Initial GABAergic expression in embryonic amphibian neuroblasts after neural induction.

At the late gastrula-early neurula stage some embryonic neuroblasts from neural plate and neural fold present apparently as a consequence of neural induction, the capability to develop in vitro into different neuronal subpopulations (cholinergic, dopaminergic, noradrenergic, somatostatinergic and some other peptidergic subpopulations without ongoing influences from the chordamesoderm (Duprat et al., 1987). Using the same in vitro model system, the aim of the present work was to delineate the abilities of these neuroblasts to develop GABAergic traits. The initial appearance and development of GABAergic phenotype has been quantitated by assaying the activity of glutamic acid decarboxylase (GAD). GAD activity was undetectable at the early gastrula stage (stage 8a) and was slightly measurable at the early neurula stage (stage 14- onset of the culture). It increased subsequently over the next 14 days in vitro. The temporal pattern of appearance and development of GAD activity in culture was in agreement with that observed in vivo. Immunocytochemical studies showed that GABA-like immunoreactivity was expressed in vitro in a subpopulation of neurons. Thus the developmental program for GAD expression and GABA phenotype maturation is acquired at least in some neuronal precursors. These data together with previously reported results on the expression of cholinergic, catecholaminergic and peptidergic phenotypes demonstrate that different neuronal subpopulations emerge near the end of gastrulation i.e. immediately after neural induction. The embryonic origin of this neuroblast heterogeneity remains to be determined.