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Notes: Abstract: The nigrostriatal dopaminergic system of rats was unilaterally lesioned with 6-hydroxydopamine. Part of the animals was grafted 2 weeks later with fetal dopaminergic cells on the lesioned side; untreated rats of the same strain served as controls. Both 3 and 12–14 months after surgery the striatal dopamine (DA) content and the in vivo rotational response following injection of d-amphetamine showed significant changes in grafted as compared to lesioned animals. At 12–14 months after transplantation, the electrically evoked release of tritiated DA and acetylcholine (ACh) in slices (preincubated with [3H]DA or [3H]choline, respectively) of striata of intact, lesioned, or grafted animals was also investigated. Electrical field stimulation of striatal slices of the lesioned side did not evoke any significant [3H]DA overflow, whereas a marked [3H]DA release was observed in slices of grafted and control striata. Moreover, both DL-amphetamine (3 μM) and nomifensine (10μM) strongly enhanced basal 3H outflow in these slices. Electrically evoked [3H]ACh release was significantly reduced in slices from all striatal tissues by 0.01 μM apomorphine. In slices from denervated striata a clearcut hypersensitivity for this action of apomorphine was present, indicating super sensitivity of DA receptors on cholinergic terminals; this hypersensitivity was significantly reduced in graft-bearing striata. Furthermore, because this hypersensitivity was unchanged in slices of lesioned striata under stimulation conditions (four pulses/100 Hz) avoiding inhibition by endogenously released DA, it is concluded that lesion-induced DA receptor super sensitivity is caused by an increase in receptor density or efficacy rather than by a decreased competition between endogenous and exogenous agonists. Both reuptake blockade of DA with nomifensine (10 μM) and release of endogenous DA by DL-amphetamine (3 μM) potently reduced [3H]ACh release only in control and grafted but not in lesioned tissue. In experiments using potassium-evoked [3H]ACh release, tetrodotoxin had no effect on the inhibitory activity of amphetamine and nomifensine, indicating that the DA receptors involved in their indirect inhibitory action are located directly on the cholinergic terminals.