Neuroimaging and autoradiography studies indicate a relationship between depression severity and reductions in serotonin transporter (SERT) binding in the amygdala [165] and neocortex [166] while 5-HT1A receptors are elevated which leads to reduced serotonin neuronal firing and release in the cortex [167, 168]. This evidence concerns the gene SLC6A4 and major depressive disorder.