Collateral sprouting and reduced activity of the rat mesocortical dopaminergic neurons after selective destruction of the ascending noradrenergic bundles

The properties of the mesocortical dopaminergic neurons projecting to the pregenual and anterior supragenual cortices were examined 3–6 months after the degeneration of ascending noradrenergic pathways caused by bilateral multiple or single microinjections of 6-hydroxydopamine made laterally to the pedunculus cerebellaris superior. In all rats and in all cortical areas examined, noradrenaline levels were reduced by more than 75%. A similar decrease in noradrenaline levels was obtained in the ventral tegmental area. As indicated by the increases in cortical levels of dopamine and in [³H]dopamine specific uptake sites as well as by histochemical analysis, these lesions induced a collateral sprouting of the mesocortical dopaminergic neurons. The intensity of the effect varied from one animal to another and even from one anteromedial hemicortex to another. When present, the increase in dopamine levels was observed in all the cortical areas investigated. As suggested by the decreased ratio of the amount of dihydroxyphenylacetic acid to dopamine in the cortex, the activity of the mesocortical dopaminergic neurons was reduced in the rats with lesions. This effect was even seen in rats in which the cortical levels of dopamine were only slightly increased. Both the collateral sprouting and the reduced activity of the mesocortical dopaminergic neurons were related to the degeneration of the noradrenergic neurons and not to a non-specific effect of 6-hydroxydopamine, since both phenomena did not occur in rats pretreated with desipramine, a treatment which prevented the decline in noradrenaline levels.Thus, a lesion of the ascending noradrenergic pathways can lead to sprouting of dopaminergic neurons in the cortex and a reduced activity of these dopaminergic neurons. The respective role of the disappearance of the noradrenergic innervation in the cerebral cortex and in the ventral tegmental area in the collateral sprouting and in the reduced activity of the mesocortical dopaminergic neurons is discussed.