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B.C. Anderson, A.R. Harrison, L.K. McLoon; Neuromuscular Junction Localization in Normal and Botox–Treated Extraocular Muscles of Rabbits . Invest. Ophthalmol. Vis. Sci. 2006;47(13):5058.
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The extraocular muscles (EOM) have extremely fast contractile properties with extremely small motor units. This study investigated the normal 3–dimensional pattern of neuromuscular junction localization in entire rectus muscles from normal adult rabbits. Botulinum toxin paralyzes neuromuscular junctions and is routinely used as a treatment for strabismus. Additionally, we examined alterations in the normal 3–dimenstional patterning of neuromuscular junction localization within 2 weeks of botulinum toxin treatment.
One randomly selected superior rectus muscle in adult rabbits received an injection of 5 Units of botulinum toxin. After two weeks, the animals were euthanized, both superior rectus muscles were removed, pinned to their in situ length, frozen and serially sectioned in the longitudinal plane. Every 10th section was immunostained with fluorescein–labeled alpha–bungarotoxin, which labels the neuromuscular junctions. Every neuromuscular junction was mapped in 3–dimensions using the Bioquant Topographer program, and 3–dimensional reconstructions were prepared.
Neuromuscular junctions were located throughout the tendon–to–tendon length of the normal rectus muscles. The density of neuromuscular junctions was approximately 2,760/mm3. Two weeks after a botulinum toxin injection, neuromuscular junctions were more homogeneously spread throughout the muscle length. They appeared smaller, but the density of neuromuscular junctions significantly increased to 5550/mm3, a two–fold increase.
Neuromuscular junctions in rabbit EOM were found throughout the tendon–to–tendon length of the muscles. Botulinum toxin injection resulted in a significant increase in the density of neuromuscular junctions two weeks after treatment. These data suggest that for maximal efficacy of botulinum toxin treatment, the drug needs to be dispersed throughout the muscle length. In addition, it may be possible to increase efficacy of botulinum toxin treatment by delaying the sprouting response and secondary up–regulation of neuromuscular junctions within the treated EOM.
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