OPRK1 and Anxiety: Partial KO of Oprk1 in the CeA (Oprk1 KO) resulted in a significant increase in anxiety-like behavior as measured by time spent in the closed arms of an EPM and a shift in the fraction of time spent in the closed versus open arms [graphed as 1-(open arm time/closed arm time) so that an increase in anxiety-like behavior is reflected by an increase in this fraction; Fig. 1G–I].