Neonatal administration of a vaccine preservative, thimerosal, produces lasting impairment of nociception and apparent activation of opioid system in rats.
Thimerosal injections cause increased mercury in brain, liver and kidney of rats and induces long-term reduction in pain sensitivity (hypoalgesia). Might this contribute to self-injurious behavior in autism?
The preservative thimerosal added to vaccines is approximately 49% mercury by weight. Once inside the body it converts to a persistent inorganic mercury. Thimerosal has been used for decades despite not undergoing sufficient safety studies. This study utilized an animal model to investigate the distribution of mercury in brain, kidney and liver after injection of thimerosal (THIM) in newborn rats at various timeframes postnatally. The schedule used was meant to be similar to that given to humans. One group of rats that had not previously gotten thimerosal as newborns (naïve group) received a single dose as an adult. It was determined that the kidneys accumulated the largest amount of mercury, followed by the liver then the brain. Individuals with autism have been known to engage in self-injurious behaviors (SIB) and to demonstrate a decrease sensitivity to pain (hypoalgesia). Studies of individuals with autism have reported a reduction in SIB when treated with naloxone. Naloxone is an opioid receptor antagonist which would allow a person to more profoundly feel their pain upon participation in SIBs. A secondary consideration of this study was to determine if thimerosal exposure modulates sensitivity to painful stimuli. Using a hot plate test, it was demonstrated that thimerosal induces hypoalgesia. The long-term impairment in pain sensitivity seen among the neonates exceeded that of the naïve adults that received a single acute dose indicating young rats are more sensitive than adults. If rats were given a dose of naloxone prior to the hot plate test, it significantly mitigated the thimerosal induced hypoalgesia thereby indicating a role of the opioid system. The authors conclude, “…our study shows that administration of THIM to suckling or adult rats causes persistent impairment of pain sensitivity, which appears to involve increased activity of endogenous opioids.”