Effects of Botox on Motoneurons
Wise Young, Ph.D., M.D.
W. M. Keck Center for Collaborative Neuroscience
Rutgers, State University of New Jersey, Piscataway, NJ 08540
Originally posted 6/22/06, updated 2/11/09
Many people have asked questions about Botox or botulinum toxin that is now commonly used to treat spasticity and other conditions.
What is Botox? Botox is the trade name of a drug derived from botulinum toxins, a powerful family of neurotoxins that selectively affects acetylcholinergic neurons, produced by a family of bacteria called clostridium bolutinum. This bacteria grows under anaerobic conditions in canned foods and can cause a condition called botulism. One of the most powerful and dangerous neurotoxins in nature, it use to kill in over 50% of people who developed botulinism before respirators were available . Ingestion, inhalation, or injection of 200-300 picograms per kilogram is a lethal dose. Because 100 grams of the toxin would be sufficient to kill every person on earth, it has been the target of some speculation as a terrorist weapon , particularly in milk  but, because the molecule is rapidly degraded when heated or exposed to air and difficult to produce in quantity, the concern has been largely theoretical .
Botulinum toxin therapy. Botulinum toxin has been studied for many years. In 1949, Justinus Burgen discovered that the toxin blocked neuromuscular transmission. In the 1950’s, it was used to eliminate facial wrinkles in actors. When injected in tiny amounts into muscle, the toxin binds to motor nerves, preventing release of neurotransmitters from the nerves to paralyze or weaken the muscle. The injection effects usually last only 3-4 months and repeated re-injections are necessary to maintain its effects for longe periods. Seven members of the botulinum toxin family (A-G) with three subtypes of the A group have been identified. In 1989, the U.S. FDA approved use of botulinum toxin A (BTX-A) to treat strabismus (eye movement), blepharospasm (spasmodic blinking), and hemifacial spasm (spasms of the face). In 2002, the FDA approved Botox use for treating facial wrinkles. BTX-A is marketed under the trade names of Botox and Dysport. Dysport is a different formulation of botulinum toxin A. Although not specifically approved for such indications, BTX-A is frequently used to treat urinary incontinence, anal fissures, spasticity, focal dystonias, and temporomandular joint pain. It has been used for treatment of neuromuscular pain, even though the toxin does not directly affect sensory nerves, because abnormal muscle activity may be associated with cramps , including bladder pain . Some animal data suggest that botulinum toxins may affect activity of nociceptive neurons. In 2000, the U.S. FDA approved the use of botulinum toxin B (BTX-B) for cervical dystonia, a condition of hyperactive muscle groups in the neck. A cheaper but less potent Chinese version of Botox is also available. Although the other versions of botulinum toxin (C-G) have not been approved, several synthetic versions and fragments of the toxin being tested .
Mechanism of action. BTX-A has three components, a heavy chain and two light chains. Injected in small amounts into muscles, the heavy chain of botulinum binds to the surfaces of motor nerves where it is endocytosed (the membrane inverts and is taken back into the cell). A recent study  suggests that a membrane protein called SV2 is protein receptor for botulinum toxin. The heavy chain has turned out to be a useful tool for getting other molecules to be transported into cells . The light chains of botulinum toxin are enzymes (proteases) that break down the SNAP-25 proteins in the SNARE family. These proteins are essential for release of acetylcholine (the neurotransmitter that activates muscles). Motor nerves treated with BTX-A tend to fill up and swell with unused acetylcholine vesicles. BTX-A and BTX-B reduce both all muscle activity, due to both voluntary and involuntary motor activity, usually within 24-48 hours and the paralysis lasts several months.
Side-effects of Botox Therapies. Botulinum toxin treatments are associated with some complications. Muscle weakness is the most commonly reported side-effect. The injection site may be sore and tender. Flu-like symptoms may occur. Pain may be present in neighboring muscles. Some 20-30% of patients may report these side effects . Injection of the toxin into the wrong muscle is of course a possibility. When injected into the neck or nearby structures, the toxin may affect ability to swallow and dry mouths (a side-effect of the toxin on nerves that stimulate saliva secretion). The beneficial effects of the drug may not appear for 7-10 days and the effects may last 4-6 weeks. When used repeatedly and more frequently than 12 weeks, the body will develop antibodies against botulinum toxins, reducing their effectiveness. Antibodies against the serum (anti-toxin) is one treatment of botulinum toxicity.
Effect of Botox on Motoneurons. BTX-A is generally considered to be a very long-acting reversible toxin. However, BTX-A may have toxic effects on motoneurons. In neonatal rats, the toxin has been reported to increase electrotonic coupling between motoneurons and other neurons . BTX-A is well known to stimulate motor axonal sprouting in muscle. This may have some unanticipated effects. For example, in 2002, Millecamps, et al.  reported that prior injection of BTX-A enhances locallly injected adenovirus retrograde gene transfer in motoneurons, apparently related to toxin-induced sprouting of the nerves and increasing the surface area of nerves exposed to virus. In 1977, Sumner  reported the BTX-A injected into the tongues of rats produced the same changes in motoneurons as cutting of the axon, with reductions in dendrite numbers and increase in abnormal dendrite appearance, increases in astrocytes around the motoneurons, and even presence of microglial cells that suggest an inflammatory response. Jung, et al.  reported changes in gene expression in rat spinal motoneurons after chemodenervation with botulinum toxin.
Effects of Botox on Muscle. Botox parayzes muscle by stopping acetylcholine release. Because the toxin is injected in small amounts that only affect the muscle in the immediate vicinity of the injection site, there is a decrease in muscle activity and strength. The toxin effect lasts for 4-6 weeks and the injections must be repeated to retain the effect. The mechanisms by which the muscle recovers strength from the toxin are not well understood but may be due to some or all of the following possibilities. First, the recovery of muscle strength may be due to sprouting axons from surrounding non-paralyzed motor nerves. After botox injections, motor axons do sprout extensively in the muscle. Second, the nerves may take several weeks to get rid of the BTX-A and transport additional SNAP-25 proteins to the end of the axon so that acetylcholine can be released. Third, surrounding muscle fibers may hypertrophy to make up for the paralysis of the muscle. Permanent weakness of muscle may occur, especially in muscles with already compromised voluntary activity.
Advantage and Disadvantage. The advantage of Botox is its ease of administration. The doctor simply injects a tiny amount of the toxin into the muscle deemed to be overactive. Depending on the vantage point, the fact that Botox’s effects are not permanent may be an advantage or a disadvantage. To doctors and the companies that make money from the procedure, it is an advantage. For the patients, the reversibility of botox effects is also an advantage, particularly if the injection is made into the wrong place. To have to receive repeated injections every 3 months, however, is disadvantageous. A second disadvantage is botox paralyzes both voluntary and involuntary (spasticity and spasms) muscle activity. For patients who use or anticipate using the muscles, the weakness due to botox may contribute further dysfunction. Finally, the side-effects and the development of immune responses to the toxin are significant drawbacks of repeated injections.
Limitations and Alternatives. Botox is useful only in situations where one or at most a few groups of muscles are overactive. It is not suitable as a treatment of widespread spasticity, spasms, and other abnormal activity affecting many muscle groups. Alternatives to botox for treatment of spasticity fall into three broad categories. The first are anti-spasticity, ant-epileptic/spasm, and other drugs that suppress central nervous system and muscle activity. These include baclofen and tizanidine. The second are surgical methods, such as lengthening tendons or even denervation. The third are chemical methods of damaging peripheral nerve connections to the muscle, such as phenol.
Of note to afficionados of spinal cord injury therapies, bacteria are a rich source of other toxins  that may be of benefit for spinal cord injury. For example, clostridia bacteria produce C2 toxin, which binds to actin . Others, such as C3, bind to proteins of the Rho family  that are responsible for mediating inhibitory effects of Nogo receptor activation on axonal growth , a mechanism that has been hypothesized to be one of the reasons why axons cannot regenerate in the central nervous system.
In summary, botox is botulinum toxin A, a potent neurotoxin made by the bacterium clostridium botulinus that grows in spoiled foods and still kills people. The toxin binds selectively to the surface of motor nerves and transfers inside the nerve where it breaks down proteins required for release of acetylcholine, the neurotransmitter responsible for muscle and secretory activity. The toxin selectively paralyzed muscles where it has been injected but the effects wear off.