Mecanismes de guidage des axones serotoninergiques du raphe dorsal: Etudes in vivo et in vitro (French text)

In the adult central nervous system (CNS), axonal regeneration is essentially abortive after a lesion and this growth limitation is generally attributed to the presence of inhibitory molecules. However, other factors, such as a reduction in the intrinsic axonal growth capacity or a lack of guidance molecules could probably contribute to this phenomenon. This work aimed at better characterizing the cellular and molecular interactions allowing axonal guidance of a particular group of neurons in the adult SNC.; Previously, serotoninergic (5-HT) axons that innervate the neostriatum of new-born or adult rat have been shown to prefer to grow into striatal rather than in ventral mesencephalic neural grafts. We first evaluated the hypothesis of an implication of the astroglial reaction, the glial scar or the expression of axon growth inhibitory molecules produced by the reactive astrocytes, in particular chondroitin sulfate proteoglycans (CSPG), in these innervation differences. We demonstrated, by immunohistochemistry, that these potentially inhibitory factors appeared following a similar pattern after neural transplantation of mesencephalic or striatal tissue, and in the adult as well as new-born rat. Thus, the differences in density of 5-HT innervation of the grafts could not be attributed to these inhibitory factors. We concluded that 5-HT axons of the neostriatum had the capacity to recognize molecules with tropic activity within areas of innervation, such as the ventral mesencephalon and the neostriatum.; We then determined the implication of astrocytes intrinsic to the grafts in the observed differences in innervation. We co-grafted astrocytes cultured from the neonatal ventral mesencephalon, neostriatum or cerebral cortex with fetal ventral mesencephalic or striatal neural tissu, into the neostriatum of adult rats. A quantitative analysis of axonal varicosities, labelled by autoradiography following uptake of tritiated 5-HT, showed that the addition of striatal or cortical astrocytes increased the 5-HT innervation of mesencephalic grafts compared to control grafts of mesencephalic tissue enriched or not with astrocytes from the ventral mesencephalon. Thus, cortical and striatal astrocytes appear to express molecules promoting the growth of adult neostriatum 5-HT axons. Astrocytes of the target regions could therefore be involved in the development of 5-HT innervations.; Finally, to better characterize this tropic activity, we adapted the explant culture in collagen gel to test the selectivity of dorsal raphe-derived 5-HT axons for these three areas of innervation. Quantitative analysis showed similar abundant innervation from the overall population of dorsal raphe 5-HT axons to ventral mesencephalon, neostriatum and neocortex. However, after having grown through a mesencephalic or striatal explant, or onto membrane substrates prepared from these regions, 5-HT axons became more selective for explants of mesencephalon and striatum, respectively. Since the explants had to be apposed to one another for such an effect, molecules associated with the extracellular matrix or the cellular membranes might be responsible of this selectivity.