Vol. 8, No. 4, 2002 Page 2&3
Male and female brains are anatomically dissimilar because they develop differently. These anatomical differences are expressed in behavior. Should we find evidence that exposure to some environmental chemical has narrowed, widened, reversed, or altered the character of these behavioral differences, we might suspect that the agent has interfered with sex differences in brain development.
Because sexual differentiation of the brain is largely under the control of gonadal or sex hormones, we can properly label such an agent as an endocrine or hormone disruptor. Endocrine disruptors captured our attention because of the insights of Dr. Theo Colborn, a scientist with the World Wildlife Fund. Her book, Our Stolen Future, written with coauthors Diane Dumanoski and J. Peterson Myers, has been translated into 22 languages [Editor's note: see review on page 5 of this issue]. Almost overnight, it thrust endocrine disruption into a leading role in how we evaluate the risks of environmental chemical contamination.
The 1996 Food Quality Protection Act, written to protect children from the hazards of pesticides, stipulated endocrine disruption as one of the factors to be considered in evaluating safety. Much of Our Stolen Future and of endocrine disruptor research is devoted to disorders of reproduction. The popular media, for example, have featured stories about declining sperm counts and testicular cancer, while much of the focus of both ecological and laboratory research has involved observations such as hermaphrodite polar bears and aberrations in male reproductive structures such as the prostate. A more subtle, probably even more important question also needs to be asked: what are the implications for nonreproductive behaviors such as cognitive abilities? For social behaviors such as aggression and nurturance?
To begin to answer such questions we have to understand how hormones guide brain development. In genetic males, testosterone in the fetal brain is converted, paradoxically, to estrogen, considered to be the female sex hormone, by an enzyme called aromatase. In rats, the resulting sex differences in brain anatomy include a nucleus in the hypothalamus that is much larger in males than in females, and a thicker layer of certain cells in the hippocampus, a key structure for memory function. Anatomical differences are accompanied by differences in the distribution of neurotransmitters such as dopamine.
One of the more intriguing differences between the male and female brain is the degree of asymmetry between its right and left halves. In rat males, the cerebral cortex is thicker on the right than on the left. In females, the left is slightly more thick than the right, but the asymmetry is not as clear. In our own research at the University of Rochester we found these anatomical differences altered in rats by prenatal exposure to quite low doses of dioxin. Operant behavior testing showed males becoming more like females and females more like males [Editor's note: see related article in this issue, Crime Times, 2002, Vol. 8, No. 4, Pages 1 & 5) Sex differences in brain development are also modified by gestational exposure to alcohol, cocaine, opiates and nicotine.
Our ability to grasp the implications of these findings for antisocial behaviors is hampered by the meager amount of data in our possession. Still, how can we expect that interference with normal patterns of brain development will yield benign outcomes for how people behave?
For further information on this topic, see: Weiss, B. Sexually dimorphic nonreproductive behaviors as indicators of endocrine disruption. Environmental Health Perspectives 2002; 110, Suppl 3:387-391.