Hundreds of chemicals are known to interfere with the vertebrate endocrine system. These endocrine disrupting chemicals (EDCs) differ from more traditional toxins in that they have myriad sublethal behavioral and physiological effects. One endpoint that is frequently affected by endocrine disruption is aggressive behavior, due to its underlying hormonal basis. Fortunately, because the endocrine system is remarkably conserved among vertebrates, we can study the effects of EDCs on aggression in relatively simple animals such as fish.
This study focused on EDCs called phytoestrogens, which are plant compounds that are structurally similar to our endogenous estrogens. Phytoestrogens are released into the environment wherever plant material is processed and concentrated, including sewage treatment plants, agricultural fields, and wood pulp mills. Humans are exposed to phytoestrogens via dietary sources such as soybeans, chickpeas, and flaxseed. My laboratory uses the fighting fish Betta splendens, a species well-known for its stereotyped aggressive displays, as a research organism with which to evaluate the effects of phytoestrogens on vertebrate behavior and physiology.
In our first experiments, my students and I exposed male B. splendens to the following concentrations of waterborne phytoestrogens using a 28-day semistatic experimental design: 17Î²-estradiol (an endogenous estrogen that we used as a positive control; 10, 100, 1000, and 10,000 ng/L), genistein (1, 10, 100, 1000 Âµg/L), equol (10, 100, 1000 Âµg/L), or Î²-sitosterol (10, 100, 1000 Âµg/L). The lower concentrations of each chemical are similar to those that animals experience in nature. Aggressive behavior in response to a mirror stimulus, a common index of agonistic behavior in fishes, was significantly decreased in several of the phytoestrogen treatment groups. These changes in behavior were not the result, however, of decreased overall activity, as we found no effects of phytoestrogens on spontaneous swimming behavior.
The decrease in 11-KT and the increases in serotonergic and dopaminergic activity are all consistent with the behavioral results we obtained.
In order to explain these changes in aggression, we examined two probable physiological mechanisms: sex steroid hormones in the bloodstream and neurotransmitters in the brain. With respect to steroid hormones, we assayed levels of the primary fish androgen, 11-ketotestosterone (11-KT), and found that fish exposed to phytoestrogens for 28 days had decreased 11-KT levels. More specifically, we found that genistein and 17Î²-estradiol (positive control) caused slight reductions in circulating 11-KT levels, while Î²-sitosterol significantly decreased concentrations of this androgen. To address the second question, we measured the concentration and metabolism of serotonin and dopamine, two monoamine neurotransmitters known to modulate aggressive behavior in vertebrates. We found that: (i) 17Î²-estradiol and Î²-sitosterol increased serotonin metabolism to its primary metabolite (5-hydroxyindoleacetic acid), (ii) equol and genistein increased dopamine levels, and (iii) Î²-sitosterol increased dopamine metabolism to its primary metabolite (3, 4-dihydroxyphenylacetic acid). The decrease in 11-KT and the increases in serotonergic and dopaminergic activity are all consistent with the behavioral results we obtained.
The significance of these findings is that low doses of phytoestrogens are capable of altering species-typical aggressive behavior in vertebrates by interfering with the normal functioning of the neuroendocrine system. The implication for humans is that elevated phytoestrogen consumption, whether through the diet or dietary supplements, has the potential to cause subtle yet significant physiological—and perhaps behavioral—changes.