Sprouting of bladder afferents after spinal cord injury: the role of growth inhibitory proteins at the lumbosacral spinal cord

• ¹ Faculty of Medicine, University of Porto, Portugal
• ² University of Zurich, Switzerland

Spinal cord injury (SCI) leads to neurogenic detrusor overactivity (NDO), reflecting abnormal lumbosacral extension of central processes of bladder afferents. Mechanisms regulating this abnormal sprouting are still unresolved but may involve changes in expression of inhibitory cues that block axonal growth, including Nogo-A and Phosphacan. It is unclear if these proteins are regulated in the lumbosacral cord after thoracic injury and if bladder afferents adjust growth accordingly. Adult female rats were divided into three groups (n=8/group). The spinal cord was left intact in one group. Two groups underwent SCI at T8/T9 spinal segments and recovered for 7 or 28 days. Seven or 28 days post-injury (7 and 28 dpi), animals underwent 1-hour cystometry under urethane anesthesia, followed by tissue collection. Lumbosacral cords (L5-S1) were processed for Western Blotting and Immunohistochemistry to evaluate GAP43, Phosphacan and Nogo-A expression levels. Control animals presented normal bladder activity while bladder reflex contractions were abolished at 7 dpi in SCI rats (p≤0.0001 vs control). At 28 dpi, NDO was evident in SCI animals, with increased frequency and amplitude of bladder contractions (p≤0.0001 vs 7 dpi animals). Lumbosacral expression of Nogo- A and Phosphacan increased 7 dpi (p≤0.05 versus spinal intact) but significantly reduced at 28 dpi (Phosphacan; p≤0.05 vs 7 dpi animals). A time-dependent increase in GAP43 expression was found. Nogo-A expression in the dorsolateral funiculus was stronger at 7 dpi than in control animals. In controls, Phosphacan was expressed in the superficial dorsal horn, dorsolateral and ventral funiculus and some motoneurons. At 7dpi, staining intensity increased. In both cases, expression returned to baseline levels at 28 dpi. GAP43 was present in the superficial dorsal horn, dorsal commissure, corticospinal tract and dorsolateral funiculus in control animals, increasing at 7 and 28 dpi. We found absence of GAP43 in Phosphacan and Nogo-A positive spinal areas. This study demonstrates that changes in repulsive cues expression are not restricted to the injury but also occur at the lumbosacral cord, indicating the presence of a widespread event likely impacting recovery and NDO emergence. Ongoing studies aim to evaluate whether bladder afferents recognize centrally expressed growth-repellent cues.