Estradiol Enhances the Development of Addition-Like Features in a Female Rat Model of Opioid Use Disorder

Rationale: Women are more vulnerable than men in many aspects of opioid use disorder (OUD); a major theory of sex differences in substance use disorders is that these differences are due to ovarian hormones with estradiol enhancing vulnerability in females. However, most of this evidence is for psychostimulants and alcohol; evidence with opioids is sparse. Objective: The goal of this study was to determine the impact of estradiol on vulnerability in females in a rat model of OUD. Method: Following self-administration training, ovariectomized (OVX) females with (E) or without (V) estradiol replacement were given extended (24h/day), intermittent-access (2, 5 min trials/h) to fentanyl for 10 days. Then, the development of three key features of OUD were assessed, including physical dependence, defined by the magnitude and time-course of weight loss during withdrawal, an enhanced motivation for fentanyl, assessed using a progressive-ratio schedule, and relapse vulnerability, assessed using an extinction/cue-induced reinstatement procedure. These later two characteristics were examined following 14 days of withdrawal when the phenotypes are known to be highly expressed. Results: OVX+E females self-administered markedly higher levels of fentanyl under extended, intermittent-access conditions and showed a longer time-course of physical dependence, a greater increase in motivation for fentanyl, and an enhanced sensitivity to the reinstating effects of fentanyl-associated cues compared to OVX+V rats. Severe health complications were also observed in OVX+E, but not OVX+V females, during withdrawal. Conclusion: These results indicate that, as with findings with psychostimulants and alcohol, estradiol enhances vulnerability in females to developing opioid addiction-like features and serious opioid-related health complications.