Responses of circulating arginine vasotocin, isotocin, and melatonin to osmotic and disturbance stress in rainbow trout (Oncorhynchus mykiss)

In teleost fish exposed to stressors AVT has been implicated in activation of the hypothalamo-pituitary-interrenal axis. Mel has been shown to counteract several behavioural and endocrine consequences of stress in mammals. These studies were undertaken to investigate the effects of either disturbance or osmotic stress, applied either separately or simultaneously, on plasma AVT, IT and melatonin in rainbow trout (Oncorhynchus mykiss). Hormones were determined in plasma by high-performance liquid chromatography preceded by solid-phase extraction. The results showed that both forms of stress caused significant increase in plasma AVT concentration, although more pronounced elevation was observed in physically disturbed fish. Conversely, neither osmotic nor disturbance stress affected plasma IT concentration. The apparent difference in response to stress by the two close related neurohypophysial nonapeptides suggests independent mechanisms controlling their synthesis and/or release and supports the idea that only AVT plays a role in physiological response to stress. Plasma Mel level was depressed in fish subjected to disturbance stress and to both stresses applied simultaneously, an effect possibly associated with the elevation of plasma AVT concentration. Results are discussed in relation to physiological interactions between hormones.