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The aim of this study was to determine whether the growth of axons along the nigrostriatal pathway from fetal dopamine cells, transplanted into the substantia nigra of young postnatal 6-OHDA-lesioned rats, is dependent on the age of the host brain. Neonatal rats were lesioned bilaterally by intraventricular injection of 6-OHDA at postnatal day 1 (P1) and received grafts of E14 ventral mesencephalon at day 3 (group P3), day 10 (group P10), or day 20 (group P20) into the right substantia nigra. One lesioned group was left untransplanted. Six months after surgery the animals were subjected to analysis of drug-induced rotation following injection of amphetamine, apomorphine, a D1 agonist (SKF38393), or a D2 agonist (Quinpirole). Animals transplanted intranigrally at day 3 and day 10 showed a strong amphetamine-induced rotational bias toward the side contralateral to the transplant. Animals transplanted into substantia nigra at P20, like the lesioned control animals, showed no rotational bias. Apomorphine and selective D1 and D2 agonists induced ipsilateral turning behavior in the P3 and P10 group, but not in the P20 or the lesion control groups. Immunofluorescence histochemistry in combination with retrograde axonal tracing, using FluoroGold injection into the ipsilateral caudate-putamen showed colocalization of tyrosine hydroxylase and FluoroGold in large numbers of transplanted neurons in the animals transplanted at postnatal day 3 and postnatal day 10, which was not observed in the group P20. The lesion control group showed a 90% complete lesion of the TH-positive cells in the substantia nigra while largely sparing the neurons in the ventral tegmental area. The results indicate that intranigral grafts can be placed accurately and survive well within the substantia nigra region at various time points during postnatal development. Furthermore, embryonic dopamine neurons have the ability to extend axons along the nigrostriatal pathway and reconnect with the dopamine-depleted striatum when transplanted at postnatal day 3 and postnatal day 10, but not at postnatal day 20., (Copyright 1999 Academic Press.)

Intrastriatal transplantation of fetal ventral mesencephalon (VM) is currently explored as a potential clinical therapy in Parkinson's disease (PD). Although providing substantial benefit for the patient, behavioral recovery so far obtained with intrastriatal VM grafts is not complete. Using the 6-hydroxydopamine lesion model of PD, we show here that near-complete restoration of the striatal dopamine (DA) innervation can be achieved by multiple intrastriatal microtransplants of fetal DA cells; nevertheless, complete recovery in complex sensorimotor behaviors was not obtained in these animals. In line with the current model of basal ganglia function, this suggests that the lesion-induced overactivity of the basal ganglia output structures, i.e., the substantia nigra (SN) and the entopeduncular nucleus, may not be completely reversed by intrastriatal VM grafts. In the present study, we have transplanted fetal VM tissue or fetal striatal tissue, as a source of DA and GABA neurons, respectively, into the SN of DA-depleted rats. Intranigral VM grafts induced behavioral recovery in some sensorimotor behaviors (forelimb akinesia and balance tests), but the effect did not exceed the recovery observed after intrastriatal VM grafts. Intranigral grafts of striatal tissue induced a pattern of functional recovery which was distinctly different from that observed after intranigral VM grafts, and recovery in coordinated forelimb use in the paw-reaching test was even more pronounced than after intrastriatal transplantation of VM cells. Combined transplantation of DA neurons into the striatum and GABA-rich striatal neurons into the SN induced additive effects of behavioral recovery observed in the forelimb akinesia test. We propose that intranigral striatal transplants, by a GABA-mediated inhibitory action, can reduce the overactivity of the host SN projection neurons and can induce significant recovery in complex motor behavior in the rat PD model and that such grafts may be used to increase the overall functional efficacy of intrastriatal VM grafts., (Copyright 1999 Academic Press.)