Paradoxical activational effects of a corticotropin-releasing factor-binding protein "ligand inhibitor" in rat brain

The corticotropin-releasing factor-binding protein is distinct from known corticotropin-releasing factor receptors, but can bind the peptide and neutralize its biological actions. Recent interest has centered about the therapeutic potential of "ligand inhibitors" of binding protein action, synthetic corticotropin-releasing factor fragments which are inactive at corticotropin-releasing factor receptors, but can displace the peptide from the binding protein, thereby increasing levels of free corticotropin-releasing factor. To identify sites of action of such ligands, the distribution of Fos expression seen following intracerebroventricular administration of rat/human corticotropin-releasing factor(6-33) (5-50 microg) was charted in relation to corticotropin-releasing factor-binding protein and receptor expression. It was expected that Fos induction would mimic aspects of the distribution of the two known corticotropin-releasing factor receptors, but the far greater correspondence was seen with that of the binding protein itself. This included neurons in the isocortex, the olfactory system, amygdala and a number of discrete brainstem cell groups; many Fos-immunoreactive neurons in each were found to co-express corticotropin-releasing factor-binding protein messenger RNA. Subsets of activated neurons co-expressed Type 1 corticotropin-releasing factor receptor messenger RNA, though these were largely limited to cell groups that also express the corticotropin-releasing factor-binding protein, and where binding protein immunoreactivity and Type 1 receptor transcripts were found to co-exist. Responsive neurons displaying Type 2 corticotropin-releasing factor receptor message were seen reliably only in the lateral septal nucleus. These findings support only a limited capacity of the ligand inhibitor to activate neurons bearing corticotropin-releasing factor receptors. The more pervasive activation seen among neurons that express the corticotropin-releasing factor-binding protein may be indicative of an unexpected role for this protein in signaling by corticotropin-releasing factor-related peptides.