We examined and compared the effects of taste stimulation by soy saponin as well as soy isoflavone and intragastric (IG) injection of both on the autonomic nerve activities and feeding behavior in rats. We found that taste stimulation by soy saponin or soy isoflavone-rich solution (SIRS) did not affect the activity of the sympathetic nerve supplying the adrenal gland in urethane-anesthetized rats; however, IG injection of SIRS, but not soy saponin, stimulated the adrenal sympathetic nerve activity (ASNA) in a dose-dependent manner. Moreover, IG injection of SIRS significantly suppressed the activity of the vagus nerve innervating the stomach, whereas sympathetic nerve outflows to brown or white adipose tissue were not affected by IG injection of SIRS. To test the involvement of the afferent autonomic nerve in the abdominal organs for regulation of the efferent ASNA by SIRS, we examined the response of the adrenal sympathetic innervation to SIRS injection in rats with ablated afferent vagus or afferent sympathetic nerves. The activating effect of SIRS on the ASNA was inhibited in sympathectomized rats but not in vagotomized rats. Thus, our data suggest that soy isoflavone might affect tissue-specific autonomic nerves through the afferent sympathetic nerve pathway.
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