Tire Chemical 6PPD-Quinone Targets Brain's Fear Center After Inhalation

6PPD-quinone (6PPD-Q) is an emerging environmental contaminant generated when 6PPD, a widely used tire rubber antioxidant, reacts with atmospheric ozone. It accumulates in tire and road wear particles and has been detected in air, water, soil, and human biological fluids including cerebrospinal fluid, serum, and urine. Elevated cerebrospinal fluid concentrations have been linked to Parkinson's disease patients, yet the toxicological consequences of inhaling airborne 6PPD-Q have remained almost entirely unstudied — the route most relevant to daily human exposure near roads and urban environments.

This study established a nose-only inhalation model in 13-week-old male ICR mice using environmentally scaled doses of 0, 0.14, and 14 mg/m³ for 2 hours per day over 28 days. Doses were derived by scaling real-world atmospheric concentrations (up to ~10,000 pg/m³) to mouse physiology using a 100× species adjustment factor. A 28-day post-exposure recovery period followed, after which tissues were collected for histopathology, transcriptomics, and behavioral analysis. Pulmonary function, cardiac imaging (echocardiography), metabolic organ weights, sperm parameters, and multi-organ histopathology were assessed alongside a battery of neurobehavioral tests including elevated plus maze (EPM), novel object recognition (NORT), and three-chamber social interaction.

The central finding was a striking hierarchy of organ vulnerability: pulmonary, cardiovascular, hepatic, renal, and splenic changes largely resolved during recovery, but neurological damage did not. Mice exposed to 6PPD-Q showed persistent anxiety-like behavior in the EPM even after 28 days of recovery, with no equivalent reversal in cognitive or social metrics indicating that the amygdala-mediated fear/anxiety circuitry was selectively and durably harmed. Nissl staining and neuronal aging markers confirmed substantial neuronal degeneration in the basolateral amygdala (BLA), paired with elevated oxidative stress markers, while the cerebral cortex showed comparatively minimal changes — underscoring region-specific vulnerability.

Transcriptomic profiling of the BLA revealed that Early Growth Response 1 (Egr1), a transcription factor critical for synaptic plasticity and stress-responsive gene regulation, was markedly downregulated at the mRNA level yet showed an upward trend in protein abundance. The authors interpret this discordance as evidence that post-transcriptional mechanisms — including altered translational efficiency, epigenetic modifications, and post-translational regulation — govern Egr1 expression under 6PPD-Q toxicity. Downstream of Egr1, solute carrier proteins and calcium-binding proteins were dysregulated, providing a plausible molecular pathway linking oxidative neuronal stress to aberrant anxiety-related neurobehavior. Critically, Egr1 protein elevation was detected in the BLA but not in the cerebral cortex, reinforcing the region-specific nature of the neurotoxic mechanism.

This study is the first to characterize inhaled 6PPD-Q neurotoxicity in a mammalian model at environmentally relevant concentrations, identifying the BLA as a sensitive target and Egr1 as a central molecular mediator. The irreversibility of behavioral outcomes after a full recovery period is particularly concerning given the ubiquitous and ongoing nature of human inhalation exposure near roadways. Limitations include use of male mice only, which precludes sex-difference analysis, the relatively short exposure window, and the translational uncertainty inherent in mouse-to-human scaling. Nonetheless, findings align with epidemiological signals linking 6PPD-Q body burden to neurological conditions and underscore the need for regulatory attention to this under-studied pollutant class.