L4 - Neuronal Inhibition & Vesicle Release Modulation

Overview & Learning Objectives

• Continuing the “people-shouting” analogy to describe synaptic influence.
  • Influence of one neuron on another originally said to depend on:
  • Distance between synapse and spike-initiating zone.
  • Size of the excitatory postsynaptic potential (EPSP) at the synapse.
  • Number of synapses connecting the two neurons.
  • Today we add two more layers of complexity:
  • “Silences” → mechanisms of inhibition mediated by chloride channels.
  • “Support staff” → mechanisms that modulate the probability of vesicle release.
• Learning goals for this video:
  1. Describe the different ways chloride ion channels generate inhibition (classical IPSPs & shunting inhibition).
  2. Detail multiple mechanisms that change the probability that an arriving action potential triggers vesicle release.

Chloride-Mediated Inhibition

• Equilibrium potential for chloride:

  • Standard intracellular vs. extracellular Cl⁻ concentrations yield E{Cl}=\frac{RT}{zF}\ln\left(\frac{[Cl^-]{out}}{[Cl^-]_{in}}\right)\approx -65\,\text{mV}.
  • E_{Cl} is very close to the resting membrane potential (RMP) of most neurons ⇒ opening Cl⁻ channels often keeps Vₘ near rest.

• Classical IPSPs (visible hyperpolarization):

  • When membrane is depolarized relative to E_{Cl}, opening Cl⁻ channels drives Vₘ back toward -65\,\text{mV} → downward deflection (IPSP).
  • EPSPs and IPSPs can summate algebraically: multiple EPSPs may depolarize enough for an AP, but concurrent IPSP(s) can offset this.
  • Example timeline:
    • Three sequential EPSPs → summate ⇒ reach threshold.
    • Insertion of an IPSP between them speeds return to RMP and may prevent threshold.

• Shunting inhibition:

  • Occurs when an inhibitory synapse opens Cl⁻ channels on a dendrite between an excitatory input and the soma.
  • Mechanism = electrical “leak”:
  • EPSP generated distally still occurs.
  • Added Cl⁻ conductance drastically lowers the dendritic length constant → charge dissipates before reaching soma.
  • Somatic recording shows no voltage deflection (flat line) even though EPSP happened distally.
  • Key feature: often no overt IPSP is seen at the soma; inhibition manifests as reduced EPSP amplitude.

Modulation of Vesicle Release Probability (Pᵣ)

Release Probability Is Variable

• Arrival of an AP at the terminal does not guarantee release.
  • Classic data from neuromuscular junction (NMJ): in low-Ca²⁺/high-Mg²⁺ saline, 18 stimulus trials produced failures (no end-plate potential) ⇒ Pᵣ < 1.
• Typical values (physiological Ca²⁺):
  • Motor neuron terminals: P_r \approx 1 (reliable transmission).
  • Spinal cord/CNS synapses: 0 \le P_r \le 1, wide diversity even between neighboring boutons.
• Plasticity lever: Neurons & circuits dynamically shift Pᵣ to tune information flow.

Axo-Axonic Presynaptic Modulation

• Specialized synapse where a regulatory axon (neuron C) contacts the presynaptic terminal of neuron A.
• Two mechanistic routes:
  1. Ionotropic: transmitter from neuron C directly opens/closes Ca²⁺ channels in neuron A’s terminal.
  2. Metabotropic: G-protein pathway alters the probability that voltage-gated Ca²⁺ channels open during an AP.

Presynaptic Inhibition

• Neuron C active → fewer Ca²⁺ channels open.
  • Observations (time-course graphs):
  • Gray: AP in axon A (unchanged).
  • Orange: Ca²⁺ current is smaller.
  • Purple: Postsynaptic potential (PSP) in neuron B is smaller than control.
  • Net result: weakened A→B connection without altering somatic excitability of A.

Presynaptic Facilitation

• Neuron C prolongs the AP in axon A.
  • Wider AP keeps membrane depolarized → Ca²⁺ channels remain open longer.
  • Residual Ca²⁺ supports more vesicle fusion events.
  • PSP in neuron B becomes larger & longer.

Paired-Pulse Facilitation (PPF)

• Relies on residual Ca²⁺ in the terminal.
• Experimental protocol: deliver two APs separated by variable Δt.
• If P_r < 1 and Δt < Ca²⁺ clearance time:
  • 2nd AP sees elevated baseline Ca²⁺ ⇒ higher vesicle fusion probability.
  • Postsynaptic current/Potential₂ > Potential₁ (facilitation).
• Magnitude of PPF:
  • Inversely related to Δt (short Δt ⇒ large facilitation; long Δt ⇒ effect decays to baseline).
  • Directly related to Ca²⁺ clearance speed (slower pump ⇒ longer facilitation window).
• Caveat: If P_r \approx 1 initially (all docked vesicles released), the second response can be smaller (synaptic depression) due to vesicle depletion.

Autoreceptors (Presynaptic Receptors)

• Located on same terminal that releases the transmitter.
• Sense cleft transmitter concentration → negative feedback onto vesicle release machinery.
  • High cleft transmitter → autoreceptor activation → lowers Pᵣ.
  • Low cleft transmitter → keeps Pᵣ high.
• Pharmacological relevance:
  • SSRIs block serotonin reuptake transporter ⇒ cleft 5-HT stays elevated.
  • Autoreceptors detect this & initially down-regulate release → clinical delay in antidepressant efficacy until autoreceptors desensitize.

Astrocytic Modulation (Tripartite Synapse)

• Astrocytes extend fine processes around axon terminals & dendritic spines.
  • Roles:
  • Neurotransmitter recycling (e.g.
    glutamate–glutamine cycle).
  • K⁺ buffering → stabilizes neuronal RMP.
  • Regulating extracellular Ca²⁺ & volume.
  • Gliotransmission: astrocytes release neuromodulators (e.g.
    ATP, D-serine) that influence presynaptic and postsynaptic elements.
• Cajal’s 19th-century drawings foreshadowed modern electron micrographs showing close astrocyte–synapse apposition.

Functional & Computational Implications

• Nervous system displays multilayered plasticity:
  • Change the number/location/type of synapses.
  • Tune vesicle release probability.
  • Adjust neurotransmitter concentration in cleft.
  • Regulate postsynaptic receptor density & subtype.
• Purpose:
  • Grants neurons the ability to specialize for diverse tasks.
  • Allows circuits to adapt over milliseconds (short-term plasticity) to years (long-term learning).
• Looking ahead: sensory & motor system lectures will illustrate how these cellular mechanisms combine into intuitive behaviors like vision, hearing, and movement control.