Voltage-gated ion channels: Na+, Ca2+ and K+

At _____ mV and above, all of the voltage-gated Na+ channels are activated

-15 mV

Peak conductance of the voltage-gated Ca2+ channel is around _____ mV

-40

The inactivation curve and activation curve intersect at

-40 mV

Until ____ mV, almost all of the voltage-gated Na+ channels are inactivated

-75 mV

The entire Voltage-gated Na+ channel is encoded by ________ gene(s)

1

2 mechanisms for peptide toxins effect on Kv channels

1. Binding to the other vestibule of the channel and occluding it (like a cork in a bottle) 2. Interacting with the votlage-sensor domain and increasing the stability of the closed state (channel becomes more difficult to open)

Mechanism of calcium channel's role in drug addiction

1. Chronic exposure to psychostimulants activate Cav1.3 channels, increasing dopamine release from the VTA 2. Increasd dopamine release in turn stimulates glutamatergic inputs 3. Glutamate activates AMPAR and NMDAR on VTA neurons 4. Cav1.3 activation and glutamate receptor activation leads to changes in gene expression --> upregulation of Cav1.2 calcium channels 5. Activation of D1Rs in the NAc activates PKA 6. PKA activation + Cav1.2 upregulation leads to long-term changes in gene expression and AMPAR insertion in the plasma membrane

Mechanism of Ca2+ channel role in Parkinsonism

1. Repetitive activation of Cav1.3 channels leads to calcium influx and calcium-dependent release from the ER. 2. High Ca2+ concentration in hte mitochondria causes the generation of reactive oxygen species 3. ROS culminates in cell damage and loss of dopaminergic neurons 2. Neurons in substantia ingra innervate D1R-expressing neurons and D2R-expressing neurons in the striatum. Loss of dopaminergic neurons decreases dopaminergic input to the striatum.

Voltage-gated Ca2+ channels have _____ domains

4

Voltage-gated Na+ channels have ___________ distinct domains

4 (DI -- DIV)

Selectivity filter site 3 contains

4 backbone carbonyls (low affinity)

Selectivity filter site 1 contains

4 carboxyl side chains (high affinity)

Selectivity filter site 2 contains

4 carboxyls and 4 backbone carbonyls (high affinity)

Voltage-gated Na+ channel domains eac have __________ transmembrane regions

6

Each domain of the voltage-gated Ca2+ channel has _____ TM regions

6; S1-S6

Dravet syndrome

A severe form of epilepsy, a rare genetic disorder that is most often not hereditary and is caused by a novel nonsense mutation in the SCN1A gene, resulting in a premature stop codon and a nonfunctional protein.

Axon hillock

A specialized part of the cell body (or soma) of a neuron that connects to the axon. This is the last site in the soma where membrane potentials propagated from synaptic inputs are summated before being transmitted to the axon

β subunit

Accessory proteins that assemble with α subunits, forming a complex that can display altered voltage dependence and cellular localization.

Coupled inactivation model

Activation of all 4 voltage sensors contributes to the opening of the channel. Inactivation results from the binding of the inactivation lid to its receptor in the pore which becomes accessible in the open state

S1 through S4 are

Alpha helices

Ancilliary Ca2+ channel subunits include

Beta1-4, gamma1-8,alpha2-delta1 through alpha2-delta4

The hypothesis is that the original ion channel must have been a ______ channel

Ca2+; Ca2+-dependent enzymes and kinases evolved a ling time ago so that the cell could communicate and interact with environmental cues

Anticonvulsants that target Na+ channels include

Carbamazepine and phenyotin

The alpha1 subunit of Ca2+ channels are encoded by the __________ genes

Cav

Chronic exposure to psychostimulants activate

Cav1.3 channels, increasing dopamine release fro the VTA which in turn stimulates glutamatergic inputs

R-type channels are found

Cerebellar granule cells, other neurons

Inward rectifier family of K+ channels

Contain 2 TM regions (S1-S2) and 1 P domain

K2P family of K+ channels

Contain 2 pairs of 2 TM regions (4 total), each flanking a pore domain Order: TM1, P1, TM2, TM3, P2, TM4

Shaker family of K+ channels

Contain 6 TM regions (S1-S6) and 1 P-domain between S5 and S6

The inactivation gate in the Na+ channel is located between

DIII and DIV

Transmembrane domains ______ of voltage-gated Ca2+ channels are hotspots for drug interactions

DIII, S6 and DIV, S6

Mutation in Dravet syndrome results in

Decrease in sodium currents and impaired excitability of GABAergic interneurons of the hippocampus. Loss of NA(V)1.1 channels was sufficient to cause the epilepsy and premature death

Anaesthetics primarily target

Domains I, II, and IV

Anaesthetics that target Na+ channels include

Etidocaine (postoperative pain)

Mechanism of Ca2+ channel role in epilepsy

Excitability of the cortical-thalamic network depends on T-type calcium channels (Cav3.1-3). Mutations in Cav3.2 that lead to increased channel function have been found in patients with congenital epilepsy. Mutations can increase cell surface expression of the channel or increase channel availability via a depolarizing shift in the steady-state inactivation curve. Both cases result in increased T-type current amplitudes, leading to increase excitability of reticular thalamic nucleus cells and seizures.

True or false? The axon hillock is the usual site of AP initiation

False; it was originally thought to be but it is now believed to be found adjacent in the initial or unmyelinated segment of the axon.

True or false? Voltage-gated Na+ channels are expressed evenly throughout the neuron

False; they are expressed unevenly

α subunit

Forms the core of the channel and is functional on its own

___________ are embedded in the axon initial segment membrane

GPI anchored proteins (thy1, lipids)

Drugs that target Cav-alpha2-delta subunits

Gabaprentin and pregabalin (for pain and epliepsy)

Nodes of Ranvier

Gaps in the myelin sheath to which voltage-gated sodium channels are confined.

The K2P family suggests it evolved from a

Gene duplication of the inward rectifiers

Examples of Na+ channelopathies include

Generalized epilepsy, Dravet syndrome, benign neonatal convulsions, familial hemiplegic migranine

Unified loss-of-function hypothesis for NaV1.1 genetic epilepsies

Icreasing severity of loss-of-function mutations of NaV1.1 channels causes progressively more severe epilepsy syndromes from familial febrile seizures to GEFS+ and finally DS,

Activation curve

Indicates the voltage range where activation occurs

Drugs that target L-type Ca2+ channels

Isradipine, nimodipine, cilnidipine (all for hypertension)

How is S4 able to sense the membrane potential?

It has 3-4 charged arginine and lysine residues that can move up and down depending on the membrane potential

Different types of Ca2+ channels

L-type P-type N-type R-type T-type

BPD medications that target Na+ channels

Lamotrigine

Sodium channels consist of

Large α subunits that associate with proteins, such as β subunits

Anaesthetic mechanism

Local anesthetics bind to a specific receptor within the pore of Na+ channels, formed by the S6 segments in domains I, II, and IV. Their binding blocks ion movement through the pore and stabilizes the inactivated state of Na+ channel

Role of Cav2.2 channels in pain

Located presynaptically where their opening allows Ca2+ entry and leads to NT release

L-type Ca2+ channels

Long-lasting; made up of genes Cav1.1, 1.2, 1.3, and 1.4

Nav1.1 mutation severity, from least to more severe

Mild missense, moderate missense, severe missense, truncation loss of function

How do the concentrations of Na+, Ca2+ and K+ differ?

Na+ : high extracellular conc, low intracellular Ca2+: low extraceullular conc K+: low extracellular, high intracellular conc

Patients with generalized epilepsy with febrile seizures plus show mutations in the ______________ channel

Nav1.1

Neuronal Na+ channel subunits include _____________________ families

Nav1.1, 1.2, 1.3, and 1.6

N-type Ca2+ channels

Neural-type; made up of gene Cav2.2

T-type channels are found

Neurons, cells that have pacemaker activity, bone (osteocytes)

Voltage-gated Na+ channels are expressed at a high density in the _______________, critical for AP propagation

Nodes of Ranvier

Dravet syndrome mutation primarily affects which types of neurons?

Parvalbumin interneurons and somatostatin interneurons

Most of the voltage-gated Na+ channel is located in the

Plasma membrane

P-type channels are found

Purkinje neurons in the cerebellum Cerebellar granule cells

P-type Ca2+ channels

Purkinje-type; made up of gene Cav2.1

R-type Ca2+ channels

Residual-type; made up of gene Cav2.3

Inactivation curve

Reveals the proportion of inactivated channels

A shift of the steady-state inactivation curve to the ________ leads to increased channel availability

Right (depolarized)

Voltage-sensitive domains in the voltage-gated Ca2+ channel

S1-S4

Segment ________ in the voltage-gated Na+ channels is the voltage sensory

S4

Voltage sensor in the voltage-gated Ca2+ channel

S4

Segments ________ in the voltage-gated Na+ channels form the pore

S5 and S6

At resting membrane potential, voltage-gated Na+ channels are

Shut but not inactivated

L-type channels are found

Skeletal muscle, smooth muscle, bone (osteoblasts), ventricular myocytes** (responsible for prolonged action potential in cardiac cell;; also termed DHP receptors), dendrites and dendritic spines of cortical neurons

Anitconvulsant mechanism

Sodium channel blockade is the most common and best-characterized mechanism of currently available antiepileptic drugs (AEDs). AEDs that target sodium channels prevent the return of the channels to the active state by stabilizing it

Ca2+ channel gamma2 KO mice are called

Stargazing

Phases of an action potential

Suprathreshold synaptic potential --> action potential --> hyperpolarization

Voltage-gated Na+ channels are ______-meric

Tetra

Voltage-gated Na+ channels are blocked by ____________ at nanomolar concentrations

Tetrodotoxin (TTX)

When S4 is in the down position this indicates

The cell is not depolarized and the ion channel does not contribute to the surface level charge

Anticonvulsants primarily target

The inactivated state of the Na+ channel, prolonging it

When S4 is in the up position this indicates

The membrane potential is positively charged

Channel activation is triggered by

The outward movement of S4 voltage sensors

Knockoff mechanism of Ca2+ permeation

The pore architecture suggests a conduction pathway involving transitions between two main states with one or two hydrated Ca2+ ions bound in the selectivity filter

Which is slower: a voltage-gated Na+ channel's inactivation or its recovery from inactivation

The recovery

What are some main targets on the voltage-gated Na+ channel that drugs act on?

The voltage sensor, upper parts of the pore, and lower parts of the pore

In cases where the AP firing is decreased (depression and cognitive dysfunction), drugs can target which aspect of K+ channels?

They can block the channel to restore normal firing

Why don't glial cells fire APs?

They have many Na+ channels but these channels are bot concentrated in a certain area such that there is no synchronized depolarization. Furthermore, there is a high concentration of K+ channels that leads to hyperpolarization of the glial cell.

Role of Cav3.2 Ca2+ channels in pain

They regulate afferent fibre excitability and contribute to Ca2+ influx in synaptic nerve terminals

N-type channels are found

Throughout the brain and PNS

The axon hillock serves a secondary function as a

Tight junction; it acts as a barrier for lateral diffusion of transmembrane proteins.

T-type Ca2+ channels

Transient-type; made up of genes Cav3.1, 3.2, 3.3

True or false? APs can also backpropagate (actively or passively) into dendrites

True

True or false? Toxins that target Na+ channels all bind in roughly the same location

True; despite binding to a similar region of the channel, each toxin has a different effect on channel gating.

Shared structural properties of toxins that interact with the voltage-sensor domain

Typically contain a cluster of hydrophobic residues on one face of the molecule that partitions into the membrane when they bind to the voltage sensor

Shared structural properties of toxins that occlude the channel

Usually small molecules, thus able to bind to the inner pore, gating hinges or the interface between alpha and beta subunits

Potassium channel subfamilies

Voltage-dependent "shaker type" Inward rectifiers Background channels "K2P"

The axon initial segment has a high density of

Voltage-gated Na+ channels

_______________ are a critical component of the AP

Voltage-gated Na+ channels

Window current

Where the activation and inactivation curves overlap, indicating steady-state activation level. Small voltage range whereT-type channels can be tonically active

Drugs often act upon the _____________ portion of the inactivation/activation curves

Window current; the area where the channels can activate and are not completely inactivated