<qa>  What is the main cause of depressive symptoms?Depressive symptoms are the hallmark major depressive disorder, dysthymia and bipolar depression within the context of bipolar disorder BD. They share clinical characteristics including depressed mood, anhedonia, and low energy and require medical treatment. The Na, K-ATPase is a major transporter in the plasma membrane which has an important role in regulating cell volume, cytoplasmic pH and Ca levels. Digitalis-like compounds DLC; steroids identified as normal constituents of human brain, plasma and other tissues, exclusively interact with the Na, K-ATPase and induce numerous biological effects. We raised the hypothesis that Na, K-ATPaseDLC system is involved in BD based on studies from ours and other laboratories, which tested the involvement of this system in the depressive state of BD. The possibility that the interaction of DLC with the Na, K-ATPase is involved in the manic state of BD was scarcely investigated. To address this issue we intend to determine circulating DLC levels in patients with BD at different states of the disease; 