A. Ratio of center time over total time in the open field in male and female mice injected with saline. B. Number of entries into the center of the open field in male and female mice injected with saline. ϕϕ interaction p<0.01, & p<0.01, # p<0.01 Saline-naloxone male (SNM), Morphine-naloxone male (MNM), Saline-naloxone female (SNF), Morphine-naloxone female (MNF)