Corticosterone facilitates retention of contextually conditioned fear and increases CRH mRNA expression in the amygdala.

The present study examined the effects of glucocorticoid administration on emotional memory and on corticotropin-releasing hormone (CRH) mRNA expression in the central nucleus of the amygdala (CeA) and the paraventricular nucleus of the hypothalamus (PVN). This was tested by administering repeated corticosterone (CORT) within a contextual fear conditioning paradigm. Rats received 2.5 mg/kg (s.c.) CORT or placebo twice a day for five and a half days and, 2 h after the last injection, rats were given one-trial contextual fear conditioning. When tested for retention of conditioned fear 6 days later, the CORT-treated rats displayed more fear-conditioned freezing in the retention test than vehicle-treated rats, which was not accounted for by an increase in footshock responsivity nor elevated plasma CORT. Another group of rats was fear conditioned prior to CORT administration, followed 24 h later by the five and a half days of CORT, and tested 6 days later; conditioned fear was not enhanced in these rats. Finally, CORT administration produced an increase of CRH mRNA in the CeA and a decrease in the PVN. The data suggest that repeated administration of CORT given before fear conditioning facilitates the acquisition of emotional memory, whereas CORT given after consolidation does not increase emotional memory.