The synthetic opioid isotonitazene induces locomotor activity and reward effects through modulation of the central dopaminergic system in mice

Abstract

Synthetic opioids, which differ from fentanyl, have recently emerged as new psychoactive substances and pose public health concerns. The pharmacological effects and drug dependency of these benzimidazole-based opioids, also known as nitazenes, remain unclear. In this study, we examined the selectivity of opioid receptors, effects on motor activity, and expression of reward effects for isotonitazene, which has been detected in many poisonings and fatalities since 2019. Isotonitazene was most selective for the μ-receptor and exhibited more potent agonist effects, with an EC₅₀ of 0.02 nM, than morphine (EC₅₀ = 34 nM) and fentanyl (EC₅₀ = 4.0 nM). In ICR mice, isotonitazene (up to 0.05 mg/kg) increased the locomotor activity in a dose-dependent manner. This effect was significantly suppressed by pretreatment with the opioid receptor antagonists naloxone (3 mg/kg) and β-FNA (1 mg/kg), the dopamine D1 receptor antagonist SCH23390 (0.5 mg/kg), and dopamine D2 receptor antagonist raclopride (6 mg/kg). The reward effects of isotonitazene, evaluated using conditioned place preference (CPP) in mice, showed that conditioning with isotonitazene produced significant dose-dependent CPP scores. Microdialysis analysis also confirmed that the isotonitazene dose that induced CPP (0.05 mg/kg) significantly increased dopamine levels in the nucleus accumbens of mice. These results suggest that isotonitazene, similar to fentanyl and morphine, is a compound with a high risk of forming drug dependence and reward effects via the dopaminergic nervous system. This study provides foundational data for biological evaluation of other nitazene compounds.