Lipopolysaccharide can induce errors in anatomical measures of neuronal plasticity by increasing tracing efficacy

Evidence suggests that activating certain components of the immune system may increase regeneration and plasticity in the injured central nervous system. Investigating the effect of lipopolysaccharide (LPS), a potent endotoxin and immune activator, on neuronal plasticity in rat models of spinal cord injury, we discovered that systemic administration of LPS can increase the number of descending motor axons that transport neuronal tracers anterogradely to the spinal cord. This effect of LPS was not observed across all motor tracts traced in two different experiments, but was significant for two different tracers administered to corticospinal tract neurons. Densitometry measurement of traced corticospinal axons within the cervical gray matter revealed that normalization to the number of traced axons is crucial to avoid false-positive reports of increased plasticity following LPS injection. These findings indicate that assessments of neuronal growth based on neuronal tracing techniques should be normalized when inflammation or immune activation is an experimental variable.