, 1998b), and a relatively small increase in trough levels could have pronounced effects on glucocorticoid signaling. In conjunction with the studies PD 332991 cited above, these results suggest that chronic stress may predispose vulnerable individuals to a variety of neuropsychiatric disorders by disrupting the circadian oscillation and especially the circadian trough, reducing the survival of newly formed synapses, and

destabilizing synapses formed early in development. Converging evidence from both clinical studies and animal models lend support to this hypothesis. Disrupted circadian glucocorticoid cycling is a relatively consistent feature in clinical studies of learn more patients with depression or PTSD (Heim et al., 2000, Holsboer, 2000, Yehuda, 2002 and Miller et al., 2007). Blunted circadian cortisol oscillations

are a feature common to both PTSD and depression (Yehuda et al., 1996). However, these two disorders appear to involve opposing changes in total cortisol secretion (decreased in PTSD, variably increased in depression): in PTSD, blunted oscillations are driven primarily by reduced circadian peaks (Yehuda et al., 1996), while in depression, they are driven primarily by elevated cortisol secretion during the circadian trough (Yehuda et al., 1996), especially in psychotic depression (Sachar et al., 1973 and Keller et al., 2006). In both disorders, blunted corticol cycling is others associated with hippocampal volume loss (Bremner et al., 1995, Bremner et al., 2000 and Sheline et al., 1996) and partially

overlapping alterations in functional connectivity (Davidson et al., 2002, Lanius et al., 2004, Greicius et al., 2007, Sheline et al., 2010, Yin et al., 2011, Qin et al., 2012 and Liston et al., 2014), which is consistent with results in animal models indicating that both peaks and troughs are necessary for balancing synaptic formation and pruning. Similarly, animal models of mood disorders provide additional support for this hypothesis. Multiple animal models of depression—including chronic unpredictable stress, chronic social defeat stress, and early life stress—recapitulate neuroendocrine abnormalities found in patients, including blunted glucocorticoid oscillations, elevated glucocorticoid activity, and disrupted circadian troughs (Willner, 1997, Meaney, 2001, Krishnan et al., 2007 and Nestler and Hyman, 2010). In at least one study, blunted circadian cycling was linked specifically to stress susceptibility: circadian rhythm amplitudes were blunted only in mice that exhibited a vulnerable behavioral phenotype in response to chronic social defeat stress, relative to resilient mice that did not develop depression-like symptoms (Krishnan et al., 2007). In other studies, circadian rhythm disturbances have been causally related to mood symptoms.