The aim of the present study is to clarify emotional property focusing on the depressive-like behaviors, and its neuronal mechanism in the raphe nuclei which are involved in the mediation of emotional functions, in rats repeatedly administered ACTH during the early postnatal period.
Tetracosactide, the N-terminal 24 amino acids of the naturally occurring ACTH, were administered once a day at dose of 100 µg to male rat pups for 5 days on the day 21 after birth (3wACTH). Saline-injected rats were subjected as a littermate control.
Adult 3wACTH (10-12 weeks old) showed the decrease of sucrose consumption in the sucrose preference test, and reduction of time spent grooming after spraying a sucrose solution to the dorsal coat in the splash test. These abnormal behaviors in 3wACTH indicated the depressive-like/anhedonic-like behaviors, which were ameliorated by repeated administration of fluvoxamine, a selective serotonin reuptake inhibitor. Moreover, immunohistochemical studies revealed that the expression of parvalbumin, a marker of GABAergic neurons, significantly reduced in the raphe nuclei of adult 3wACTH.
These findings suggest that pharmacological stress during early postnatal period might produce depressive-like behaviors, which possibly implicated in dysfunction of the GABAergic neuronal systems in the raphe nuclei.