Receptors and Ion Channels (DONE) Flashcards
Name the four types of ion channels and their characteristics
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
What influences the function of an ion channel or receptor?
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
What are the general aims of sodium channels?
Physiology- exciting role in the generation of an action potential
Pathophysiology- channelopathies e.g. epilepsy, periodic paralysis etc.
Pharmaceutical relevance- local anaesthetics, antiepileptics, analgesics
What are the general aims of calcium channels?
Physiological- structure and function, neurotransmitter release etc. of calcium channels
Pathophysiology- channelopathies e.g. epilepsies, Timothy syndrome
Pharmaceutical relevance- antiepileptics, analgesics
How do electrochemical gradients dictate the movement of ions?
At equilibrium the membrane potential approaches that of the reversal/ equilibrium potential of the permeant species
What defines excitability in any cell?
The presence of sodium channels
However, in some cases its role is taken over by Ca channels
The voltage dependent sodium channel
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
Characteristics of generalised epilepsy with febrile seizures
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
Other sodium channelopathies
CNS- intractable childhood epilepsy, Dravet’s syndrome
PNS- familial primary erythermalgia
Muscle- periodic paralysis
Cardiac- long QT
Describe the structure of the native sodium dependent channel
Sodium channel from mammalian brain is a complex of three subunits: alpha, beta 1, beta 2
Stoichiometry is 1:1:1
Inactivation
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
Name four major drug classes that target the voltage dependent sodium channels
Local anaesthetics
Anti-epileptics
Neuropathic pain
Anti-arrhythmics
Local anaesthetics
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
Anti-epileptics
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
Voltage gated calcium channels
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
Can there be excitation without sodium ions?
Invertebrate muscle does not use sodium channels for its action potential- instead uses calcium
Voltage gated calcium channels theory of excitation
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
How do cells regulate intracellular calcium?
Cells tightly regulate intracellular calcium
Regulation is achieved by both antiport and uniport mechanisms
As a result, intracellular calcium is kept very low
What are the three subfamilies of the calcium channel?
L- skeletal muscle, cardiomyocytes, endocrine cells and retina
T- cardiomyocytes, neuronal cell bodies, dendrites
PQRN- nerve terminals, dendrites and neuronal cell bodies
What are the two components of calcium current?
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
Describe the sub-units of the voltage gated calcium channel
A1 sub-unit: pore forming sub-unit (major physiological characteristics)
A2D sub-unit: drug binding
B and G subunits: modify trafficking and biophysical properties
How can we target voltage gated calcium channels?
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
Ziconotide (Prialt)
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
Pregabalin (Lyrica)
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
Chilli channels
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
What is a receptor?
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
Nicotinic ACh receptor
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
Ligand gated ion channel families
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
Ionotropic receptor family sub-units have different structures
Nicotoid receptors- pentameric
Glutamate receptors- tetrameric
P2X receptors- trimeric
Describe the ionotropic receptor structure
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)
What can channels be affected by?
Agonists, antagonists, channel blockers and allosteric modulators
Name some current and potential therapeutic applications of AChR active compounds
Tobacco addiction General anaesthetics Alzheimer's disease Parkinson's disease Schizophrenia ADHD Neuropathic pain
Describe some characteristics of 5-HT receptors
Ubiquitous in PNS
Trigger exocytosis, reflex bradycardia, hypotension, pain sensation
CNS- cortex, limbic and brainstem, fast transmitter release
Therapeutic applications of 5-HT3 receptor modulators
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
How can the arrangement of subunits (stoichiometry) affect the activity of ligands?
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
How do benzodiazepines act positively, negatively or as pure antagonists of the allosteric modulation?
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
Allosteric modulation of GABA-A receptors
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
How do neurosteroids allosterically modulate GABA-A receptors?
High doses may directly activate the receptor
Positive modulator: allopregnanolone
Negative modulator: pregnenolone sulphate
Direct activation: allopregnanolone (higher dose)
Indications for modulating GABA-A receptors
Agonists/antagonists- only used experimentally
Benzodiazepine modulation- anxiolytics, sedatives, antiepileptics
Barbiturates- general anaesthetics, antiepileptics
Neurosteroidal modulation- post-partum depression, premenstrual tension, catamenial epilepsy
Glycine receptor
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
Other ionotropic receptor families
Nicotoid receptors
Glutamate receptors
P2X receptors
Glutamate receptors
Each subunit consists of three TMs and a re-entrant loop
NMDA receptors- N-methyl-D-aspartate
non-NMDA receptors- AMPA, kainate
AMPA glutamate receptors
Widespread throughout CNS, regional specificity for subtypes, responsible for fast synaptic excitation
Kainate glutamate receptors
Widespread throughout CNS, regional specificity for subtypes, responsible for fast synaptic excitation
NMDA receptors
Throughout CNS/PNS, excitatory, structurally complex
Used in CNS development, memory, learning, apoptosis
Involved in Alzheimers, Parkinsons, Huntingtons, schizophrenia, epilepsy, encephalitis, stroke
Regulation of NMDA receptors
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
How do zinc and polyamine regulate the NMDA receptors?
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
P2X receptors
Fast synaptic transmission
Neuron-smooth muscle transmission
ATP-mediated lysis of antigen-presenting cells
