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 sympathetic nerves is not entirely lost, and firing of other nerves continues unabated or is increased. This study was done to evaluate the importance 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 was recorded in anesthetized rats, and peripheral afferent influences were eliminated by dorsal rhizotomy from T4 to L2. Activity of renal and mesenteric nerves was well maintained after high cervical and thoracic (T4) cord transections. Rhizotomy had no effect on sympathetic discharge in rats with intact neuraxes but decreased renal nerve activity significantly (-25%) in spinal rats. Because rhizotomy decreased mesenteric discharge in only three of six spinal rats, mean mesenteric nerve discharge was not decreased significantly. The decreased renal nerve discharge after dorsal rhizotomy could not be attributed to input from any specific spinal segment, and ipsilateral input was no greater than contralateral input. After rhizotomy, both renal and mesenteric nerves had substantial excitatory drive from the transected, deafferented spinal cord. These findings demonstrate that dorsal root afferent influences on spinal neurons can contribute to the generation of tonic discharge in some sympathetic nerves in spinal animals.