GCG and major depressive disorder: The papers in the present review were generally selected under the scope of EECs and endocrine molecules including serotonin, glucagon-like peptide 1 (GLP1), glucagon-like peptide 2 (GLP2), peptide YY (PYY), as well as their involvement in particular neurological psychiatric disorders, such as PD, schizophrenia, visceral pain, neuropathic pain, and depression.