Time course of neocortical graft innervation by AChE-positive fibers

Long-term survival and development of fetal ventral spinal grafts into the motoneuron-depleted rat spinal cord: role of donor age

Fetal spinal cord (SC) tissue grafts can survive and develop into the lesioned SC, but no conclusive data are available concerning the long-term fate of transplanted material and the relation between the graft fate and the donor embryo age. Here, pre-labelled suspensions of ventral SC from E12 or E17 rat fetuses were grafted to the lumbar SC of adult rats with motoneuron depletion induced by perinatal injection of volkensin. E12 and E17 are presumably the stages when motoneuron development starts and terminates, respectively. Four or 10months post-grafting, SCs were analyzed to check the graft survival rate and to follow the differentiation and spatial distributions of grafted cells. Neurotoxic lesion produced a 61% motoneuronal loss in the lumbar SC. In transplanted animals, all E12 fetal grafts survived until the observed time-points and developed various mature cell phenotypes. Many motoneuron-like labelled cells were found within the graft area or adjacent to it. Conversely, none of the E17 fetal grafts survived, since no graft-derived elements with neuronal morphology were found either in the site of graft placement or adjacent to it. The present findings indicate that spinal neuroblasts can survive for a long time and develop within the motoneuron-depleted SC, and that the donor embryo age is crucial for successful engraftment.

Extracellular amyloid formation and associated pathology in neural grafts

Amyloid precursor protein (APP) processing and the generation of beta-amyloid peptide (Abeta) are important in the pathogenesis of Alzheimer's disease. Although this has been studied extensively at the molecular and cellular levels, much less is known about the mechanisms of amyloid accumulation in vivo. We transplanted transgenic APP23 and wild-type B6 embryonic neural cells into the neocortex and hippocampus of both B6 and APP23 mice. APP23 grafts into wild-type hosts did not develop amyloid deposits up to 20 months after grafting. In contrast, both transgenic and wild-type grafts into young transgenic hosts developed amyloid plaques as early as 3 months after grafting. Although largely diffuse in nature, some of the amyloid deposits in wild-type grafts were congophilic and were surrounded by neuritic changes and gliosis, similar to the amyloid-associated pathology previously described in APP23 mice. Our results indicate that diffusion of soluble Abeta in the extracellular space is involved in the spread of Abeta pathology, and that extracellular amyloid formation can lead to neurodegeneration.

Enhancement of cortical plasticity by behavioral training in acetylcholine-depleted adult rats

Trimming all whiskers except two on one side of an adult rat's face results in cortical plasticity in which the spared whiskers, D2 and one D-row surround whisker (either D1 or D3), evoked responses containing more spikes than the response evoked by the cut whisker (called whisker pairing plasticity). Previously we have reported that acetylcholine (ACh) depletion in cortex prevents surround D-row whisker plasticity from developing within the barrel cortex. In this study we examined whether the animal's active use of its two intact whiskers can restore some aspects of plasticity in the ACh-depleted cortex. To achieve this goal, ACh was depleted from barrel field cortex, and 14 days after the depletion surgery, whiskers were trimmed and animals were trained on a whisker-dependent gap crossing task. After 7 days of training, animals were anesthetized with urethan and prepared for single-unit recording. Training the ACh-depleted, whisker-paired animals resulted in a significant enhancement of responses to paired surround whiskers: the D-paired whisker-evoked response contained more spikes than the D-cut evoked response. We conclude that training whisker paired rats has a positive impact on response properties of neurons in S1 cortex, even in ACh-depleted animals.

Role of the basal forebrain cholinergic projection in somatosensory cortical plasticity

Trimming all but two whiskers in adult rats produces a predictable change in cortical cell-evoked responses characterized by increased responsiveness to the two intact whiskers and decreased responsiveness to the trimmed whiskers. This type of synaptic plasticity in rat somatic sensory cortex, called "whisker pairing plasticity," first appears in cells above and below the layer IV barrels. These are also the cortical layers that receive the densest cholinergic inputs from the nucleus basalis. The present study assesses whether the cholinergic inputs to cortex have a role in regulating whisker pairing plasticity. To do this, cholinergic basal forebrain fibers were eliminated using an immunotoxin specific for these fibers. A monoclonal antibody to the low-affinity nerve growth factor receptor 192 IgG, conjugated to the cytotoxin saporin, was injected into cortex to eliminate cholinergic fibers in the barrel field. The immunotoxin reduces acetylcholine esterase (AChE)-positive fibers in S1 cortex by >90% by 3 wk after injection. Sham-depleted animals in which either saporin alone or saporin unconjugated to 192 IgG is injected into the cortex produces no decrease in AChE-positive fibers in cortex. Sham-depleted animals show the expected plasticity in barrel column neurons. In contrast, no plasticity develops in the ACh-depleted, 7-day whisker-paired animals. These results support the conclusion that the basal forebrain cholinergic projection to cortex is an important facilitator of synaptic plasticity in mature cortex.

Selective innervation of embryonic hippocampal transplants by adult host dentate granule cell axons

Fragments containing different cytoarchitectonic fields were dissected out of late embryonic rat hippocampal primordia and transplanted into the hippocampus or septum of adult syngeneic hosts. Field CA3 transplants contained clusters of large, angular (pyramidal) cell bodies surrounded by a radiating corona of dendrites. These cells stained selectively with our monoclonal antibody Py, and a proportion were labelled by [3H]thymidine administered on the 15th day of embryonic life. Field CA1 transplants contained smaller, angular, Py-negative cells, which formed elongated laminae rather than globular clusters. The ability of the host dentate granule cells to project to the transplants was examined by (1) the Timm stain for mossy fibres, (2) electron microscopy of Golgi-impregnated CA3 pyramidal neurons in the transplants, and (3) quantitative electron microscopic assessment of the proportions of large mossy fibre terminals in the synaptic population of the transplants. The Timm stain showed that CA3 transplants received a projection from host dentate granule cells when the transplants were placed in direct contact with the axons in the host mossy fibre pathway. As in the normal host field CA3, the ingrowing mossy fibres terminated selectively on the juxtacellular regions of the dendritic tree and ignored the major part of the dendrites in the radiating corona. The electron micrographs showed that within this territory the host mossy fibres formed synaptic terminals with all the complex features typical of normal mossy fibres, and were presynaptic to complex spines arising from the juxtacellular region of Golgi-impregnated donor CA3 pyramidal cells. The quantitative electron microscopic study demonstrated that the mossy fibre-innervated juxtacellular regions of the field CA3 transplants had up to 20% of the normal density of mossy fibre synapses found in the stratum lucidum of field CA3 in situ. CA3 transplants which were placed in the septum, remote from the host mossy fibres, had either trivial numbers of mossy fibre synapses or none. This confirmed that the abundant mossy fibre terminals in the intrahippocampal CA3 transplants were of host origin, and not due to donor dentate granule cells inadvertently included in the grafts. The selectivity of the host dentate projection for field CA3 transplants was demonstrated by the observation that CA1 transplants in the same locations received only slight mossy fibre projections in the Timm stain, and in electron micrographs their synaptic population had only insignificant numbers of large mossy fibre terminals.(ABSTRACT TRUNCATED AT 400 WORDS)

Dissimilar responses of adult thalamic monoaminergic and somatosensory afferent fibers to implantation of thalamic fetal cells

It is generally accepted that transplanted fetal neurons can, after several weeks to months, establish connections with the host CNS. Host afferent systems seem, however, to show different types of responses to the presence of grafted fetal neurons. The present study is a preliminary step to identify mechanisms involved in the reactions of adult axons to transplanted fetal neurons. The right ventrobasal thalamus of adult rats was depleted of neurons by in-situ injection of kainic acid and cell suspensions from homotopic thalamic embryonic primordia which were injected into the lesioned area. After various post-implantation delays, ranging from five to 30 days, two types of experiments were performed: (i) noradrenaline and serotonin immunohistochemistry with specific antibodies on alternate sections; and (ii) anterograde tracing using wheat germ agglutinin conjugated to horseradish peroxidase from the dorsal column nuclei and the principal sensory trigeminal nucleus. Five days after transplantation, host monoaminergic fibers (either noradrenergic or serotoninergic) had already grown into the transplants. Ingrowing fibers were thin and poorly varicose, exhibiting endings morphologically similar to the growth cones observed during axogenesis. Seven days after grafting, growth cones were no longer visible and monoaminergic fibers exhibited either normal-sized or very large varicosities. Large varicosities progressively decreased in number and, after three weeks, the fibers displayed a normal adult morphology, forming a dense network all over the transplants. In contrast, host somatosensory afferents, labeled by anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, did not grow into the transplants. Intermingling of somatosensory afferents and transplanted cells was observed only after 10 days, when grafted neurons extended outside the original transplantation site into the neuron-depleted area containing the somatosensory afferents. The present results demonstrate that adult monoaminergic and somatosensory afferents, when deprived of their usual target, do not react in a similar way to the addition of fetal neurons. It is proposed that adult monaminergic fibers have the ability to regain morphological (and probably functional) immature forms which were considered to be restricted to the period of axogenesis or to lesion-induced regeneration. In contrast, fetal transplants do not seem to induce, by themselves, a similar alteration of genetic expression in adult somatosensory neurons. It has been proposed that "diffuse" and "point-to-point" axonal systems may be differentiated in the CNS on anatomical bases. The present results add to the identification of two different systems by demonstrating that, in the thalamus, they present dissimilar responses to the implantation of fetal cells.

Expression of cholinergic markers in transplants of immature mouse neocortex into adult mouse parietal cortex

Cerebral cortex of embryonic and newborn mouse was removed and transplanted into adult mouse neocortex to investigate whether the development of cholinergic markers would proceed normally after transplantation. It is shown that both AChE and muscarinic receptors developed in transplanted mouse neocortex. However, while muscarinic receptor binding increased to adult levels in transplants from both pre- and postnatal donors AChE staining intensity only achieved adult levels in tissue from postnatal donors. These results are in accordance with the normal developmental time course of the two different cholinergic markers, since muscarinic receptors appear prenatally in cortex while AChE becomes histochemically detectable after birth. These data suggest that the prenatal donor tissue, unlike the postnatal transplants, lacks an environmental signal for the appearance of AChE stained elements, that is, presumably host AChE fiber ingrowth. Transplantation of immature donor CNS tissue into adult host thus appears to be a useful paradigm to study the regulation of differentiation processes.