Central L-cysteine induces sleep, and D-cysteine induces sleep and abnormal behavior during acute stress in neonatal chicks

ABSTRACT L‐Cysteine (L‐Cys) is a non‐essential and glycogenic amino acid. Previously, we reported that the intracerebroventricular (i.c.v.) injection of L‐Cys induced sedative effects under isolation‐induced stress in neonatal chicks. L‐Cys has an optical isomer, D‐Cys. The aim of this study was to clarify the effect of L‐Cys and D‐Cys during a stressful condition in chicks. The i.c.v. injection of L‐Cys and D‐Cys (0.84 µmol) decreased both distress vocalization and spontaneous activity induced by isolation. However, the two cysteine isomers induced different behaviors. L‐Cys increased sleep‐like behavior while D‐Cys caused abnormal behavior including syncope as well as sleep‐like behavior. In conclusion, while both L‐Cys and D‐Cys caused a sedative effect when injected i.c.v, D‐Cys caused abnormal behavior and may be detrimental to neonatal chicks.