Several models have been developed to explain the respective roles of REM sleep alterations and negatively-biased amygdala activity in the pathology of MDD, however the possible interaction between these two potential risk-factors remains uncharted.
Second, the most consistent heritable biological markers of major depression are proposed as biological endophenotypes for genetic studies: REM sleep abnormalities, functional and structural brain abnormalities, dysfunctions in serotonergic, catecholaminergic, hypothalamic-pituitary-adrenocortical axis, and CRH systems, and intracellular signal transduction endophenotypes.
As patients with an acute episode of a major depression, the group of the healthy relatives exhibited signs of a hyperactive hypothalamic-pituitary-adrenocortical system verified by the combined dexamethasone corticotropin-releasing hormone (DEX/CRH) test, as well as a slow wave sleep deficit in the first sleep cycle and an increased amount of rapid eye movements during REM sleep.
Shortened REM latency and related sleep neurophysiological disturbances have also been reported to characterize so-called 'borderline' personality disorder even when examined in the absence of concomitant major depression.