Voluntary oral fentanyl intake produces dose- and sex-dependent physical dependence in mice without overt affective disturbances.

The ongoing opioid epidemic underscores the need for scalable and translational preclinical models of voluntary opioid intake and dependence. We therefore sought to establish and validate a voluntary two-bottle choice drinking-in-the-dark (DID) model of oral opioid intake in mice and to determine relationships between experimental parameters and behaviors during and after withdrawal. Male and female C57BL/6J mice were given daily access to two bottles during the dark phase for 24 drinking sessions over 5 weeks. Control mice received two bottles containing water. Experimental mice received one water bottle and one bottle containing oxycodone (0.1-1 mg/mL) or fentanyl (10-100 μg/mL) under varying session durations and concentrations. On the final day, physical dependence was assessed using naloxone-precipitated withdrawal and then a behavioral battery to assess negative affect was performed in the following week. Mice voluntarily consumed both oxycodone and fentanyl without taste adulteration and maintained drug preference across most concentrations. Oxycodone intake produced minimal withdrawal symptoms. In contrast, fentanyl intake resulted in naloxone-precipitated withdrawal that was modulated by session duration and concentration. Four-hour sessions produced stronger withdrawal than two-hour sessions at equivalent concentrations. Escalating high-concentration fentanyl exposure revealed emerging sex differences, with females exhibiting greater intake and withdrawal at higher concentrations. Affective behavioral assays following withdrawal revealed minimal persistent alterations in any cohort. These findings establish key parameters for a scalable voluntary fentanyl model that produces dose- and session-dependent physical dependence in male and female mice. This paradigm provides a cost-effective and straightforward platform for future investigations of opioid use and dependence.