Wk5 L12,13,14,15 - Ion Channels

What are GPCRs and their role in neurotransmission?

GPCRs (G-protein coupled receptors) are integral membrane proteins that participate in slow synaptic transmission and long-lasting modulation of neuronal excitability. They play critical roles in sensory perception, hormone release, and neurotransmission.

What is the general function of GPCRs?

GPCRs activate associated G-proteins upon ligand binding, which modulate ion channels or enzymes to trigger cellular responses, leading to changes in neuronal excitability, gene expression, and cellular metabolism.

Describe the structure of GPCRs.

GPCRs are composed of a single polypeptide chain that spans the membrane seven times, forming seven transmembrane helices.

What are the key G-protein subunits?

G-proteins consist of three subunits: Alpha (α): 20 isoforms, Beta (β): 4 isoforms, Gamma (γ): 7 isoforms.

How are G-proteins activated and inactivated?

G-proteins are activated when bound to GTP and inactivated when bound to GDP. Upon receptor activation, GDP is exchanged for GTP, leading to dissociation of the Gα subunit from the βγ dimer.

What is signal amplification in GPCR signaling?

GPCRs amplify signals by activating multiple G-proteins from a single ligand binding event, producing robust and prolonged signaling effects.

What are the main intracellular messengers in GPCR signaling?

The main intracellular messengers are:

• cAMP: Activates PKA, regulates ion channels.

• cGMP: Regulates smooth muscle relaxation and visual signaling.

• Calcium (Ca²⁺): Activates calmodulin and CaMKs, influencing processes like muscle contraction and gene expression.

How are cAMP, cGMP, and calcium regulated?

• cAMP increases via adenylyl cyclase and is degraded by phosphodiesterase (PDE).

• cGMP is produced by guanylyl cyclase and degraded by PDE.

• Calcium levels increase through voltage-gated or ligand-gated channels and are pumped out by calcium pumps.

How does cAMP function as a second messenger?

cAMP activates Protein Kinase A (PKA), which phosphorylates target proteins, affecting metabolism, gene expression, and ion channel regulation.

What is the role of calcium as a second messenger?

Calcium ions activate calmodulin and CaMKs, influencing processes such as neurotransmitter release, muscle contraction, synaptic plasticity, and gene expression.

How does light transduction in photoreceptors involve GPCRs?

Light absorption triggers a conformational change in rhodopsin, leading to a cascade that affects Na⁺/Ca²⁺ conductance, amplifying the signal and resulting in hyperpolarization of the photoreceptor.

What are the effects of GPCR subtypes, such as the muscarinic acetylcholine receptors?

Muscarinic receptors (M1–M5) have distinct signaling pathways. For example, M2 receptors inhibit adenylyl cyclase (via Gαi) and affect heart rate, while M1 receptors activate PLC (via Gαq) and influence neuronal excitability.

What role do protein kinases play in GPCR signaling?

Activation of GPCRs often leads to the activation of protein kinases, which phosphorylate target proteins on serine, threonine, or tyrosine residues, modulating their functions.

What is the process of receptor desensitisation?

GRKs phosphorylate GPCRs after prolonged ligand binding, recruiting arrestins, which prevent further G-protein activation and mediate receptor internalization, reducing sensitivity.

How do mutations in GPCRs contribute to diseases?

Mutations in GPCRs or G-proteins can drive disease progression, such as cancer, cardiovascular diseases (hypertension, heart failure, arrhythmias), and neurodegenerative diseases (Parkinson's, Alzheimer's, Huntington's).

What is the difference between GPCR agonists and antagonists?

• Agonists activate GPCRs (e.g., dopamine for dopamine receptors).

• Antagonists block GPCR activation (e.g., beta-blockers for β-adrenergic receptors in heart disease).

What is biased agonism in GPCR pharmacology?

Biased agonism refers to activating specific signaling pathways without affecting others, which can be useful for designing drugs with fewer side effects.

How do allosteric modulators affect GPCRs?

Allosteric modulators bind to sites other than the active site, modifying receptor activity and offering a way to fine-tune signaling responses without directly activating or blocking the receptor.

What are the three types of chemical signaling?

• Synaptic signaling: Communication between neurons via neurotransmitters at synapses.

• Paracrine signaling: Involves secretion of chemicals onto nearby target cells or nerve endings.

• Endocrine signaling: Hormones are secreted into the bloodstream to affect target cells throughout the body.

What are the major intracellular messengers involved in signal transduction?

• Cyclic AMP (cAMP)

• Cyclic GMP (cGMP)

• Calcium ions (Ca²⁺)

• Inositol phosphates (IP₃)

• Diacylglycerol (DAG)

How are IP₃ and Diacylglycerol formed and what do they do?

• Phosphatidylinositol bisphosphate (PIP₂) is cleaved by phospholipase C, producing:

  • Inositol trisphosphate (IP₃): Mobilizes Ca²⁺ from the ER.

  • Diacylglycerol (DAG): Activates protein kinase C (PKC).

• Phospholipase C is activated by G-proteins and Ca²⁺.

What are the effects of activating phospholipase C?

• IP₃ releases Ca²⁺ from the endoplasmic reticulum, activating PKC and Ca²⁺-regulated signals, including ion channels.

• DAG activates protein kinase C (PKC).

What role do second messengers play in neuronal signaling?

Second messengers like cAMP, cGMP, IP₃, DAG, and Ca²⁺ amplify and propagate signals inside the cell, contributing to cellular responses to neurotransmitters.

What factors influence a neuron's response to neurotransmitters?

• Receptor location

• Receptor type and quantity

• Receptor subunit composition

• Intracellular messengers

• Kinases and phosphatases

• Target proteins available

Why is neuronal signaling complex?

• Amplification: Small stimuli can lead to large responses.

• Multiple levels of control: Ensures precise regulation.

• Pathway interactions: Different signaling pathways influence each other.

How does light transduction work in photoreceptors?

• Light activates rhodopsin, triggering a cascade involving transducin and phosphodiesterase (PDE).

• This process leads to the breakdown of cGMP, closing ion channels and hyperpolarizing the cell.

What happens during long-term responses to GPCR activation?

Prolonged activation can lead to changes in gene expression through the activation of transcription factors, resulting in lasting changes in cellular function.

How does CREB regulate gene transcription?

When CREB is phosphorylated, it binds to CRE sites on DNA, enhancing transcription of genes like c-Fos, BDNF, and neuropeptides, contributing to long-term synaptic plasticity.

What are the different categories of cellular receptors?

• Channel-linked receptors (ionotropic): Ligand-gated ion channels.

• G-protein-coupled receptors (GPCRs): Metabotropic receptors.

• Enzyme-linked receptors:

  • Tyrosine kinase receptors: Activated by growth factors.

  • Guanylyl cyclase receptors: Increase cGMP levels.

• Intracellular receptors: For lipophilic molecules like steroid hormones.

What is the structure of a Tyrosine Kinase Receptor?

• Extracellular domain: Binds ligands.

• Intracellular C-terminal region: Contains tyrosine kinase domain responsible for intrinsic kinase activity.

How does NGF activate tyrosine kinase receptors?

• NGF binds to TrkA receptors, causing receptor dimerization.

• This leads to auto- and cross-phosphorylation of tyrosine residues, recruiting adapter proteins and activating key pathways like Ras-MAPK, PI3K-Akt, and PLCγ-PKC.

How does NGF influence pain perception?

NGF sensitizes sensory neurons through pro-inflammatory mediators (e.g., prostaglandins, histamine), increasing pain response and leading to hyperalgesia.

What is the role of Nitric Oxide (NO) in signaling?

• NO is a small gas that acts as a neurotransmitter, activating soluble guanylyl cyclase and producing cGMP.

• It can diffuse across cell membranes, influencing nearby cells and affecting processes like blood vessel dilation.

How is arachidonic acid involved in signalling?

Arachidonic acid is derived from membrane phospholipids and can be converted into signalling molecules that regulate inflammation, often through activation of phospholipase A₂.

What are the long-term effects of receptor activation on gene expression?

Intracellular messengers alter the phosphorylation of transcription factors, leading to changes in gene expression and long-lasting modifications in neuronal function.

What are amino acid neurotransmitters and their role in synaptic transmission?

Amino acid neurotransmitters are essential for synaptic transmission in the CNS.

• They include glutamate, GABA, and glycine, each playing a role in excitatory or inhibitory signaling.

What is the role of Glutamate in the CNS?

Glutamate is the major excitatory neurotransmitter in the mammalian CNS and is responsible for most fast excitatory synaptic transmission.

What is the function of GABA in the CNS?

GABA is the major inhibitory neurotransmitter in the CNS, regulating neuronal excitability by inhibiting neuron firing.

What is the role of Glycine in neurotransmission?

Glycine is the major inhibitory neurotransmitter in the brainstem and spinal cord, involved in inhibitory signaling.

How do Glutamate and GABA regulate neuronal excitability?

Glutamate facilitates excitatory transmission, while GABA inhibits neuronal activity, balancing overall neuronal excitability in the brain.

How is Glutamate synthesised?

Glutamate is synthesised from glucose metabolism, with intermediates from glycolysis and the Krebs cycle. It also serves as a precursor to GABA in inhibitory neurons.

How is Glycine synthesized?

Glycine is synthesized from serine by the enzyme serine hydroxymethyltransferase (SHMT) and is primarily used as an inhibitory neurotransmitter in the brainstem and spinal cord.

Describe Glutamate neurotransmission and release.

Glutamate is synthesized locally from glucose or recycled from glutamine in mitochondria. It is transported into synaptic vesicles via the VGLUT transporter and released via calcium-dependent exocytosis.

How is Glutamate removed from the extracellular space?

Glutamate is removed by excitatory amino acid transporters (EAATs), which co-transport it with Na+ into glial cells or neurons. It is converted to glutamine by glutamine synthetase in glial cells.

What is Glutamine recycling?

Glutamine, formed in glial cells, is transported back into neurons via the SAT2 transporter for recycling into glutamate.

What are Ionotropic Glutamate Receptors?

Ionotropic glutamate receptors include AMPA, NMDA, and Kainate receptors.

• They mediate fast excitatory synaptic transmission, with specific roles in ion flux and synaptic plasticity.

What is the function of AMPA receptors?

AMPA receptors are permeable to Na+ and K+ ions and mediate fast excitatory transmission. They consist of GluR1-4 subunits, and their activation results in rapid depolarization of the postsynaptic membrane.

What is the function of NMDA receptors?

NMDA receptors are permeable to Na+, K+, and Ca²⁺ ions and play a critical role in synaptic plasticity, learning, and memory. They require glycine as a co-agonist and are blocked by magnesium at negative membrane potentials.

What makes NMDA receptors unique?

NMDA receptors are both agonist- and voltage-sensitive. Magnesium blocks the receptor at negative membrane potentials, and depolarization removes this block to allow ion influx.

What is the role of GluR2 subunit in AMPA receptors?

The GluR2 subunit of AMPA receptors determines calcium permeability. AMPA receptors lacking GluR2 are permeable to Na+, K+, and Ca²+, while those with GluR2 limit calcium entry.

What are Kainate receptors?

Kainate receptors are similar to AMPA receptors but less widely distributed. They are less common in the CNS and mediate excitatory synaptic transmission.

What are metabotropic glutamate receptors (mGluRs)?

mGluRs are G-protein coupled receptors that modulate signaling pathways. They are classified into three groups (I, II, III) based on their function, with Group I activating PLC and Groups II and III inhibiting cAMP production.

How do glutamate receptors contribute to EPSPs?

AMPA and NMDA receptors mediate excitatory postsynaptic potentials (EPSPs).

• When glutamate binds to these receptors, they open ion channels, leading to membrane depolarization and action potential generation.

What is the role of NMDA receptors in synaptic plasticity?

NMDA receptors play a key role in synaptic plasticity, which underlies learning and memory. The influx of Ca²⁺ through NMDA receptors triggers long-lasting effects on synaptic strength.

What is the typical postsynaptic current generated by glutamate?

The postsynaptic current consists of a fast component mediated by AMPA receptors and a slow component mediated by NMDA receptors. AMPA receptors mediate rapid depolarization, while NMDA receptors contribute to slower, sustained depolarization.

How is GABA synthesized?

GABA is synthesized from glutamate by the enzyme glutamic acid decarboxylase (GAD) in GABAergic neurons, requiring the co-factor pyridoxal phosphate (vitamin B6).

How is GABA neurotransmission regulated?

GABA is transported into synaptic vesicles by VIAAT and is removed from the synaptic cleft by GABA transporters (GATs), which return it to neurons and astrocytes.

What is the function of GABA and Glycine in IPSPs?

GABA and glycine mediate inhibitory postsynaptic potentials (IPSPs) by hyperpolarizing the postsynaptic membrane, reducing the likelihood of action potential generation.

What are the two main ways ions traverse the cell membrane?

• Transporter/pump proteins

• Ion channels that form pores through the membrane.

What is the selectivity of ion channels?

Many ion channels are highly selective for particular ions (e.g., K+, Na+, Cl-, Ca++).

Some ion channels are less selective and allow several types of ions to pass.

What role do K+ channels (Leak Channels) play in the cell?

They are involved in setting the resting membrane potential.

What is the function of voltage-gated ion channels?

Voltage-Gated Sodium Channels: Involved in the upstroke of the action potential.

Delayed Rectifier Potassium Channels: Involved in the downstroke and after-hyperpolarization of the action potential.

Voltage-Sensitive Calcium Channels (N-type): Mediate transmitter release from nerve terminals.

Calcium-Activated Potassium Channels: Responsible for long after-hyperpolarizations following some action potentials.

What are the classifications of ion channels?

Functional Basis: Measured by the ions they pass, conductance, voltage-dependence, etc.

Gene Families: Categorized based on the genes encoding their subunits.

What are some examples of ion channel gene families?

Ligand-Gated Ion Channels: 5-HT3, GABAA, Glycine, Glutamate (NMDA, Kainate, AMPA), Nicotinic ACh, P2X.

Voltage-Gated Ion Channels: Sodium, Calcium, Potassium, H+ channels.

Other Ion Channels: Aquaporins, Zinc-activated, TRPs.

What are the ligand-gated ion channels?

• Nicotinic Acetylcholine Receptors (nAChR): Excitatory; permeable to Na+ (and sometimes K+ and Ca++).

• GABAA Receptors: Inhibitory; permeable to Cl-.

• Glycine Receptors: Inhibitory; permeable to Cl-.

• 5-HT3 Receptors: Excitatory; permeable to Na+.

• Ionotropic Glutamate Receptors: NMDA, Kainate, AMPA – Excitatory; permeable to Na+, K+, and Ca++.

• P2X Receptors: Excitatory; permeable to Na+.

What is the structure of voltage-gated sodium channels and their subtypes?

Structure: 4 fused subdomains, each containing 6 transmembrane domains.

Subtypes: Nav1.1 to Nav1.9, each with distinct roles, especially in the nervous system.

Example: Nav1.7 – critical for pain perception. Mutation causes primary erythermalgia or loss of pain sensation (congenital insensitivity to pain).

What is the structure of voltage-gated calcium channels and their subtypes?

Structure: The α1 subunit with 4 fused subdomains, each with 6 transmembrane domains.

Types: L-type (smooth muscle), N-type, P/Q, R, and T-type.

Function: N-Type calcium channels are responsible for transmitter release at synapses.

What are Na+ and Ca++ ion channel drugs and their effects?

• Lidocaine (Lignocaine): Blocks Na+ channels, used as a local anesthetic and for neuropathic pain.

• Tetrodotoxin: Irreversible blocker of Na+ channels, used in laboratories.

• Ziconotide: Blocks N-type Ca++ channels, used for chronic pain relief.

What types of K+ channels exist and their functions?

• Voltage-Sensitive K+ Channels: Involved in delayed rectifier and transient outward currents.

• Calcium-Sensitive K+ Channels: Involved in long-after hyperpolarization in neurons.

• Tandem Pore Domain Channels: Set the resting membrane potential (leak channels).

• Inward Rectifying K+ Channels (Kir): Involved in inhibitory G-protein coupled receptor responses.

What are TRPv1 channels and how are they activated?

TRPv1 is a non-selective cation channel activated by capsaicin, heat, and other stimuli.

It passes Ca++, Na+, and K+ and plays a crucial role in pain sensation and temperature detection.

TRPv1 can be sensitized by bradykinin, prostaglandins, ATP, and NGF, which lead to phosphorylation.

How do mutations in ion channels lead to diseases? (SCN9A gene)

Mutations in SCN9A gene can cause:

• Primary Erythermalgia: Burning pain when warm.

• Paroxysmal Extreme Pain Disorder: Loss of channel inactivation, leading to chronic pain.

• Congenital Insensitivity to Pain: Loss of pain sensation due to mutation in Nav1.7.

What is the interaction between neurotransmission and ion channels?

• G-Protein Coupled Receptors (GPCRs): Trigger second messenger cascades modulating ion channels.

• Ionotropic Receptors (Ligand-Gated Ion Channels): Mediate fast synaptic potentials by forming their own pores in the membrane.

Why are ion channels important in pharmacology?

Ion channels are significant drug targets, especially GPCRs, which represent ~50% of all medical drug targets.