Mixture more imposing on brain messenger than lone chemicals.
Andersen, IS, OA Voie, F Fonnum and E Mariussen. 2009. Effects of methyl mercury in combination with polychlorinated biphenyls and brominated flame retardants on the uptake of glutamate in rat brain synaptosomes: a mathematical approach for the study of mixtures. Toxicological Sciences 112(1):175-184.
A mixture of different environmental contaminants can add up to a have a bigger effect on an important brain chemical called glutamate than any one of them alone.
Risk assessments of the harmful effects of chemicals typically focus on assessing each chemical individually. Yet, we encounter a sea of chemicals in everyday life. Chemical combinations can potentially have additive (1 + 1 = 2), synergistic (1 + 1 > 2), or antagonistic (1 + 1 < 2) effects. A growing body of evidence shows that looking at the effects of combinations of chemicals that are likely to be encountered may be a more appropriate way to assess health risk.
Norwegian scientists examined the effects of three different classes of common environmental contaminants on the neurochemical messenger glutamate in rat brain nerves. The contaminants – methyl mercury, polychlorinated biphenyls (PCBs), and brominated flame retardants (BFRs) – were chosen because researchers believe they affect glutamate in similar ways.
Glutamate is the brain's most common neurotransmitter – a chemical that carries signals between nerve cells in the brain. Glutamate has vital roles in learning and memory as well as numerous other brain functions. Typically, a nerve cell releases glutamate, and it crosses the tiny space between neurons (the synapse) to the receiving neuron. Then it returns and is taken up again by the releasing neuron and surrounding cells where it can be recycled for future release.
In this laboratory study, the combined chemicals acted together and more strongly inhibited the nerve cells that released glutamate from reabsorbing it. The effects were additive; that is, the effects from each contaminant added together to produce a greater negative effect than that caused by exposure to one chemical at a time.
Researchers tested the chemicals on simple preparations of nerves from rat brains. The next step will be to evaluate similar changes on glutamate re-uptake in the brains of live animals.
This study emphasizes that, when estimating risk, it may be more important to evaluate combinations of chemicals that are most likely to be encountered through environmental exposure in order to gain a more complete picture of the overall effects of the contaminants.
