Inhibition Of Terminal Sprouting Triggered By Botulinum Toxin Type A In Rats Poised With Acrylamide

Background and purpose: Botulinum toxin type A (BTXA) is one of the bacterial exotoxins produced by Clostridium Botulinum during its growth and propagation. BTXA is a potent muscle relaxation, which blocks transmission and release of acetylcholine at neuromuscular junction through cleaving the SNAP-25 protein in presynaptic membrane. Intramuscular injection of BTXA has served as an effective and safe therapy for local dystonia. However, muscular weakness is temporary and muscular power could recover in 3-4 months. BTXA not only induces muscle paralysis but also triggers in vivo a profuse sprouting from motor nerve terminals which may contribute to the functional recovery of neuromuscular transmission. Nerve terminal sprouting increased in number, length and complexity after toxin administration. So nerve terminal sprouting triggered by BTXA limits BTXA's efficacy and how to inhibit nerve terminal sprouting is a challenge to BTXA's application.Acrylamide (ACR) is a neurotoxic monomer, which leads to a "dying-back" degeneration neuropathy. Morphologic analysis of peripheral neuropathy reveals that the most severe site of neurotoxicity is Ranvier's node and the neurotoxic area includes neuromuscular junction. Toxicant mechanism consists of axonal transport compromise, modification of neurofilaments, microtubules, energy-generating metabolic enzymesand motor proteins.Here we study the inhibition of terminal sprouting triggered by BTXA in SD rats poised with acrylamide. We evaluate terminal sprouting through examination of single fiber electromyography (SFEMG) and morphologic nerve fiber analysis. Meanwhile, we evaluate functional recovery of neuromuscular junction through examination of muscular power and SFEMG.Methods: SD rats were divided randomly into four groups: A (BTXA+ACR), B (BTXA + Saline), C (Saline + ACR) and D. 5u of BTXA was injected into right gastrocnemius of the rats in both group A and group B and the saline in group C. ACR was injected into abdominal cavity of rats in both group A and group C and the saline in group B. Rats in group D weren't treated with BTXA, saline or ACR. Accumulative dosage of ACR was 105-131mg/kg. The examination of muscular power of right lower limb and SFEMG (FD+MCD) and the morphologic nerve fiber analysis were carried out in group A, B and C at 1, 2, 3, 4, 6, 8, 10 and 12 weeks after injection of BTXA. The same examination and analysis was carried out in group D once and the results of the examination and analysis acted as normal range.Results: The results of fiber density (FD) and the morphologic nerve fiber analysis revealed terminal sprouting after injection of BTXA and inhibition of terminal sprouting by ACR. The results of mean consecutive difference (MCD) and muscular power revealed the recovery of neuromuscular junction in 12 weeks after injection of BTXA and inhibition of the recovery by ACR. The results of group C were normal. Conclusion: Motor nerve terminal sprouting contributes to the functional recovery of neuromuscular junction after injection of BTXA. ACR inhibits terminal sprouting both in the sprouting number and the functional recovery.