NEUROCHEMICAL AND PHARMACOKINETIC CHARACTERIZATION OF FENTANYL-INDUCED REWARD MECHANISMS IN MICE

Fentanyl is a highly potent opioid receptor agonist which induces sedation and analgesia. Due to its strong euphoric effects, fentanyl has become a major contributor to opioid-related overdose, raising public health concerns. Although pharmacological and pharmacokinetic (PK) properties of fentanyl have been partly characterized, integrated understanding of reward mechanisms remains limited. This study provides the comprehensive framework for the rewarding effect of fentanyl by connecting PK parameters to molecular mechanisms. μ-opioid receptor (MOR) binding assay identified the high binding affinity of fentanyl (K_i=3.1nM). To evaluate the rewarding effect of fentanyl, conditioned place preference (CPP) test was conducted. Especially, 50 μg/kg of fentanyl significantly increased the CPP score. Western blot analysis and enzyme-linked immunosorbent assay (ELISA) were used to elucidate neurochemical changes on the reward pathway of fentanyl. Mice were injected 50 μg/kg fentanyl following the CPP schedule, after which ventral tegmental area was collected. ELISA showed that the gamma-aminobutyric acid (GABA) level of fentanyl group decreased compared to the vehicle injected group. In the Western blot analysis, fentanyl reduced MOR, GAD, β-arrestin2, p-ERK and p-JNK. To determine PK parameters of fentanyl, mice were treated 0.1 mg/kg (IV) or 5 mg/kg (PO). Fentanyl showed low plasma clearance (4.51 mL/min/kg), low volume of distribution (136.9 mL/kg) and high K_p,brain (1.29). These findings provide an integrated perspective that fentanyl is highly distributed to the brain, altering GABA release and MOR-associated signaling pathways, thereby contributing to the rewarding effect. [Supported by the NRF (RS-2025-16067597) and the MFDS (23212MFDS218) of Korea.]