Case released. ACh is transported in synaptic vesicles

Case
History:
Kelly, a 12-year-old girl, has significant gait abnormalities
resulting from cerebral palsy. She walks on her toes and exhibits a
scissor gait, with her legs strongly adducted with each step. Kelly
has shown no significant improvements in gait with standard therapy,
including exercises, gait training, and training in activities of
daily living. Her physicians now want to inject a small amount of
botulinum toxin into the gastrocnemius and adductor magnus muscles of
both legs in an effort to reduce involuntary muscle activity and
improve gait.

By
what mechanism could injection of botulinum toxin reduce involuntary
muscle activity?

ACh
is a neurotransmitter that is synthesised into acetyl coA and
chlorine, catalyzed by choline acetyltransferase (CAT).

In
a normal neuron (in this case at the neuromuscular junction), the
arrival of an action potential at the axon terminal opens
voltage-gated calcium channels, allowing
calcium
to enter the neuron. The entry of calcium causes ACh to be released.
ACh is transported in synaptic vesicles to the presynaptic terminal.
Three SNARE proteins allows the ACh molecules to be released into the
synaptic cleft (synaptic fusion complex) by binding the synaptic
vesicle onto the presynaptic membrane. ACh is released into the
synaptic cleft. It then binds to the receptors on the post-synaptic
terminal causing a depolarization of the sarcolemma via T tubules,
which carry the action potential along their surface. This opens that
voltage-dependent calcium channels in the terminal cisternae.
Troponin can then bind to the calcium, resulting in tropomyosin
acting on the actin filament. This uncovers the myosin binding site
on the actin, and causes muscle fibre contraction for as long as
there is calcium available.

Botulinum
toxin disrupts this process. It is a neurotransmitter antagonist that
works by inhibiting the release of acetylcholine (ACh) into the
synaptic cleft from the presynaptic neuron at the neuromuscular
junction.1
The
botulinum toxin molecule
consists of a heavy chain and light chain. The heavy chain binds
irreversibly to the presynaptic
cholinergic
neuron. The light chain detaches and then cleaves one of the three
SNARE proteins which prevents the binding to the synaptic vesicle
containing
ACh to
the presynaptic membrane, thereby preventing the release of ACH into
the synaptic cleft. The sarcolemma is then not depolarized and
therefore the action potential is not propagated, and the muscle
cannot contract, reducing spasticity.

In
the case of Kelly,
botulinum toxin
would work locally in the muscles that have been injected, preventing
the contraction of gastrocnemius and adductor magnus via the
mechanisms mentioned above, which in turn would lessen her
spasticity. Clinical trials show good response to
botulinum toxin
in children with cerebral palsy, with gait velocity showing
significant increases.
2,3

At
the neuromuscular junction, ACh acts via a ligand-gated receptor. Is
the action of ACh on the nicotinic, ligand-gated receptor the same
as its action on the muscarinic, G-protein–mediated receptor?

Broadly
speaking, no. The action of ACh on ligand-gated receptors is that it
causes a fast signal transmission at the synapse and a rapid
post-synaptic response by opening ion channels directly. In nicotinic
receptors, it acts as an excitatory neurotransmitter. These are
normally found in the neuromuscular junction, autonomic ganglia and
different areas of the CNS.4
The
action on muscarinic receptors tends to be a slower process that the
action on ligand-gated receptors. It can either act as
an inhibitory or excitatory neurotransmitter. On muscarinic
receptors, it activated G-proteins
which increase intracellular calcium as part of a intracellular
cascade. The binding of ACh to the receptors causes them to change
shape, which causes the G-protein to then convert guanosine
triphosphate to guanosine diphosphate,
hence becoming activated. This protein then stimulates enzymes to
catalyse further intracellular reactions via a secondary messenger
system. Muscarinic receptors generally are involved in autonomic
effects on the heart (regulation of heart rate in cardiac muscle) and
the brain, and can have longer-lasting effects especially concerning
gene expression. Interestingly, ACh is a neuromodulator that allows
improved encoding of new stimuli5
and therefore enhance the encoding of new memories.6

References:

Nigam
PK, Nigam A. Botulinum toxin. Indian journal of dermatology.
2010;55:8-14.

Baker
R, Jasinski M, Maciag-Tymecka I, et al. Botulinum toxin treatment of
spasticity in diplegic cerebral palsy: a randomized, double-blind,
placebo-controlled, dose-ranging study. Developmental Medicine
and Child Neurology. 2002;44:666-675

Wissel
J, Heinen F, Schenkel A, et al. Botulinum toxin A in the management
of spastic gait disorders in children and young adults with cerebral
palsy: a randomized, double-blind study of “high-dose”
versus “low-dose” treatment. Neuropediatrics.
1999;30:120.

Lundy-Eckman
L. Neuroscience: fundamentals for rehabilitation. 4th
Ed. S.l.: Saunders; 2012.

Dannenberg
H, Young K, Hasselmo M. Modulation of Hippocampal Circuits by
Muscarinic and Nicotinic Receptors. Frontiers in Neural Circuits.
2017;11. doi:10.3389/fncir.2017.00102.

Hasselmo
ME. The role of acetylcholine in learning and memory. Current
Opinion in Neurobiology. 2006;16:710-715.