N371 Ion Channels IV + V

What are the main roles of voltage-gated Ca²⁺ channels (VGCCs) in neurons?

They allow Ca²⁺ entry that acts as a second messenger to activate protein kinases, enzymes, and proteases, and they trigger neurotransmitter release at presynaptic terminals.

Why do neurons keep intracellular Ca²⁺ concentration ([Ca²⁺]ᵢ) low?

• Prevents “noise” → ensures precise signaling.

• Maintains a large concentration gradient (high [Ca²⁺]ₒ / low [Ca²⁺]ᵢ) → allows fast, sensitive Ca²⁺ signaling responses.

How does intracellular Ca²⁺ affect neuronal excitability?

Elevated [Ca²⁺]ᵢ activates BK and SK K⁺ channels, which modulate bursting and limit repetitive spiking.

What are the 2 main types of Ca2+ channels?

HVA, LVA

How are voltage-gated Ca²⁺ channels structured?

Similar to Naᵥ and Kᵥ channels: one large α₁-subunit with four domains (DI–DIV), each containing six transmembrane segments (S1–S6).

What are the roles of S1–S4 vs. S5–S6 regions in Ca²⁺ channels?

• S1–S4: voltage-sensing region.

• S5–S6: forms the pore and selectivity filter.

What auxiliary subunits associate with Ca²⁺ channels?

• α₁-subunit: forms the pore; determines subtype (L, N, P, Q, R, T).

• β-subunit & α₂δ-subunit: modulate kinetics (present in all except T-type).

• γ-subunit: found mainly in L-type channels (especially in muscle).

How are Ca²⁺ channels classified?

By electrical activation properties (high or low voltage) and pharmacology (specific blockers).

What defines HVA Ca²⁺ channels?

They activate near the AP threshold (~–40 mV) and require strong depolarization to open.
Subtypes: L, N, P, Q, R channels.

What are the key properties of L-type Ca²⁺ channels?

HVA, slowly inactivating, and produce large, long-lasting currents.

Where are L-type Ca²⁺ channels found and what do they do?

In skeletal, cardiac, endocrine, and neuronal cells; mediate excitation–contraction coupling, hormone secretion, and gene regulation.

What drugs block L-type Ca²⁺ channels?

Dihydropyridines (DHPs) such as nifedipine or verapamil.

What are the properties of N-type Ca²⁺ channels?

Intermediate activation voltage (between L and T types); mainly HVA, located at presynaptic terminals.

What is the primary function of N-type Ca²⁺ channels?

Trigger neurotransmitter release at presynaptic terminals.

How are N-type Ca²⁺ channels modulated by G-proteins?

Direct + indirect inhibiton

What is direct inhibition in the context of N-Type channels?

Gβγ subunits bind directly to the channel → lower open probability (fast, voltage-sensitive, reversible).

What is indirect inhibition in the context of N-Type channels?

via second messenger cascades → slower, voltage-insensitive, longer-lasting inhibition.

What toxin blocks N-type Ca²⁺ channels?

ω-conotoxin GVIA.

What are the key properties of T-type Ca²⁺ channels?

Low-voltage activated (LVA) → open near RMP (~–70 mV), fast inactivation, and require hyperpolarization to recover.

What is the main function of T-type Ca²⁺ channels?

Generate rhythmic bursts and slow oscillations in pacemaker and thalamic neurons, especially during sleep; also involved in absence seizures.

What is the “window current” of T-type channels?

A narrow voltage range where some channels are open and others inactivated → produces a small steady depolarizing current that drives excitability even near rest.

How do T-type channels contribute to burst firing?

• Hyperpolarization → channels recover.

• Depolarization → T-channels open → small Ca²⁺ influx → depolarization triggers Na⁺ AP burst.

• Ca²⁺ inactivates T-channels → K⁺ channels repolarize → cycle repeats.

What drugs block T-type Ca²⁺ channels?

Ethosuximide, used to treat absence seizures by suppressing thalamic rhythmic activity.

What kind of ion channels are TRPs?

Nonselective cation channels that act as cellular sensors for physical and chemical stimuli.

What is polymodal gating in TRPs?

The ability to be activated by multiple stimuli — temperature, chemicals, mechanical force, and pain.

TRP channel structure — how many transmembrane domains?

6 transmembrane domains (S1–S6).

What forms the ion pore in TRP channels?

The region between S5–S6.

Where is the activation gate located in TRP channels?

Near the end of S6.

Do TRPs have a voltage-sensing domain?

They have an S1–S4 region resembling one, but they’re voltage-modulated, not voltage-gated.

Explain “voltage adjusts the dial but doesn’t flip the switch.” in TRP channels?

Voltage can modulate TRP activity but cannot directly open the channel — unlike true voltage-gated channels.

What is the overall structure of TRP channels?

Tetrameric (4 subunits form the full channel).

What happens when TRP channels open?

Na⁺ and Ca²⁺ influx → neuron depolarization → sensation (heat, cold, pain).

What is capsaicin?

The active compound in chili peppers that activates TRPV1, producing a burning pain sensation.

What is capsazepine?

A competitive antagonist of capsaicin (TRPV1 blocker). can cause hyperthermia or tumorigenesis.

How was TRPV1 identified?

By gain-of-function screening:
mRNA from DRG neurons → make cDNA library → insert genes into HEK cells → test with calcium imaging for capsaicin response.

What technique is used to detect Ca²⁺ changes in these experiments (how was TRP1 discovered?)

Calcium imaging with fluorescent indicators.

What does “gain of function screening” mean?

Introducing genes into non-responsive cells and seeing which gene makes them respond to a stimulus.

TRPV1 stimuli and function?

Activated by heat, capsaicin, and protons (acid) → detects noxious heat + chemical pain.

TRPV1 I–V curve features?

Reversal ≈ 0 mV, nearly linear (weak outward rectification), conducts inward and outward currents efficiently.

What happens to TRPV1 during prolonged activation?

It desensitizes rapidly (ms–s) — protective mechanism.

Example of mild TRPV1 desensitization in real life?

Hot foot bath — gentle activation causes temporary desensitization (pain relief).

TRPV1 loss-of-function mutation effect?

No pain response even with high capsaicin concentration.

TRPV1 agonist drug example and use?

Capsaicin (Qutenza patch) — desensitizes nerves → pain relief.

TRPM8 stimuli and function?

Activated by cooling and menthol → detects non-noxious cold.

TRPM8 I–V curve features?

Reversal ≈ 0 mV, strong outward rectification → current flows better outward at depolarized voltages.

What does “outward rectifying” mean for TRPM8?

Conducts more current outward (during depolarization) and less inward at rest.

What happens to TRPM8 with prolonged cold or menthol exposure?

Desensitizes — protects neurons from overactivation.

What happens to menthol-induced current at high temperature?

It is inactivated (TRPM8 shuts off).

What technique measures single-channel activity?

Patch clamp recording.

Inside-out patch clamp tests what?

Effects of intracellular molecules (e.g., Ca²⁺, ATP, signaling compounds).

Outside-out patch clamp tests what?

Effects of extracellular factors (e.g., drugs, neurotransmitters, capsaicin).

What is the paradoxical heat sensation?

Very cold temperatures can activate TRPV1, making extreme cold feel hot or burning.