Behavioral Assessment of the Synergistic Analgesic Effects of Thymus vulgaris Essential Oil and Cerium Oxide Nanoparticles in a Rat Formalin-Induced Pain Model

Abstract

Pain is a biphasic response mediated by Aδ and C fibers and orchestrated within central neural circuits. Opioid therapy remains constrained by
adverse effects and addiction risks, underscoring the need for safer alternatives. Compared with conventional approaches, this study
investigated the analgesic and anti-inflammatory activities of Thymus vulgaris (thyme) essential oil and cerium oxide nanoparticles (CeO₂-
NPs). Thirty-six male Rattus norvegicus rats were used. Pain was induced by subplantar injection of 100 μL formalin (2%) into the right hind
paw. Animals were allocated into nine groups: negative control (needle prick), positive control (formalin), and treatment groups receiving
CeO₂-NPs (600 μg/mL; 100 μL intraplantar or intraperitoneal), thyme oil (100%; 100 μL topically or as pre-treatment), or a CeO₂-NPs and
thyme oil combination (200 μL intraplantar or intraperitoneal) In certain groups, pre-treatment with the assigned agents was performed 60
minutes prior to formalin injection. Rats were observed for 60 minutes in transparent chambers, and paw-licking counts were recorded
continuously. The session was divided into five phases, and total scores (0–60 minutes) were assessed as indices of nociception and therapeutic
efficacy. Treated groups exhibited significant reductions in nociceptive responses compared with the positive control. The combined
formulation achieved superior effects in the late phase, particularly in groups 4 and 6 (p ≤ 0.05). Conversely, groups 7 and 9 displayed elevated
licking responses during the early phase, reflecting route- and time-dependent variations in action. In conclusion, thyme oil and CeO₂-NPs
showed synergistic analgesic effects, highlighting their promise as safer options for pain management, considering the route of administration
marked analgesic, with synergistic late-phase effects, supporting their promise as safer alternatives in pain management.