Glucocorticoids, Noradrenergic Arousal, and the Control of Memory Retrieval.

Glucocorticoids and noradrenaline can enhance memory consolidation but impair memory retrieval. Beyond their effects on quantitative memory performance, these major stress mediators bias the engagement of multiple memory systems toward "habitual" control during learning. However, if and how glucocorticoids and noradrenaline may also affect which memory system is recruited during recall, thereby affecting the control of retrieval, remain largely unknown. To address this question, we trained healthy participants in a probabilistic classification learning task, which can be supported both by cognitive and habitual strategies. Approximately 24 hr later, participants received a placebo, hydrocortisone, yohimbine (an α2-adrenoceptor antagonist increasing noradrenergic stimulation), or both drugs before they completed a recall test for the probabilistic classification learning task. During training, all groups showed a practice-dependent shift toward more habitual strategies, reflecting an "automatization" of behavior. In the recall test, after a night of sleep, this automatization was even more pronounced in the placebo group, most likely due to offline consolidation processes and with beneficial effects on recall performance. Hydrocortisone or yohimbine intake abolished this further automatization, preventing the shift to a more efficient memory system and leading, in particular in the hydrocortisone group, to impaired recall performance. Our results suggest that glucocorticoids and noradrenergic stimulation may modulate the engagement of different strategies at recall and link the well-known stress hormone-induced retrieval deficit to a change in the system controlling memory retrieval.