Neurogenic detrusor overactivity (NDO) emergence after spinal cord injury (SCI) has been linked to neuroplastic changes in lumbosacral cord neuronal pathways, particularly sprouting of bladder afferents at the lumbosacral cord. Our preliminary observations suggest sprouting is association with changes in expression of growth inhibitory cues in response to thoracic injury, such as Nogo-A and Phosphacan. It is presently unclear whether bladder afferents recognize these repulsive cues and respond to them by regulating the expression of their receptors, thus adjusting their growth. Here, we investigated whether Nogo-A and Phosphacan and their specific receptors NgR1/p75, NgR1/NgR3 and Rptp/Lar are regulated at the lumbosacral cord and dorsal root ganglia (DRG) neurons, respectively, following SCI.

Adult female Wistar rats were divided in groups (n=8/group). The spinal cord was left intact in the control group. Two other groups underwent largely incomplete spinal cord transection at T8/T9 spinal segments and were left to recover for 7 or 28 days. At each timepoint, lumbosacral spinal cord segments (L5-S1) and respective DRG neurons were collected. Lumbosacral cords were processed by Western Blotting to evaluate the expression of Nogo-A, Phosphacan and GAP43, a marker of axonal sprouting. Total RNA from lumbosacral DRG neurons was extracted, reverse-transcribed and analyzed by qPCR to evaluate the expression of Nogo-A receptor complex NgR1/p75 and Phosphacan receptor complexes NgR1/NgR3 and Rptp/Lar.

Lumbosacral cord expression of Nogo-A and Phosphacan increased 3-fold at 7 dpi, compared to controls (Phosphacan: p≤0.05 vs. controls; Nogo-A: p0.001 vs. controls), returning to baseline values at 28 dpi (Nogo-A: p≤0.001 vs 7 dpi). A time-dependent increase in GAP43 expression at the lumbosacral cord was also found post-SCI. In lumbosacral DRG neurons, levels of Nogo-A receptor complex NgR1/p75 decreased in a time-dependent manner, compared to controls (NgR1: p≤0.05 controls vs. 7 dpi, p≤0.001 controls vs. 28 dpi). Phosphacan receptor complexes NgR1/NgR3 and Rptp/Lar also showed time-dependent decreases in expression, compared to controls (NgR1: p≤0.05 controls vs. 7 dpi, p≤0.001 controls vs. 28 dpi; NgR3: p≤0.05 7 dpi vs. 28 dpi; Rptp: p≤0.05 controls vs. 28 dpi; Lar: p≤0.05 controls vs. 28 dpi).

Here, we observed alterations in expression of known inhibitors of axonal growth, Nogo-A and Phosphacan, at the lumbosacral cord following thoracic SCI. Both proteins were overexpressed at the lumbosacral cord 7 dpi, even though axonal sprouting continued to occur at this site, indicated by GAP43 levels. Despite the presence of a highly repulsive spinal environment, bladder sensory afferents were able to continue to grow, reflecting a time-dependently downregulation of the expression of receptors for those repulsive cues.

These results indicate that NDO emergence, which has been linked to aberrant sprouting of sensory afferents, reflects changes in the molecular mechanisms regulating axonal growth.