Osmoregulatory responses of expression of vasotocin, isotocin, prolactin and growth hormone genes following hypoosmotic challenge in a stenohaline marine teleost, tiger puffer (Takifugu rubripes)

To examine possible roles of vasotocin (VT), isotocin (IT), prolactin (PRL) and growth hormone (GH) in osmoregulation of a stenohaline marine teleost, tiger puffer (Takifugu rubripes), changes in expression levels of these genes following hypoosmotic challenge, were examined in two experiments. Fish were transferred from 100% seawater (SW) to 33% SW, 10% SW and fresh water (FW), and left for 3 days in experiment I. In experiment II, fish were transferred to FW, and left for 1 day. Changes in plasma osmolality, concentrations of Na+ and Cl−, and Na+/K+-ATPase activity in the gills and kidney were examined. Changes in the absolute amounts of VT, IT, PRL and GH mRNAs were determined by real-time PCR. In experiment I, almost all fish survived over 3 days of acclimation. The plasma parameters decreased on day 1, and remained at similar levels until day 3. The renal Na+/K+-ATPase activity significantly increased in 10% SW and FW on day 1. The amounts of VT and IT mRNAs tended to decrease in the hypoosmotic conditions on day 1. The amounts of PRL mRNA significantly increased in the hypoosmotic conditions, whereas those of GH mRNA decreased in FW. In experiment II, the amount of VT mRNA significantly decreased in FW concomitantly with the changes in PRL and GH mRNAs. The present results suggest that the hyperosmotic responses may be regulated by neuroendocrine factors such as VT, PRL and GH in tiger puffer, as in case of euryhaline teleosts. Particularly, the present study first shows that the expression of VT gene may be down-regulated following hypoosmotic challenge in the stenohaline marine fish.