Central functions of L-arginine and its metabolites for stress behavior

L-Arginine is an essential amino acid for birds, carnivores and young mammals and a conditionally essential amino acid for adults. L-Arginine can be catabolized by four sets of enzymes in mammalian cells, resulting in the production of urea, L-ornithine, L-proline, L-glutamate, polyamines, nitric oxide, creatine, agmatine, etc.. Unlike mammals, birds lack carbamyl phosphate synthetase, one of the urea cycle enzymes necessary for the synthesis of L-citrulline from L-ornithine in the liver and kidney. Therefore, it is impossible to synthesize L-arginine in birds, and L-arginine is classified as an essential amino acid for birds. In this chapter, we introduce recent studies about central functions of L-arginine and its metabolites for stress behavior. In particular, the functions in avian species are focused upon. In neonatal chicks, centrally injected L-arginine induces sedative and hypnotic effects under social separation stress. Among L-arginine metabolites, L-ornithine, L-proline and L-glutamate would especially contribute to these effects.