GDNF and depressive symptom measurement: For the treatment of depression, targeting signaling transduction by BDNF and its receptor, tropomycin receptor kinase B (TrkB), is essential.143,144 Recent research has shown a link between decreased hippocampal neurogenesis and low levels of BDNF and glial-derived neurotrophic factor (GDNF) in the brains of depressed individuals.145 Under normal conditions, astrocytes release various nutrients and cytokines.