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Elevated dietary intake of L-tryptophan counteracts the stress-induced elevation of plasma cortisol in rainbow trout (Oncorhynchus mykiss)

Olivier Lepage, Olof Tottmar and Svante Winberg^*

Evolutionary Biology Centre, Department of Comparative Physiology, Uppsala University, Norbyvägen 18A, SE-752 36, Sweden

View larger version (17K): [in a new window]   Fig. 1. Food intake, as percentage of body mass, of isolated juvenile rainbow trout after being transferred to observation aquaria. On day 7, the commercial trout feed was exchanged for feeds supplemented with L-tryptophan to a level corresponding to two (2x, open squares), four (4x, open circles), or eight (8x, open triangles) times of that of the non-supplemented control feed (1x, filled triangles). Values are means ± S.E.M. from 5-7 individuals.

View larger version (17K): [in a new window]   Fig. 2. Plasma levels of cortisol in isolated juvenile rainbow trout fed feeds supplemented with L-tryptophan to a level corresponding to two (2x), four (4x) and eight (8x) times the L-tryptophan content of the non-supplemented control feed (1x). Stressed fish (filled squares) were exposed to a standardized stressor (lowering the water level), whereas non-stressed fish were kept undisturbed (open triangles). Different letters indicate a significant difference at the P<0.05 level (LSD post-hoc test). Values are means ± S.E.M. from 5-7 individuals.

View larger version (22K): [in a new window]   Fig. 3. The amount of L-tryptophan (TRP) in the telencephalon (A), hypothalamus (B), optic tectum (C) and brain stem (D) of isolated juvenile rainbow trout fed feeds supplemented with L-tryptophan to a level corresponding to two (2X), four (4X) and eight (8X) times the L-tryptophan content of the non-supplemented control feed (1X). Stressed fish (filled square) were exposed to a standardised stressor (lowering the water level); non-stressed fish were undisturbed fish kept isolated (open triangle). Different letters indicate a significant difference at the P<0.05 level (LSD post-hoc test). Values are means ± S.E.M. from 5-7 individuals.

View larger version (22K): [in a new window]   Fig. 4. The [5-HIAA]/[5-HT] ratios in the telencephalon (A), hypothalamus (B), optic tectum (C) and brain stem (D) of isolated juvenile rainbow trout fed feeds supplemented with L-tryptophan to a level corresponding to two (2x), four (4x) and eight (8x) times of that of the non-supplemented control feed (1x). Stressed fish (filled squares) were exposed to a standardised stressor (lowering the water level); non-stressed fish were undisturbed fish kept isolated (open triangle). Different letters indicate a significant difference at the P<0.05 level (LSD post-hoc test). Values are means ± S.E.M. from 5-7 individuals.