Escalation of intravenous fentanyl self-administration and assessment of withdrawal behavior in male and female mice.

Rationale: The rise in overdose deaths from synthetic opioids, especially fentanyl, necessitates the development of preclinical models to study fentanyl use disorder (FUD). While there has been progress with rodent models, additional translationally relevant models are needed to examine excessive fentanyl intake and withdrawal signs.

Objective: The current study aimed to develop a translationally relevant preclinical mouse model of FUD by employing chronic intravenous fentanyl self-administration (IVSA).

Methods: The study performed intravenous self-administration (IVSA) of fentanyl in male and female C57BL/6J mice for 14 days. Mechanical pain sensitivity during withdrawal was assessed using the von Frey test. Anxiety-like behavior was evaluated via the open field test one week into abstinence, and drug seeking behavior after extended abstinence was assessed at four weeks abstinence.

Results: Both male and female mice demonstrated a significant escalation in fentanyl intake over the 14 days of self-administration, with significant front-loading observed in the final days of self-administration. Mice showed increased mechanical pain sensitivity at 36 and 48hours withdrawal from fentanyl. At 1-week abstinence from fentanyl, mice exhibited increased anxiety-like behavior compared to naive mice. Four weeks into abstinence from fentanyl, mice maintained lever-pressing behavior on the previous reward-associated active lever, with significantly higher active lever pressing compared to inactive lever pressing.

Conclusions: The study establishes a translationally relevant mouse model of IVSA of fentanyl, effectively encapsulating critical aspects of FUD, including escalation of drug intake, front-loading behavior, withdrawal signs, and drug-seeking behavior into extended abstinence. This model offers a robust basis for further exploration into behavioral and neurobiological mechanisms involved in fentanyl dependence and potential therapeutic strategies.