Acetylcholine

Acetylcholine (ACh)

Overview

This section provides a comprehensive overview of acetylcholine, including its discovery, functions, and clinical implications. The structure of the notes follows the main themes presented in the transcript.

Discovery

• Otto Loewi’s Experiment (1921): Acetylcholine was first identified as a chemical messenger through Loewi's "Vagusstoff" experiment. This discovery was foundational in establishing the concept of chemical neurotransmission within the nervous system.

Big Picture: Roles in Brain and Body

Acetylcholine plays multifaceted roles in both the central nervous system (CNS) and the peripheral nervous system (PNS).

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• Central Nervous System (CNS)

  • Regulates Arousal and Attention: ACh is crucial for modulating arousal and is involved in attention, learning, and memory processes.

  • Modulates Reward and Motivation: ACh influences circuits related to reward and motivation, contributing to behavioral reinforcement.

  • Balances Excitatory and Inhibitory Tone: ACh helps maintain a balance between excitatory and inhibitory signals in cortical and hippocampal networks.

• Peripheral Nervous System (PNS)

  • Neuromuscular Junction: ACh triggers muscle contraction at the neuromuscular junction, essential for voluntary movement.

  • Autonomic Nervous System:

    • Parasympathetic Functions: Slows heart rate, constricts pupils, and enhances digestion.

    • Sympathetic Functions: Transmission via nicotinic ACh receptors (nAChRs) in autonomic ganglia.

Acetylcholine Cycle

• Synthesis:
  The synthesis of ACh occurs from choline and acetyl-CoA through the enzyme choline acetyltransferase (ChAT):
  $ext{Choline} + ext{Acetyl-CoA} <br>ightarrow ext{ACh}$

• Storage:
  ACh is packed into vesicles through the vesicular acetylcholine transporter (VAChT), with a concentration near 100 mM.

• Release:
  ACh is released via calcium-dependent exocytosis.

• Breakdown:
  Once released, ACh is broken down by acetylcholinesterase (AChE) into acetate and choline.
  $ext{AChE} <br>ightarrow ext{acetate} + ext{choline}$

• Recycling:
  Choline is recycled back into the presynaptic neuron through a high-affinity choline transporter (CHT1).

Sources of Choline

• Main Sources:

  • Dietary Phosphatidylcholine (commonly found in lecithin) and Free Plasma Choline are the primary sources utilized for synthesizing ACh.

• Blood-Brain Barrier Transporters:

  • CTL1 (SLC44A1): Intermediate-affinity, high-capacity, Na⁺-independent transporter.

  • CTL2 (SLC44A2): Low-affinity transporter with a mitochondrial and excretory role, Na⁺-independent.

  • MFSD7c: Facilitates choline export from the brain and regulates its levels.

  • Organic Ion Transport (OCT).

ACh Synthesis Process

ACh synthesis involves several steps. The acetate anion combines with choline in the presence of Acetyl-CoA, catalyzed by ChAT. This synthesis can be summarized as follows:

• Choline and Acetyl-CoA act as substrates.

• The reaction releases coenzyme A, forming Acetylcholine:
  $ext{Choline} + ext{Acetyl-CoA} <br>ightarrow ext{ACh}$

Vesicular Packaging

• Transport into Vesicles:

  • Mediated by VAChT, which is part of the SLC18 family. It utilizes the proton (H⁺) electrochemical gradient created by the V-type H-ATPase on synaptic vesicles.

  • For every ACh molecule loaded, protons are exchanged ( ext{H}^+/ACh antiport).

  • VAChT can be blocked by vesamicol, preventing vesicular loading without affecting ACh synthesis.

ACh Catabolism

• Breakdown Mechanism:

  • ACh is broken down into acetate and choline by AChE, and the reuptake process then allows choline to return to the presynaptic terminal for recycling.

Reuptake Process

• The reuptake mechanism involves the following components:

  • Choline and acetate efforts (via AChE, nAChR, and mAChR) allow efficient recycling back into the neuronal terminal and maintain synaptic efficiency.

Acetylcholine Receptors

ACh receptors are categorized into two primary classes: nicotinic receptors (nAChRs) and muscarinic receptors (mAChRs).

Nicotinic Receptors (nAChRs)

• Structure and Function:

  • Ionotropic, forming pentameric cation channels leading to fast (1-5 ms) Na⁺ and K⁺ flux.

  • Locations include the neuromuscular junction, autonomic ganglia, and CNS (notable subtypes include α4β2 and α7).

  • Roles: Muscle contraction, attention, and reward processing.

Muscarinic Receptors (mAChRs)

• Structure and Function:

  • Metabotropic GPCRs (G protein-coupled receptors) divided into M1–M5 subtypes.

  • Subtypes M1, M3, M5 trigger excitatory Gq pathways, while M2 and M4 engage inhibitory Gi pathways.

  • Locations: CNS, heart (M2), and smooth muscle (M3), modifying heart rate, cognitive functions, and parasympathetic activity.

Structural Features of ACh Receptors

• General Features:

  • Pentameric “Cys-loop” receptor structure contributes to a central ion pore.

  • Each subunit has an extracellular domain for ligand-binding and four transmembrane helices (M1–M4).

  • The cytoplasmic loop regulates trafficking and phosphorylation, and the M2 helices form the ion gate which undergoes conformational change during activation.

  • Agonist binding occurs at α-β or α-α interfaces, key to initiating receptor activity.

Activation and Desensitization Mechanisms

• Activation Process:

  • Two ACh molecules bind at α–β interfaces, leading to conformational changes that open the hydrophobic gate of the ion channel, allowing cation passage (Na⁺, K⁺, Ca²⁺).

  • The result is rapid depolarization within 1-5 ms.

• Desensitization:

  • Following activation, the channel may return to a closed state or enter a desensitized state, which is ligand-bound yet non-conducting.

Pathologies Related to ACh Dysfunction

• Nicotinic Receptor Pathologies:

  • Myasthenia Gravis: Autoantibodies target nAChRs at the NMJ, resulting in muscle weakness.

  • Congenital Myasthenic Syndromes: Genetic mutations impair nAChR channel function.

  • Cognitive Disorders: Alterations in the α4β2 or α7 subunits can lead to dysregulated neuronal excitability.

  • Nicotine Addiction: Chronic nicotine intake up-regulates nAChRs but causes receptor functional desensitization.

  • Neurodegeneration: Loss of α7 nAChRs is associated with cognitive decline in Alzheimer's disease.

• Muscarinic Receptor Pathologies:

  • Alzheimer’s Disease: Loss of cholinergic neurons leads to decreased M1 signaling.

  • Schizophrenia: Reduced expression of M1/M4 receptors affects dopaminergic activity.

  • Parkinson’s Disease: Overactivity of M1 receptors causes movement disorders.

  • Autonomic Dysfunction: Defects in M2/M3 receptors can lead to various organ system dysfunctions.

  • Poisoning from Organophosphates: Can lead to hyperstimulation of muscarinic receptors, causing severe autonomic crises.

Cholinergic Tone and Behavioral Aspects

• Interplay between reward and aversion systems regulated by cholinergic tone, with pathways through the lateral habenula (LHb) and ventral tegmental area (VTA) contributing to dopaminergic signaling and motivational outputs.

Nicotine and Addiction Mechanism

• Activation:

  • Nicotine stimulates nAChRs on dopamine neurons within the VTA, leading to dopamine release and reinforcing behaviors associated with reward.

• Neuroadaptation:

  • Chronic exposure prompts nAChR up-regulation while inducing functional desensitization, affecting the cholinergic tone.

• Genetic and Clinical Factors: Variants in CHRNA5/A3/B4 genes can increase susceptibility to addiction and may manifest in associated attention or anxiety disorders.

Therapeutic Approaches

• Cholinesterase Inhibitors:

  • Drugs like Donepezil, Rivastigmine, and Galantamine increase ACh availability and are documented for treating Alzheimer's disease and investigating other conditions like stroke.

• ACh Receptor Agonists/Modulators:

  • Direct receptor stimulation is achieved through nicotine or other ligands, selectively targeting various receptor subtypes.

• ACh Release Inhibitors:

  • Utilized for therapeutic interventions (e.g., botulinum toxin) to reduce muscle overactivity in dystonia or spasticity.

Future Directions in ACh Research

• Agonists for α7 nAChRs:

  • Potential to reduce neuroinflammation and improve outcomes in neurodegenerative disorders.

• M1/M4 Modulators:

  • Aims to restore balance of neurotransmission in Alzheimer's and schizophrenia.

• Vagus Nerve Stimulation:

  • Engaging cholinergic pathways for recovery in stroke or traumatic brain injury.

• RNA-based Regulation:

  • Use of microRNAs like miR-132 and miR-124 for fine-tuning AChE expression.

Summary of Key Points

• The identification of acetylcholine as the first neurotransmitter brought significant advances in neuroscience.

• ACh functions in rapid synaptic transmission and modulatory pathways influencing cognition and behavior.

• Its synthesis, release, degradation, and associated pathologies underline its importance in various neurological disorders.

• Interventions targeting the cholinergic system open avenues for new therapeutic mechanisms.

Questions for Further Exploration

1. How does ACh signaling compare between the CNS and PNS?

2. What are the desensitization and up-regulation mechanisms associated with chronic nicotine exposure?

3. What therapeutic benefits can arise with selective modulation of α7 or M1/M4 receptors in neurodegenerative diseases?

4. How might future treatments utilize RNA-based regulation or accessory proteins like VILIP-1?

Sources

• Ojiakor, O., & Rylett, R. J. (2020). Choline transport and acetylcholine synthesis in the nervous system. Neurochemistry International, 136, 104714.

• Dani, J. A. (2015). Neuronal nicotinic acetylcholine receptor structure and function. Neuron, 86(5), 901-914.

• Purves, D., et al. (2017). Neuroscience (6th ed.). Oxford University Press.

• Winek, K., Soreq, H., & Meisel, A. (2021). Regulators of cholinergic signaling in disorders of the central nervous system. Journal of Neurochemistry, 158(6), 1425–1438. https://doi.org/10.1111/jnc.15332

• Muscarinic acetylcholine receptors: novel opportunities for drug development (Nature Reviews Drug Discovery, 2014).