receptors and chemical mediators

Question 1

what did John Newport Langley do?

Answer 1

he tried to figure out how the poisons pilocarpine and atropine work
through animal experiments he figured out pilocarpine lowered heart rate (and increased salivation), but atropine blocked this if taken before
he concluded they both must affect the same thing

he did similar stuff with chicken legs, using nicotine (causes muscle contraction) and curare which prevents this. curare applied first prevents the action of later added nicotine

Question 2

define signal transduction and some key ideas as to how it works

Answer 2

1. extracellular signal
2. binds to specific receptors on target cells
3. initiates intracellular signalling that alters cell behaviour
   *most extracellular signals act on multiple cells as the receptor is expressed on multiple cell types
   This allows wide spread coordinated responses in multiple organs e.g. adrenaline
   Cascade signalling causes amplification

Question 3

describe contact dependent signalling and its therapeutic uses

Answer 3

this is the shortest range, important in development, using a membrane bound signalling molecule
Immune response - T cells - CAR T immunotherapy - uses contact dependent signalling - engineered receptor designed to recognise cancer cells, inserted into patient genome, kills cancer cells

Question 4

define and describe paracrine and autocrine signalling

Answer 4

paracrine - one signalling cell with a few surrounding target cells
a subsection of this is autocrine signalling, the cell releasing the signal also has receptors for that signal, in a feedback system to let it know when there’s enough
these act locally and use different chemical mediators

Question 5

what kind of chemical mediators are used in paracrine signalling and what for?

Answer 5

Uses different kinds of chemical mediators :
Stored - e.g. in vesicles, proteins and growth factors, amino acid derivatives like histamine

Synthesised on demand - stuff that’ll just go straight across plasma membranes like lipid based substances and gases. Used in inflammation, cell proliferation and wound healing

Question 6

how are paracrine and autocrine signalling used in pharmacology?

Answer 6

Antihistamines work on receptors - mast cells detect allergens, release histamine causing itch and redness (vasodilation) antihistamines block the receptors.
Paracetamol - work via intracellular signalling - prevents production of eicosanoids to stop inflammation.
Nitric oxide - diffuses right into the cell, causes production of cyclic GMP, relaxes smooth muscle cells, vasodilation. Viagra prevents breakdown of cyclic GMP

Question 7

broadly, how does neuronal signalling work?

Answer 7

neurotransmitters, diffuse across the synapse.
Can be long or short distance, super quick.
drugs/medicines can affect pre or postsynaptic terminal i.e.
production of neurotransmitters (cause or prevent)

Question 8

how does endocrine signalling work and what are some common pharmacological uses?

Answer 8

endocrine cells release hormones, have to then travel through the bloodstream, can be local but often quite far, not selective of which cells they go to, but are selective of the receptor.
Uses =
Epi-pens - adrenaline works on a different pathway to the allergen but causes the opposite effects physiologically.
Insulin therapy (synthetic insulin for type 1 or increased signalling through insulin receptor for type 2) etc…

Question 9

how does endocrine signalling work and what are some common pharmacological uses?

Answer 9

endocrine cells release hormones, have to then travel through the bloodstream, can be local but often quite far, not selective of which cells they go to, but are selective of the receptor.
Uses =
Epi-pens - adrenaline works on a different pathway to the allergen but causes the opposite effects physiologically.
Insulin therapy (synthetic insulin for type 1 or increased signalling through insulin receptor for type 2) etc…

Question 10

what did Otto Loewi do? and Henry Dale?

Answer 10

two hearts, one with nerves removed (recipient), the other not (donor).
Stimulated vagus nerve of the donor heart, collected the fluid around the nerves and transferred it to the recipient heart, which also slowed down, proving there is something, a chemical or whatever. Called the fluid Vagusstoff

Henry Dale worked on motor neurons and muscles and realised it was acetylcholine in both cases - vagusstoff = acetylcholine

Question 11

what are the three experimental criteria for defining a chemical mediator?

Answer 11

1 .It is released from cells in sufficient amounts to produce a biological action on target cells within an appropriate time frame.

2. Application of an authentic sample of the mediator reproduces the original biological effect.
3. Interference with the synthesis, release or action (eg. Using receptor selective drugs, enzyme inhibitors, knock-down or knock-out techniques) stops or modulates the original biological response

Question 12

what are the main two ways chemical mediators are synthesised?

Answer 12

Typically specific enzymes are used to edit amino acids, so only cells that express these enzymes can produce the mediator. E.g. molecule ‘dopa’ converted to dopamine via DOPA decarboxylase.

Peptides produced via transcription, stored in vesicles, released on demand e.g. NO or prostaglandins (lipid based) for injuries - inflammation etc…

Question 13

what are the two usual ways of storing chemical mediators?

Answer 13

1. Pre-formed, stored in vesicles, ‘docked’ at the membrane, released by exocytosis.
   Allows for rapid communication.

Need to remove neurotransmitters from receptors quickly, most efficient is to take them back into the cell by endocytosis.

2. Produced on demand, released by diffusion or constitutive secretion, this means they take longer to act.

Ca2+ incredibly important in both scenarios - stimulates exocytosis and stimulates enzymes that form the on-demand mediators

Question 14

how did early studies of neurotransmitters lead to the idea of vesicles?

Answer 14

peaks in excitatory potentials occurred every 0.4 seconds (at even intervals) telling us that neurotransmitters are being released in set quantities hence the idea of pre packaged vesicles - this has been confirmed by electron microscopy

Question 15

how do vesicles respond to Ca2+?

Answer 15

vesicles involved in regulated exocytosis have a calcium sensor protein called synaptotagmin

Question 16

what are some ways of terminating neurotransmitter action?

Answer 16

enzymes to break down the NT like acetylcholinesterase
common way is to use transporters in the presynaptic membrane to take in the NTs and repackage them, as this is more efficient than synthesising from scratch

Question 17

what are two pharmacological links to termination of NT action, one to do with a prescribed drug, another to do with an illegal class of drugs?

Answer 17

1. fluoxetine (Prozac) targets transporters to prevent reuptake of NTs (serotonin) to prolong the signal (its an antidepressant)
2. amphetamines hijack the transporter that loads noradrenaline, occupying its vesicles, as a result the noradrenaline is homeless, so gets forced out of the cell and causes responses where there shouldn’t be any

Question 18

what is the active ingredient in strepsils and how does it work?

Answer 18

lignocaine - it inhibits Na+ channels so that the cell wont become depolarised, preventing nerve impulses (so less pain)

Question 19

what common drug for epilepsy reduces abnormal neural activity in the brain and how does it do so?

Answer 19

gabapentin targets Ca2+ voltage gated channels involved in NT exocytosis

Question 20

define a ligand and the two categories

Answer 20

anything that binds to a receptor

agonist = causes a change that provokes a response in the cell
antagonist = prevents or inhibits a response

Question 21

explain an example of an agonist and antagonist (the one from Langley’s experiments)

Answer 21

in the heart, an agonist is pilocarpine, a drug that decreases heart rate
its antagonist is atropine, it occupies the receptors in the heart preventing the action of pilocarpine, in fact atropine would cause a slight increase in heart rate by preventing the body’s acetylcholine binding tot these receptors and lowering heart rate)

Question 22

dextrorphan is an antagonist for nicotinic acetylcholine receptors
why is it not okay to use this to treat nicotine addiction?

how can we get around this?

Answer 22

nicotinic acetylcholine receptors are involved in reward pathways, but also neuromuscular junctions - you don’t want your drug used to treat nicotine addiction to be effecting neuromuscular junctions as well

receptor families have subtypes - exploiting the differences in these subtypes is commonly used in pharmacology to ensure drug specificity

Question 23

what are the four superfamilies of receptors?

Answer 23

ligand gated ion channels /ionotropic
G protein coupled receptors / metabotropic
kinase linked receptors
nuclear receptors

Question 24

giving an example, describe the structure, response time and how to stop ligand gated ion channels

Answer 24

example = nicotinic ACh receptors
structure = a pore, inside lined with +ve ions means it allows in -ve ions and vice versa
takes milliseconds
responds to endogenous agonists like glutamate and ACh, to stop a response the agonist is removed or the receptors become desensitised

Question 25

giving an example, describe the structure, response time of G protein coupled receptors

Answer 25

example = muscarinic ACh receptors
structure = a single protein that spans the membrane 7 times, between the 5th and 6th transmembrane domain is the loop connecting to the G protein - the alpha subunit of this G protein is release upon ligand binding to cause a downstream effect
take seconds

Question 26

giving an example, how do kinase linked receptors work and how long do they take, including structural information?

Answer 26

insulin uses kinase linked receptors
ligand activates an intracellular catalytic domain, resulting in some kind of phosphorylation of a protein that leads to gene transcription to produce something (like insulin)
takes hours
structure includes a 20-25 A.a. transmembrane domain and an extracellular domain

Question 27

what do nuclear receptors do and what quality is required of their chemical mediators? give an example

Answer 27

regulate gene transcription, they are found in the cytoplasm until their ligand binds, resulting in translocation to the nucleus to bind to DNA and regulate transcription

the mediators must be lipophilic as they have tog et into the cell
an example is oestrogen

Question 28

how is myasthenia gravis treated?

Answer 28

its an autoimmune disease that causes destruction of ACh receptors, these cannot be replaced so instead anti-cholinesterases are used to prevent breakdown of ACh and give it more of a chance to activate the remaining receptors

Question 29

how does the structure of G proteins allow for variation?

Answer 29

G proteins are heterotrimeric with three different subunits, alpha, beta and gamma ( a GDP is bound tot he alpha subunit)

there are 30 different versions of these subunits, and there are different combinations, resulting in many possible G proteins that will interact differently with their receptors

so receptors that are the same could have different effects depending on their coupled G protein, or different receptors may have the same effect if they have the same G coupled protein

Question 30

explain what happens to the G protein when an agonist binds to the receptor

Answer 30

at rest, the alpha subunit has a bound GDP
binding of a ligand causes a conformational change that promotes the exchange of GDP for GTP
the G protein leaves the receptor, then the BY subunit separate from the a-GTP subunit, these go on to cause different downstream effects

Question 31

how does a G protein return to resting?

Answer 31

the alpha subunit is in itself a GTPase, so it removes a phosphate from the bound GTP, making it GDP again, and from their the whole thing can return to resting

Question 32

what are the possible effectors of GPCRs?

Answer 32

ion channels
membrane bound enzymes like adenylyl cyclase that use second messengers to initiate a signalling cascade

Question 33

what cascade does the G protein subunit αs (s for stimulatory) cause?

Answer 33

activates adenylyl cyclase
converts ATP to cAMP
activates protein kinase A
phosphorylates several possible things

Question 34

what does G protein with the alpha subunit αi cause?

Answer 34

i is for inhibitory, the subunit inhibits adenylyl cylcase

Question 35

what does G protein with the alpha subunit αi cause?

Answer 35

i is for inhibitory, the subunit inhibits adenylyl cyclase

Question 36

including structural details, how does protein kinase A work?

Answer 36

two catalytic domains, two regulatory subunits that keep the catalytic activity low, cAMP causes separation of the catalytic and regulatory domains, so they can phosphorylate loads of stuff

Question 37

adrenoreceptors are GPCRs - how do they work?

Answer 37

Ligand causes GDP to swap to GTP
G alpha ‘s’ released
Adenylyl cyclase
cAMP
Protein kinase A - phosphorylates calcium channels - Ca2+ influx - increases contraction - so assists in fight/flight response

Question 38

how do opioids reduce pain?

Answer 38

opioid receptors are GPCRs
their B-Y subunit directly effects ion channels, specifically GIRK, a K+ channel, increasing its permeability so that K+ leaves the cell, hyperpolarising it, reducing action potentials and therefore reducing pain

Question 39

how does the G protein with alpha subunit ‘q’ work?

Answer 39

Once bound to GTP, moves along and activates phospholipase C
This enzyme breaks down PIP 2, leaving its lipid chains in the membrane which are now called DAG - diacylglycerol

This then activates protein kinase C, phosphorylates stuff

The other part of PIP 2 left is IP3, IP3 is a ligand for a ligand gated ion channel - conformational change upon binding, opens pore, allows Ca+ out of the ER into the cytosol

An example is histamines causing this response, the Ca+ results in contraction of smooth muscle

Question 40

what are the general properties of adrenoreceptors and their three ligands?

Answer 40

typically sympathetic
agonists mimic fight or flight (antagonists do the opposite)
adrenaline, noradrenaline and isoadrenaline (last one is not endogenous)

Question 41

medically, what can adrenaline be used for?

Answer 41

adrenaline is typically the strongest of the three ligands and can therefore be used to reset the system, like in cardiac arrest or anaphylactic shock

Question 42

B1 adrenoreceptors - where are they located, what alpha subunit does it have and therefore what does it cause when activated?

Answer 42

located in the nodal and muscle tissue of the heart
has an alpha ‘s’ subunit so is stimulatory
alpha s subunit means adenylyl cyclase - cAMP - protein kinase A - Ca2+ channels open for longer, increasing muscle contraction and heart rate

Question 43

B2 adrenoreceptors - where are they located, what alpha subunit does it have and therefore what does it cause when activated?

Answer 43

located in the bronchi
g protein is GS (stimulatory)
adenylyl cyclase - more cAMP - PKA phosphorylates contractile machinery in smooth muscle cells causing it to relax - bronchodilation

Question 44

A1 adrenoreceptors - where are they located, what alpha subunit does it have and therefore what does it cause when activated?

Answer 44

found in heart muscle and blood vessels
coupled to Gq: phospholipase C - PIP3 to DAG and IP3 - more Ca2+
this means it usually causes muscle contraction

Question 45

A2 adrenoreceptors - where are they located, what alpha subunit does it have and therefore what does it cause when activated

Answer 45

located in smooth muscle
have Gi protein so inhibit adenylyl cyclase and reduce cAMP levels, so are involved in vasodilation

Question 46

define sympathomimetic

Answer 46

s drug that mimic the effects of the sympathetic nervous system - they hijack transporter vesicles for noradrenaline, compete for space which results in the noradrenaline being pushes out and causing a sympathetic response

Question 47

A1 adrenoreceptor antagonists are used to treat what? (give example)

Answer 47

example = prazosin
used to treat hypertension (as it would prevent any agonists causing vasoconstriction)

Question 48

how can heart failure be treated using adrenoreceptor antagonists?

Answer 48

the drug carvedilol is an antagonist for a and b receptors, its non selective
it lowers BP by targeting a receptors and lowers heart rate by targeting b receptors

Question 49

what does the drug propranolol used to treat anxiety do?

Answer 49

propranolol is b1 and b2 adrenoreceptor selective to alleviate physical symptoms of anxiety like increased heart rate and difficulty breathing

Question 50

both carvedilol and propranolol are both non-selective
why does this mean you must be careful who takes them?

Answer 50

they can cause unwanted side effects
B2 receptors when activated cause bronchodilation, antagonists would prevent this so carvedilol is avoided in asthma patients for example
also, too much cardiac depression must be avoided in the elderly and can cause fatigue and cold extremities

Question 51

what is salbutamol and what is it used to treat?

Answer 51

salbutamol is a B2 selective agonist and so causes bronchodilation, so is used to treat asthma
other B2 selective agonists are used in nasal decongestants