A comparison of two immediate-early genes, c-fos and NGFI-B, as markers for functional activation in stress-related neuroendocrine circuitry.

The promoter regions of the rat corticotropin-releasing factor (CRF), oxytocin (OT), and vasopressin (AVP) genes contain sequences similar to the cis-acting response element identified for NGFI-B, an immediate-early gene structurally related to the steroid hormone receptor superfamily. Combined immuno- and hybridization histochemical approaches were used to determine whether challenges that influence the synthesis and secretion of CRF, OT, and/or AVP result in altered expression in neurosecretory neurons of NGFI-B and another immediate-early gene, c-fos, which is widely used as a marker for functionally activated neurons. NGFI-B mRNA was found to be expressed at constitutively high levels in the telencephalon, but not in the endocrine hypothalamus, of unperturbed controls; basal levels of c-fos expression were uniformly low throughout the CNS. NGFI-B and c-fos mRNAs, and Fos protein, were induced with a similar time course and in similar neuroendocrine cell types in response to acute hypotensive hemorrhage (15% reduction in blood volume), intravenous injection of interleukin-1 beta (IL-1 beta; 1.87 micrograms/kg), chronic salt loading (7 d maintenance on 2% saline), and acute bilateral adrenalectomy. c-fos mRNA and Fos protein were readily demonstrable in afferent pathways that have been implicated as mediating the neuroendocrine responses in the three stress paradigms; these include medullary catecholaminergic cell groups in response to IL-1 beta and hemorrhage, and cell groups lining the lamina terminalis in response to salt loading. Challenge-specific induction of NGFI-B expression was detectable in these extrahypothalamic cell groups, though with a lesser sensitivity than that required to reveal NGFI-B induction in the hypothalamus, or c-fos expression in these related afferents. These results establish NGFI-B as a useful adjunct to c-fos, for revealing synaptic and/or transcriptional activation in the magno- and parvocellular neurosecretory systems. Differences in the sensitivity of the two markers in revealing functionally related activation in extrahypothalamic regions speak to general issues concerning the use of immediate-early genes in mapping functional circuitry in the CNS.