A major hurdle to developing new and effective treatments for drug addiction is better understanding how exactly it manifests itself before, during and after chronic use. In a paper published online in the April 21, 2021 issue of the journal eNeuro, an international team of researchers led by scientists at University of California San Diego School of Medicine describe the creation of two unique collections of more than 20,000 biological samples collected from laboratory rats before, during and after chronic use of cocaine and oxycodone.