 150 mg/kg and benserazide 37.5 mg/kg with (+)amphetamine 2 mg/kg resulted in stereotyped climbing behavior, that was not observed with modafinil 10-80 mg/kg. The profound akinesia induced by reserpine (4 mg/kg s.c.; 5 h before testing) was reversed by (+)amphetamine 2 mg/kg but not by modafinil 40 mg/kg. Finally, on synaptosomes prepared from mouse striata preloaded with 3H]dopamine, modafinil 10(-5) M did not increase the spontaneous 3H]dopamine release whereas (+)amphetamine, at the same concentration, doubled it. From all these differences between the two drugs, it is concluded that the mechanism underlying the modafinil induced stimulant locomotor effect differs completely from that of (+)amphetamine.