Lecture 2- Drug Actions at Ion Channels

Question 1

5 receptor subtypes

Answer 1

1. Lipid-soluble ligand
2. Enzymatic reaction
3. Tyrosine kinase activation
4. Ion channel activation
5. GPCRs

Question 2

Ion channels

Answer 2

• 3-6 similar membrane spanning proteins or structural sub-units
• provide a permeable route for charged ions to move through the hydrophobic lipid membrane
• consists of a chain of amino acids folded into complex structures
• pore opens and closes in response to how the ion channel is gated (voltage-gated, ligand gated, signal gated)

Question 3

Properties of ion channels

Answer 3

1. Selective for ions: Na+, K+, Cl-
2. Non-selctive: TRP channels
3. Gating: open and close in response to specific chemical, electrical, or mechanical signals
4. Conduct ions across the plasma membrane
5. Regulates the membrane potential of cells

Question 4

Types of ion channels

Answer 4

Question 5

Mechanically-gated ion channels

Answer 5

• actual pressure or deformation of cell itself, causes opening

Two types:

1) Direct gating :by physical opening of channel

• there is compression/ deformation of cell membrane
• shear stress: can be physical deformation or hypertonicity (ex: if cell starts to swell)
• differntial stiffness between ECM and ICM: membrane sensitive to tension: TRP channels

2) Indirect gating

• indirect activation of GPCRs then opening of TRP channels
• mechanical deformation, activates GPCRS, leads to opening of various TRP channels —> RELEASE of specific second messengers (PLC) —> cause reorganization of cytoskeleton around channel (change in architecture), leads to opening of pore

Question 6

Voltage-gated ion channels

Answer 6

• Nat+, K+, Ca2+, Cl-
• 6 transmembrane segments (S1-S6)
• open in response to changes in membrane potential

Question 7

Ligand-gated ion channels

Answer 7

• 3 major superfamilies
• based on folded architecture and # of subunits
• transmembrane domains important for pore formation and ion selectivity
• extracellular domains –> important for ligand binding
• ligand binds to receptor, ion channel opens, ions flow by their concentration gradient, cellular effect

Question 8

Mechanogated ion channels can differentiate between soft and noxious stimuli –> why?

Answer 8

Mild mechanical stimulation

• small # of ion channels opening
• few APs
• interpreted in CNS as mild tactile stimulus

Noxious stimulation

• a lot more mechanogated ion channels opening
• more APs
• increased firing of mechanosensory nerves interpreted as pain

We can differentiate between tactile and noxious touch based on the number of OPEN mechanogated ion channels

Question 9

Resting membrane potential

Answer 9

• -70mV
• more + extracellular compared intracellular

Question 10

Na+/K+ ATPase

Answer 10

• pumps 3 Na+ out, 2 K+ in
• helps maintain resting membrane potential

Question 11

The action potential

Answer 11

1. Stimulus, reach threshold
2. VG Na channels open: depolarization
3. VGSC rapidly inactivated, with NA/K ATPas get depolarization
4. overshoot of ATPase= refractory
5. Back to resting state

Question 12

Curare

Answer 12

• found on tips of poisoned darts
• kills by blocking respiratory muscle activity: prey suffocates
• blocks NMJs in respiratory muscles

Question 13

Cys-Loop Channel example

Answer 13

Nicotinic Acetylcholine receptor

Pentamer

• two 𝜶
• one 𝜷
• one 𝜸
• one 𝜹

MW: 43,000- 50,000 kDa

Requires two ACh molecules for activation

Pore Open: Na+ influx

Functions:

• skeletal muscle contraction
• smooth muscle relaxation
• inhibits cardiac muscle contraction
• nerve depolarization

Question 14

Depolarizing blockade

Answer 14

• excess ACh and nicotinic receptor agonists
• continuous activation of N receptors at NMJ –> brief period of muscle excitation
• followed by fasiculations in myocytes
• then flaccid paralysis
• prolonged agonist bound nAChr’s desenstizees neurons = less likely to fire
• continuously depolarized membrane affects inactivation of voltage-gated sodium channels

Question 15

nACh receptors and pain

Answer 15

• located in DRGs and dorsal horn of spinal cord
• ACh sensitizes 50% of c fibres for heat but not mechanical pain

Analgesics

• 𝛂7 blockers
• botulinum toxin
• epibatidine
• nicotine
• inhibits inflammatory neuropeptide and cytokine release
• DRG= cell body of sensory nerves
• dorsal horn= relays pain info to CNS/brain
• sensitization with respect to heat pain, Ach doesn’t have effect on mechanical pain
• can reduce pain by inhibiting inflammatory neuropeptides and inflammatory cytokines

Question 16

GABA A Receptors

Answer 16

• cys-loop receptor
• anionic receptor: inhibitory, causes passage of Cl- into the cell
• heteropentameter with variable sub-unit composition
• activated by gamma-immunobutyric acid (GABA)

Question 17

GABA A receptors and pain

Answer 17

• found in dorsal horn of spinal cord
• inhibits pain transmission by hyperpolarizing second order neurons
• inhibits substance P release from primary afferent neurons
• Benzodiazepines and barbiturates potentiate GABA-mediated synaptic inhibition

Question 18

Benzodiazepines and Barbiturates Allosterically Modulate GABAAR by Two Different Mechanisms

Answer 18

GABA normally binds to 𝛂-𝛃 subunit interface

Benzodiazepines

• GABA binds to 𝛂-𝛄
• increase the probability of opening

Barbiturates

• interacts with pockets in the transmembrane domain
• increase the open time of GABA A receptor