Connectivity of striatal grafts implanted into the ibotenic acid-lesioned striatum--I. Subcortical afferents

Intrinsic Organization and Connectivity of Intrastriatal Striatal Transplants in Rats as Revealed by DARPP-32 Immunohistochemistry: Specificity of Connections with the Lesioned Host Brain

Intrastriatal grafts of tissue obtained from the striatal or neocortical primordia of rat fetuses have been studied with respect to their intrinsic organization and connectivity using antibodies to DARPP-32 in combination with acetylcholinesterase (AChE) histochemistry, tyrosine hydroxylase (TH) immunocytochemistry, and anterograde and retrograde axonal tracing techniques. The striatal grafts were characterized by distinct patches of DARPP-32-immunoreactive neurons, which were identical to the densely AChE-positive patches stained in adjacent sections from the same specimens. The non-patch areas possessed only few DARPP-32-positive neurons and contained only sparse AChE-positive fibres. The cortical grafts, by contrast, contained no neurons with clear-cut DARPP-32-positivity and they exhibited a sparse, evenly distributed AChE fibre network, similar to that seen in the non-patch areas of the striatal grafts. The host dopaminergic afferents, as revealed by TH immunostaining, had grown selectively into the DARPP-32-positive patches in the striatal grafts, where they formed a dense terminal network around the DARPP-32-positive cell bodies. The non-patch areas, as well as the cortical grafts, received only sparse TH innervation. By contrast, the host cortical afferents, labelled by Phaseolus vulgaris leucoagglutinin from the host frontal cortex, were seen to extend into both the patch and non-patch areas of the striatal grafts. Transplant neurons projecting into the host brain were labelled by Fluoro-Gold injections into the ipsilateral host globus pallidus. These injections labelled large numbers of medium-sized neurons within the striatal grafts and the vast majority of them (over 85%) were confined to the DARPP-32-positive patches. Similar Fluoro-Gold injections labelled only few graft neurons in the cortical grafts. The results indicate that the striatal grafts are composed of a mixture of striatal and non-striatal tissue, and that the striatal graft compartment selectively establishes afferent and efferent connections with the host nigro-pallidal system. These graft connections demonstrate a remarkable specificity in the formation of graft - host connectivity. The results, moreover, suggest that developmental properties of the grafted striatal primordium are retained and expressed in the implanted cell suspension, and that the neuronal systems of the lesioned adult host brain, at least to some extent, remain responsive to growth regulating mechanisms normally operating during ontogenetic development.

Connectivity of striatal grafts implanted into the ibotenic acid-lesioned striatum--II. Cortical afferents

Grafts of fetal striatal tissue, implanted into the ibotenic acid-lesioned caudate-putamen in adult recipient rats, have previously been shown to receive extensive afferent inputs from various subcortical areas in the host. In the present study, the formation of graft afferents from the host neocortex has been analysed, 3-12 months after transplantation, by means of the anterograde axonal tracer Phaseolus vulgaris leucoagglutinin, iontophoretically injected at multiple sites into the host frontal cortex, in combination with intra-transplant injections of the fluorescent retrograde tracer, Fluoro-Gold. From the cortical injection sites, fibres labelled with Phaseolus vulgaris leucoagglutinin could be traced through the corpus callosum, along the myelinated fascicles of the internal capsule, into the host caudate-putamen. The labelled axons passed in large numbers across the host-graft border zone to form a rich terminal plexus that covered large parts of the grafted tissue, in close association with the grafted neurons. The labelled corticostriatal terminal network was dense, particularly in the peripheral portions of the grafts, although it did not reach the density seen in the spared portions of the host caudate-putamen. Injections of Fluoro-Gold into the depth of the striatal grafts labelled large numbers of neurons in the frontoparietal cortical regions. Consistent with previous findings, labelled neurons were also found, ipsilateral to the graft, in the intralaminar nuclear complex of the thalamus, basolateral amygdala, substantia nigra and dorsal raphe. The host neocortical neurons labelled by Fluoro-Gold injections into the graft had the same laminar and regional distribution as the neurons labelled from the homotopic area of the intact striatum, indicating that the cortical afferents to the graft were derived from those corticostriatal neurons which projected to the area of the ibotenic acid-lesioned caudate-putamen into which the graft had been placed. The number of Fluoro-Gold-labelled cortical neurons ipsilateral to the graft reached in the best cases about 1/3 of the number labelled by identical Fluoro-Gold injections into the intact caudate-putamen. Labelled neurons occurred, with a similar distribution, also in the contralateral frontoparietal cortex, but they were considerably fewer in number. The results demonstrate that fetal striatal tissue, implanted into the previously neuron-depleted adult caudate-putamen, receives innervations from all normal principal cortical and subcortical striatal afferent systems, thus pointing to a remarkable capacity of the implanted fetal striatal neurons to become anatomically integrated into the lesioned host striatal circuitry.

Release of acetylcholine and its dopaminergic control in slices from striatal grafts in the ibotenic acid-lesioned rat striatum

Tritium accumulation during incubation with 3H-choline, and the efflux as well as the electrically evoked overflow of tritium during subsequent superfusion, were investigated in slices from unilateral striatal suspension grafts 16 to 20 weeks after implantation into the previously ibotenic acid-lesioned rat striatum. Slices from non-operated animals, from striata contralateral to grafts, and from animals with acute ibotenic acid lesions of the striatum were studied in parallel. The accumulation of tritium and the overflow of tritium in response to electrical stimulation (2 min, 3 Hz) were markedly impaired in acutely lesioned striata. In graft slices, tritium accumulation and the subsequent electrically evoked overflow were greater than in slices obtained after acute lesions, but were still smaller than in non-operated animals or in the contralateral striata. The dopamine D2-receptor agonist quinpirole inhibited the electrically evoked overflow of tritium in grafts, but only to a small extent. The D2-receptor antagonist sulpiride increased, whereas the dopamine uptake inhibitor nomifensine and the dopamine releasing drug amphetamine decreased the evoked overflow in slices from non-operated rats and from striata contralateral to grafts, but had no significant effect in grafts. As in graft slices, the release of acetylcholine in striata from animals in which the mesostriatal dopamine pathway had been lesioned by 6-hydroxy-dopamine was not changed by sulpiride and amphetamine, and was only minimally decreased by nomifensine. Our data show that striato-striatal grafts can partly restore the impaired choline accumulation and acetylcholine release in excitotoxin-lesioned striata. Functional D2-receptors are present on graft cholinergic cells, but are not activated by endogenous dopamine under the present in vitro conditions.

Rapid growth of host afferents into fetal thalamic transplants

Fetal cell suspension grafts grow and differentiate when implanted into adult rat CNS areas previously neuron-depleted using an excitotoxin. There is some controversy in the literature concerning the timetable of establishment and possible extent of host-graft connections in these experimental conditions. The present study was undertaken to analyze the development of adult host monoaminergic afferents into a transplant formed by fetal thalamic neurons in the previously excitotoxically lesioned thalamus. It is demonstrated that both norepinephrin- and serotonin-immunoreactive fibers are present in the transplant as soon as 8 days after grafting. At those times, immunoreactive fibers exhibit morphological characteristics typically associated with immature stages. After longer survival time, up to 4 weeks after grafting, immunoreactive fibers are numerous in the transplant and exhibit morphological features comparable to those observed in the adult thalamus. These results demonstrate the rapid ingrowth of some fiber systems of the adult host into the transplant and suggest that grafted fetal cells can be functionally integrated into the host circuitry as soon as a few weeks after grafting.

Morphological alteration of thalamic afferents in the excitotoxically lesioned striatum

Excitotoxic lesions of the neostriatum cause anatomical and biochemical changes resembling those occurring in Huntington's disease. One major characteristic of these lesions is that they acutely spare axons of passage and afferent fibers. However, evidence is accumulating that afferent axonal systems decrease their fiber density in the long-term excitotoxic lesion. Ultrastructural changes of neuron-deprived terminals may also occur. A parallel study considering changes in afferent fibers to the excitotoxically lesioned thalamus showed that, a few weeks after neuron-depletion, specific 'point-to-point' systems formed regenerating axonal growth cone-like structures. The present study used the anterograde transport of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) to determine whether specific thalamostriatal afferents form the same kind of regenerating structures following excitotoxic lesion of their target neurons. Thalamostriatal afferents decreased in density over months after lesion, but some were still labeled as long as 4 months after ibotenic acid injection. Remaining afferents formed axonal growth cone-like structures, identified at both light and electron microscopic levels, similar to those observed in the lesioned thalamus. These results demonstrate that in the striatum as in the thalamus, neuron depletion is followed by a long-term alteration of the morphology of some afferent fibers which form regenerating growth cone-like structures. These results are discussed with regard to the possible functional integration of fetal neurons transplanted into previously excitotoxically lesioned areas.