Methods Dyn KO mice were generated by replacement of the entire coding region of the prodynorphin gene [2] and backcrossed onto C57bl/6N. Age and testing experiencematched female intact and ovariectomized (OVX) Dyn KO and wildtype (WT) mice at 3-8 months age were tested in all experiments. Anxiety (open field test, OF; elevated plus maze test, EPM; light dark test, LDT) and stressrelated behaviour (forced swim test, FST; tail suspension test, TST) was investigated in correlation to the estrous cycle in intact female WT and Dyn KO mice and in OVX WT and Dyn KO mice treated with the general estrogen receptor (ER) agonist 17β-estradiol (E2), and specific agonists for ERα (PPT), ERβ (DPN) or GPER (G1) two hours before testing.