The nucleus accumbens (NAc) has a significant role in addiction.  In the nucleus accumbens of mice, repeated cocaine exposure resulted in reduced TET1 messenger RNA (mRNA) and reduced TET1 protein expression. Similarly, there was a ~40% decrease in TET1 mRNA in the NAc of human cocaine addicts examined postmortem.
As indicated above in learning and memory, a short hairpin RNA (shRNA) is an artificial RNA molecule with a tight hairpin turn that can be used to silence target gene expression via RNA interference.  Feng et al. injected shRNA targeted to TET1 in the NAc of mice.  This could reduce TET1 expression in the same manner as reduction of TET1 expression with cocaine exposure.  They then used an indirect measure of addiction, conditioned place preference.  Conditioned place preference can measure the amount of time an animal spends in an area that has been associated with cocaine exposure, and this can indicate an addiction to cocaine.   Reduced Tet1 expression caused by shRNA injected into the NAc robustly enhanced cocaine place conditioning.