Chronic fentanyl induces sex-specific anxiolytic-like effects in opioid-dependent rats

Fentanyl is used as a treatment for severe pain, and it is a major contributor to opioid misuse and overdose fatalities. While it has been established that opioids are widely misused for their euphoric effects, little is known about the effects of chronic fentanyl exposure on anxiety-like behavior. We determined the effects of chronic fentanyl on anxiety-like behavior, locomotor activity, dependence, and body weight in adult male and female Wistar rats (n = 8 per sex and treatment) that received saline or fentanyl (0.6 mg/kg/day) via minipumps. Locomotor activity was measured in a small open field (SOF) before pump implantation. After implantation, rats were retested in the SOF, evaluated for anxiety-like behavior in the elevated plus-maze (EPM) and large open field (LOF), and it was determined if they were opioid dependent by counting somatic withdrawal signs. Fentanyl increased locomotor activity in the SOF and the LOF. In the EPM, fentanyl increased open-arm time in males but not in females. In the LOF, fentanyl increased entries into, and shortened the latency to enter, the anxiogenic inner zones, most prominently in females. Naloxone precipitated a somatic withdrawal syndrome in fentanyl-treated rats of both sexes, and fentanyl prevented body weight gain in males and attenuated it in females. To provide insight into the behavior across the LOF and EPM, an additional correlational analysis was conducted. In saline-treated males, but not females, anxiety-related measures were correlated between the two tests. Rats that made more open-arm entries in the EPM made more entries into, and spent more time in, the center zone of the LOF (r = 0.78 and 0.75), and the percentage of time on the open arms correlated with time spent in the middle zone of the LOF (r = 0.73). In conclusion, fentanyl decreases anxiety-like behavior in opioid dependent animals. These findings have significant implications as they suggest that dependent individuals might continue to misuse fentanyl for its anxiolytic effects. Furthermore, patients treated with chronic fentanyl for severe pain and comorbid anxiety might continue to benefit from the drug's anxiolytic effects.