Modeling spontaneous opioid withdrawal in male and female outbred mice using traditional endpoints and hyperalgesia.

Opioid withdrawal significantly impacts drug dependence cycles as hyperalgesia associated with withdrawal is often a reason for continued drug use. Animal models of addiction are important tools for studying how drug dependence and withdrawal impact not only normal neurocircuitry but also the effectiveness of potential treatments for dependence and withdrawal. We conducted a study of the time course of spontaneous morphine withdrawal in outbred male and female mice that can be used to examine sex differences in male and female mice using both traditional somatic endpoints and mechanical hyperalgesia as an endpoint of withdrawal. Male and female national institute of health (NIH) Swiss mice were made dependent upon morphine using an escalating dosing schedule. Injections were stopped after 5 days. Withdrawal behavior was assessed at time intervals up to 106 h after the final injection. Numbers of forepaw tremors, wet-dog shakes, jumps and other behaviors were scored to create a global score. Paw pressure readings were then also taken to track changes in sensitivity to a painful stimulus over time. Male and female mice had approximately similar withdrawal severity peaking at 24 h after the final injection as measured by composite global scores. Females did exhibit an earlier and greater frequency of tremors than males. Although males and females showed similar hyperalgesia during withdrawal, females recovered faster. Spontaneous opioid withdrawal peaking at 24 h was demonstrated in male and female NIH Swiss mice. We also successfully demonstrated that hyperalgesia is an endpoint that varies over the course of withdrawal.