Acute Corticosterone Treatment Elicits Antidepressant-Like Actions on the Hippocampal 5-HT and the Immobility Phenotype

Corticosterone (CORT) has long been shown to modulate 5-HT system, and to alter hippocampal functions in various physiological and pathological conditions. However, CORT-elicited changes in the hippocampal 5-HT transmission and the immobility phenotype had not been fully addressed. The current study sought to explore effects of acute CORT subcutaneously injected at 10, 20, 40 mg/kg on the extracellular 5-HT in the hippocampus and the immobility time in male CD-1 mice. Following an injection of CORT or vehicle, time course of the extracellular 5-HT in the hippocampal CA3 was determined using in vivo microdialysis. The immobility time was measured at 0 min, 60 min, 120 min, 180 min in the forced swimming test (FST) and the tail suspension test (TST), respectively. Results showed that the vehicle, used to dissolve CORT, did not affect the dialysate 5-HT, nor the immobility time, by comparing the pre- and post-injection. CORT was found to dose-dependently increase the dialysate 5-HT and decrease the immobility time when compared to their vehicle-treated controls. The peak increase in the dialysate 5-HT and the decrease in the immobility time were both obtained at the 120 min following the CORT injection. Furthermore, a negative correlation was detected between the immobility time and the peak increase in the dialysate 5-HT. Our results indicated that acute CORT injection elicits antidepressant-like actions on the hippocampal 5-HT and the immobility time. The study suggested that hippocampal 5-HT responses may be one of the neurochemical bases for the immobility phenotype following CORT injection.