The neurobehavioral effects of phytoestrogens in male Syrian hamsters

We used a phytoestrogen (PE) and a phytoestrogen-free (PE-Free) diet to determine whether or not diet can have neurobehavioral effects on intermale aggression in Syrian hamsters (Mesocricetus auratus). In Experiment 1, 20 adult male hamsters were pre-tested for aggression and then placed on a PE (n=10) or a PE-Free diet (n=10) for 4 weeks in isolation. During week 5, experimental hamsters were exposed to a group-housed, nonaggressive opponent (NAO) for 5 min in a neutral cage arena. PE-fed hamsters exhibited more attacks (33.4+/-6.1) toward the NAO compared to the PE-Free-fed hamsters (18.1+/-4) (p<0.05). Interestingly, testosterone in the blood serum was higher in the PE-fed group (11.01+/-1.48 ng/ml) compared to the PE-Free group (6.5+/-0.87 ng/ml). In Experiment 2, 16 juvenile hamsters were weaned onto a PE (n=8) or a PE-Free diet (n=8). After 7 weeks on the diet, experimental hamsters were exposed to a NAO for 5 min in a neutral cage arena. Although the PE group exhibited higher levels of aggressive behavior, there were no statistically significant differences between groups. However, the PE group had higher levels of testosterone (9.0+/-0.95 ng/ml) compared to the PE-Free group (4.6+/-0.98 ng/ml) (p<0.05). In addition, analysis of the brains from both experiments revealed differences in binding for vasopressin 1A (V1A) receptors. Optical densities were converted to disintegrating units per min/mg. The PE-Free group had higher levels of V1A receptor binding (2689.93+/-254.8 dpm/mg) compared to the PE group (1907.32+/-136.3 dpm/mg) in the lateral septum (p<0.05). In addition, there were differences in the lateral hypothalamus, but the PE group had higher receptor binding (2550.9+/-63.59 dpm/mg) when compared to the PE-Free group (2011.9+/-174.14 dpm/mg) (p<0.05). In sum, these data present the first evidence that phytoestrogens can affect aggressive behavior and, concurrently, alter hormonal status and stimulate changes in the brain of male hamsters.