Spinal Central Effects of Peripherally Applied Botulinum Neurotoxin A in Comparison between Its Subtypes A1 and A2

Abstract

Because of its unique ability to exert long-lasting synaptic transmission blockade, botulinum neurotoxin A (BoNT/A) is used to treat a wide variety of disorders involving peripheral nerve terminal hyperexcitability. However, it has been a matter of debate whether this toxin has central or peripheral sites of action. We employed a rat model in which BoNT/A1 or BoNT/A2 was unilaterally injected into the gastrocnemius muscle. On time-course measurements of compound muscle action potential (CMAP) amplitudes after injection of BoNT/A1 or BoNT/A2 at doses ranging from 1.7 to 13.6 U, CMAP amplitude for the ipsilateral hind leg was markedly decreased on the first day, and this muscle flaccidity persisted up to the 14th day. Of note, both BoNT/A1 and BoNT/A2 administrations also resulted in decreased CMAP amplitudes for the contralateral leg in a dose-dependent manner ranging from 1.7 to 13.6 U, and this muscle flaccidity increased until the fourth day and then slowly recovered. Immunohistochemical results revealed that BoNT/A-cleaved synaptosomal-associated protein of 25 kDa (SNAP-25) appeared in the bilateral ventral and dorsal horns 4 days after injection of BoNT/A1 (10 U) or BoNT/A2 (10 U), although there seemed to be a wider spread of BoNT/A-cleaved SNAP-25 associated with BoNT/A1 than BoNT/A2 in the contralateral spinal cord. This suggests that the catalytically active BoNT/A1 and BoNT/A2 were axonally transported via peripheral motor and sensory nerves to the spinal cord, where they spread through a transcytosis (cell-to-cell trafficking) mechanism. Our results provide evidence for the central effects of intramuscularly administered BoNT/A1 and BoNT/A2 in the spinal cord, and a new insight into the clinical effects of peripheral BoNT/A applications.

Retrograde transport of radiolabelled botulinum neurotoxin type A to the CNS after intradetrusor injection in rats

“…In compartmentalized cultures of rat sympathetic neurons, there was retrograde movement of BoNT/A into cell bodies in the distal compartments [31]. Further studies have shown axonal transport of peripherally injected BoNT/A by detection of cleaved SNAP-25 fragments [27,[32][33][34].…”

In vivo onset and duration of action varies for botulinum neurotoxin A subtypes 1-5

“…With the continually growing number of BoNT variants, a potential treasure trove for discovery of new information on these toxins is presented to the botulinum neurotoxin community. The presented and described comparative studies of BoNT/A1–5 already have identified BoNT/A2 as a variant that has potential pharmacological benefits over the currently used BoNT/A1 (16, 17, 22, 23, 25). Further in depth functional studies of these and additional BoNT variants will increase our molecular understanding of these toxins, particularly when combined with structural studies.…”

Clinical differences between A1 and A2 botulinum toxin subtypes

“…They injected 10u of A1LL or A2NTX in the hind limb of a rat, which was sacrificed 4 days later, and dissected out spinal cord segments which were processed for quantitative assessment of the cSNAP25 (Fig.1) 15 . fibers terminate, underscoring its potential to inhibit painful stimuli.…”