Nigrostriatal dopamine actions on the D₂ receptors mediate methamphetamine effects on the striatonigral substance P system

Multiple administrations of methamphetamine to rats produced elevated concentrations of substance P-like immunoreactivity within the substantia nigra, an effect which is believed to be mediated by increases in dopaminergic activity induced by methamphetamine. The results reported here demonstrate that the effect of methamphetamine is mediated specifically by the dopamine pathway in the nigrostriatum. Thus, extensive destruction of dopamine neurons in the nigrostriatum by lesions induced by 6-hydroxydopamine or blockade of synthesis of dopamine with α-methyl-p-tyrosine prevented the increases in substance P-like immunoreactivity in the nigra, induced by methamphetamine. Similarly, the concurrent administration of the D₂ receptor antagonist, sulpiride, also prevented the effects of methamphetamine on substance P-like immunoreactivity in a dose-dependent way. In addition, multiple administrations of the D₂ agonist, RU24926, elevated, whereas the D₁ agonist, SKF38393, decreased the content of substance P-like immunoreactivity in the nigra. The effects of methamphetamine were not altered in rats with lesions induced by 5,7-dihydroxytryptamine. Thus, these findings suggest that the effect of methamphetamine on the striatonigral substance P pathway is mediated by nigrostriatal dopaminergic actions on D₂ receptors.