Effects Of BTX-A On Expressions Of CGRP,c-fos In Intestinal Wall And Spinal Cord Of CRD And Sodium Channels Of DRG In Rats

[Background and aims]Botulinum toxin type A(BTX-A)has recently been used in visceral pain.As we known,it is a potent inhibitor of acetylcholine release in the neuromuscular junction,BTX-A was initially thought to provide pain relief by reducing muscular activity.But many researches suggest that BTX-A may have a more complex mechanism of action on the pain system.BTX-A-mediated antinociceptive effects have been investigated using animal models.In these studies,BTX-A has been shown to decrease visceral pain by reducing expressions of neuropeptide in sensory neurone and spinal cord.At the same time,the study of visceral pain has been made a progress in iron channels by the patch clamp technique.Some researches found the visceral pain was in relation to the sodium channel of dorsal root ganglion(DRG).The purpose of this study to investigate the antinociceptive effect of BTX-A and its underlying mechanism in colorectal distention(CRD)model and to observe the influence of BTX-A on sodium channels of DRG in rats.[Methods]Male Wistar rats of postnatal eight weeks were divided into four groups:BTX-A(5U,10U,15U)and vehicle(NS.,0.08ml)was injected in colorectal wall of group B,C,D and A rats.Rats were challenged repeatly with colorectal balloon distention 4 weeks and 8 weeks post injection.Abdominal writhing behaviors were monitored in 30 minutes as indicators of pain.The samples of colorectal wall and spinal cord were stained by the immunohistochemistry for CGRP and c-fos.The expression of CGRP and c-fos were analyzed by computing image analysis system of image-pro plus 5.0 semi-qualitively.DRG cells were obtained by the dissection and dissociation of adult rat DRG neurons.The cells were grouped into the control group with no drug in the culture medium,and the BTX-A group with the ultima concentration 30U·ml^-1BTX-A.The record of the voltage dependent sodium channels in DRG was taken by whole-cell patch-clamp techniques.Holding potential at-100mV, eliciting sodium currents by voltage-step at 10mV increment from -60mV to +10mV and the duration at 400ms to observe the impact of BTX-A on sodium channels currents and explore the changes of characteristics of the ion channels. Data were packed up by SPSS 11.5 statistical software with p＜0.05 considered significant.[Results]At the end of week 4^thafter the injection,total writhing test scores in 30 minutes of group B,C and D were decreased significantly compared with control group(group A)(P＜0.05).The expression of CGRP,c-fos in colorectal walls and spinal cord of BTX-A pretreated groups were lower than group A signiflcantly.At week 8^th,the writhing sores and the expression of CGRP,c-fos in colorectal wall and spinal cord of group C and D were lower than group A as well. But all the effects were not detected in group B.Compared with the control group, BTX-A groups decreased the peak amplitude of Na⁺ current 37.26%(n=5,P＜0.01) and raised the curve of sodium channels' current-voltage(n=5,P＜0.01).[Conclusions]Injection ofbotulinum toxin type A in colorectal wall can reduce colorectal distention nociceptive behaviors significantly,decrease the expression of CGRP and c-fos in colorectal wall and spinal cord and the sodium current of DRG cell in rats.Taken together,such an antinociceptive effect of BTX-A may be postulated with the inhibition of the express of CGRP and c-fos,and the restraint of the open in sodium channel in DRG cell in rats.