Effects of l-tryptophan-supplemented dietary on growth performance and 5-HT and GABA levels in juvenile Litopenaeus vannamei

A 60-day feeding trial was conducted to evaluate the effects of dietary l-tryptophan level on the growth performance and neurotransmitter, including 5-hydroxytryptamine (5-HT) and γ-aminobutyric acid (GABA) of juvenile Litopenaeus vannamei. Six experimental diets were formulated to supplement with graded levels of tryptophan, resulting in different dietary tryptophan levels at 0 (control), 0.12, 0.24, 0.36, 0.48 and 0.60 %, respectively. A total of 960 shrimps (0.63 ± 0.01 g) were divided into six treatments and fed with the respective tryptophan diets in an indoor recirculating system. The results showed that the body weight, body length, live weight gain, specific growth rate, protein efficiency ratio and survival rate were significantly increased by dietary tryptophan levels, and the maximum were all obtained at 0.60 % tryptophan-supplemented diet. Feed conversion ratio was significantly decreased as dietary tryptophan level increased. No significant difference was found in hepatopancreas index and body proximate composition among all the treatments. High-performance liquid chromatography analysis showed that 5-HT level increased first and then decreased in all examined tissues except in thoracic ganglia with dietary tryptophan level increased, while GABA level first elevated and then decreased in X organ–sinus gland, brain, stomach and intestinal tract, and on the contrary, in thoracic ganglia, hepatopancreas, but declined in hemolymph with higher dietary tryptophan level. qRT-PCR results exhibited 5-HT1 mRNA expression was similar to 5-HT level variation. Based on these results, the dietary tryptophan supplemented 0.36 % dry diet or 1.72 % protein diet is found to be optimum to improve L. vannamei growth performance. In conclusion, it suggested that diet supplemented with tryptophan was beneficial to improve growth performance possibly by mediating 5-HT as well as GABA pathway in L. vannamei. These data provide new insight into the physiological functions of tryptophan in shrimp.