Evidence concerning the involvement of 5-hydroxytryptamine in the locomotor activity produced by amphetamine or tranylcypromine plus L-DOPA.

1 Pretreatment of rats with p-chlorophenylalanine (PCPA; 2 X 200 mg/kg) decreased the concentration of 5-hydroxytryptamine (5-HT) in the brain. It also decreased the locomotor activity produced by tranylcypromine plus L-DOPA administration 24 h after the second dose of PCPA. 2 Pretreatment with p-chloroamphetamine, which produced a similar decrease in brain 5-HT concentrations did not decrease the locomotor response to tranylcypromine and L-DOPA. 3 PCPA pretreatment decreased the rise in the concentration of DOPA and dopamine in the brain following tranylcypromine and L-DOPA, suggesting its effect on the dopamine-induced locomotor activity was the result of this drug diminishing dopamine formation in the brain, probably by inhibiting L-DOPA uptake. 4 The locomotor activity produced by tranylcypromine and L-DOPA was not decreased by pretreatment 6 h earlier with disulfiram (400 mg/kg). This argues against the locomotor activity being due to noradrenergic stimulation. 5 PCPA pretreatment did not alter amphetamine-induced stereotypy or the circling behaviour in unilateral nigro-striatal lesioned rats.