In vitro and in vivo study of butyrylfentanyl and 4-fluorobutyrylfentanyl in female and male mice: Role of the CRF1 receptor in cardiorespiratory impairment

Background and Purpose: Fentanyl analogues have been implicated in many cases of intoxication and death with overdose worldwide. The aim of this study is to investigate the pharmaco-toxicology of two fentanyl analogues: butyrylfentanyl (BUF) and 4-fluorobutyrylfentanyl (4F-BUF). Experimental Approach: In vitro, we measured agonist opioid receptor efficacy, potency, and selectivity and ability to promote interaction of the mu receptor with G protein and beta-arrestin 2. In vivo, we evaluated thermal antinociception, stimulated motor activity and cardiorespiratory changes in female and male CD-1 mice injected with BUF or 4F-BUF (0.1-6 mg<middle dot>kg(-1)). Opioid receptor specificity was investigated using naloxone (6 mg<middle dot>kg(-1)). We investigated the possible role of stress in increasing cardiorespiratory toxicity using the corticotropin-releasing factor 1 (CRF1) antagonist antalarmin (10 mg<middle dot>kg(-1)). Key Results: Agonists displayed the following rank of potency at mu receptors: fentanyl > 4F-BUF > BUF. Fentanyl and BUF behaved as partial agonists for the beta-arrestin 2 pathway, whereas 4F-BUF did not promote beta-arrestin 2 recruitment. In vivo, we revealed sex differences in motor and cardiorespiratory impairments but not antinociception induced by BUF and 4F-BUF. Antalarmin alone was effective in blocking respiratory impairment induced by BUF in both sexes but not 4F-BUF. The combination of naloxone and antalarmin significantly enhanced naloxone reversal of the cardiorespiratory impairments induced by BUF and 4F-BUF in mice. Conclusion and Implications: In this study, we have uncovered a novel mechanism by which synthetic opioids induce respiratory depression, shedding new light on the role of CRF1 receptors in cardiorespiratory impairments by mu agonists.