DEVELOPMENTAL-CHANGES IN THE INDUCIBILITY OF FOS-LIKE IMMUNOREACTIVITY IN PRIMARY EMBRYONIC SPINAL-CORD CULTURES

The immediate early gene (IEG) transcription factor c-fos coordinates changes in the pattern of long term gene expression and, therefore, it may be involved in mediating epigenetic control during neurodevelopme nt. We used pharmacological treatments mimicking various environmental and intracellular signals and assessed the inducibility of fos-like i mmunoreactivity (LIR) at various stages of neurodifferentiation in a p rimary embryonic spinal cord culture system by immunohistochemistry. C onstitutive fos LIR exclusively found in neurons, was driven by the on set and extent of spontaneous electrical activity, as it was blockable by tetrodotoxin (TTX) at all developmental stages. Phorbol myristate 13 acetate (PMA) increased the number of fos-LIR cells equally effecti vely at all stages, but the predominant cellular localization of fos-L IR changed through ontogeny. The effect of veratridine, kainate and se rum-derived factors in significantly inducing fos-LIR was restricted t o the earliest developmental stage (4 days in vitro; DIV) investigated ; whereas forskolin, the GABA(A) antagonist picrotoxin and NMDA failed to induce fos-LIR at this stage, but increased the number of fos-LIR neurons at later stages. Dihydropyridine agonists of the voltage-sensi tive calcium channels (VSCC) raised the number of fos-LIR neurons and also prevented ?TX-mediated down-regulation; whereas antagonists marke dly reduced fos-LIR at all ages. Either type of NMDA antagonists (AP5 and MK801) and the GABA(A) agonist muscimol significantly reduced fos- LIR at all ages. These findings demonstrate that the inducibility of f os-LIR is substantially different in embryonic neurons than in adult o nes and that inducibility by various first and second messengers is de pendent on the developmental stage.