Indirect Mechanisms of Synaptic Transmission Notes

Indirect Mechanisms of Synaptic Transmission

• Involve metabotropic receptors and second messenger cascades, leading to slower but longer-lasting effects compared to ionotropic receptors.

1. Ionotropic vs. Metabotropic Transmission

• Ionotropic Receptors:
  • Mediate direct gating (fast transmission).
  • Are ion channels themselves.
• Metabotropic Receptors:
  • Mediate indirect gating (slow transmission).
  • Trigger second messenger cascades.
  • Receptors are not channels.
• Properties of Indirect Synaptic Transmission:
  • Similar to direct synaptic transmission on the presynaptic side.
  • Receptors differ; they are not channels.
  • Slow synaptic transmission can be the only form of transmission in some synapses.
  • In some synapses, both fast and slow synaptic transmission occur, with the slow transmission modulating the fast transmission.

Two Postsynaptic GABA Effects: Fast and Slow IPSPs

• Fast IPSP:
  • Mediated by chloride (Cl) ions.
  • Involves GABA_A receptors, which are ligand-gated receptor/channels.
• Slow IPSP:
  • Mediated by potassium (K^+) ions.
  • Involves GABA_B receptors, which are G protein-coupled receptors.
  • GABA{R1} and GABA{R2} are components of the GABA_B receptor.
  • G proteins act on K^+ or Ca^{2+} channels.

2. Structure and Function of G Protein-Coupled Receptors

• G protein-coupled receptors have 7-transmembrane regions.
• Extracellular Side:
  • NH_2 terminus.
• Cytoplasmic Side:
  • COOH terminus.
  • G protein-binding domain.
  • Phosphorylation sites for B-Adrenergic receptor kinase and cAMP-dependent protein kinase.

Neurotransmitters Binding to G Protein-Coupled Receptors

• Many different neurotransmitters can bind to G protein-coupled receptors:
  • mAChR (Acetylcholine)
  • GABA_B (GABA)
  • Adrenergic (Norepinephrine)
  • Peptidergic (Peptides)
  • mGluR (Glutamate)
  • Odorant Receptors (smelly stuff)
• Acetylcholine:
  • Muscarinic: Metabotropic receptors named after muscarine, which elicits a large response (inhibits heart rate by opening K^+ channels, leading to hyperpolarization).
  • Nicotinic: Ionotropic receptors named after nicotine, which elicits a large response (neuromuscular junction, excitation).
• Localization at the synapse:
  • Ionotropic receptors are usually found at the center of the synapse.
  • Metabotropic receptors are generally found in the perisynaptic regions or even presynaptically.

Synaptic Second Messenger Systems

• Share a common scheme.
• Targets: Voltage and ligand-gated channels, vesicle proteins, enzymes.

3. Metabotropic Receptors Acting Through G-Proteins

• Desensitization of receptors: Phosphorylation of the receptor by G-protein-coupled receptor kinase (GRK), binding of arrestin prevents interaction with G proteins.
• GDP (guanosine diphosphate) is involved in G-protein activation.

Activation of a G-Protein Coupled Receptor

• Site of Action:
  • Presynaptic: Affect the amount of transmitter released.
  • Postsynaptic: Affect ionotropic receptors and postsynaptic potential.
  • Cell soma: Affect the excitability of the cell.

4. Primary Effectors of G-Proteins

• G protein subunits can directly modulate ion channels.
• Muscarinic AChR in the heart causes hyperpolarization that is mediated by the direct effect of G protein on K^+ channels.
  • ACh slows the heart.

ACh and Potassium Channels

• ACh increases the open probability of K^+ channels.
• Known as the M-current, characterized by Bertil Hille.

G Protein Inhibition of Calcium Channels

• Auto-inhibition of neurotransmitter release by alpha-2 adrenergic receptors (Lipscombe et. al. 1989).
• Norepinephrine blocks Ca^{2+} channel activity.

G Protein Activation of Cytoplasmic Second Messenger Systems

• Involves a receptor, transducer, and primary effector.
• Cytoplasmic side:
  • Second messengers act on secondary effectors like ion channels and kinases.
  • Can affect gene expression.

The cAMP System

• Example: Fight or flight response
  • Norepinephrine binding to β-adrenergic receptors on cardiac muscle cells causes an increase in the rate and force of contraction of the heart.
  • Cardiac action potential is mediated by voltage-dependent Ca^{2+} channels.

Examples of G Protein Activation of Cytoplasmic Second Messenger Systems

• β-adrenergic receptors activate calcium channels via a G protein-adenylyl cyclase (AC) pathway.
• Adenylyl Cyclase converts ATP to cAMP, which then activates Protein Kinase A (PKA).
• PKA phosphorylates calcium channels causing their activation.
• Agonist does not need to be added to the patch pipette.
• Amplification in signal transduction pathways.

Membrane Phospholipids as Second Messengers

• G-protein activation of phospholipase C.
• Glutamate can also activate this pathway.
• Slower, but longer-lasting effects.
• Second messenger cascades can activate transcription factors, which can alter gene expression.

Calcium as an Intracellular Second Messenger

• Intracellular levels of calcium are tightly regulated (10-100 nM) by:

  • Regulating entry and exit through the plasma membrane.
  • Release from and uptake into intracellular stores.
  • Ca^{++} binding proteins.

• Calcium is spatially restricted.

• Calcium-activated K^+ channel.

• Different signals can cause different spatiotemporal patterns of calcium changes in the cell.

• Different effectors will be activated depending on the spatiotemporal pattern of the calcium transient.

GABAergic Interneurons and Calcium Waves

• Immature neurons have depolarizing GABA responses, while mature neurons have hyperpolarizing GABA responses.
• KCC2 and NKCC1 are chloride co-transporters involved in establishing chloride gradients.
• High intracellular chloride ([Cl^-]_i) leads to depolarizing GABA.
• Development and pathologies can affect these processes (Ben-Ari, 2021).

Zinc Ions as Neuronal Messengers

• Zinc ions are neuronal messengers co-released with glutamate that bind to mZnR/GPR39 (Sunuwar et al. 2017, Hershfinkel 2023).
• mZnR/GPR39 activation causes intracellular Ca^{++} release, which enhances KCC2 activity.

KCC2 Activity and Chloride Concentration

• KCC2 activity results in a decrease in [Cl^-]i, leading to stronger inhibitory drive and a more negative E{Cl}.
• Synaptically released Zn^{++} triggered increases in KCC2, which could be a negative feedback mechanism to limit seizures in epilepsy (Sunuwar et al. 2017).

Nitric Oxide Signaling

• Nitric Oxide Synthase makes Nitric Oxide (NO), a retrograde neurotransmitter.
• NO diffuses freely, activating Guanylate Cyclase, which makes cGMP.
• cGMP activates Protein Kinase G, which can phosphorylate Ca^{++} channels, reducing Ca^{++} currents, leading to decreased neurotransmission and blood vessel relaxation.

Summary of G-Protein Signaling Pathways

• Slow, indirect transmission can modulate neuronal function.
• Main players: 7 transmembrane receptors, G proteins, effector proteins, second messengers (cAMP, IP3, Ca^{++}).
• Short and long-term effects.