NPPA and anxiety disorder: An abundance of high-quality research addressed the neurobiological causes of anxiety disorders, including research in neuroimaging, genetics, neurochemistry (neurotransmitters such as serotonin, norepinephrine, dopamine or GABA, neuropeptides such as cholecystokinin, neurokinins, atrial natriuretic peptide, or oxytocin, the HPA axis, neurotrophic factors such as NGF and BDNF, immunology, CO2 hypersensitivity, neurophysiology and neurocognition) [5, 6]; however, at present, none of the putative biomarkers has proved sufficient and specific as a diagnostic tool for anxiety disorders.