Comparison of centrally injected tryptophan-related substances inducing sedation in acute isolation stress-induced neonatal chicks

In the present study, we first focused on the function of L-tryptophan (TRP) metaboliteswhich are synthesized in different metabolic pathways, namely, the kynurenine (KYN) pathway and serotonin (5-HT) pathway during an acute isolation stress. When L-TRP metabolites were intracerebroventricularly injected on an equimolar basis (100 nmol), 5-HT induced a sedative effect in neonatal chicks. Additionally, plasma corticosterone, dopamine, 5-HT, and its metabolite 5-hydroxyindoleacetic acid concentrations were increased in the diencephalon of the 5-HT treated group compared with other groups. Second, the two doses (400 or 800 nmol) of L- and D-TRP were compared under a corticotrophin-releasing hormone-augmented social isolation stress. When comparing the efficacy between L- and D-TRP against stress behavior, both amino acids had a similar effect and quickly suppressed distress vocalizations. Finally, D-amino acid levels in the diencephalon and telencephalon weremeasured but D-TRPwas not found. These results indicate that L- and D-TRP induce the same effect in attenuating stress but the mode of action of TRP derivatives, namely 5-HT differs during an acute isolation stress in neonatal chick. The absence of D-TRP in the diencephalon further suggests that instead of being an endogenous factor it may play role as a pharmacological factor.