Using specific antisera to neutralize ACTH in sturgeon: A method for manipulating the interrenal response during stress

Interrenal function and the magnitude of the stress response were assessed in green sturgeon (Acipenser medirostris) passively immunized with antisera directed against adrenocorticotropic hormone (ACTH). The nucleotide sequence encoding ACTH was determined using reverse transcriptase polymerase chain reaction (RT-PCR). We identified two isoforms of ACTH that differ at a single site (position 26) in the 39 AA peptide. Both forms of green sturgeon ACTH (gsACTH1-39) display 100% homology with both sequences of white sturgeon ACTH (wsACTH1-39). The N-terminal portion of gsACTH also shares absolute identity with the comparable portion of human ACTH (hACTH). However, we identified considerable sequence divergence in the C-terminal domain between gsACTH and hACTH. Species-specific anti-ACTH sera were generated by vaccinating sheep against the C-terminal portion of gsACTH (gsACTH26-39). The peptide was covalently linked to a carrier protein (keyhole-limpet-hemocyanin [KLH]) to further enhance its immunogenicity. The anti-gsACTH sera recognized gsACTH1-39 and the immunogenic peptide (gsACTH26-39), but did not interact with hACTH1-39. To assess the impact of the antisera, fish were passively immunized with anti-gsACTH26-39 sera or anti-KLH sera and challenged with a hACTH1-39 injection on day 1 followed by a 1-min air emersion stressor on day 2. The magnitude and duration of the secretory response induced by hACTH did not differ (P > .05) between groups. Conversely, the magnitude of cortisol secretion induced by air emersion was significantly attenuated (P < .05) in fish passively immunized against gsACTH26-39. Collectively, these data demonstrate that the targeted antisera used in this study can discriminate between mammalian and green sturgeon ACTH and moderate the in vivo response to a stressor.