Site and Mechanism of Action of Botulinum Toxin Type-A in the Urinary Bladder

Description of projects available to graduate students:
The symptoms of bladder overactivity include urgency, frequency and urge incontinence. For many years the only therapeutic intervention has been antimuscarinic drug treatment. These drugs are not effective for all patients and have a number of unpleasant side effects including dry mouth and constipation, which reduce patient compliance. However, more recently injections of the toxin Botulinum A (BTX-A) directly into the bladder wall have been used effectively to alleviate bladder overactivity. The toxin is known to specifically target cholinergic nerves and inhibit the vesicular release of acetylcholine (ACh) at the neuromuscular junction. However, the exact site of action is unknown and there are a number of possibilities, other than efferent nerves, including the urothelium, smooth muscle and afferent nerves. Accordingly our project aim is to determine the site and mechanism of action of BTX-A in the overactive bladder.
In this project we use both control mice and a spinal cord transected (SCT) mouse model of bladder overactivity to investigate the action of the BTX-A. The SCT mouse bladder becomes hypertrophied and shows spontaneous bladder contractions as seen in many patients with bladder overactivity. BTX-A is applied in one of two ways to the bladder, either via in vivo intra-serosal injections or addition to the recording chamber once the bladder has been excised. We employ a number of techniques to study the possible sites.
BTX-A could block the sensitization of afferent nerves that can occur in overactive bladders. To study the role of afferents we use a mouse in vitro pelvic nerve-bladder sheet preparation from which we can measure tension of the bladder and record both multi and single unit afferent activity. Mechanically sensitive afferents within the bladder can be activated by stretch of the bladder, as in vivo, using our stepper-motor and the effect of BTX-A on these responses can be studied.
The spontaneous activity observed in overactive bladder may be enhanced by release of factors, such as ACh and ATP, from the urothelium upon stretch. These factors can act on interstial cells within the sub-urothelium which in turn may activate smooth muscle or afferent nerves to increase spontaneous contractions. We hypothesize that BTX-A could act by blocking this process. To study the role of the urothelium we use our unique optical mapping approach whereby Ca2+ and voltage transients can be mapped across the urothelium in a bladder sheet preparation or across the bladder wall in a cross-section preparation while recording bladder tension.
Afferent recording and optical mapping can be performed simultaneously on a single preparation providing us with a powerful technique to investigate the mechanism of bladder overactivity and how this can be modulated by treatments such as BTX-A.

Techniques graduate student will learn:
See above.