DORSAL-ROOT AFFERENT INFLUENCES ON TONIC FIRING OF RENAL AND MESENTERIC SYMPATHETIC-NERVES IN RATS

After spinal cord transection in cats and rats, the activity of many s ympathetic nerves is not entirely lost, and firing of other nerves con tinues unabated or is increased. This study was done to evaluate the i mportance of dorsal root afferent discharge on the generation of tonic sympathetic activity in renal and mesenteric postganglionic nerves in spinal rats and in rats with intact neuraxes. Sympathetic discharge w as recorded in anesthetized rats, and peripheral afferent influences w ere eliminated by dorsal rhizotomy from T4 to L2. Activity of renal an d mesenteric nerves was well maintained after high cervical and thorac ic (T4) cord transections. Rhizotomy had no effect on sympathetic disc harge in rats with intact neuraxes but decreased renal nerve activity significantly (-25%) in spinal rats. Because rhizotomy decreased mesen teric discharge in only three of six spinal rats, mean mesenteric nerv e discharge was not decreased significantly. The decreased renal nerve discharge after dorsal rhizotomy could not be attributed to input fro m any specific spinal segment, and ipsilateral input was no greater th an contralateral input. After rhizotomy, both renal and mesenteric ner ves had substantial excitatory drive from the transected, deafferented spinal cord. These findings demonstrate that dorsal root afferent inf luences on spinal neurons can contribute to the generation of tonic di scharge in some sympathetic nerves in spinal animals.