**2. Results**

### *2.1. Basic Mechanism of Action of BTX-A*

Inactive BTX-A is a single-chain polypeptide of 150 kDa. When BTX-A is pharmacologically activated, it is cleaved to a 100-kDa heavy chain and a 50-kDa light chain that are connected by a single disulfide bond as well as noncovalent bonds [1,6]. BTX-A inhibits or reduces muscle contractions by blocking vesicular neurotransmitter release at neuroglandular and neuromuscular junctions. Two types of presynaptic cell membrane surface receptors for BTX-A have been identified—gangliosides and the synaptic vesicle-associated protein-2 (SV2) family. BTX-A binds to nerve terminals because of the high affinity of its heavy chain for SV2 allowing the toxin to be endocytosed into synaptic vesicles [7]. The light chain of BTX-A is translocated across the vesicle membrane in an acidic environment, and is then released into the cytosol by reduction of the interchain disulfide bond. Following its release from vesicles, the light chain is able to cleave synaptosomal-associated protein 25 (SNAP25) proteins, a part of a heterotrimeric soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex, thereby inhibiting the fusion of vesicles with the nerve terminal membrane, and ensuring the blockade of neurotransmitter release and consequent smooth muscle contractions [8].

When using BTX-A to treat lower urinary tract diseases, the net effect results in: (1) the paralysis of low-grade contractions of the unstable detrusor to increase bladder capacity and reduce detrusor pressure during filling and resting phases, and (2) the preservation of high-grade contractions of the detrusor to initiate micturition [9–11]. In addition to this effect, a significant reduction in the sensation of urinary urgency has been reported by patients with OAB, suggesting a sensory effect on the bladder [12]. The effects on sensory feedback loops explain the mechanism of BTX-A activity in relieving symptoms of detrusor overactivity as well as suggest a potential role for BTX-A in the relief of hyperalgesia-associated lower urinary tract disorders, such as IC/BPS and chronic pelvic pain syndrome [9].
