Sensitivity of Sympathetically Correlated Spinal Interneurons, Renal Sympathetic Nerve Activity, and Arterial Pressure to Somatic and Visceral Stimuli after Chronic Spinal Injury

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Abstract
In the chronic stage of spinal cord injury in humans, both innocuous and noxious somatic and visceral stimuli can elicit severe autonomic dysreflexia characterized by potentially dangerous, sympathetically mediated, increases in arterial pressure. We hypothesized that a similar sympathetic hyperexcitability would be manifested in spinal sympathetic networks of chronically spinally transected rats. To test this hypothesis, we compared the responses of sympathetically correlated spinal interneurons and arterial pressure to both innocuous and noxious stimuli in acutely and chronically spinally transected rats. Experiments were conducted in anesthetized female rats, either within hours of T3 spinal transection (rats with acute spinal transection) or one month after T3 spinal transection (rats with chronic spinal transection). Sympathetically correlated spinal interneurons were identified by cross correlating their ongoing activity with simultaneously recorded renal sympathetic nerve activity. Cutaneous stimuli (either light brushing or noxious pinch) were delivered to a wide area of the ipsilateral side of the rat. Colorectal distension was used as a noxious visceral stimulus. The activity of sympathetically correlated interneurons was increased by stimulation of more of the body surface and decreased by stimulation of less of the body surface in rats with chronic spinal transection than in rats with acute spinal transection. Colorectal distension elicited greater increases in arterial pressure in chronically than acutely spinally-transected rats without exciting significantly more interneurons in those rats. These results suggest that spinal circuits undergo significant plastic changes in the chronic stage of spinal cord injury, and they provide a mechanism for the observation, in some human patients, that many stimuli, both noxious and non-noxious, applied caudal to the site of spinal injury increase sympathetic activity and arterial pressure.