TRYPTOPHAN ENRICHED DIETS INCREASE THE SEROTONIN LEVEL IN THE BRAIN OF THE TELEOST Totoaba macdonaldi AND ACTIVATES THE NEGATIVE FEEDBACK MECHANISM AFTER AN EVENT OF ACUTE STRESS

Central serotonergic system plays a pivotal role in the regulation of HPI axis in fish¹ and could be involved in neuroendocrine loops adjusting body homeostasis and improving ability to cope stress in fish through an inhibitory effect on HPI activity, under stress conditions. The aim of this study was to determine the effect of dietary tryptophan level on serotonergic system and the acute stress response in juvenile of Totoaba macdonaldi.

A group of totoaba juveniles (90 ±5.0 g) were distributed in twelve 500 L tanks in a recirculating system. Four diets with different tryptophan levels (0.5, 1.09, 1.69 and 2.28%) were formulated. At the end of experiment fish from each tank were divided into three groups and each group was exposed to one of following stress conditions: Control (unrest) Handling (persecution) and hypoxia (1 mg L^-1O₂). After stress, each condition fish were anesthetized and bled. Later, they were euthanized by decapitation and telencephalon and diencephalon were extracted. Plasma cortisol level (EIA 1887 DRG), 5-HT and 5-HIAA (HPLC-Electrochemical detector) were determined. A two way ANOVA was performed using the Sigma Plot 12.5 packet with a significance level α = 0.05 to evaluate the possible effects.

A significant increase (P =0.001) of 5-HT was observed in telencephalon of undisturbed fish fed with D1.69 and D2.28 compared against D0.5. However, after both stress exposure, no differences were observed. In contrast 5-HIAA levels were significant increase in fish fed with D0.5 and D1.09 (P <0.001) by handling compared against to undisturbed and then, 5-HIAA levels were restored with D1.69 and D2.28. This data suggest that higher 5-HT content in fish fed with higher level of dietary trp could mediate a negative feedback after acute stress². Cortisol response was significant higher in D1.09 compared to D0.5 (P <0.001) and then decreased with D1.69, and D2.28.

Our data demonstrate that serotonergic activity in telencephalon and plasmatic cortisol are modulated by stress, and dietary tryptophan could play an important role as a modulator in totoaba for induced hatchery stressors like handling. Likewise other captive bread fish could respond in a similar way.

Acknowledgements: The authors would like to thank staff from both the Nutrition Lab and the Fish Biotech Unit (FCM-UABC). Also to Dr. Veronica M. Rodriguez from the Institute of Neurobiology (UNAM-Juriquilla) for their collaboration during the experimental phase and analytical work.

Rerefences: ¹Medeiros and McDonald. Comparative Biochemistry and Physiology, Part A. 164 (2013) 612-621. ²Jacobs and Formal. Seminars in the neurosciences. 7. (1995) 401-408.