Consumption of soy diet before nerve injury preempts the development of neuropathic pain in rats

Background: A previous report using a partial sciatic nerve ligation (PSL) model for neuropathic pain in rats demonstrated that consumption of soy-containing diets preoperatively and postoperatively suppressed development of mechanical and heat allodynia, as well as hyperalgesia. The current study examined whether dietary soy suppresses these neuropathic sensory disorders when consumed either before or after PSL injury. Methods: Male Wistar rats were grouped into seven different feeding regimens. These rats were fed SOY (RMH-1000, PMI Feeds, St. Louis, MO), a diet containing 85% soy protein since weaning, and were then switched to noSOY (Bio-Serv Co., Frenchtown, NJ), a diet devoid of soy at certain time points before PSL injury (14, 7, 1 days, or 15 and 0 h). Postoperatively, these rats were fed SOY or noSOY diets. Levels of mechanical and heat allodynia and hyperalgesia were determined preoperatively and 3, 8, and 14 days after PSL injury. Results: Compared with groups fed preoperative noSOY, consumption of SOY before PSL injury significantly blunted postoperative levels of allodynia and hyperalgesia. Administering the SOY diet both before and after PSL injury provided no additional suppression of neuropathic pain. No pain suppression was noted in rats fed a noSOY diet preoperatively and SOY diet after PSL injury, Switching from SOY to noSOY feeding within 15 h of PSL injury was sufficient to allow for the full development of allodynia and hyperalgesia. Conclusions: Consumption of a soy-containing diet suppressed the development of neuropathic pain after PSL injury. The pain-suppressing properties of dietary soy were the result of a preemptive effect (i.e., when consumed preoperatively), but not a palliative effect (i.e., when consumed postoperatively). This effect of soy-containing diets appears to be short-lived, since switching to a noSOY diet 15 h before ligation abrogated the suppressive effect of soy.