Dopamine and cholecystokinin immunoreactive neurons in mesencephalic grafts reinnervating the neostriatum: evidence for selective growth regulation.

Pieces of embryonic mesencephalic tissue rich in dopamine and cholecystokinin immunoreactive neurones were grafted to the dorsal surface of the caudate-putamen of adult host rats subjected to unilateral dopamine depleting lesions. After 3 months, neuronal survival in the graft and fibre outgrowth into the host brain were studied by tyrosine hydroxylase and cholecystokinin immunohistochemistry, both in serial sections and by elution and restaining of the same sections. Both dopamine- and cholecystokinin-containing neurones as well as neurons containing both compounds survived the transplantation process. The ratio of neurones in which dopamine and cholecystokinin-like immunoreactivity occurred independently and in coexistence was similar in the grafts to that seen in the intact ventral mesencephalon. This suggests that the grafted cells maintain and express at least some of their normal chemical characteristics in the ectopic cortical location. Only those fibres which contained tyrosine hydroxylase but apparently lacked the cholecystokinin-like peptide showed extensive reinnervation of the host neostriatum. The cholecystokinin-positive fibres were found in a narrow zone immediately adjoining the graft. These results indicate that the dopaminergic reinnervation of the denervated neostriatum is preferentially carried out by the population of grafted mesencephalic dopamine neurones apparently lacking the cholecystokinin-like peptide. This suggests the presence of growth regulating mechanisms in the denervated neostriatum which selectively favour the ingrowth of fibres from the appropriate dopaminergic neuronal subset. The transplantation technique may therefore provide a powerful tool for the study of neurone-target interactions in the establishment of neuronal connections, and of the possible role of peptidergic coexistence in the development and organization of monoaminergic pathways and their innervation patterns.