An animal model of early life exposure to thimerosal finds it adversely impacts learning and memory and may be associated with the development of autism in susceptible individuals.
Effect of thimerosal on the neurodevelopment of premature rats.
Thimerosal is a preservative used in vaccines and contains approximately 49% mercury. This animal model study was undertaken to investigate the neurodevelopmental effect of exposure to thimerosal in premature rats. Four groups of premature rats received four different doses of thimerosal (32.8, 65.6, 98.4 or 131.2 μg/kg) injected into the gluteus maximus on postnatal day 1. A fifth group served as a control and received a saline injection. Tests were performed 44-48 days post injection to determine spatial learning and memory function. On day 49 the animals were euthanized and prepared for immunohistochemical staining to determine the expression of the dopamine D4 receptor (DRD4), serotonin 2A receptor (5-HT2AR) and apoptosis (programmed cell death) in the prefrontal cortex. The DRD4 receptor is associated with memory function while the 5-HT2AR receptor correlates with episodic memory. Previous rat studies have demonstrated that early life exposure to thimerosal alters the dopamine and serotonin systems. Similarly, this study found a significant decrease in the expression of DRD4, 5-HT2AR and learning at the highest dose of thimerosal. The decrease in receptor expression correlated with increased levels of apoptosis. In all but the lowest exposure group, memory function was significantly decreased when compare to the control group. The authors close by saying, “In conclusion, our results are consistent with previous studies in mice, rats, rhesus macaques, and humans, demonstrate that exposure to mercury from thimerosal- containing vaccines in susceptible populations, such as premature infants, may be associated with neurodevelopmental disorders like autism.”