Acetylcholine

Acetylcholine (ACh) is a neurotransmitter responsible for facilitating communication between neurons and other cells, including muscles and glands. 

It is important in various physiological processes such as cardiac function, gastrointestinal activity, and muscle contraction. 

Synthesized from choline and acetyl-CoA by the enzyme choline acetyltransferase within cholinergic neurons, ACh is released into the synaptic cleft upon an action potential-induced calcium influx. 

This release enables ACh to bind to nicotinic and muscarinic receptors, triggering fast and slow synaptic transmissions, respectively. 

Nicotinic receptors, found in muscles and the autonomic nervous system, are primarily excitatory and crucial for muscle contraction and neurotransmitter release modulation. Muscarinic receptors, which are G protein-coupled, mediate slower responses affecting heart rate, smooth muscle contraction, and cognitive functions. 

Acetylcholine's action is terminated by the enzyme acetylcholinesterase, which rapidly degrades it into choline and acetate. 

The availability of choline, sourced from diet and liver production, is essential for continuous ACh synthesis. 

ACh's significance extends to clinical contexts, particularly in neurodegenerative diseases like Alzheimer’s where cholinergic deficits contribute to cognitive decline, and conditions such as myasthenia gravis, where impaired ACh signaling leads to muscle weakness.

What is Acetylcholine?  [17.]

Acetylcholine (ACh) is a neurotransmitter that facilitates communication between neurons and other cells, including muscle cells and glandular tissues. It is involved in various physiological processes such as cardiac function, gastrointestinal activity, and muscle contraction.

Tissues responsive to ACh are called cholinergic. Anticholinergics are chemicals that inhibit ACh’s action.

It is synthesized from choline and acetyl-CoA by the enzyme choline acetyltransferase within the cytoplasm of cholinergic neurons.

ACh release is triggered by an action potential leading to calcium influx.  The calcium influx stimulates ACh release into the synaptic cleft, where it binds to receptors on target cells.

Acetylcholinesterase (AChE) rapidly breaks down ACh in the synaptic cleft into choline and acetate, terminating its action.

Acetylcholine Receptors  [17., 21.] 

When an action potential travels down a cholinergic neuron, acetylcholine is released from the axon terminal into the synaptic cleft. It can then bind to and activate two main classes of receptors:

Nicotinic Acetylcholine Receptors 

These are ligand-gated ion channels that mediate fast synaptic transmission when activated by acetylcholine.  They are primarily excitatory, affecting muscles and autonomic ganglia.

Their key roles include muscle contraction at the neuromuscular junction and modulating neurotransmitter release in the brain.

Nicotinic receptors are divided into two subtypes: muscular (N1) and neuronal (N2). 

The N1 receptors are located on the surface of muscle cells at the neuromuscular junction, facilitating muscle contraction. 

In contrast, N2 receptors are found in the peripheral and central nervous systems, including the adrenal medulla, postsynaptic cell bodies within the sympathetic and parasympathetic nervous systems, and various brain regions such as the ventral tegmental area, hippocampus, prefrontal cortex, amygdala, and nucleus accumbens. 

Muscarinic Acetylcholine Receptors 

These are G protein-coupled receptors that mediate slower metabolic responses that can be both excitatory and inhibitory, affecting heart, smooth muscles, and various CNS regions.

These are important for regulating many peripheral functions like heart rate, smooth muscle contraction, and glandular secretions.  They are also involved in cognitive processes like learning and memory in the brain.

Muscarinic receptors consist of five subtypes: M1, M2, M3, M4, and M5. 

The M1, M3, and M5 subtypes are generally stimulatory, activating phospholipase C to produce IP3 and DAG, which increase intracellular Ca2+ levels and activate protein kinase C. 

Conversely, the M2 and M4 subtypes are inhibitory, reducing cAMP levels by inhibiting adenylate cyclase. 

Muscarinic receptors are widely distributed, with M1 receptors found in the cerebral cortex, salivary, and gastric glands; M2 receptors in smooth muscle and cardiac tissue; M3 receptors in the smooth muscles of the bronchioles, iris, bladder, and small intestines; and M4 and M5 receptors in various brain regions, including the hippocampus and substantia nigra. 

Unlike the fast-acting nicotinic receptors, muscarinic receptors mediate slower, prolonged responses through their complex signaling pathways.

After binding its receptors, acetylcholine is rapidly degraded by the enzyme acetylcholinesterase to terminate its signaling. The breakdown products are recycled to synthesize new acetylcholine molecules.

Choline as a Requirement for Acetylcholine  [8.] 

Choline is naturally found in foods including egg yolks, liver, some meats, seeds of various vegetables, and legumes.  It is also produced by the liver.  [17.] 

Choline from the diet or produced by the liver crosses the blood-brain barrier via sodium-dependent uptake channels.  In neurons, it is acetylated by choline acetyltransferase (CAT) to form acetylcholine (ACh).

The SLC44A family is a primary group of main choline transporters, with SLC44A1 (CTL1) being the main choline transporter.  CTL1 is involved in choline transport across the plasma membrane and mitochondria in various tissues including muscles, liver, and cancer cells.

CTL1/SLC44A1 plays a significant role in transporting choline into cells where it is used for ACh synthesis, particularly in the nervous system and possibly non-neuronal tissues.

CTL1/SLC44A1 polymorphisms can lead to muscle damage in choline-deficient conditions and are associated with altered choline metabolism in diseases like rheumatoid arthritis and cancer.

Acetylcholinesterase (AChE)  [15.] 

AChE is an essential enzyme in both vertebrate and invertebrate nervous systems that degrades the neurotransmitter acetylcholine (ACh) into choline and acetic acid. 

This enzyme is mainly found at neuromuscular junctions and cholinergic synapses in the central nervous system, where its role is to stop synaptic communication by breaking down ACh.

Organophosphates (OP) and carbamates (CR) are environmental toxins that inhibit AChE, leading to the accumulation of ACh.  This accumulation disrupts nervous system function, causing death in pests and mammals. 

The binding between OPs and AChE is quasi-irreversible, resulting in complete enzyme inactivation.

AChE activity can serve as a reliable bioindicator for pesticide toxicity.  One study suggested that the inhibition of AChE depends on factors such as the chemical structure of the pesticide and its ability to bind to specific amino acid residues on the enzyme.  [15.] 

Acetylcholine Functions

Acetylcholine has functions in the autonomic, peripheral, and central nervous systems.  While its primary functions are considered to occur at the neuromuscular junction and within the autonomic nervous system, it also has some functions in the central nervous system.

Autonomic Nervous System  [17., 21.]

In the autonomic nervous system, acetylcholine (ACh) acts as the neurotransmitter for both preganglionic sympathetic and parasympathetic neurons. 

ACh is also involved in neurotransmission at the adrenal medulla and all parasympathetically innervated organs. 

Additionally, ACh serves as the neurotransmitter at the sweat glands and piloerector muscles within the sympathetic nervous system.

Peripheral Nervous System  [17., 21.] 

Within the peripheral nervous system, ACh is the primary neurotransmitter at the neuromuscular junctions, where it facilitates communication between motor nerves and skeletal muscles, leading to muscle contraction.

Central Nervous System  [17., 20.] 

In the central nervous system, ACh is predominantly found in interneurons and a few significant long-axon cholinergic pathways.  These pathways include those that are often degenerated in Alzheimer's disease. 

Additionally, most subcortical areas are innervated by cholinergic neurons originating from the ponto-mesencephalic region.

Acetylcholine’s Functions in Various Body Systems  [17.] 

• Cardiovascular: ACh induces vasodilation and decreases heart rate and contraction force.

• Gastrointestinal: Enhances tone and secretory activity through vagus nerve stimulation.

• Respiratory: Causes bronchoconstriction and stimulates chemoreceptors.

• Urinary: Increases bladder emptying pressure and ureteral peristalsis.

• Exocrine Glands: Stimulates secretion from glands such as salivary and sweat glands.

• Eye: Causes pupil constriction and lens accommodation.

• Reproductive System: Induces erection.

Clinical Significance of Acetylcholine as a Biomarker

Neurodegenerative Diseases

Acetylcholine deficiency plays a significant role in the pathogenesis of neurodegenerative diseases, particularly those characterized by cognitive impairment and neuropsychiatric symptoms.

Alzheimer's Disease  [6., 11.]

Alzheimer's disease (AD), the leading cause of dementia affecting millions worldwide, is characterized by cognitive decline due to synaptic loss and the accumulation of neurofibrillary tangles and amyloid plaques. 

A crucial aspect of this decline is the degeneration of cholinergic neurons in the basal forebrain, particularly those in the nucleus basalis of Meynert, which significantly contributes to memory and attention deficits. 

Acetylcholine’s release and rapid breakdown by acetylcholinesterase (AChE) are central to cholinergic signaling. 

Current treatments for AD include cholinesterase inhibitors (donepezil, galantamine, rivastigmine) and the NMDA receptor antagonist memantine, which offer symptomatic relief but do not alter disease progression. 

Emerging therapies aim to modulate cholinergic receptors, reduce amyloid-beta (Aβ) production, and enhance neuroprotection, potentially modifying the disease course by preserving cholinergic neurons and improving cognitive function.  

Parkinson’s Disease and Lewy Body Dementias  [3., 4., 11., 18.]

Parkinson's disease dementia (PDD) and dementia with Lewy bodies (DLB) are also associated with significant cholinergic deficits due to the degeneration of the nucleus basalis of Meynert (NBM), a key cholinergic nucleus in the basal forebrain that provides cholinergic projections to the cortex. [11., 18.] 

In vivo neuroimaging studies have revealed reduced cortical cholinergic activity and loss of cholinergic neurons in the basal forebrain of PDD and DLB patients compared to healthy controls. [5., 11., 18.] 

This growing body of evidence provides a rationale for the use of cholinesterase inhibitors as part of a treatment plan in this population.  [9.]

Autoimmune Conditions

Myasthenia Gravis  [1., 14.] 

Acetylcholine plays a central role in the pathogenesis of myasthenia gravis, an autoimmune disorder affecting the neuromuscular junction. 

At the junction, ACh is released from motor neuron terminals and binds to nicotinic acetylcholine receptors (AChRs) on the muscle membrane, triggering muscle contraction.  However, in MG, autoantibodies are produced against these nicotinic AChRs, leading to antibody-mediated destruction and accelerated degradation of the receptors. 

The resulting reduction in available AChRs impairs neuromuscular transmission, as there are fewer receptors for ACh to bind and activate the muscle. 

This causes the characteristic muscle weakness and fatigue in MG patients, as the muscles cannot sustain contraction due to the ACh/AChR deficiency. 

Acetylcholinesterase inhibitor drugs like neostigmine are used to treat MG by inhibiting the enzyme that breaks down ACh, thereby increasing ACh availability at the neuromuscular junction. 

This partially compensates for the AChR deficiency by providing more ACh molecules to bind to the remaining functional receptors and activate muscle contraction. 

Lambert-Eaton Myasthenic Syndrome (LEMS)  [10.]

Lambert-Eaton myasthenic syndrome is an autoimmune disorder that impairs acetylcholine signaling at the neuromuscular junction, resulting in muscle weakness and fatigue. 

In LEMS, antibodies are produced against presynaptic voltage-gated calcium channels on the motor nerve terminals. These anti-VGCC antibodies inhibit the entry of calcium into the nerve terminals, which is required for the release of acetylcholine from synaptic vesicles. 

Consequently, there is less acetylcholine available to bind and activate receptors on the muscle membrane postsynaptically. This impairs neuromuscular transmission, as acetylcholine is the key neurotransmitter that triggers muscle contraction. 

The deficiency in acetylcholine at the neuromuscular junction leads to the characteristic proximal muscle weakness, fatigue, and other autonomic symptoms seen in LEMS patients. 

LEMS can occur in isolation or in association with certain cancers like small cell lung cancer. 

Treatment strategies involve immunosuppression, plasmapheresis to remove pathogenic antibodies, and drugs like amifampridine that enhance acetylcholine release from nerve terminals. 

Respiratory Diseases

Emphysema  [2.] 

Acetylcholine (ACh) plays an anti-inflammatory role in the lungs.  In a study using a mouse model of emphysema, it was found that deficiency in ACh due to reduced vesicular acetylcholine transporter (VAChT) levels led to increased lung inflammation. 

VAChT-deficient mice exhibited heightened levels of macrophages, neutrophils, lymphocytes, and eosinophils, along with elevated inflammatory cytokines and oxidative stress markers. 

Despite this increased inflammation, the severity of emphysema and lung function impairment was similar to that in wild-type mice. These findings suggest that while ACh deficiency exacerbates lung inflammation, it does not worsen emphysema progression, highlighting ACh's protective role in pulmonary health.

COVID-19  [12.] 

COVID-19 can disrupt the acetylcholine system (AChS), leading to various neurological and neuromuscular symptoms. 

The virus can invade the central nervous system directly or through blood vessels, causing inflammation and activating immune cells.  This disrupts the cholinergic anti-inflammatory pathway, leading to an overproduction of pro-inflammatory cytokines and nitric oxide, which in turn inhibits acetylcholinesterase, prolonging acetylcholine activity and causing synaptic dysfunction. 

These disruptions can result in symptoms like brain fog, muscle weakness, and cognitive impairments. 

Additionally, a "cytokine storm" and oxidative stress from nitric oxide can damage acetylcholine receptors, leading to autoimmune responses and chronic inflammation. 

This complex interplay underscores the need for targeted treatments to restore AChS function in post-COVID rehabilitation.

Key Pharmacological Applications 

Cholinesterase Inhibitors

Used in treating conditions like Alzheimer's and MG by preventing ACh breakdown.

Botulinum Toxin 

Prevents ACh release, used for muscle spasticity and cosmetic applications.

Black Widow Venom 

Causes excessive ACh release, leading to muscle contraction and potential paralysis.

Acetylcholine Deficiency

It is very challenging to measure acetylcholine levels as they are rapid-acting and then rapidly broken down by acetylcholinesterase.  An acetylcholine deficiency is typically caused by an autoimmune condition that impairs the actions of acetylcholine, and it is usually first identified symptomatically.  

Symptoms of Acetylcholine Deficiency

ACh has many functions in the nervous system, especially in the autonomic nervous system and at neuromuscular junctions.  The symptoms of ACh deficiency can include:  [17.] 

Cognitive Impairments:

• Memory loss (short-term and long-term)

• Difficulty in learning and retaining new information

• Attention deficits

• Impaired decision-making

• Confusion

Neuromuscular Symptoms:

• Muscle weakness (especially after repeated use)

• Fatigue

• Rapid muscle weakening

• Paralysis in severe cases

Cardiovascular Symptoms:

• Decreased heart rate (bradycardia)

• Reduced cardiac contraction force

• Lowered speed of conduction in the heart (affecting sinoatrial and atrioventricular nodes)

Respiratory Symptoms:

• Bronchoconstriction

• Difficulty in breathing (due to bronchoconstriction)

Gastrointestinal Symptoms:

• Reduced gastrointestinal motility

• Constipation

• Reduced secretory activity in the stomach and intestines

Urinary Symptoms:

• Difficulty in bladder emptying

• Reduced ureteral peristalsis

Ocular Symptoms:

• Blurred vision (due to impaired lens accommodation)

• Pupil dilation issues (miosis)

Exocrine Gland Symptoms:

• Dry mouth (reduced salivary secretion)

• Reduced tear production (dry eyes)

• Reduced sweat production

Other Symptoms:

• Impaired stress response

• Reduced arousal and motivation

Psychiatric Symptoms:

• Apathy

• Depression

Laboratory Testing for Acetylcholine Levels

Acetylcholine Receptor Antibody Levels

Often, testing associated with ACh involves testing for the presence of antibodies against ACh receptors in the case of myasthenia gravis or of calcium-binding antibodies in the case of Lambert-Eaton Myasthenic Syndrome.  

These are blood tests that require a venipuncture for sample collection.  Typically, no special preparation is required.  

Direct measurement of Acetylcholine levels can be challenging due to its rapid degradation by acetylcholinesterase (AcetylcholineE) and the low concentrations present in biological samples.

Interpreting Acetylcholine Antibody Test Results

Optimal Levels of Acetylcholine Antibodies

The presence of anti-acetylcholine receptor antibodies or anti-calcium channel antibodies indicates the presence of an autoimmune condition, respectively myasthenia gravis or Lambert-Eaton Myasthenic Syndrome.  

Optimal levels of either of these antibodies is undetectable.  

Increasing Acetylcholine Levels

Maintaining optimal acetylcholine (Acetylcholine) levels is essential for various physiological functions, and strategies to increase Acetylcholine levels may have therapeutic potential in conditions associated with cholinergic deficiency. 

Acetylcholine Precursors and Supplements

Choline is a precursor for Acetylcholine synthesis, and supplementation with choline or choline-rich compounds may increase Acetylcholine levels in the body.

Alpha-glycerylphosphorylcholine (Alpha-GPC)  [20.]

A naturally occurring compound that can cross the blood-brain barrier and serve as a precursor for Acetylcholine synthesis in the brain. It has been studied as a potential treatment for cognitive impairment and Alzheimer's disease.

Citicoline  [19.] 

A compound that can be converted to choline and cytidine, both of which are involved in Acetylcholine synthesis.  It has been investigated for its potential neuroprotective and cognitive-enhancing effects.

Acetylcholinesterase Inhibitors

Acetylcholinesterase is the enzyme responsible for breaking down Acetylcholine in the synaptic cleft.  Inhibiting Acetylcholinesterase can prolong the action of Acetylcholine and increase its availability.

Donepezil, Rivastigmine, and Galantamine are FDA-approved Acetylcholinesterase inhibitors used in the treatment of Alzheimer's disease to improve cognitive function by increasing Acetylcholine levels in the brain.  [7.] 

Dietary Choline  [16.]

Certain foods are rich in choline, which can contribute to Acetylcholine synthesis in the body.

Egg yolks, beef liver, soybeans, and cruciferous vegetables (broccoli, cauliflower) are good dietary sources of choline.

While these approaches may help increase Acetylcholine levels, it is important to consult with a healthcare professional before starting any supplements or medications, as they may have potential side effects or interactions with other treatments.

Acetylcholine FAQ

What is acetylcholine and what is its function?

Acetylcholine is a neurotransmitter in the brain and nervous system that plays a crucial role in muscle activation, memory, attention, and arousal. It is essential for many neural processes and cognitive functions.

What are acetylcholine supplements, and do they work?

Acetylcholine supplements are designed to increase levels of acetylcholine in the brain. These supplements may contain precursors like choline or compounds that inhibit the breakdown of acetylcholine. Their effectiveness can vary, and it's important to consult with a healthcare provider before starting any new supplement.

What are the signs of acetylcholine deficiency?

Signs of acetylcholine deficiency can include memory problems, cognitive decline, fatigue, and difficulties with concentration and learning. Severe deficiencies can lead to neurological disorders.

How can I increase acetylcholine levels naturally?

You can increase acetylcholine levels naturally by consuming foods rich in choline (such as eggs, fish, and dairy), exercising regularly, and ensuring adequate sleep. Some people also use supplements like alpha-GPC or CDP-choline to boost acetylcholine levels.