Are neuronal precursor cells committed to coexpress different neuroactive substances in early amphibian neurulae?

Considering the initial expression of neurotransmitters and neuropeptides immediately after neural induction in amphibian embryos, we previously pointed out that a neuronal cell population emerges from neural plate (NP) and neural fold (NF) expressing very early specific cholinergic, catecholaminergic, GABAergic and peptidergic traits. The purpose of the present work was to investigate the extent to which the neuroblasts that are present in the neurectoderm immediately after gastrulation are committed to give rise to multiple subsets of neurons containing various combinations of neuroactive transmitters rather than to different subpopulations of neurochemically homogeneous neurons. By means of double immunocytochemical localization with a monoclonal TOH-antibody and polyclonal antibodies against GABA or somatostatin, no coexistence of neurotransmitters and neuropeptide was ever found in neuronal subpopulations arising in vitro from NP or NF. The early emergence, under the same conditions, of distinct neuronal subpopulations as a consequence of neural induction strongly suggests that, at the gastrula stage, the neural precursor population most probably does not constitute a homogeneous set of cells.