ADRENOCORTICOTROPIC HORMONE IN RELATION TO INTERRENAL FUNCTION DURINGSTRESS IN TILAPIA (OREOCHROMIS-MOSSAMBICUS)

This study examines ACTH-like immunoreactivity in the pituitary pars d istalis and pars intermedia of the freshwater teleost Oreochromis moss ambicus (tilapia). Two products (tACTH(A) and tACTH(B)) were present i n both lobes. These two products also accounted for the majority of th e ACTH i.r. when in vitro pars distalis incubation medium was analyzed by HPLC. In a homologous bioassay the two tilapia ACTH-like molecules and human ACTH(1-39) possessed similar corticotropic potency. The pep tides were quantified using a newly validated radioimmunoassay, which was also used to measure ACTH in plasma of unstressed and stressed fis h. Short-term (<12 min) stress rapidly increased plasma cortisol, reac hing levels of around 300 ng/ml in 5 min. Surprisingly, this initial e levation was not accompanied by a rise in plasma ACTH levels. A more p rolonged (3 hr) confinement in pairs resulted in high plasma cortisol and ACTH levels in one fish of every pair. The second fish had control ACTH levels and only marginally elevated cortisol levels. Therefore, in this species social interactions seem to influence the reaction of the pituitary-interrenal axis to stress. The short-term cortisol respo nse to disturbance could be abolished completely by pretreating fish i n vivo with cortisol for 48 hr. This treatment did not alter circulati ng ACTH levels. It is concluded that tilapia did not rely on circulati ng ACTH for a rapid elevation of plasma cortisol levels. Both neuronal mechanisms and cortisol feedback may regulate the pituitary-interrena l axis at the level of the interrenal. (C) 1994 Academic Press, Inc.