This study is designed to examine SV2A density in MDD and PTSD as a correlate of synaptic density, and to determine whether ketamine administration will reverse the synaptic loss in vivo in human subjects. To our knowledge, this is the first human study to examine SV2A in vivo in MDD and PTSD and to use the first known drug (ketamine) that rapidly reverses synaptic loss to determine whether ketamine administration could restore some of the structural changes associated with depression and PTSD.