Tropism of serotonergic neurons towards glial targets in the rat ependyma.

During development, recognition mechanisms between neurons and their targets are necessary for the formation of the neuronal network. Neural connections are synaptic or non-junctional. Both types of communication can be found between neurons and glial elements in the periventricular walls. Serotonergic fibers form synaptic contacts on the specialized ependymocytes of the subcommissural organ, a structure which forms the roof of the third ventricle at its junction with the aqueduct. A network of non-junctional fibers containing both GABA and serotonin spread between the cilia of the classical ependymocytes in the ventricles. These anatomical, morphological and biochemical features suggest a tropism and specific recognition mechanisms between glial elements and serotonergic neurons. This hypothesis can be tested by the study of the innervation of the subcommissural organ and the classical ependyma by grafted embryonic neurons after a chemical destruction of the serotonergic endogenous innervation. Solid implants or cell suspensions prepared from embryonic metencephalon were transplanted to either the third ventricle or the periventricular gray matter in 5,7-dihydroxytryptamine denervated rats. Grafted serotonergic neurons were able to reinnervate the classical ependyma and the subcommissural organ. The fibers forming the supraependymal plexus were non-junctional and contained both serotonin and GABA while those innervating the subcommissural organ formed synaptic contacts and contained only serotonin. The signals capable of inducing the ependymal innervation were specific for serotonergic neurons since catecholaminergic neurons present in the grafts were unable to innervate either classical or specialized ependymocytes. These results demonstrate that glial cells are targets for serotonergic neurons and that the morphological and biochemical characteristics of the serotonergic innervation are closely related to the target cell phenotype.

Migration of purified embryonic motoneurons grafted into adult mouse CNS.

Embryonic motoneurons were fluorescently-labelled with carbocyanine (diI) by means of retrograde transport and then grafted into the adult mouse spinal cord (L2) and brain (striatum) for 2-10 weeks. The motoneurons were grafted either following purification on the fluorescence-activated cell sorter or in the presence of embryonic glial cells and interneurons from the spinal cord. In both conditions of grafting, motoneurons were found to survive and develop in both grey and white matter and were found to migrate long distances in both regions of the central nervous system. Migration of neurons after grafting remains a controversial issue, therefore we have discussed the work of other groups that have described the same phenomenon.

Grafting of embryonic motoneurons into spinal cord and striatum of adult mice.

The aim of the present work was to determine whether embryonic motoneurons could survive in the adult CNS and whether they display different survival and growth characteristics in their natural site (the spinal cord) as compared to an ectopic site (the striatal region of the brain). Specific labelling of embryonic motoneurons was obtained by retrograde transport of a fluorescent tracer (carbocyanine) followed by partial purification of the dissociated motoneurons on a density gradient. This technique offers the advantage that only the motoneurons in the cell suspension used for the grafts contain the fluorescent tracer. This study demonstrates that these motoneurons can survive and differentiate in the white and grey matters, not only in the adult spinal cord, but also in the brain. Furthermore, these motoneurons can migrate approximately 2 mm in the spinal cord and 4 mm in the brain.