Serotonin as a Regulator of the Hypothalamic-Pituitary-Interrenal Axis in Rainbow Trout (Oncorhynchus mykiss)

Abstract

Although empirical evidence suggests that interactions occur between serotonin (5- HT) and the hypothalamic-pituitary-interrenal (HPI) axis teleost fish, the mechanisms of serotonergic regulation of the HPI axis require elucidation. The hypothesis that 5-HT regulates the stress response in rainbow trout (Oncorhynchus mykiss) by acting at multiple levels in the HPI axis through tissue-specific 5-HT receptor subtypes was examined. Messenger RNA for 5-HT1A, 5-HT2 and 5-HT4 receptor subtypes was expressed in all HPI axis tissues. Administration of 5-HT in vivo in cannulated trout caused significant increases in plasma cortisol and glucose concentrations. In vitro head kidney preparations revealed a stimulatory effect of 5- HT, acting through the 5-HT4 receptor, on cortisol production. Collectively, these data suggest that 5-HT plays a role in HPI axis activation in rainbow trout, and that at the head kidney level, these effects likely are mediated by the 5-HT4 receptor.