Mechanism of drug action and drug targets Flashcards
Name the four main families of receptors
Ligand gated ion channels
G-protein coupled receptors
Tyrosine/cytokine receptors
Nuclear/ steroid hormone receptors
Describe what happens following a NT binding to a Ligand-Gated Ion Channel
Causes an immediate conformational change that opens the channel portion of the protein, this allows ions to cross the membrane changing the membrane potential to change within 0.1-2 milliseconds.
What ion channels are opened through binding of ligands to excitatory receptors
Cation channels; Na+ and K+
What ion channels are opened through binding of ligands to inhibitory receptors
Chloride channels (Cl-), leads to hyper polarisation of the post synaptic membrane
What are the NT receptors utilised in slow synapses?
G protein coupled receptors
What are the NT receptors utilised in fast synapses?
Ligand gated ion channels
Describe the process of NT binding to G protein receptors
Binding of a NT leads to a conformational change in the receptor that results in activation of a specific G protein. G protein may directly activate or inhibit ion channels… or G protein activates or inhibits adenylate cyclase or phospholipase C, this triggers rise in cytosolic cAMP or Ca2+ respectively. These second messengers in turn affect the ion conductance of a linked ion channel or trigger a kinase cascade that results in phosphorylation or dephosphorylation of target proteins.
What are synapses where Ach is NT termed?
Cholinergic synapses
What are acetylcholine receptors that cause excitatory responses lasting only milliseconds called?
Nicotinic acetylcholine receptors.
What kind of receptors are nicotinic acetylcholine receptors?
Ligand gated ion channels
Name another kind of Ach receptor (not nicotinic)
Muscarine
What kind of receptor are muscarine receptors?
G proteins coupled receptors
Describe affinity
A measure of how tightly a drug binds to it’s receptor. Low affinity means the drug does not bind well and action will be shorter
How can you numerically measure affinity
By using the dissociation constant KD. This is the concentration of the drug when 50% of receptors are occupied
Describe potency
A measure of how much of a drug is required to produce a particular effect
How to measure potency
Using a concentration response curve and the EC50- the concentration of the drug required to produce 50% of maximal response
What is the effect of a drug with high potency
Only a small amount of the drug is required to induce a larger effect
Describe efficacy
The ability of a drug to produce a response
Describe efficacy and affinity relating to agonists, partial agonist and antagonists
Agonist has affinity and efficacy
Partial agonist has affinity but do not produce maximal response (even at 100% binding)
Antagonist has affinity but no efficacy
Describe ways in which enzymes can be utilised as drug targets
- competitive inhibitors
- Drug acts as a false substrate disrupting normal enzyme action
Describe how carrier proteins can be used as drug targets
Blocking transport by utilising recognition site specific to carrier protein.
List drug targets
Ion channels Receptors Enzymes Carrier/ transport molecules DNA
Describe the three types of ion channels
- Ligand gated ion channels
- Second messenger gated ion channels (can be opened directly by G proteins or downstream by molecules such as cAMP)
- Voltage gated ion channels- open in response to voltage changes across the cell membrane
Describe how local anaesthetics work
By blocking voltage gated ion channels preventing the AP that carries that pain signal from the site of injury to the brain
What is an inverse agonist
An agent that binds to the same receptor as an agonist but produces the opposite pharmacological effect.
Four steps of neurotransmission
Neurotransmission synthesis
Neurotransmitter release
Action on receptors
Inactivation
Describe the steps of an action potential passing from a pre-synaptic to postsynaptic neurone
1) Na+ channel brings choline into the cell and ACh is made using AcCoA, the Ach is packaged into vesicles
2) Action potential in pre-synaptic neurone activates voltage dependant Ca2+ channels
3) Ca2+ allows the Ach vesicles to fuse with the cell membrane
4) Ach enters synapse and binds to receptors in the post synaptic cell membrane
5) Ach broke down in synapse by Ach esterase to stop NT, choline recycled.
Describe ligand gated ion channels (five points)
- Mediate fast signal transmission
- All are multi sub unit complexes
- Activated in response to specific ligands
- Conduct ions through otherwise impermeable membrane
- Selective among ions
Describe the nicotinic acetylcholine receptor
- Stimulated by nicotine. Site see both ACh and nicotine
- 5 subunits around central receptor channel
- Each subunit has 4 transmembrane domains
- Two ACh binding sites on each channel always at the interface of an alpha subunit
- Binding sites sit between two subunits
- Binding to both sites need to occur for channel opening
What causes ion selectivity in the ion channel receptor pore
Amino acids in the transmembrane domain
What do most excitatory NT cause in increase in permeability in and why?
Na+ and K+
The inside of the cell becomes more positive (depolarised) and has an increased probability of action potential
Three examples of ligand gated (inotropic) ion channels that are used as receptors for drug targets
GABA2
Glutamate
Nicotinic
What drugs work on the GABA2 receptor
Benzodiazepines and barbituates
What drugs work on the glutamate receptors
Ketamine
What drugs work on nicotinic receptors
Nicotine and pancuronium (antagonist)
In G protein coupled receptor family what do Gs and Gi protein receptors do
Gs stimulates adenelate cyclase
Gi inhibits adenelate cyclase activates or inhibits cAMP
What do Gq protein receptors do
Stimulate phospholipase C leading to increased IP3 and DAG and then activate cascade
What are the G protein coupled receptors activated by
ACh or muscarine
What is the aim or pharmacology in terms of selectivity in g protein receptors
To create drugs that target specific M receptors (1-5) greater selectivity means less adverse effects.
Describe the pre-synaptic receptor
M2 receptor, usually Gi linked, when lingered binds leads to a blocking of edentate cyclase and less cAMP.
Causes inhibition of Ca2+ channels resulting in reduced release of ACh into synaptic cleft. Negative feedback
What are the effects of blocking the M2 receptor
Dramatic increase in the amount of NT released. Increased activity of the neuron.
Name a couple of other receptors that a GPCR
Opioid receptors, seretonin receptors
Describe tyrosine kinase receptors
Ligands include VEGF, EGF.
Generally have a single transmembrane domain which has a ligand binding site extracellularly which activates the intracellular tyrosine kinase.
Kinase phosphorylates proteins activating them.
Mediate the actions of growth factors, cytokines, and certain hormones.
What kind of receptor is the vascular endothelial growth factor receptor
A tyrosine kinase receptor
What does binding of VEGF to a tyrosine kinase receptor result in biologically (5 things)
- Endothelial cell survival
- Endothelial cell proliferation
- Endothelial cell migration
- NO and PGi2 production
- Increased vascular permeability
What are the four mechanisms of drugs binding to receptors
Van der Waals forces- weak
Hydrogen binding -stronger
Ionic interactions- between atoms with opposite charges- stronger
Covalent binding- irreversible
If you have a high Kd does the drug have a low or high affinity
Low because Kd is describing the concentration of the drug needed to obtain 50% occupancy of receptors. If you need a higher concentration of the drug to achieve this (high Kd) this shows that the drug has a low affinity.
In a log scare of Kd would the left of right hand curve have a higher affinity
Left as [LOG] L on X axis, lower concentration needs to achieve 50% binding, higher affinity
Why is the concentration/ response curve not a good measure of affinity?
Because the relationship between receptor occupancy and response is not strictly proportional e.g. there are many other factors that impact on final response.
How is biological response measured?
Through concentration response curves.
How can the response to a drug be classified
EC50 and Emax
Will a high potency drug have a low or high EC50?
Low as lower concentrations needed to produce the effect. E.g. the drug is very potent.
What is a reversible competitive antagonist
A drug which can reversible bind to a receptor with no efficacy (i.e. its point is to block endogenous ligand binding)
In a dose response curve with a reversible competitive antagonist present will the curve stay the same shift to right or left?
Parallel rightwards shift as higher concentrations of the agonist will compete with antagonist and get same response
In a dose response curve with a irreversible antagonist present what will happen to the curve?
Downward shit as same concentrations of the agonist will have a lower response as the irreversible antagonist has blocked some of the sites.
Why does the NT need to be inactivated once in the synapse
Because otherwise there would be constant stimulation of the post synaptic cell
What are the two mechanisms for inactivation
- Transport back to the pre-synaptic membrane
- Enzymic degradation
What is the enzyme that results in ACh degradation
Acetylcholine esterase
What happens when you administer an irreversible inhibitor of AChE
Extreme cholinergic response. E.g. nerve gas.
What happens with reversible AChE inhibitors
Increase cholinergic function. Used to treat myasthenia graves which causes break down of ACh binding sites… the AChE allows more ACh to be available.
How is serotonin inactivated
Through transporters that bind to serotonin and return it back to the presynaptic membrane
What is the effect of a selective serotonin re-uptake inhibitor
Bind to serotonin transporters and preventing the transport of serotonin back to presynaptic cell. Enhanced serotonin activity, enhanced mood.
