Regulation of the neuroendocrine stress axis in response to ammonia exposure in rainbow trout: Pharmacological and transcriptional evidence implicating serotonin and multiple hypophysiotropic peptides

Abstract

Ammonia is neurotoxic and exposure to high environmental ammonia (HEA) activates the hypothalamic-pituitary-interrenal (HPI) axis in teleosts. To gain insight into the neural factors that regulate the HPI axis in response to this environmental stressor, as well as elucidate potential interactions between these factors, we exposed rainbow trout to one of three ammonia levels (0, 650, 1000 μM NH₄Cl) for 24 or 96 h and assessed the gene expression and circulating levels of key determinants of HPI axis activity. In parallel with circulating ammonia concentrations, plasma cortisol levels increased dose-dependently after 24 h of HEA exposure and partially recovered after 96 h. HEA exposure also elicited dose-, time-, and brain region-specific changes in components of the central serotonergic (5-HTergic; tph2, htr1aa, htr1b, htr2c), corticotropin-releasing factor (crfb), arginine vasotocin (avt, avtr1a, avtr2), and isotocin (it, itr) signaling systems. Moreover, while intraperitoneal injections of 5-HT_1B and 5-HT_2C receptor antagonists reduced basal cortisol levels, treatment with 5-HT_1A and 5-HT_2C receptor antagonists blocked the increase in plasma cortisol elicited by HEA. Finally, treatment with specific 5-HT receptor antagonists blunted the HEA-induced increases in brain preoptic area crfb, avt, and it expression. These findings implicate 5-HT and multiple peptidergic systems in the hypophysiotropic regulation of the HPI axis in response to HEA exposure and provide novel insight into the multifactorial neural circuitry mediating the neuroendocrine stress response in fishes.