Structural characterisation of a cyprinid (Cyprinus carpio L.) CRH, CRH-BP and CRH-R1, and the role of these proteins in the acute stress response.

We elucidated the structure of the principle factors regulating the initiation of the acute stress response in common carp: corticotrophin-releasing hormone (CRH), CRH-receptor 1 (CRH-R1) and CRH-binding protein (CRH-BP). Phylogenetic analyses reveal that these proteins are evolutionarily well conserved in vertebrates. CRH and CRH-BP expression are not co-localised in the same hypothalamic perikarya. On the contrary, CRH-BP expression is limited to the perimeter of the nucleus preopticus (NPO), but is abundant in other regions, including an area directly rostral from, and in close proximity to, the NPO. Despite the lack of co-expression, the nerve fibres projecting onto both the rostral pars distalis (rPD) as well as the large fibre bundles projecting onto the pars intermedia (PI) contain CRH as well as CRH-BP, suggesting that both ACTH release from the rPD as well as the release of PI melanotrope content is regulated via CRH and CRH-BP. Finally, we show via real-time quantitative PCR that expression of hypothalamic CRH and CRH-BP following a 24 h restraint significantly increases, whereas PD CRH-R1 expression decreases; this reflects desensitisation of the PD for hypothalamic CRH output. We conclude that these factors are actively involved in the regulation of acute stress responses in the teleost fish.