Mediators of neuromuscular transmission in rat bladder strips were dissected pharmacologically to examine their susceptibilities to inhibition by botulinum neurotoxins (BoNTs) and elucidate a basis for the clinical effectiveness of type A in alleviating smooth muscle spasms associated with over-active bladder. BoNT/A, /C1 or /E reduced peak and average force of muscle contractions induced by electric field stimulation (EFS) in dose-dependent manners by acting only on neurogenic, tetrodotoxin-sensitive responses. BoNTs that cleave vesicle-associated membrane protein (VAMP) proved much less effective. Acetylcholine and ATP were found to provide virtually all excitatory input because EFS-evoked contractions were abolished by the muscarinic receptor antagonist, atropine, combined with either a desensitising agonist of P2X1 and P2X3 or a non-selective ATP-receptor antagonist. Both transmitters were released in the innervated muscle layer and, thus, persisted after removal of urothelium. Neither atropine nor a desensitiser of the P2X1 or P2X3 receptors altered the rate at which muscle contractions were weakened by BoNT/A. Moreover, though cholinergic and purinergic signalling could be partially delineated using high frequency EFS (which intensified a transient, largely atropine-resistant, spike in muscle contractions that was reduced following P2X-receptor desensitisation), they proved equally susceptible to BoNT/A. Thus, equi-potent blockade of ATP co-released with acetylcholine from muscle efferents likely contributes to the effectiveness of BoNT/A in treating bladder over-activity, including non-responders to anti-cholinergic drugs. As purinergic receptors are known mediators of sensory afferent excitation, inhibition of efferent ATP release by BoNT/A could also help to ameliorate acute pain and urgency sensation reported by some recipients.
- Received April 16, 2010.
- Revision received June 23, 2010.
- Accepted June 23, 2010.
- The American Society for Pharmacology and Experimental Therapeutics