AQP4 and major depressive disorder: Astrocytes in MDD subjects display altered K+ and water homeostasis: Kir4.1 channels are down-regulated in the hippocampus of depressed patients (Medina et al., 2016); AQP4 immunoreactivity and mRNA levels decrease in the grey matter of orbitofrontal cortex, where the coverage of blood vessels by astrocytic endfeet is reduced (Rajkowska et al., 2013), in locus coeruleus and in the hippocampus of MDD patients, compared to psychiatrically-normal control subjects (Bernard et al., 2011, Medina et al., 2016).