EFFECT OF CONFINEMENT STRESS ON CIRCULATING LEVELS OF GROWTH-HORMONE AND 2 PROLACTINS IN FRESH-WATER-ADAPTED TILAPIA (OREOCHROMIS-NILOTICUS)

The aim of the present study was to assess a potential link between co nfinement stress and prolactin (PRL), the hormone responsible for adap tation to a hypoosmotic environment in freshwater-adapted tilapia (Ore ochromis niloticus). The effect of stress on plasma levels of the two tilapia PRL forms, tiPRL(I) (or tiPRL(188)) and tiPRL(II) (or tiPRL(17 7)), was examined along with the effects on plasma levels of cortisol and growth hormone (GH). In a preliminary study, various sampling prot ocols (immediate sampling; sampling one by one; anesthesia at 0.5, 1, 2 ml/liter phenoxyethanol) were tested for their ability to modify bas al plasma PRL and cortisol. In fish sampled within 1 min of capture (i mmediate sampling), no changes in the plasma levels of these hormones were observed, whereas when fish were sampled one at a time, PRL level s did not change but cortisol levels were modified. The immediate samp ling protocol was used to study the effects of 1 hr confinement stress , which induced a large increase in plasma cortisol levels as well as increases tiPRL(1) and tiPRL(II) levels with kinetics similar to those of cortisol. In contrast, plasma tiGH levels significantly decreased after 1 hr confinement. When this stress situation was removed, plasma cortisol and tiPRL levels decreased and plasma GH levels increased. T wo and one-half hours later, values were not significantly different f rom those measured in control fish. In tilapia exposed to 24 hr confin ement stress, similar changes in hormone levels were observed. However , after 24 hr confinement, only cortisol levels were significantly dif ferent from those measured in control fish. None of these stress condi tions significantly changed plasma chloride levels. Together, these re sults indicate that both PRL and GH have important roles in the adapti ve response of freshwater-adapted tilapia to confinement stress. (C) 1 997 Academic Press.