a comparison between the effects of estrogen and soy extract on chronic pain in male rats

in addition to its effect on reproductive organs, estrogen exhibits complex effects on brain structure, function and behavior. in this regard, the influence of gonadal hormones especially estrogens on nociception has recently been accepted. soybean is known as an important phytoestrogen containing plant that is used as an alternative for estrogen replacement therapy. in this study, the influences of soy extract and estrogen on formalin-induced nociception in male rats were examined. in the first experiment, the effect of soy extract added to drinking water (60 mg/kg per day) on the nociception was assessed and compared with a control group (with soy-free drinking water). there was a significant decrease in nociception in rats that received soy extract for two weeks. furthermore, different doses of soy extract (50, 150 and 200 mg/kg) were injected subcutaneously 30 min before formalin test to assess its effect on acute and chronic pain perception. it was observed that different doses of soy extract had similar antinociceptive effects that were significantly different from saline injected control group. in other experiment, rats treated with a single dose of 150 ?g of 17-? estradiol (s.c.) 48 h prior to the formalin test, to be compared with a control group just received estrogen vehicle (dmso). acconding to our results estradiol did not show any significant effect on pain suppression. these results suggest that the antinociceptive effect of soy extract on chronic pain is not aresult of its phytoestrogenic activitiy, it might atherefore be related to other substances within soy extract.