Partial lesions of the dopaminergic nigrostriatal system in rat brain: biochemical characterization.

Various doses of 6-hydroxydopamine injected into the rat substantia nigra produced partial, dose-dependent lesions of the dopaminergic nigrostriatal tract. The resulting reduction in striatal dopamine concentrations and tyrosine hydroxylase activities tended to be proportional, allowing these measurements to serve as indices for lesion severity in any particular animal. Lesions destroying two-thirds or more of the nigrostriatal neurons accelerated dopamine's synthesis in, and release from, surviving neurons, as indicated by increased striatal levels of the dopamine metabolites dihydroxyphenylacetic acid and homovanillic acid. Formation of these metabolites was also enhanced in dendrites of dopaminergic neurons in the substantia nigra. Supersensitivity of striatal postsynaptic receptors, as judged by induction of rotational behavior after apomorphine or L-DOPA administration, occurred when 90% or more of the nigrostriatal neurons had been destroyed. In contrast, rotational behavior could be induced by amphetamine in animals with only 50% of these neurons destroyed.