Chronic Pain Induced by Social Defeat Stress in Juvenile Mice Depends on TLR4

A significant portion of adolescents suffer from mental illnesses and persistent pain due to repeated stress. It remains unclear which components of the nervous system are involved in the linkage between stress and pain at an early age. Prior studies in adult mice implicated the innate immune system, specifically Toll-like receptors (TLR), as critical for inducing long-term anxiety and pain-like behavior in social defeat stress (SDS) model. In this work, we investigated the pain and anxiety behavioral phenotypes of wild-type and TLR4-deficient juvenile mice subjected to repeated SDS and evaluated the engagement of TLR4 by measuring dimerization in the spinal cord, dorsal root ganglia, and prefrontal cortex. Male juvenile (4-week-old) mice (C57BL6/J or Tlr4-/-) underwent six social defeat sessions with adult CD1 aggressors. In WT mice SDS promotes chronic mechanical allodynia by von Frey testing and thermal hyperalgesia by Hargreaves test. In parallel, the stressed WT mice exhibited transient anxiety-like behavior and long-lasting locomotor activity reduction in the open-field test. Tlr4-/- stressed animals were resistant to the induction of pain-like behavior but had a remnant of anxious behavior, spending less time in the center of the arena. In WT SDS, there were concordant robust increases of TLR4 dimerization in dorsal root ganglia macrophages and spinal cord microglia, indicating activation. These results suggest that the chronic pain phenotype and locomotor impairment induced by SDS in juvenile mice depends on TLR4 engagement evidenced by dimerization in immune cells of the dorsal root ganglia and spinal cord.