Impact of acute environmental challenges on the expression of peptide hormone-encoding genes in the zebrafish caudal neurosecretory system

The caudal neurosecretory system (CNSS) is a neuroendocrine complex unique to fish, first described in 1955. Since then, it has been hypothesized to contribute to several physiological processes, but its real functions remain largely unclear. However, so far, the many studies devoted to it agree that it could play an important role in osmoregulation. More recently, it has also been suggested that it could be involved in thermal adaptation. The zebrafish (Danio rerio) is a well-established model organism for functional studies. Yet so far, the functions of the CNSS have not been thoroughly investigated in this species. As a first step in such investigations, the present study aimed to identify environmental factors whose variations induce changes in CNSS endocrine activity. For this purpose, juvenile zebrafish were submitted to acute (2, 8, and 24 h) pH, salinity, and temperature challenges. As indicators of the CNSS endocrine activity, the expression levels of peptide hormone-encoding genes known or suspected to be synthesized in the CNSS were measured using quantitative PCR. The genes selected for this study were as follows: corticotropin-releasing hormone b (crhb), oxytocin (oxt), proenkephalin (penka and penkb), parathyroid hormone-like hormone (pthlha and pthlhb), stanniocalcin 2 (stc2a and stc2b), urotensin 1 (uts1) and urotensin 2 (uts2a and uts2b). Our findings revealed that the pH challenge affected the expression of three genes - crhb, penka, and stc2b - and the salinity challenge altered four genes - oxt, uts1, uts2a, and uts2b - while the temperature challenge modified the expression of all genes of our panel. These results indicated that the zebrafish CNSS is sensitive to changes in these environmental parameters and support the use of the zebrafish as a relevant model for studying the functions of the CNSS.