Comparison of the expression of c-fos, nur77 and egr1 mRNAs in rat hypothalamic magnocellular neurons and their putative afferent projection neurons: cell- and stimulus-specific induction.

Hypothalamic magnocellular neurons and their afferent inputs provide a model system in which to study the regulation of inducible transcription factors in the brain in vivo. Osmotic stimulation of rats produced by graded infusions of saline at different tonicities was found to lead to the induction of c-fos, nur77 and egr1 mRNAs in magnocellular neurons, as well as in putative afferent neurons, including those in structures of the forebrain (subfornical organ, median preoptic nucleus and organum vasculosum of the lamina terminalis). The results presented suggest that stronger levels of osmotic stimulation recruit additional afferents from the forebrain and brainstem that can act on magnocellular neurons via alternative receptors. A single systemic injection of the peptide cholecystokinin produced robust induction of c-fos and nur77 mRNAs in afferent neurons of the brainstem nucleus tractus solitarii and in magnocellular neurons. Despite the fact that these two neuronal populations are clearly electrically active, egr1 was not induced by this stimulus, providing examples of cell- and stimulus-specificity of its expression. This study re-emphasizes that the induction of transcription factors is largely dependent on the nature of the afferent input and does not correlate necessarily to the electrical activity of the neuron.