Evaluation of rewarding effects of nitazene analogs: results from conditioned place preference tests and in vivo microdialysis experiments in mice

In illicit drug markets, the most recently expanding new synthetic opioid subclass is benzimidazoles, also known as nitazenes, which were originally developed as analgesics in the 1950s. The emergence of this classical, potent drug family has attracted extensive research interest in the field of forensic toxicology; however, information on their psychological and physical dependence is very limited. Herein, we evaluated the rewarding effects of four nitazene analogs using a battery of in vivo experiments, with a positive control drug (isotonitazene). The four test materials, metonitazene, etodesnitazene, metodesnitazene, and flunitazene, were administered to male C57BL/6J mice by i.p. administration at 0.5, 2, 20, and 20 mg/kg, respectively. In comprehensive behavioral observation tests, representative opioid-related physiological and behavioral states, including analgesia, stereotypic circling behavior, hyperlocomotion, and Straub tail response, were observed. A set of conditioned place preference tests revealed that all the four analogs induced palatability in mice. Furthermore, measurements of dopamine levels in the nucleus accumbens shell by in vivo microdialysis resulted in significant elevations in all test material-treated groups, suggesting that the nitazenes elicit the rewarding effect through a neural circuit originating from the μ-opioid receptor activation at the ventral tegmental area. Our findings add important data regarding the psychological dependence of nitazenes and highlight the abuse potential of these four materials and other prevailing nitazene analogs.