Peptidergic properties expressed in vitro by embryonic neuroblasts after neural induction.

As an immediate consequence of neural induction, some neuroectodermal cells acquire the ability to develop a number of characteristic neuronal features, without requiring any subsequent embryonic cues (Duprat et al. 1987). Thus, adrenergic, cholinergic and gabaergic traits are expressed in cultures of neural fold and neural plate isolated from amphibian embryos immediately after induction and grown in a defined medium. The aim of the present study was to determine, using the same in vitro model, their abilities to develop peptidergic phenotypes. Using immunocytochemical techniques, we show that substance P-, enkephalin- (leu-enkephalin, metenkephalin), and somatostatin- like immunoreactivities are expressed in subpopulations of neurones grown in vitro, whereas VIP (vasoactive intestinal polypeptide) is not detected under the same conditions. The appearance and development of the somatostatinergic phenotype has been quantified by RIA both in cell extracts and in the culture medium. Somatostatin-like immunoreactivity (SLI) undetectable at the late gastrula stage, can be measured in cells after 4 days of culture and continues to increase over the next 10 days. In culture medium, SLI is present at a constant level from day 4 up to day 14. These data reveal that some neuronal precursor cells acquire, during neural induction, the potentiality to biosynthesize, store and release neuropeptides. Furthermore, the expression of these peptidergic phenotypes in distinct subpopulations of neurones suggests that certain neuronal precursors become committed to different metabolic pathways at the earliest steps of neurogenesis.