Release of glutamate produces calcium transients in Xenopus laevis NeuroD expressing neuroblasts
Résumé
NeuroD is a proneural gene for primary and secondary neurogenesis. Recent advances in transgenesis technique have allowed identification of posmitotic living neuroblasts and it appeared that their electrical activity (ionic channels and neurotransmitter receptors) is a prerequisite for they proper migration and differentiation. Our laboratory has developed a transgenic xenopus line which expresses EGFP under the control of NeuroD promoter and this gave us the possibility to study the properties of EGFP expressing living neuroblasts, either in the context of living tadpole or in brain slices. The spatial expression pattern of NeuroD+ cells in vivo was found to decrease from whole brain expression at stage 40 to restricted areas at stage 48 (olfactory bulb, pallium, optic tectum, cerebellum, rhombencephalon). No colocalization of EGFP and the glial marker SR-101 was found, confirming that EGFP cells are not astrocytes. Some functional properties of these cells were determined using time laps calcium imaging of acute brain slices with confocal microscopy. NeuroD+ cells were found to exhibit more calcium transients when Mg2+ was removed from the medium, and this effect is reversed by addition of APV (a NMDA receptor antagonist). Moreover, NeuroD+ cells respond more to bath application of glutamate or NMDA than NeuroD- cells. Taken together, these results suggest that a spontaneous release of glutamate produce calcium transients in NeuroD+ cells, and that activity is triggered by NMDA receptors.