Opioids drugs, like oxycodone, are often used as medicines because they can relieve the pain, but can also cause euphoria and are thus commonly abused and addictive. Research suggests that there are sex differences in the reward value of certain drugs. For instance, negative affective states are reported more frequently in women than men, with women often initiating drug use as a coping strategy to deal with anxiety, low self-esteem, depression, and feelings of isolation (Becker & Chartoff 2019). Less is known about sex differences in oxycodone reward.
In rodents, the reward value of drugs, like oxycodone, can be studied using a procedure called conditioned place preference (CPP). The CPP apparatus consists of two chambers, a black and a white one. During the training period, one chamber is paired with oxycodone injections while the other is paired with neutral saline injections. These chamber-injection pairings occur once per day for eight days, on alternating days. During a subsequent testing session, which is drug-free, the animal’s preference for each chamber is determined by allowing the animal to run between the two sides (Cunningham 2006). The amount of time that mice spent in each chamber reflected their preference for that chamber and therefore for oxycodone.
Our primary hypothesis was that female mice are more likely to spend more time in the chamber in which they received the oxycodone injection than males. Our results indicate that male and female mice both preferred the oxycodone-paired chamber, compared to control mice, F(1,39) = 11.86, p<0.01. Although many studies state that females become addicted to drugs more easily, our results suggested that males’ preference may have been slightly stronger than females (p=0.08). One reason for these results might be that females have an estrous cycle, during which their hormone levels change; drug self-administration has been shown to change across the estrous cycle. This potential confound is also one reason that males are still so commonly used in scientific studies (Zucker & Beery 2011). Future projects will analyze preference by females’ estrous stage.