Effects of retinoic acid on rat forebrain cells derived from embryonic andperinatal rats

All-trans retinoic acid (RA), a potent inducer of neural development in non -committed neuroectodermal precursors and also: a teratogenic agent for ear ly prosencephalic development is reported to promote the survival and diffe rentiation of embryonic forebrain neurons, in vitro. In cultures of embryon ic (E13, E15) rat forebrain cells, long-term (2-5 days) treatment with RA i ncreased the number of neurons and the overall neurofilament immunoreactivi ty. Treatment with RA for periods longer than 1 h resulted in enhanced bind ing of the non-competitive NMDA-receptor antagonist, TCP, by embryonic and fetal (E17, E18) cells, but not by cells derived from perinatal(E19, PO) fo rebrains. As TCP binding-sites are localised within the channel-complex, tr eatment with RA was thought to result in an opening of the NMDA receptor ch annel. In direct binding assays, however, RA bad no delectable effect, whil e conditioned media taken from RA-treated embryonic or fetal cells increase d the TCP-binding, immediately. Analyses on conditioned media taken from co ntrol cultures of cells with various in vivo or in vitro ages revealed a st able extracellular glutamate level ([Glu](e)) of 1-3 mu M. This basal [Glu] (e) was restored within 24 h after addition of 100 mu M exogenous glutamate . In the presence of RA, however, [Glu](e) was stabilised at an approximate ly three-fold higher (4-10 mu M) level by cells derived from embryonic and fetal brains. RA-treatment did not influence the [Glu](e) in cultures of pe rinatal cells. The RA-induced rise in the neurofilament-immunoreactivity of embryonic brain cell cultures was prevented by simultaneous treatment with APV, a competitive antagonist of NMDA-receptors. The data suggest that a R A-induced shift in the set-point of extracellular glutamate-balance plays a n important role in the promotion of survival and maturation of developing neurons, in culture. (C) 1999 Elsevier Science Ltd. All rights reserved.