Grafts of Embryonic A9 Dopamine Neurons in a Rat Organotypic Culture Model of Parkinson's Disease Indicates the Potential and Limits of Nigro-Striatal Circuitry Reformation.

Primary dopaminergic cell transplantation is a potential therapeutic strategy for Parkinson's disease. However, the procedure commonly involves placing donor neurons heterotopically into the host striatum. To gain insights into the potential to reform circuitry between the substantia nigra and striatum, a sagittal explant system, including the striatum, medial forebrain bundle (MFB) and ventral mesencephalon (VM), was generated from postnatal rats. This ex vivo system provided a platform to test the potential for heterotopic or homotopic grafts of whole embryonic day 14 (E14) rat VM or sub-dissected regions of the A9 or A10 cell groups to reform midbrain dopaminergic circuitry. Results showed that homotopically placed whole VM tissue and sub-dissected A10 donor tissue were able to extend tyrosine hydroxylase+ (TH+) projections along the full rostro-caudal extent of the MFB. However, homotopically grafted A9 dopamine neurons showed little capacity to extend TH+ neurites beyond the graft region. In addition, heterotopic placement of A9 tissue adjacent to the host striatum revealed selective infiltration of TH+ projections into the dorsal striatum, where they displayed functional electrophysiological responses in host striatal neurons. This was in opposition to A10 heterotopic grafts, where TH+ projections were more random with no evidence of synaptic coupling in the host. Such results illustrate the fundamental utility of this system for exploring dopaminergic circuitry reconstruction ex vivo and indicate that the reformation of nigrostriatal circuitry may be limited for the A9 group in isolation, but that the A10 neurons can project along the full extent of the MFB.