Molecular mechanisms of depression: Perspectives on new treatment strategies

Depression is a multicausal disorder and has been associated with the risk to develop cancer, dementia, diabetes, epilepsy and stroke. As a metabolic disorder depression has been associated with obesity, diabetes, insulin sensitivity, neuropeptide Y, glucose regulation, poor glycemic control, glucagone-like peptide-1, cholezystokinin, ghrelin, leptin, the endocannabinoid system, insulin-like growth factor and gastrin-releasing peptide. As a cardiovascular disease a close relationship exists between depression and blood pressure, heart rate, norepinephrine, sympathetic tone, vascular resistance, blood viscosity, plasma volume, intima thickness and atherosclerosis. Additionally blood coagulation, fibrinolysis, D-dimers, plasminogen activator inhibitor-1 protein, platelet activation, VEGF, plasma nitric oxide and its synthase are changed in depressed patients. As an endocrinological and stress disorder depression has been connected with the concentration of free T ₄ , TSH, CRH, arginine vasopressin, corticotrophin, corticosteroid release and ACTH. Depression as an inflammatory disorder is mediated by pro-inflammatory cytokines, interleukin-1, interleukin-6, TNF-alpha, soluble interleukin-2 receptors, interferon-alpha, interleukin 8, interleukin-10, hs-CRP, acute phase proteins, haptoglobin, toll like receptor 4, interleukin-1beta, mammalian target of rapamycin pathway, substance P, cyclooxygenase-2, prostaglandin-E2, lipid peroxidation levels and acid sphingomyelinase. Nutritional factors might influence depression risk, i.e. the consumption of folate, omega-3 fatty acids, monounsaturated fatty acids, olive oil, fish, fruits, vegetables, nuts, legumes, vitamin B6 and vitamin B12. The neurodegenerative hypothesis of depression explains decreased hippocampal volumes in depressed patients and changes of neurotrophic support by BDNF, erythropoietin, GDNF, FGF-2, NT3, NGF and growth hormone. In this context, a fast neuroprotective and antidepressant effect has also been observed by ketamine, which acts via the glutamatergic system. Hence, GABA, AMPA, EAAT, NMDA- and metabotropic glutamate receptors (mGluR1 to mGluR8) have gained interest in depression recently. Alternative, causative or also easy available treatment strategies beyond serotonin and noradrenaline reuptake inhibition might be a major topic of future psychiatric care. In this review, an attempt is made to overview concepts of the disease and search for perspectives on antidepressant treatment strategies beyond approved medications.