Effects of noradrenergic stimulation on memory in patients with major depressive disorder.
Major depressive disorder (MDD) has been associated with alterations in the noradrenergic system and impaired memory function. In turn, enhanced memory function has been associated with noradrenergic stimulation. In this study, we examined whether noradrenergic stimulation would differentially improve memory function in patients with MDD and healthy controls. In a placebo-controlled crossover study, 20 patients with MDD and 18 age- and sex-matched healthy controls received either placebo or 5 mg of yohimbine, an alpha-2-adrenoceptor antagonist that causes increased noradrenergic activity, orally before memory testing. A word list paradigm was used to test memory consolidation. Furthermore, the autobiographical memory test assessing memory retrieval and a working memory test were administered. Salivary alpha-amylase and blood pressure were measured. Yohimbine improved memory consolidation (word list learning) across groups (main effect of yohimbine: p = 0.05). This effect was more prominent in depressed patients compared with controls (post hoc t-test: MDD p = 0.01, controls p = 0.77). Memory retrieval (autobiographical memory specificity) and working memory were not affected by yohimbine. Across groups, yohimbine administration resulted in an increase in blood pressure and alpha-amylase. In sum, these results further support the hypothesis that noradrenergic stimulation enhances memory consolidation. The mechanism by which yohimbine leads to stronger memory consolidation in depressed patients compared with healthy controls remains to be elucidated.