Tetrodotoxin-resistant sodium channels Nav 1.8/SNS and Nav 1.9/NaN in afferent neurons innervating urinary bladder in control and spinal cord injured rats

Tetrodotoxin-resistant (TTX-R) sodium channels Na-v 1.8/SNS and Nav 1.9/NaN are preferentially expressed in small diameter dorsal root ganglia (DRG) neurons. The urinary bladder is innervated by small afferent neurons from L6/S1 DRG, of which similar to75% exhibit high-threshold action potentials that are mediated by TTX-R sodium channels. Following transection of the spinal cord at T8, the bladder becomes areflexic and then gradually hyper-reflexic, and there is an attenuation of the TTX-R sodium currents in bladder afferent neurons. In the present study, we demonstrate that Na-v 1.8 is expressed in both bladder and non-bladder afferent neurons, while Na, 1.9 is expressed in non-bladder afferent neurons but is rarely observed in bladder afferent neurons. In spinal cord transected rats 28-32 days following transection, there is a decreased expression of Na-v 1.8 sodium channels in bladder afferents, but no change in the expression of Na, 1.8 in non-bladder afferent neurons. Both bladder and non-bladder afferent neurons exhibit limited increases in Na-v 1.9 expression following spinal cord transection. These results demonstrate that the expression of TTX-R channels in bladder afferent neurons changes after spinal cord transection, and these changes may contribute to the increased excitability of these neurons following spinal cord injury. (C) 2002 Elsevier Science B.V. All rights reserved.