Immunolocalization of corticotropin-releasing factor (CRF) andcorticotropin-releasing factor receptor 2 (CRF-R2) in the developinggut of the sea bass (Dicentrarchus labrax L.)

Our previous data indicated an important role for adrenocorticotropic (ACTH)-like moleculesco-operating with macrophages to control the modifications in body homeostasis during the firstperiod of the life of sea bass (up to 30 days post-hatching) before the lymphoid cells have reachedcomplete maturation. The aim of the study was to determine the immunolocalization of corticotropinreleasingfactor (CRF), which is a very important mediator of stress-related responses. Our data showedthat immunostaining for CRF is localized already at 8 days after hatching in nerve fibers of thegastrointestinal tract wall from the pharynx to the anterior gut, when the larvae are still feeding onyolk. This pattern of immunolocalization appeared similar to that in 24-day-old larvae, but at this stagethere were also large cells immunopositive to CRF located in the wall of the midgut and hindgut.Lipopolysaccharide (LPS) treatment, which is a known stimulator of stress hormone responses, did notmodify the CRF immunostaining pattern, though it did affect the immunolocalization of the peripheralCRF receptor, i.e. CRF-R2. Immunolocalization of CRF-R2 appeared in nerve fibers of the gut wall inlarvae fixed 1 h after the end of lipopolysaccharide (LPS) treatment. The present results suggest that CRFplays important autocrine and/or paracrine roles in the early immune responses at the gut level in thelarval stages of sea bass (Dicentrarchus labrax L.) as already proposed for ACTH. Moreover, our studiestaken together with other research on fish, in comparison with mammals, suggest a phylogeneticallyold role of CRF in immune–endocrine interactions.