Development of D1 and D2 dopamine receptors and associated second messenger systems in fetal striatal transplants

Stereotactic implantation of fetal brain regional anlage into adult host brain (“brain transplantation”) appears to be an increasingly viable strategy for therapy of neurodegenerative diseases. We have studied implantation of fetal striatum into adult striatum, previously lesioned by neurotoxic amino acid injection, as a model for transplantation therapy of Huntington's disease. The beginning of behavioral recovery to apomorphine is not apparent until 6.5 months after implantation. By 4 months after implantation cerebral blood flow through the implants appears equal to that in the intact contralateral striatum. At this time, cerebral glucose utilization is reduced in the implants but increases following apomorphine treatment. The development of D1 and D2 dopamine (DA) receptors is markedly deficient in the striatal grafts at both 4 and 6.5 months after implantation. Very little D2 radioligand binding was observed in the grafts at either time point; D1 receptors appeared in a patchy fashion by 6.5 months at densities approaching normal striatum. In situ hybridization of D2 dopamine receptor mRNA demonstrated robust hybridization signal in normal striatum and accumbens but no signal in 6.5-month-old striatal grafts. Adenylate cyclase (AC) activity, examined with high-affinity [ 3H]forskolin binding, also appeared in patches similar to D1 receptors at 6.5 months. In contrast, protein kinase C activity, labeled with [ 3H]phorbol ester, was very apparent in the grafts at both time points. Higher and generally homogenous densities of muscarinic cholinergic receptors, assessed with [ 3H]QNB binding, develop in the grafts, but there appear to be few functioning cholinergic terminals, as measured by [ 3H]hemicholinium binding. Our data demonstrate that very few D1 or D2 DA receptors develop in fetal striatal neurons removed from E15–16 embryos and subsequently implanted into adult ibotenic acid-lesioned striata. The lack of D2 receptor development appears to derive from lack of transcription of the D2 receptor gene. By 6.5 months, D1 receptor sites and AC activity both appear in discrete patches, consistent with the known in vitro association of D1 receptors with stimulation of AC activity. In this model, the majority of graft protein kinase C activity, to the extent that it is labeled with [ 3H]phorbol ester, is not associated with DA receptors. DA receptor development in fetal striatal implants may require coimplantation of DA neurons.