AB318. SPR-45 Decentralization reduces nicotinic receptor-mediated canine bladder contractions in vitro

Objective: Bladder function depends upon several complex signaling pathways that induce either contraction or relaxation. We performed nerve re-routing surgery on bladder of decentralized dogs (reinnervated) as a model with the goal of restoring bladder function in spinal cord injured patients. The neuromuscular nicotinic receptor blocker, succinylcholine blocks spinal root-stimulated bladder contraction in vivo in reinnervated dogs, but not in sham-operated dogs. Our lab explored the function and location of nicotinic receptors involved in bladder contraction, with emphasis on their possible role in the release of other neurotransmitters such as acetylcholine or ATP in sham-operated, 12-month-decentralized and immediate-reinnervated bladders.

Methods: Smooth muscle strips (mucosa denuded) were isolated from the region just rostral to the trigone and suspended in muscle baths. Strips were ranked based on their contractile response to a 3-min exposure to 120 mM KCl and sorted so that the average response in each group was equal. Strips were incubated with either 1 µM atropine (ATR), 1 µM tetrodotoxin (TTX), 10 µM alpha, beta-methylene ATP (α,β-ATP) or vehicle (water) and then induced to contract with the nicotinic receptor agonists 1,1 dimethyl-4-phenyl-piperazinium iodide (DMPP, 100 µM), TC2559 (100 µM) or epibatidine (10 µM) or 1 mM nicotine itself.

Results: The DMPP-induced contraction was not different between sham, reinnervated or decentralized bladders (11%, 3.3%, and 3.6% of KCl contraction respectively). While the epibatidine-induced contraction in shams was not different relative to that in the reinnervated (41% vs. 27% KCl respectively), it was significantly greater than that in decentralized bladders (13% of KCl). TC2559 did not induce bladder contractions. Nicotine-induced contractions in sham-operated controls were 16% of KCl. ATR completely blocked nicotine-induced contraction while α,β-ATP had no statistically significant effect in shams. TTX had no significant inhibitory effect on DMPP, epibatidine or nicotine-induced contraction in any group.

Conclusions: Nicotinic receptors mediate contraction in sham, reinnervated and decentralized bladders. This nicotinic receptor-mediated contraction is decreased after decentralization. TTX does not block nicotinic receptor-mediated contractions, indicating that action potentials are not required to induce contraction. In sham-operated dog bladders, the nicotine-induced contraction is blocked by ATR, suggesting that these nicotinic receptors are located on cholinergic nerve terminals and induce the release of acetylcholine, which activates muscarinic receptors on the smooth muscle.

Funding Source(s): NIH-NINDS NS070267

Keywords: In vitro contractility; innervation; neural plasticity; dog; nicotinic receptors