Fig. 7.

Proposed quantitative trait mechanism linking decreased Zhx2 expression with increased brain concentration of OMOR and increased OXY-induced behaviors. We hypothesize that the insertion of a MERV into Zhx2 leads to decreased protein expression and, in turn, changes in expression of liver enzymes that metabolize OXY. The functional consequence is an increase in brain concentration of the highly potent metabolite OMOR, which serves as both a potent unconditioned and conditioned stimulus that facilitates both state-dependent learning and state-dependent memory of OXY reward and possibly other OXY-induced behaviors. Created with BioRender.com.