GABA-INDUCED CHEMOKINESIS AND NGF-INDUCED CHEMOTAXIS OF EMBRYONIC SPINAL-CORD NEURONS

During CNS development, neuroblasts proliferate within germinal zones of the neuroepithelium, and then migrate to their final positions. Alt hough many neurons are thought to migrate along processes of radial gl ial fibers, increasing evidence suggests environmental factors also in fluence nerve cell movement. Extracellular matrix molecules are though t to be involved in guiding neuronal migration, and molecules such as NGF and GABA exert trophic effects on immature neurons. The nature of the signals that initiate and direct neuroblast migration, however, is unknown. In vitro, NGF and GABA promote neurite outgrowth from cultur ed cells, and NGF induces axonal chemotaxis (directed migration along a chemical gradient). At earlier developmental stages, these molecules could influence neuroblast movement. Therefore, we investigated wheth er these molecules induce embryonic neuronal migration. Using an in vi tro microchemotaxis assay, we show that rat embryonic spinal cord neur ons migrate toward picomolar NGF and femtomolar GABA beginning at embr yonic day 13 (E13). Cells exhibit chemotactic responses to NGF while G ABA stimulates chemokinesis (increased random movement). GABA effects are mimicked by muscimol and inhibited by bicuculline and picrotoxin, suggesting GABA motility signals are mediated by GABA receptor protein s. Expression of GABA receptors by embryonic cord cells has been previ ously reported (Mandler et al., 1990; Walton et al., 1993). We used po lymerase chain reaction analysis to demonstrate the presence of NGF an d trk mRNA in E13 and E14 cord cells, indicating the cells express mes sage for both NGF and high-affinity NGF receptors. Immunohistochemistr y of E13 spinal cord sections indicates that NGF and GABA colocalize i n fibers close to the target destinations of migrating neurons, sugges ting diffusible gradients of these molecules provide chemoattractant s ignals to migratory cells. Thus, in vitro, neuroblast migration is ind uced by specific signaling molecules that are present in the developin g spinal cord, and may stimulate migration of embryonic neurons prior to synaptogenesis.