Sleep-associated memory disorders in depression

Sleep is thought to be critically involved in memory consolidation, particularly for the procedural learning of motor skills. In depression, the sleep-dependent components of memory consolidation have not been investigated until recently. Considering the well documented sleep-aberrations due to depression and antidepressant pharmacotherapy, it seems obvious that this kind of memory consolidation might be altered in patients with depression. Testing this hypothesis, we found that age and depression synergistically impair the sleep-dependent consolidation of motor skills. In several follow-up studies, we search for the underlying mechanisms of this consolidation failure. In contrast to a prevalent theory associating procedural memory consolidation with REM sleep, time patients spend in REM sleep is not correlated with memory impairments, while patients receiving REM-suppressing antidepressants show even better memory consolidation than patients receiving REM-increasing medication. Depriving healthy subjects of REM sleep by manual awakenings or pharmacological treatment with antidepressants also does not seem to impair sleep-dependent memory consolidation. However, multiple sclerosis patients receiving high-dose corticosteroid therapy show consolidation impairments similar to that seen in depression. We therefore hypothesize that sleep-associated memory impairments in depression are more probably related to HPA-dysfunction than to alterations in sleep per se.