Phenotype of Striatal Cells Expressing c‐Fos Following Amphetamine Treatment of Rats with Intrastriatal Dopaminergic Grafts

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Abstract
Activation of the nigrostriatal dopaminergic system by psychostimulants such as amphetamine increases c‐Fos expression in the striatum, mostly in the striatonigral substance P‐ergic pathway. This effect is greatly reduced in the neostriatum deprived of dopaminergic afferents. Dopaminergic grafts implanted into the denervated neostriatum restore the reactivity of the striatum to amphetamine. However, the number of striatal neurons expressing c‐Fos is greatly increased in the graft‐bearing striatum compared with the normal striatum. We examined whether this increase in the number of c‐Fos‐expressing neurons corresponds to the recruitment of a new neuronal population, or whether it reflects an increase in the proportion of substance P‐ergic neurons exhibiting activation of c‐Fos. Adult rats received a unilateral 6‐hydroxydopamine lesion of the ascending dopaminergic mesotelencephalic pathway, and a suspension of embryonic mesencephalic neurons was subsequently implanted into the denervated neostriatum. Three months after implantation, animals were injected with d‐amphetamine (5 mg/kg) and killed 2 h later. In the first experiment, striatal sections were processed to visualize both c‐Fos protein, by immunohistochemistry, and preproenkephalin A or substance P, by in situ hybridization. In the second experiment, c‐Fos and neuropeptide Y were visualized on the same sections. In addition, some sections incubated with anti‐c‐Fos antibody were counterstained with toluidine blue in order to determine whether cholinergic neurons were expressing c‐Fos following amphetamine treatment. The density of neurons expressing c‐Fos following amphetamine treatment was three‐fold higher in the graft‐bearing striata than in the striata of control animals. Approximately 75% of the c‐Fos expressing cells were substance P‐ergic in control animals whereas 6% were enkephalinergic and only a few were neuropeptide Y‐ergic or cholinergic. Similar proportions were found in the graft‐bearing striatum, signifying that the pattern of activation of c‐Fos following amphetamine administration is not changed by the graft. Thus, the increased expression of c‐Fos predominantly reflects a graft‐induced increase in the proportion of neurons expressing c‐Fos within the same population of neurons which normally expresses c‐Fos in the striatum, i.e. the striatonigral substance P‐ergic neurons; there is no recruitment of a new neuronal population. This increased activation of the striatonigral substance P‐ergic pathway may underlie the abnormal behavioural reactions brought about by amphetamine‐induced stimulation of the implanted dopaminergic neurons.

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