To further elucidate pituitary adrenal interactions during development we studied the organogenesis of the interrenal organ, the teleost homologue of the mammalian adrenal gland in zebrafish. To this end we compared wild-type zebrafish interrenal development with that of mutants lacking pituitary cell types including corticotrophs. In addition, we studied the effects of Acth receptor (Mc2r) knockdown and dexamethasone (dex) on interrenal development and pituitary feedback. Until 2 days post fertilization (2dpf) interrenal development assessed by transcripts of key steroidogenic genes (cyp11a1, mc2r, star) is independent of proopiomelanocortin (Pomc) as demonstrated in aal/eya1and lia/fgf3 mutants. However, at 5dpf lack of pituitary cells leads to reduced expression of steroidogenic genes at both the transcriptional and the protein level. Pituitary control of interrenal development resides in corticotrophs, as pit1 mutants lacking pituitary cells except corticotrophs have a phenotype similar to that of wild-type controls. Furthermore, development in mc2r knockdown morphants does not differ from aal/eya1 and lia/fgf3 mutants. Inhibition of steroidogenesis by mc2r knockdown induces up-regulation of pomc expression in the anterior domain of pituitary corticotrophs. Accordingly, dex suppresses pomc in the anterior domain only, leading to impaired expression of steroidogenic genes commencing at 3 dpf and interrenal hypoplasia via reduced interrenal proliferation. In contrast, negative feedback on pituitary corticotrophs by dex is evident at 2 dpf and precedes effects of Pomc on the interrenal primordium.

These data demonstrate a gradual transition from early pituitary-independent interrenal organogenesis to developmental control by the anterior domain of pituitary corticotrophs acting via Mc2 receptors.