5-Hydroxytryptamine involvement in the locomotor activity suppressant effects of amphetamine in the mouse.

d-Amphetamine, in doses lower than required to increase motor activity, reduced mouse spontaneous locomotor activity when this was assessed using cages equipped with photocell units, using treadwheels, or the measurement of spontaneous climbing behaviour. Acute treatments with the serotonergic agonists quipazine and 5-hydroxy-DL-tryptophan also reduced wheel running activity, spontaneous locomotor activity assessed using photocell cages, and spontaneous climbing behaviour; fenfluramine caused a similar effect. Pretreatment with 5-hydroxy-DL-tryptophan enhanced the inhibitory effects of d-amphetamine. A 3-day treatment with fenfluramine, or lesions of the median raphe nucleus (but not the dorsal raphe nucleus) abolished the ability of d-amphetamine to reduce motor activity in the three test systems. It is concluded that low doses of d-amphetamine can reduce locomotor activity and that the effects may be mediated via an enhancement of the release of 5-hydroxytryptamine from the system arising in the median raphe nucleus.