Opiod Receptors Flashcards
What proves that the opioid pain pathway has been arround for a while?
Components of the innate opioid pain defence mechanism can be found in jawless fish
Idicates that it has evolved for >450 million years
What are examples of opium alkaloids (opiates)?
Morphine
Codeine
Thebaine
Oripavaine
What is the link betweem opium and heroin?
Opium can be refined to produce heroin (dacetyl-morphine, aka diamorphine)
What are examples of endogenous opioids?
Endorphins (beta-endorphin)
Enkephalins (met-enkephalin, dynorphin, endomorphins)
What is the structure of an opioid receptor?
GPCR: has 7 alpha-helix transmembrane domains
What is the function of an opioid receptor?
Ligand binds, inhibitory Gi/o protein is activated > transduces effect to effector proteins > adenylyl cyclase decreases, decreased Ca entry, increased K exit > reduces function of the cell/neuron the receptor is located on > effect is based on location of receptor
Name 4 opioid receptors
Their gene
Selective agonist
G protein
MOP (mu)
OPRM1
DAMGO (synthetic peptide)
Gi/o
DOP (delta)
OPRD1
DPDPE (synthetic peptide)
Gi/o
KOP (kappa)
OPRK1
U50,488-H (synthetic peptide)
Gi/o
NOP (not an opioid receptor, but very much like one)
ORL1
Nociceptin (endogenous)
Gi/o
What kind of receptors do opioids activate?
Metabotropic receptors (GPCRs) only
G proteins are the second messengers
What effects do opioids have on behaviour?
Analgesic (decreases pain)
What is the ascending pain pathway?
Injury > primary afferent nociceptors >
What is hyperalgesia?
Great sensitivity to pain
Opioids and tolerance
The body develops a tolerance to opioids
Using opioid analgesics causes a tolerance to develop
- The curve of analgesia (y) against opioid concentration (x) shifts to the right after weeks of opioid use (more is opioid is needed for the same analgesic effect)
- Shows a loss of potency
What is an analgesic?
A medication that reduces or eliminates pain
What is Opioid Induced Hyperalgesia (OIH)?
The pain worsens after the opioid effect wears off
Plotting a graph of pain intensity (y) against stimulus intensity (x) shows that after weeks of opioid use, the graph shifts to the left (people feel pain sooner than they used to)
What are some side effects of opioid use?
Reward pathway leads to addiction
Respiratory depression
Immunosuppression
Constipation
Hyperalgesia
Tolerance/dependence
Mu-opioid Receptors (MOPrs) endocytosis and signalling via beta-errestin
G-protein coupled receptor kinase (GRK) gets displaced by beta-arrestin 2 (beta-arr2)
The GPCR gets endocytosed > dephosphorylates by phosphatase > can either undergo lysosomal degradation OR recycled and put back in the plasma membrane
beta-arr2 activates the following pathways: MAPK, Src, Akt
Arrestins
Arrestins 2 and 3 are better known as beta-arrestin-1 and beta-arrestin-2 as they interact with beta-adrenergic receptors
Unbound the c terminus of beta-arrestin is buried
Bound, the c terminus is exposed
Mouse model of morphine analgesia and tolerance
Place the tail of a mouse in warm/hot water and time how long it takes for the mouse to remove their tail (tail withdrawal latency)
- Can test the development of tolerance
Beta-arrestin-2 knockout mice
- Show almost no effect of tolerance (beta-arrestin-2 is necessary for tolerance)
The influence of beta-arrestin2 on opioid receptors
Has an effect on:
- Basal analgesia mediated by MOPrs
- Morphine tolerance/dependence
- Locomotion
- Reward
(For locomotion and reward beta-arr2 may actually exert their effect on the dopamine receptors rather than MOPrs)
What is the point of perceiving pleasure?
Being rewarded for things that are good for the survival of the species
How are the rewarding effect of opioids measured?
- Selfadministration paradigm
- Conditioned place preference
(MOPrs knockout mice do not respond the same as WT mice)
WT - show preference for light
MOPrs KO - no preference
Beta-arr2 KO - more preference for light than WT
Dopamine and its receptors
- DA regulates locomotion, cognition, reward, emotions, endocrine function
- DA dependent disorders: schizophrenia, bpd, depression, Parkinson’s, drug abuse
- DA is synthesised by tyrosine hydroxylase (TH) in specific neurons
- DA receptors: D1-D5
D1 like: D1, D5
D2 like: D2, D3, D4
In what brain pathways are DA neurons found?
Nigostratial, mesolimbic, mesocortical, tuberoinfundibular, tuberohypophysial
Nigostratial - DA neurons originate from the substantia nigra and VTA, projecting to the caudate putamen and NA respectively
DA and second messengers
D1 like - activate adenylyl cyclase
D2 like - inhibit adenylyl cyclase
DA receptors and morphine conditioned place preference (CPP)
D2 receptors required for morphine CPP
Beta2-adrenergic receptor
Conformational change of the receptor depends on the agonist that binds
Characteristics of the ‘ideal’ opiod
- Activates Gi/o with the same efficacy
- Poorly recruits beta-arrestin
Can do computational screening to see if existing drugs that aren’t known for their opioid receptor activation, to see if they could possibly activate opioid receptors in the right way
Screen their propensity for possible side effects, their activation of Gi, activation of Go, and recruitment of beta-arr2
What is the barcode hypothesis?
That beta-arr2 is a biased agonist
Argues that beta-arr2 can be recruited by itself or with a whole lot of proteins/signalling molecules attached
- It can therefore do different things, one may be detrimental, and the other not
What is a mechanism that could account for the barcode hypothesis?
Differential phosphorylation of the GPCRs by GRKs
Ie. one G protein-biased ligand cause GRK2/3 phosphorylation resulting in beta-arrestin recruitment without associated signalling molecules. Ligands that cause GRK5/6 lead to beta-arr2 mediated signalling
What does ‘in vivo’ mean?
Within a living organism
What does ‘in vitro’ mean?
In an artificial environment outside a living organism
What are the 3Rs of animal testing?
Replacement (can other things be used?)
Reduction (use as little animals as possible)
Refinement (cause as little stress as possible)
What can in vitro opioid testing/pharmacology show?
Binding
Activity
Signalling
Regulation
What can in vivo opioid testing/pharmacology show?
Antinociception
Respiratory depression
Opioid Induced Hyperplasia and Tolerance
What are experimental considerations for in vitro experiments?
Signal amplification
Ceiling/floor effects
Kinetics
Cellular context
What are experiment considerations for in vivo experiments?
3Rs of animal testing
Strain sensitivities
Methods and cut-offs
Tissue receptor reserves
