Neurotransmission and Acetylcholine Metabolism Notes

Introduction to Neurotransmission

• Neurotransmitters bind to receptors on the neuron's membrane and open ion channels.
• This causes an influx of ${\text{Na}}^+$ ions, making the inside of the cell less negative, which disrupts the resting potential of the target cell.
• If the membrane potential remains negative after initial ion influx, the effect is transient; however, if enough ${\text{Na}}^+$ enters and the membrane potential reaches a threshold, it rapidly depolarizes.
• If the cell hyperpolarizes (becomes more negative than resting potential, e.g., from $V_{rest} \approx -70\ \mathrm{mV}$ to a more negative value), it makes an action potential less likely.
• Conversely, if the cell depolarizes significantly (shifts toward a more positive value from $V_{rest}$), it can generate an action potential.
• An action potential is a rapid, all-or-none electrical signal that travels as a current along the neural membrane and triggers a physiological response (e.g., excitation of the next neuron in a pathway).
• Key concept: transmission requires propagation of an electrical signal to a synapse, chemical signaling across the cleft, and a postsynaptic response.

Excitatory vs Inhibitory Neurotransmitters

• Excitatory neurotransmitters include: $\text{ACh}, \text{aspartate}, \text{dopamine}, \text{histamine}, \text{norepinephrine}, \text{epinephrine}, \text{glutamate}, \text{serotonin (5-HT)}$.
• Neurons that release excitatory transmitters tend to raise the likelihood of an action potential in postsynaptic neurons by causing depolarization, primarily through the opening of ${\text{Na}}^+$ channels, which allows positive ions to enter the cell.
• Cortical pyramidal neurons are excitatory and project locally or over long distances within cortical areas.
• Glutamate is the main excitatory neurotransmitter in the central nervous system.
• Excitatory transmission causes depolarization of postsynaptic neurons, enabling unidirectional or bidirectional information flow and activation of ${\text{Na}}^+/{\text{K}}^+$ channels in the postsynaptic neuron.
• Inhibitory neurotransmitters include: $\text{glycine}, \text{taurine}, \text{GABA}$ .
• Inhibitory neurons prevent or reduce postsynaptic firing by making depolarization less likely, often by opening chloride $({\text{Cl}}^-)$ channels, allowing negative ions to enter, or potassium $({\text{K}}^+)$ channels, allowing positive ions to exit, thus hyperpolarizing the cell or stabilizing the resting potential.
• Inhibitory neurons tend to project within localized regions and regulate excitatory activity.
• Inhibitory signaling commonly involves opening chloride channels (e.g., GABA_A receptors).
• Inhibitory neuron types include: stellate, chandelier, basket cells.

Stages in Neurotransmission (Resting, Excitation, Termination)

• Stage 1: Resting
  • Presynaptic and postsynaptic membranes maintain a resting potential around $V_{rest} \approx -70\ \mathrm{mV}$ (inside is more negative than outside), maintained by the ${\text{Na}}^+/{\text{K}}^+$ ATPase pump and differential ion permeability.
  • The diagrammatic representation shows a resting presynaptic terminal with vesicles and a postsynaptic receptor channel awaiting activation.
• Stage 2: Excitation
  • An action potential arrives at the presynaptic terminal, causing the rapid opening of voltage-gated ${\text{Ca}}^{2+}$ channels.
  • ${\text{Ca}}^{2+}$ influx triggers fusion of synaptic vesicles with the presynaptic membrane and release of neurotransmitter into the synaptic cleft.
  • The neurotransmitter binds to a ligand-gated (chemically gated) receptor on the postsynaptic membrane, opening ion channels and allowing ${\text{Na}}^+$ influx, leading to local depolarization.
  • If this local depolarization reaches a threshold, it triggers rapid opening of voltage-gated ${\text{Na}}^+$ channels, generating an Excitatory Postsynaptic Potential (EPSP) that can lead to an action potential propagating downstream. Repolarization then occurs due to the opening of voltage-gated ${\text{K}}^+$ channels.
• Stage 3: Termination
  • Neurotransmitters are cleared or degraded to terminate signaling and allow the postsynaptic neuron to recover.
  • In the cleft, acetylcholine is rapidly degraded by acetylcholinesterase; serotonin and catecholamines (dopamine, norepinephrine) can be reuptaken into the presynaptic neuron or glial cells for degradation (e.g., by MAO or COMT enzymes).
  • Termination mechanisms also include inactivation of voltage-gated ${\text{Na}}^+$ channels, the absolute and relative refractory periods, and reuptake/enzymatic clearance of neurotransmitters at synapses.

Selective Serotonin Reuptake Inhibitors (SSRIs)

• SSRIs are a class of medicines used to treat depression and are often first-line pharmacotherapy for depression and other psychiatric disorders.
• Mechanism: SSRIs inhibit reuptake of serotonin from the synaptic cleft back into the presynaptic neuron by blocking the serotonin transporter (SERT), thereby increasing serotonin availability at the postsynaptic receptors.
• Clinical notes: generally considered safe, efficacious, and tolerable. They are also used to treat anxiety disorders, obsessive-compulsive disorder (OCD), and panic disorder, though their therapeutic effects may take several weeks to manifest.

Glial Cells: Astrocytes, Microglia, Oligodendrocytes versus Multiple Sclerosis

• Glial cells support and maintain normal nervous system physiology, outnumbering neurons in some brain regions.
• Oligodendrocytes
  • Produce fatty myelin sheaths that insulate axons in the central nervous system.
  • Myelin increases the speed and efficiency of action potential propagation through saltatory conduction.
  • Loss or dysfunction of axonal connections (demyelination) can contribute to Multiple Sclerosis (MS), which causes movement and balance disruptions as well as vision problems due to impaired nerve signal transmission.
• Astrocytes
  • Are star-shaped glial cells that are integral parts of synapses and regulate molecules needed for inter-neuron communication.
  • Clear neurotransmitters like glutamate from the synapse to terminate signaling and prevent excessive transmission, which can be neurotoxic.
  • Release neural growth factors that support neuron maintenance and repair.
  • Can uptake monoamine neurotransmitters (e.g., serotonin, dopamine) for clearance, contributing to neurotransmitter homeostasis.
• Microglia
  • Are the brain’s resident immune cells, acting as macrophages to fight infections and clear cellular debris.
  • Respond to injury by changing morphology and releasing inflammatory mediators that can damage neurons if overactivated or chronically stimulated.
  • Microglia activity is linked to neuropathic pain when hyperstimulatory signals affect neighboring neurons, leading to chronic sensitization.
• Rasagiline (a MAO-B inhibitor) can modulate microglia and astrocyte dopamine metabolism by inhibiting monoamine oxidase B, helping maintain dopamine levels in Parkinson's disease by reducing its breakdown.

Neuron-Glial Cell Association (Overview)

• Neurons function in close association with glial cells; astrocytes, microglia, and oligodendrocytes collectively influence neuronal signaling, metabolism, and protection.
• Glial cells contribute to neurotransmitter clearance, neuroinflammation regulation, myelination, and metabolic support, forming a crucial neuroglial network for brain function.

Role of Microglia Activation (Additional Notes)

• Microglia respond to injury or disease by activating inflammatory pathways, potentially contributing to neuronal damage if chronically activated in conditions such as neurodegenerative diseases.
• Therapeutic strategies may target microglial MAO-B or inflammatory pathways to protect neurons (e.g., rasagiline in PD, which reduces oxidative stress and neuroinflammation).

Role of Acetyl CoA in Metabolism

• Acetyl-CoA links various metabolic pathways and serves as a key two-carbon donor in multiple processes:
  • Tricarboxylic Acid (TCA) cycle entry and energy production within mitochondria.
  • Transacetylation reactions in various biosynthetic pathways.
  • Isoprenoid synthesis, precursors for cholesterol and steroids.
  • Fatty acid synthesis, crucial for lipid production.
  • Cholesterol synthesis, essential for cell membranes and steroid hormones.
  • Neurotransmitter synthesis (as a donor of acetyl groups in acetylcholine production).

Synthesis and Degradation of Acetylcholine (ACh) and Inhibitors of Acetylcholinesterase and Acetylcholine Receptors

• Synthesis of acetylcholine

  • Enzyme: Choline acetyltransferase (ChAT) catalyzes the transfer of an acetyl group from acetyl-CoA to choline in the cytoplasm of cholinergic neurons.
  • Chemical reaction:

  ${\text{Choline}} + {\text{Acetyl-CoA}} \xrightarrow{\text{ChAT}} {\text{Acetylcholine}} + {\text{CoA}}$

• Packaging and release

  • Acetylcholine is packaged into synaptic vesicles by a vesicular ACh transporter (VAChT) in the presynaptic neuron.
  • Upon nerve stimulation (action potential arrival and ${\text{Ca}}^{2+}$ influx), ACh is released into the synaptic cleft via exocytosis and binds to postsynaptic receptors, triggering ${\text{Na}}^+/{\text{K}}^+$ channel opening and postsynaptic depolarization.

• Termination of action

  • Acetylcholine is rapidly hydrolyzed in the cleft by acetylcholinesterase (AChE) into choline and acetate, quickly ending the signal.
  • Chemical reaction:

  ${\text{Acetylcholine}} + {\text{H}}_2{\text{O}} \xrightarrow{\text{AChE}} {\text{Choline}} + {\text{Acetate}}$

  • AChE turnover is extremely fast to allow rapid recovery from depolarization, ensuring precise control over cholinergic signaling.

Inhibitors of Acetylcholinesterase

• Physostigmine and neostigmine inhibit AChE by carbamoylating the active site serine residue, forming a temporary, reversible covalent bond that slows ACh hydrolysis.
• DIPF (diisopropyl phosphofluoridate) is an organophosphate that reacts irreversibly with the active site serine, covalently modifying AChE and leading to its permanent inactivation (unless reactivated by an oxime, like pralidoxime).
• Receptor antagonism at nicotinic acetylcholine receptors occurs with agents such as tubocurarine (a competitive antagonist) and alpha-bungarotoxin (and cobra toxins), which bind tightly and competitively with acetylcholine at the receptor binding site, thereby blocking ACh's effects.
• These inhibitors and antagonists are important pharmacological tools for studying cholinergic systems and have clinical relevance in treating diseases (e.g., myasthenia gravis, Alzheimer's disease) or causing toxicity (e.g., nerve agents, pesticides).

Summary and Key Takeaways

• Neurotransmitters are broadly classified as excitatory (e.g., glutamate, ACh, dopamine, norepinephrine, epinephrine, serotonin, histamine, aspartate) and inhibitory (e.g., GABA, glycine, taurine).
• Excitatory transmitters promote action potential firing by depolarizing postsynaptic membranes; inhibitory transmitters dampen neuronal activity by hyperpolarizing or stabilizing the resting potential.
• Nerve transmission proceeds through three stages: resting, excitation (involving ${\text{Ca}}^{2+}$ influx and neurotransmitter release), and termination (via enzymatic degradation or reuptake), each with distinct voltage dynamics and clearance mechanisms.
• Serotonin reuptake inhibition by SSRIs increases serotonergic signaling in the synaptic cleft and is a cornerstone of depression treatment, with a favorable safety/effectiveness profile, also used for anxiety and OCD.
• Glial cells (astrocytes, microglia, oligodendrocytes) play critical roles in neurotransmitter clearance, immune defense, myelination, and metabolic support; dysregulation can contribute to disease states such as MS (demyelination) and PD (neuroinflammation and dopamine metabolism disruption).
• Acetyl-CoA serves as a central metabolic branch point, feeding energy metabolism and the synthesis of acetylcholine, isoprenoids, fatty acids, and cholesterol.
• Acetylcholine synthesis uses ChAT to transfer an acetyl group from acetyl-CoA to choline; ACh is rapidly degraded by AChE in the synaptic cleft (into choline and acetate), enabling quick termination of the signal.
• Pharmacological targets include AChE inhibitors (e.g., donepezil, galantamine, rivastigmine for Alzheimer's) and SSRIs, illustrating how therapeutic manipulation of neurotransmitter signaling can treat neurological and psychiatric disorders.
• Understanding these processes helps explain normal nervous system function and the basis for treating neurological and psychiatric disorders.