Endogenous dipeptide cycloprolylglycine shows selective anxiolytic activity in animals with manifest fear reaction

Experiments on two mouse strains with opposite reactions to emotional stress showed selectivity of the anxiolytic effect of endogenous dipeptide cycloprolylglycine. In the open field test cycloprolylglycine (0.01-0.10 mg/kg intraperitoneally) dose-dependently (1.8-2.1-fold) increased motor activity of BALB/c mice with manifest fear reaction and had no effect on C57Bl/6 mice with active behavior. The content of endogenous cycloprolylglycine in mouse brain correlated with the type of emotional stress reaction: its content in the brain of C57Bl/6 mice 1.5 times surpassed that in BALB/c mice. It is concluded that cycloprolylglycine is involved in the endogenous regulation of fear reaction.