Receptors and Ion Channels (DONE)

Question 1

Name the four types of ion channels and their characteristics

Answer 1

Resting potassium channel- always open
Voltage gated channel- opens transiently in response to change in the membrane potential
Ligand gated channel- opens in response to a specific extracellular neurotransmitter
Signal gated channel- opens in response to a specific intracellular molecule

Question 2

What influences the function of an ion channel or receptor?

Answer 2

Structure- ligand binding site, antagonist binding sites, ion channel domains, recognition sites for extra- and intra-cellular modulators, sites of contact for regulatory proteins
Localisation- how much of each channel/ receptor is found where in the body

Question 3

What are the general aims of sodium channels?

Answer 3

Physiology- exciting role in the generation of an action potential
Pathophysiology- channelopathies e.g. epilepsy, periodic paralysis etc.
Pharmaceutical relevance- local anaesthetics, antiepileptics, analgesics

Question 4

What are the general aims of calcium channels?

Answer 4

Physiological- structure and function, neurotransmitter release etc. of calcium channels
Pathophysiology- channelopathies e.g. epilepsies, Timothy syndrome
Pharmaceutical relevance- antiepileptics, analgesics

Question 5

How do electrochemical gradients dictate the movement of ions?

Answer 5

At equilibrium the membrane potential approaches that of the reversal/ equilibrium potential of the permeant species

Question 6

What defines excitability in any cell?

Answer 6

The presence of sodium channels

However, in some cases its role is taken over by Ca channels

Question 7

The voltage dependent sodium channel

Answer 7

Role- rising phase of the action potential, voltage dependent gating leads to depolarisation
Subtypes- nine functional members of the family
Expression- tissue specific expression and distribution

Question 8

Characteristics of generalised epilepsy with febrile seizures

Answer 8

Autosomal dominant epilepsy syndrome
Normal age range six months to six years
Short generalised tonic-clonic seizures <5 minutes
Febrile seizures usually abate by puberty but can go on to have afebrile seizures
May be caused by mutations in sodium channels

Question 9

Other sodium channelopathies

Answer 9

CNS- intractable childhood epilepsy, Dravet’s syndrome
PNS- familial primary erythermalgia
Muscle- periodic paralysis
Cardiac- long QT

Question 10

Describe the structure of the native sodium dependent channel

Answer 10

Sodium channel from mammalian brain is a complex of three subunits: alpha, beta 1, beta 2
Stoichiometry is 1:1:1

Question 11

Inactivation

Answer 11

Sodium channels have hinged lid mechanisms to go from open to inactivated
The intracellular loop connecting domains III and IV of the channel forms the hinged lid

Question 12

Name four major drug classes that target the voltage dependent sodium channels

Answer 12

Local anaesthetics
Anti-epileptics
Neuropathic pain
Anti-arrhythmics

Question 13

Local anaesthetics

Answer 13

Action is brought about by reversible sodium channel blockade (cytoplasmic side)
Lipophilicity is important for permeation and charge is important for block
Mode of entry depends on drug
Block AP in all sensory, motor and autonomic fibres
Use-dependent block- the more active the channel is, the more likely it will be blocked

Question 14

Anti-epileptics

Answer 14

Lamotrigine, phenytoin and carbamazepine bind preferentially to open sodium channels during inactivation
Little effect on normal AP generation in neurons at normal resting potentials
Voltage and use-dependent block
The voltage and use-dependent block only come into play during periods of aberrant activity

Question 15

Voltage gated calcium channels

Answer 15

Ubiquitously expressed
When gated they are a source of charge and calcium ions
When gated they generate an intracellular signal
Calcium can manipulate life and death of the cell
The key signal transducers of electrical excitability

Question 16

Can there be excitation without sodium ions?

Answer 16

Invertebrate muscle does not use sodium channels for its action potential- instead uses calcium

Question 17

Voltage gated calcium channels theory of excitation

Answer 17

The electrical activity of calcium channels is even more positive than sodium channels
They translate electrical signals into chemical signals
Regulation of intracellular events possible

Question 18

How do cells regulate intracellular calcium?

Answer 18

Cells tightly regulate intracellular calcium
Regulation is achieved by both antiport and uniport mechanisms
As a result, intracellular calcium is kept very low

Question 19

What are the three subfamilies of the calcium channel?

Answer 19

L- skeletal muscle, cardiomyocytes, endocrine cells and retina
T- cardiomyocytes, neuronal cell bodies, dendrites
PQRN- nerve terminals, dendrites and neuronal cell bodies

Question 20

What are the two components of calcium current?

Answer 20

Some calcium channels are inactive
Different classes of calcium channel are activated over different voltage ranges
T-tiny/threshold/transient- microscopic or single channel currents
L-large/long lasting- macroscopic, population or summed currents

Question 21

Describe the sub-units of the voltage gated calcium channel

Answer 21

A1 sub-unit: pore forming sub-unit (major physiological characteristics)
A2D sub-unit: drug binding
B and G subunits: modify trafficking and biophysical properties

Question 22

How can we target voltage gated calcium channels?

Answer 22

Epilepsy- e.g. zonisamide, gabapentin
Pain- e.g. gabapentin, pregabalin
Hypertension- e.g. nifedipine, amlodipine
Arrhythmias IV- e.g. diltiazem, verapamil
Angina- e.g. diltiazem, verapamil
Migraine- e.g. nimodopine

Question 23

Ziconotide (Prialt)

Answer 23

Derivative of peptide
Used in post operative analgesia
N-type calcium channel bloackade in presynaptic terminals of nociceptive neurons
Prevention of calcium influx
Prevents neurotransmitter release, blocking signal

Question 24

Pregabalin (Lyrica)

Answer 24

Related to gabapentin
Used in neuropathic pain and epilepsy
Binds to A2D subunits of both N-type and P/Q type Ca channels
Prevention of calcium influx
Prevents neurotransmitter release, blocking signal

Question 25

Chilli channels

Answer 25

Transient receptor potential channels TRPV (V=vanilloid) activated by capsacin TRPV1 antagonists are very effective at killing pain including neuropathic pain in animal models TRPV1 tells the body to cool itself down- hence why sweating when eating chillies

Question 26

What is a receptor?

Answer 26

A receptor is the site of competition for agonists and antagonists and the vehicle for the transmission of the stimulus of agonist interaction to the cell for the production of a physiological response

Question 27

Nicotinic ACh receptor

Answer 27

The nACh-R was the first ion channel to be purified, have a complete amino acid sequence, be functionally reconstituted and to have single channel recordings (understanding of function) Found in unusually high amounts in the electric organ of electric fish Fluorescent and radiolabelled toxin binds with high specificity to the protein

Question 28

Ligand gated ion channel families

Answer 28

``` Three families (nicotinoid, glutamate and P2X) but there is little structural similarity between them Increased diversity of receptors increases the chance of being able to target one on its own ```

Question 29

Ionotropic receptor family sub-units have different structures

Answer 29

Nicotoid receptors- pentameric Glutamate receptors- tetrameric P2X receptors- trimeric

Question 30

Describe the ionotropic receptor structure

Answer 30

Membrane pore Each subunit has 4 trans-membrane spanning helices Channel made up of five subunits M2 subunit lines the pore Charged amino acids at the top and bottom create the selectivity A ring of leucines for the gate which is closed in the absence of a ligand and determine the pore size (which determines the size of the ion that passes through)

Question 31

What can channels be affected by?

Answer 31

Agonists, antagonists, channel blockers and allosteric modulators

Question 32

Name some current and potential therapeutic applications of AChR active compounds

Answer 32

``` Tobacco addiction General anaesthetics Alzheimer's disease Parkinson's disease Schizophrenia ADHD Neuropathic pain ```

Question 33

Describe some characteristics of 5-HT receptors

Answer 33

Ubiquitous in PNS Trigger exocytosis, reflex bradycardia, hypotension, pain sensation CNS- cortex, limbic and brainstem, fast transmitter release

Question 34

Therapeutic applications of 5-HT3 receptor modulators

Answer 34

Agonists may alter mood- varenicline (lamotrigine may be inverse agonist) Antagonists: nausea and vomiting, chemotherapy, radiotherapy, post-surgical, post-operative neuropathic pain, headache and migraine

Question 35

How can the arrangement of subunits (stoichiometry) affect the activity of ligands?

Answer 35

There are often binding sites between subunits, creating different shapes Benzodiazepines modulate the action of GABA at certain receptors, they are allosteric modulators, so regulate by binding to alternate site

Question 36

How do benzodiazepines act positively, negatively or as pure antagonists of the allosteric modulation?

Answer 36

Full positive- can maximise small GABA stimulated Cl- currents at many GABA-A receptor subtypes e.g. diazepam Selective positive- maximise small GABA stimulated Cl- currents in selected GABA-A receptor sybtypes e.g. zolpidem Partial positive- amplify GABA stimulated Cl- current to limited extent in most subtypes

Question 37

Allosteric modulation of GABA-A receptors

Answer 37

Different modulator sites Barbiturates can positively allosterically modulate GABA-A receptors High doses of barbiturates may also directly activate the receptor in the absence of GABA The size of interaction is different from that used by benzodiazepines and neurosteroids

Question 38

How do neurosteroids allosterically modulate GABA-A receptors?

Answer 38

High doses may directly activate the receptor Positive modulator: allopregnanolone Negative modulator: pregnenolone sulphate Direct activation: allopregnanolone (higher dose)

Question 39

Indications for modulating GABA-A receptors

Answer 39

Agonists/antagonists- only used experimentally Benzodiazepine modulation- anxiolytics, sedatives, antiepileptics Barbiturates- general anaesthetics, antiepileptics Neurosteroidal modulation- post-partum depression, premenstrual tension, catamenial epilepsy

Question 40

Glycine receptor

Answer 40

Widely distributed in CNS and inhibitory Glycine used in foods, buffering agent in cosmetics, drugs and toiletries Herbicide Required co-agonist for NMDA glutamate receptors Strychnine (poison) is an antagonist

Question 41

Other ionotropic receptor families

Answer 41

Nicotoid receptors Glutamate receptors P2X receptors

Question 42

Glutamate receptors

Answer 42

Each subunit consists of three TMs and a re-entrant loop NMDA receptors- N-methyl-D-aspartate non-NMDA receptors- AMPA, kainate

Question 43

AMPA glutamate receptors

Answer 43

Widespread throughout CNS, regional specificity for subtypes, responsible for fast synaptic excitation

Question 44

Kainate glutamate receptors

Answer 44

Widespread throughout CNS, regional specificity for subtypes, responsible for fast synaptic excitation

Question 45

NMDA receptors

Answer 45

Throughout CNS/PNS, excitatory, structurally complex Used in CNS development, memory, learning, apoptosis Involved in Alzheimers, Parkinsons, Huntingtons, schizophrenia, epilepsy, encephalitis, stroke

Question 46

Regulation of NMDA receptors

Answer 46

Glutamate, glycine and magnesium binding sites Important for receptor activation and gating of the ion channel (glycine is an essential binding component to NR1) Zinc and polyamine sites are not needed for receptor activation, affect the efficacy of the channel

Question 47

How do zinc and polyamine regulate the NMDA receptors?

Answer 47

Zinc blocks the channel in a voltage-independent manner Polyamine site- binds compounds such as spermine or spermidine, potentiating or inhibiting depending on the combination of subunits

Question 48

P2X receptors

Answer 48

Fast synaptic transmission Neuron-smooth muscle transmission ATP-mediated lysis of antigen-presenting cells