Physiological roles of tryptophan in teleosts: current knowledge and perspectives for future studies

Abstract

Tryptophan is an essential amino acid with a huge functional versatility, in addition to its participation in protein synthesis. Because of the complexity of its metabolism, and the functional relevance of several of its metabolites, it directly or indirectly participates in a wide array of physiological pathways. This amino acid is a precursor for the synthesis of the neurotransmitter/neuromodulator serotonin (5HT), the hormone melatonin and kynurenine and related compounds such as kynurenic acid, quinolinic acid or niacin. Because of this, it has a key role in the regulation of processes ranging from the neuroendocrine to the immune system in vertebrates. In aquaculture, extensive research has been performed to optimize the levels of tryptophan in the commercial diets for many fish species. Providing adequate levels of this amino acid is critically important for fish growth but also for fish welfare, as tryptophan has been shown to modulate fish behaviour, stress responses, and antioxidant and immune systems. Currently, available data suggest a wide variation in tryptophan requirements of different species ranging 0.3–1.3% of dietary protein level, but recent evidence also shows that fish tryptophan requirements can greatly vary depending on the rearing conditions of the fish. We also review here the participation of tryptophan and related metabolites in different physiological functions that are crucial for fish welfare. The review covers the involvement of tryptophan in 5HT‐ and melatonin‐mediated functions, along with its participation in the regulation of the immune system and its role as an antioxidant and antitoxic agent in fish.

Effects of dietary tryptophan levels and fish stocking density on immunological and antioxidant responses and bactericidal activity againstAeromonas hydrophilain rainbow trout (Oncorhynchus mykiss)

“…Similar to present results, Taheri Mirghaed et al () reported that high stocking density significantly increased oxidative stress characterized by MDA elevation. Trp has antioxidant ability by direct reaction with pro‐oxidant molecules and/or stimulation of the antioxidant enzymes (Hoseini et al, ). The present results showed that 5 g Trp per kg diet potentiated to suppress oxidative stress both under the LD and HD conditions.…”

“…The present results showed Trp at 5 g per kg diet significantly improved the fish immune strength, but 10 g Trp per kg diet significantly suppressed the immune strength of the fish. Immunomodulatory effects of Trp have been assessed in several studies; however, the results have been variable and contradictory [reviewed by Hoseini et al (2019b)]. Several studies have shown Trp administration significantly mitigated immunosuppression caused by chronic thermal (Akhtar, Pal, Sahu, & Ciji, 2013b;Akhtar et al, 2013a;Kumar et al, 2018), osmotic (Akhtar et al, 2013b(Akhtar et al, , 2013a and pollution (Ciji, Sahu, Pal, & Akhtar, 2015) stresses in different species.…”

“…Therefore, there is a need to find out methods mitigating adverse effects of high stocking density on rainbow trout immune and antioxidant system. Tryptophan (Trp) is an essential amino acid in fish nutrition with many physiological roles, including modulation of stress response, immune response and antioxidant system [reviewed by Hoseini, Pérez-Jiménez, Costas, Azeredo, and Gesto (2019b)]. Modulation of immune system by Trp administration has been examined by different researchers, which found such a modulatory effects depend on the stress nature, Trp administration levels and duration, and fish species [reviewed by Hoseini et al (2019b)].…”

The Internal Organs Weight of 6-Weeks Old Native Chickens after Supplement Addition With L-threonine and L-tryptophan in the Feed

“…Threonine contributes to bone formation, fat metabolism and digestive function; controls the body's health and immune system, liver and nerves and maintains body protein (Qaisrani et al, 2018). Tryptophan, as reported by Iwuji et al (2014) and Hoseini et al (2019), triggers the production of serotonin, stimulates niacin production, increases appetite and feed efficiency, stimulates growth, and increases body weight. The production of meat and eggs in the grower, pullet and layer phases is largely determined by growth in the starter phase.…”

“…For chicken, threonine and tryptophan are the third critical amino acids after methionine and lysine (Qaisrani et al, 2018). Threonine affects feed intake and body weight (Estalkhzir et al 2013), while tryptophan contributes to protein biosynthesis, builds muscle mass and stimulates immune responses, enhances the immune system, maintains internal proteins, induces growth and increases antibodies, and acts as serotonin precursors and control aggressiveness (Younis et al, 2016;Hoseini et al, 2019). Supplementation of 0.23 -0.31% tryptophan in feed was reported to increase broilers' weight (Opoola, 2017).…”