Week 7: Presynaptic Processes and Neurotransmitter Release

Presynaptic Processes: Neurotransmitters, Ca2+ channels, vesicular release, and presynaptic proteins

  • Purpose of notes: Provide a comprehensive, exam-ready synthesis of presynaptic neurotransmission from the provided transcript, with emphasis on definitions, mechanisms, components, and practical implications.

What is a neurotransmitter and why?

  • Neurotransmitters are chemical signals released from presynaptic nerve terminals into the synaptic cleft to affect a postsynaptic cell.

  • Historical framing: the “Soup vs Spark” controversy highlighted the chemical nature of signaling at synapses (as opposed to purely electrical transfer).

  • Key historical milestones:

    • John Eccles vs Henry Dale debate and Otto Loewi’s frog heart experiments (1921) demonstrated chemical transmission by vagal stimulation releasing acetylcholine (ACh) that slowed heart rate.

    • Loewi and Dale awarded the 1936 Nobel Prize for chemical transmission of nerve impulses.

    • Eccles, Hodgkin, Huxley awarded the 1963 Nobel Prize for ionic mechanisms in synaptic transmission.

  • Practical takeaway: neurotransmitters are chemical messengers released from presynaptic terminals to influence postsynaptic physiology.

Neurotransmitter criteria

  • A transmitter must satisfy a set of criteria to be defined as such, with acknowledged exceptions:

    1. Synthesis: produced within the neuron.

    2. Storage: stored within the neuron.

    3. Release: released upon depolarization of the neuron.

    4. Receptor: causes a biological effect on a postsynaptic receptor when released.

    5. Inactivation: there is a mechanism for inactivation (uptake or degradation).

    6. Postsynaptic mimicry: if applied to the postsynaptic membrane, should mimic the effect of release.

  • Note: some substances do not fit all criteria in every context, leading to nuanced classifications.

Classical (small molecule) vs Non-classical (large peptides) neurotransmitters

  • Classical (small molecules):

    • Amino acids: Glutamate, γ-aminobutyric acid (GABA), Glycine

    • Monoamines: Noradrenaline (norepinephrine), Dopamine, Adrenaline (epinephrine), 5-hydroxytryptamine (5-HT, serotonin), Acetylcholine (ACh)

  • Non-classical (large peptides):

    • Peptides: Substance P, Vasoactive intestinal peptide (VIP), Somatostatin, Neuropeptide Y (NPY), Enkephalin (Enk), Kisspeptin

    • Gases: Nitric oxide (NO), Carbon monoxide (CO)

    • Lipids: Anandamide

  • Conceptual distinction: Classical transmitters tend to act rapidly and locally; non-classical transmitters (peptides, gases, lipids) can have slower, longer-lasting, or broader signaling roles.

Comparison: Classical vs Non-classical neurotransmitters

  • Classical vs Non-classical (summary):

    • Size: Small molecules vs Neuropeptides (4–100 amino acids) for non-classical.

    • Synthesis: Uptake/enzymes for classical vs Protein synthesis for neuropeptides.

    • Vesicles: Small vesicles for classical; large dense-core vesicles for neuropeptides.

    • Duration of action: Fast and short for classical vs slow and long for non-classical.

  • See detailed comparison figure in transcript for specifics.

Summary of neurotransmitter concepts

  • Neurotransmitter: a substance released at a synapse by one neuron that modulates a postsynaptic cell in a specific manner.

  • Primary classes: classical (small molecules) vs non-classical (peptides, gases, lipids).

  • Exceptions exist; the classification helps predict release mechanisms and postsynaptic effects.


Presynaptic processes: basic process and nerve terminal organization

  • Goal: Describe how neurotransmitter release is organized at the presynaptic terminal.

  • Sequence of events (neurotransmission):

    1. Transmitter synthesized and stored.

    2. Action potential arrives.

    3. Depolarization opens voltage-gated Ca2+ channels (VGCCs).

    4. Ca2+ influx through channels.

    5. Ca2+ triggers vesicle fusion with presynaptic membrane.

    6. Transmitter released into the synaptic cleft by exocytosis.

    7. Transmitter binds postsynaptic receptors.

    8. Opening of postsynaptic channels.

    9. Post-synaptic current alters excitability (EPSP/IPSP).

    10. Reuptake and/or degradation terminate the signal.

  • Organization of the nerve terminal includes a reservoir of vesicles, the active zone, and a dense network of proteins coordinating release.

Voltage-gated Ca2+ channels (VGCCs) and roles in release

  • VGCCs couple membrane depolarization to transmitter release.

  • Major VGCCs involved in fast synaptic transmission are high-voltage-activated channels:

    • N-type (Cav2.2): classic blocker: conotoxin (from cone snails)

    • P/Q-type (Cav2.1): classic blocker: agatoxin (from spiders)

  • Calcium entering through VGCCs binds to vesicle-release proteins to trigger vesicle fusion and transmitter release.

  • Reference: Gambardella (2014) Prog Neurobiol, 213:87-96.

Transmitter release in packets: quantal release

  • Concept: neurotransmitter release occurs in packets called quanta.

  • Quantum content: the amount of transmitter in one vesicle; at the neuromuscular junction, ~10,00010{,}000 molecules of acetylcholine per vesicle.

  • Postsynaptic response to a single quantum is the miniature end-plate potential, mEPP.

  • mEPP size is about 1/100 of the end-plate potential (EPP) produced by whole-nerve stimulation: mEPP ≈ 1100 EPPmEPP \,\approx \,\frac{1}{100} \, EPP

  • Consequence: normal neurotransmission largely results from the release of many vesicles simultaneously.

  • Historical note: Bernard Katz and colleagues contributed foundational work on quantal release; Katz received the Nobel Prize in 1970.

  • Postsynaptic receptor interactions in quantal events: ~2,0002{,}000 receptors interact with the ACh in the neuromuscular junction example.

  • This quantal framework links vesicle content, release probability, and postsynaptic response.

Co-transmitters: co-release vs co-transmission

  • Co-transmitters: some terminals release more than one transmitter type.

  • Low-frequency stimulation:

    • Preferentially raises near-membrane Ca2+ leading to release from small clear-core vesicles (classical transmitters).

  • High-frequency stimulation:

    • Increases global Ca2+ and can trigger release of neuropeptides from large dense-core vesicles in addition to small-molecule transmitters.

  • Definitions:

    • Co-release: two transmitters packaged in the same vesicles and released together.

    • Co-transmission: transmitters packaged in distinct vesicles with differential Ca2+ sensitivity or spatial segregation, allowing selective release depending on activity patterns.

  • Reference: Varga et al., 2014 Curr Opin Neurobiol. 29:25-32.

Co-release and co-transmission: practical distinctions

  • Co-release scenario (same vesicles): both neurotransmitters are released together when the terminal depolarizes.

  • Co-transmission scenario (distinct vesicles): differential release from vesicle populations based on Ca2+ sensitivity; possible spatial segregation to different boutons or targets.

  • Implications: enables nuanced signaling, targeting different postsynaptic receptors, and allowing complex modulation of neural circuits.

Relationship between synaptic vesicle exocytosis and quantal transmitter release

  • Visualization and evidence:

    • Electron micrograph showing the arrangement of Ca2+ channels near vesicle fusion sites and vesicle fusion events (freeze-fracture EM).

    • Manipulating AP duration (e.g., with 4-AP) changes the number of vesicle fusions and quanta released.

    • Schematic of frog presynaptic terminals shows vesicles arranged in rows and connected to each other.

  • Practical takeaway: the probability and pattern of vesicle fusion govern the quantal output of a synapse.

Co-transmitters and vesicle pools: a mechanistic view

  • At low activity, selective Ca2+ microdomains near the membrane favor release from small clear-core vesicles (classical transmitters).

  • At higher activity, global Ca2+ elevation triggers release from large dense-core vesicles carrying neuropeptides and other modulators.

  • This dynamic allows a single neuron to switch signaling modes based on activity patterns.

Presynaptic vesicles and their roles

  • Vesicle membrane surface is densely populated with proteins (only a subset shown in illustrations).

  • Roles of vesicle-associated proteins include tethering, docking, priming, fusion, recycling, and maintenance of vesicle pools.

  • Example: Synapsin proteins help tether vesicles to the cytoskeleton, maintaining a reserve pool by crosslinking vesicles to each other and to actin.

Proteins in vesicle recycling: docking, priming, fusion, and beyond

  • Key steps and protein players:

    • Docking: involves GTP-binding proteins and SNAREs (to position vesicles at the membrane).

    • Priming: SNAREs, NSF, and SNAPs prepare vesicles for fusion.

    • Fusion: Synaptotagmin 1 acts as the Ca2+ sensor; SNAREs mediate fusion.

    • Coating: Clathrin, synaptotagmins, synaptobrevin, NSF, SNAPs regulate vesicle coat formation.

    • Budding: Dynamin, clathrin, actin drive vesicle formation from membranes.

    • Uncoating: Clathrin, Hsc-70, auxilin, synaptojanin remove coats after vesicle formation.

Structure and function of the SNARE complex

  • The SNARE complex is the core fusion machinery (“SNAP receptor”) and is essential for vesicle fusion.

  • Core components:

    • Synaptobrevin (vesicular SNARE)

    • Syntaxin (plasma membrane SNARE)

    • Snap25 (plasma membrane SNARE)

    • Synaptogamin (vesicular Ca2+-binding protein) acts as the Ca2+ sensor (in many contexts) and interacts with the SNARE complex to promote fusion.

  • The SNARE complex brings vesicle and plasma membranes into close apposition to enable fusion pore formation and transmitter release.

Synaptic vesicle recycling: models

  • Three main models of vesicle recycling discussed:

    • Kiss-and-run: vesicles transiently fuse, release transmitter through a transient pore, then reseal and are reused without full collapse into the membrane.

    • Clathrin-mediated endocytosis: vesicles fuse and collapse into the membrane; new vesicles are reformed at a distance via clathrin coats.

    • Ultrafast endocytosis (new model, Science 2015): rapid internalization of membrane, vesicle membrane delivered to an endosome, followed by clathrin-mediated regeneration of synaptic vesicles from the endosome.

  • Key takeaway: vesicle recycling can occur via multiple pathways, potentially coexisting and operating at different timescales depending on activity.

Summary of vesicle biology and neurotransmission (recap)

  • The surface of synaptic vesicles is densely populated with proteins, most of which contribute to vesicle recycling and cycling.

  • Many vesicle-associated proteins function as tethering or anchoring components; exact roles for some remain incompletely understood.

  • The SNARE complex is central to vesicle fusion.

  • There are three recognized models of vesicle recycling—clathrin-mediated, kiss-and-run, and ultrafast endocytosis with endosomal regeneration.

Connections to broader concepts and implications

  • Electrical signals are transduced into chemical signals at the synapse, enabling synaptic density and network-level computation through transmitter diversity, vesicle pools, and receptor subtype distribution.

  • Activity-dependent release patterns (frequency, duration) shape which transmitters are released (classical vs peptide) via distinct vesicle populations and Ca2+ microdomains.

  • Understanding vesicle recycling pathways informs pharmacology and potential therapeutic targets for synaptic dysfunction.

  • The historical Nobel Prize-context anchors the significance of chemical neurotransmission and the discovery of ion-channel mechanisms in neural signaling.

Key terms to remember

  • Neurotransmitter, postsynaptic receptor, EPSP, IPSP, mEPP, EPP, quantal release, quantum, vesicle, dock, priming, fusion, SNARE, Synaptotagmin, SNARE complex, VGCC, Cav2.2 (N-type), Cav2.1 (P/Q-type), conotoxin, agatoxin, Kiss-and-run, clathrin-mediated endocytosis, ultrafast endocytosis, endosome, dense-core vesicles, small-clear-core vesicles, co-release, co-transmission, TAC (transmitter-associated cortex placeholders if needed).

References and notes

  • Loewi, Otto (1921) demonstration of chemical transmission; Nobel Prize 1936.

  • Nobel Prize context: Eccles, Hodgkin, Huxley (1963) for ionic mechanisms of synaptic transmission.

  • Katz, Bernard (Nobel Prize 1970) on quantal release at the neuromuscular junction.

  • Varga et al. (2014) Curr Opin Neurobiol. Co-release and co-transmission explored.

  • Gambardella (2014) Prog Neurobiol, 213:87-96 on VGCC roles.

  • Science (2015) on ultrafast endocytosis and vesicle recycling models.