The underlying molecular mechanism of depression is not understood, but the activation of CREB, which drives the expression of brain-derived neurotrophic factor (BDNF) to promote neuronal survival and other growth factors that promote neurogenesis, is a core signalling pathway that is suppressed in mouse models of depression and up-regulated in response to anti-depressants [95,96]. Here, BDNF is linked to depressive symptom measurement.