In 2008, some experiments were made on rats at the Instituto di Neuroscienze in Pisa, Italy, where botulinum toxin A was injected into their jaw.
One would find out if botulinum toxin A could spread to the brain.
As the toxin itself in botox and botox-like preparations is in a very small amount, the researchers looked for indirect evidence that the poison had spread instead. Namely, botulinum toxin has the property of degrading a specific protein in the body called SNAP-25 protein.
Three days after the rats received botulinum injections, evidence was found that the SNAP-25 protein had been degraded in the animal’s brain stem.
It shows that the nerve poison had spread to the brain, the researchers believe. They believe the toxin has spread through the nerve pathways.
A longer scientific article from 2008 published in The Journal of Neuroscience , which explains how the toxin can spread from the periphery to the central nervous system: