Opioids Flashcards
What are opioids
Endogenous or synthetic substance with morphine-like effects, antagonized by naloxone
‘Opiate’ synthetic morphine-like drug with non-peptide structure
Opioid actions include analgesia, respiratory depression, euphoria and sedation
Led to discovery of receptor subtypes
Morphine analogues
Agonists - morphine, diamorphine (heroin), codeine
Antagonists – naloxone
Synthetic derivatives: pethidine, fentanyl, methadone
What are agonists, partial agonists and antagonists
Opioids produce their effect by acting at the opioid receptors in the nervous system
-opioid receptor most important
Agonists
bind to the receptor and stimulate physiological activity
Partial agonists
bind to the receptor but do not produce maximum stimulation
Antagonists
have no intrinsic pharmacological effect, but bind to the receptor and can block the action of an agonist
describe the opiate chemical structure
form an alkaloid
add two acetyl groups to morphine you get heroine
CH3C=O and CH2=CH groups
opioid peptide and receptor families
the first thing to say is there are four opioid receptor types.
Three classical receptors mu, delta and kappa and a forth, originally identified by homology screening after the cloning of the receptors that identified a related receptor, orginally called the opioid receptor like 1 receptor.
Proopiomelanocortin (B- endorphin—- MOP and DOP)
proenkephalin (enkephalin) acts on DOP
prodynorphin (dynoprhin) acts on KOP
pronociceptin (nociceptin) acts of NOP
in each of these, the opioid peptides act on the opioid receptors
It not only shared homology but was also a classical Gi/Go coupled 7 transmembrane spanning receptor and it turned out to have an endogenous ligand that shared sequence homology with classical endogenous opioid peptide ligands that had already been identified as acting at the mu, delta and kappa opioid receptors.
MOP, DOP, KOP and NOP! The four receptors have four families of endogenous ligands that act at them.
POMC that gives rise to b-endorphin, Proenkephalin producing at least four distinct enkephalins, Prodynorphin producing two dynorphins and two neo-endorphins and pronociceptin that produces nociceptin and two other biologically active compounds, one nocistatin which is an antagonist at the NOP receptor.
Structure of opiate receptors:
Mu opiate receptor:
intracellular and extracellular domain
Comparison of the amino acid sequences of the cloned mouse deta and kapa and rat mu receptor. The blue indicated common amino acid sequences. G protein couple receptors.
Describe the structural homology of opioid receptors
Mu opioid receptors: intracellular and extracellular domain
Comparison of the amino acid sequences of the cloned mouse deta and kapa and rat mu receptor. The blue indicated common amino acid sequences. G protein couple receptors.
Met-enkephalin Tyr-Gly-Gly-Phe-Met
Leu-enkephalin Tyr-Gly-Gly-Phe-Leu
Dynorphin A Tyr-Gly-Gly- Phe-Leu-Arg-Arg-Ile-Arg-Pro-Lys-Leu-Lys-Trp-Asp-Asn-Gln
ß-endorphin Tyr-Gly-Gly-Phe-Met-Thr-Ser-Glu-Lys-Ser-Gln-Thr-Pro-Leu-Val-Thr-Leu…
Nociceptin(OFQ) Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln
Endomorphine No precursor found
How can radiographic sections show us where opiate receptors are located
radographic coronal sections
shows where mu opioid receptor is localied and distributed, shown in red
high levels of mu opioid receptor in regions of award- nucleus accumbens
explains why opioids are addictive- due to high density in areas associated w award
high areas in periaquedutal grey- associated w modulation of pain. explains why opioids are analgesics beause they bnd to regions associated w pian
These are coronal sections from fore to the beginning of the hindbrain and all of the images I will show you conform to the same colour coding. Blue is low, with increasing receptor levels from green through yellow to red and black. There is widespread expression in many brain regions, including the rostral to caudal cortex, motor regions such as the caudate, limbic structures such as the amygdala, stria terminalis, reward areas such as the accumbens and of course pain processing structures such as the thalamus and the colliculi.
pharmacological effects of opioids (factors to consider)
Consider:
Analgesia
Supraspinal
Spinal
Peripheral
Respiratory depression
Pupil constriction
Reduced gastric motility
Euphoria
Dysphoria
Sedation
Physical dependenceEffects of opioids on respiratory depression
Respiratory depression
reduced sensitivity of respiratory centre to CO2
Mediated by mu receptors
Most troublesome side effect, occurs at therapeutic doses
Commonest cause of death in opiate poisoning
Morphine used as reference compound
Euphoria likened to orgasm and sudden ‘rush’
usually when you have too mucb co2 signal is sent to your lungs from brain to lungs to hyperventilate and expel co2. this is desensitised by opioids
means co2 levels increase in blood and death occurs due to asphyxia
Describe opioids as analgesics
Enalodine, U50488 (KOP)/low dependence liability BUT psychotomimetic effect (withdrawn)
Peripheral acting KOP for peripheral pain and itching
DOP agonists poor analgesics but have antidepressant properties. However, proconvulsant effects
codeine reduces coughing
opioids induce nausea and vomiting initially as opioid receptors are found in emesis centre of brain and in chemoreceptor trigger zone
other actions of opioids on systems?
Cough reflex
Codeine used in cough medicine to suppress cough
Mechanism unclear, can occur at subanalgesic doses
Nausea and vomiting (Q. how?)
Occurs in 40% of patients
Opioid action in CTZ () and vomiting centre ()
Pupil constriction
Pinpoint pupils used as diagnostic feature in opiate poisoning
Other causes of respiratory depression & coma cause pupil dilatation
GI tract
tone and motility, constipation
Other actions
Morphine releases histamine from mast cells - itching
effector mechanisms of opioids
Opioid receptors: mu delta kappa/ inhibition
G-protein coupled receptors (Gi/Go)
increased K+ channel opening, hyperpolarization
decreased opening of voltage-gated Ca2+ channels
Reduce neuronal excitability (K+) and reduce neurotransmitter release (Ca2+)
Inhibition of adenylyl cyclase, decrease cAMP, PKA
mu delta and kappa receptors are all gpcrs- gi or go
so if heroine binds, receptor couplingw a Gi/go protein occurs
activates different 2nd messengers
beta gamma subunit of the g protein opens potassium ions causing them to leave (more conc in the neurone than outside)
causes hyperpolarisation of neurone
inhibits calcium channels- inhibits release of neurotransmitters from neurones
Mechanism for how opioids block pain?
Shows how opioids block pain- how they act as an analgesic
so let’s look at how pain is transmitted from periphery to brain
AFFERENT neurones have cell body in dorsal root ganglion with 2 projections: one projection to periphery and other towards dorsal horn of spinal cord
pain receptors called nociceptors are activated by mechanical/chemical/inflammatory stimulation
nociceptors carry action potential when activated along neurone, leading to dorsal horn of spinal cord
leads to release of neurotransmittes: substance P and glutamate
acts on neighbouring neurons which project from dorsal horn of spinal cord ot the brain where pain is detected
so this is how pain transmission NORMALLY works
anything that blocks the transmission of neurotransmitters from the periphery to the brain can be used as an analgesic
opioids by acting on opioids receptors in the periphery, dorsal horn of spinal cord as well as the brain act as ideal analgesics can block the transmission of pain , acting on 3 LEVELS of transmission making them great analgesics
Describe the dorsal spinal cord
afferent neurones project from periphery, carrying pain info
release substance p and glutamate
transmit to neighbouring neurone, projecting to brain
mu opioids receptors found post synaptically on cell body of neurones
also found pre-synaptically on afferent neurones , also post-synaptically on dorsal horn
morphine acting pre-synaptically would inhibit the release of substance p and glutamate (pain inducing neurotransmitters)
therefore pain is transmitted from periphery to brain
Describe the descending control system
Opioids excite the PAG neurons and neurons in the nucleus reticularis paragigantocellularis (NRPG) stimulating neurons in the nucleus raphe magnus (NRM)
The 5-HT and enkephalin neurons of the NRM run to the dorsal horn and inhibit transmission
Opioids can act directly on the dorsal horn (and on peripheral terminals)
NAergic neurons of locus coeruleus also inhibits dorsal horn
the periaqueductal- PAG area of the brain is activated by opioids
stimulates descending inhibitory neurones
PAG to DLF
stimulates other inhibitory neurones like serotonin and enkephalin
release in dorsal horn of spinal cord
suppress information from periphery to dorsal horn of spinal cord to block transmission of pain
List examples of opioid analgesics
Diamorphine (MOP)
Used as analgesic in UK
Tissue injury, tumour growth
Oxycontin
Pethidine (MOP)
Favoured for analgesia during labour as no in uterine contraction
Pethidine slowly eliminated in the neonate, naloxone may be needed to reverse respiratory depression
Fentanyl (MOP)
More rapid onset and shorter duration than morphine
Main uses: anaesthesia, patient-controlled infusion, severe pain
Codeine
Readily absorbed, only 20% potency of morphine
Used as oral analgesic for mild pain
No euphoria, constipation
Dihydrocodeine
Useful in 10% of population who are resistant to codeine – lack demethylating enzyme converting codeine to morphine
Tramadol
- opioid agonist and weak NA reuptake inhibitor
Naloxone, antagonist which do not cross BBB for opioid constipation
Describe the uses of opioid receptor antagonists
Naloxone use to reverse opioid-induced overdose
Naltrexone use for treatment of opioid addiction and alcoholism
Why has opioid overdosing increased and what is the opioid crisis
doctors are prescribing more opioids
then after 2012 when they realised the prescriptions were a problem this reduced
people becoming tolerant causing them to switch from prescription based opioids to somethng stronger like heroine or phentayl
21-29% of patients prescribed opioids for chronic pain misuse them
8-12% develop an opioid use disorder
4-6% who misuse prescription opioids transition to heroin
80% of people who use heroin first misused prescription opioids
Addiction” among chronic pain patients varies (3%-40%) depending differences in treatment duration, assessment of addiction
Properly managed short term medical use of opioid analgesics rarely cause addiction but physical dependence is more common
Risk of addiction increases when used in other ways as prescribed (high doses, different route of administration, combined with alcohol, history of comorbid psychiatric disorders, genetic polymorphisms, age (adolescents, older adults)
Describe tolerance to opioids- opioid disinhibition effect
dopaminergic neurones projecting from VTA to NA
when you activate reward system dopamine gets released into NA induces award
in VTA there are also inhibitory GABA neurones
when GABA released, inhibits dopamine release in NA
opioid receptors in synaptic bouton of GABA
heroine is an opioid- has inhibitory response- inhibits release of GABA
increase of mesolimbic pathway- increasing dopamine in NA- reward.
Effect of morphine in MOP knockout mice
conditional place preference test
mu opioid receptor KO mouse do not seek morphine anymore
microdialysis study show that mechanism is by incrasing dopamine release in NA
These KO mice were immediately used to identify the role of these receptors and peptides in opioid reinforcement. These are the first studies done in Kieffers Mop KO mice demontrating that MOP are the primary molecular target for the rewarding effects of morphine. In order to investigate the rewarding effects of morphine in WT and MOP KO annimals, they used the CPP paradigm which can see here, is a paradigm based on pair associating of the drug with one compartment and saline with the other. We can see here that the WT mice seen in yellow has very high preference to the morphine paired compartment, but in the knockout mice that preference was completely abolished suggesting that the rewarding, the positive reinforcement effect of opioid is mediated via the MOP. Further freely moving microdialysis studies were carried out in the nucelus accumbens which is this region here of morphine treated miceWT and MO KO mice. As you can see here 20 mg/kg of morphine increased dopamine in the nucleus accumbens of WT mice but not in Ko mice. This really shows that activation of MOP induces DA release. Together these data suggest that the MOP is central in mediating the positive reinforcement.
Effect of morphine on KOP knockout mice
mu receptors increase dopamine
kappa receptor activation decreases dopamine in NA - cause aversion
Now lets look a the situation in mice lacking the KOP. Again these were done in Briggite Kieffers KOP KO. As you can see the CPP was used in order to investigate the motivational properties of KOP agoinists. As we can see here KOP agonist U50488H compound induced a place aversion in the CPP paradigm in the WT mouse. It actually spent 400 secs more in the chamber not associated with the KOP agonist injection. This effect was completely abolished in the KOP KO mouse suggesting that activation of the KOP system produces an aversive/dysphoric effect. Further freely moving microdialysis data in the nucelus accumbens of naïve WT, het and KO KO mice. As you can see there were significantly higher levels of dopamine in KO mice vs WT indicating that KOP activation decrease dopamine release in the brain. Together this data indicates that activation of the KOP system produce an aversive/dysphoric/antirewarding effect. Indeed this is a well known effect of KOP agonist is the prime reason that enalodine which was developped as a KOP selective analgesic failed in clinical trials together with its psychotomimetic effects.
how do opioids act on a molecular level
morphine binds on opioid receptor
g protein
depolarisation
if you take morphone for prolonged period of time
bombard mu opioid receptor
g receptor kinase will deposit phosphate group
arrestins and beta arrestins will be recruited to receptor and DESENSITISE it, stopping it from producing
some receptors get internalised or get fused with lysosome and recycledback to membrane
some opioids such as morphine do not internalise
evidence that opioid receptors demonstrate agonist selective endocytosis
after giving morphine, no internalisation of receptors
but chronic administation of damgo internalises the opioid receptor
Opioid Receptors Demonstrated Agonist-Selective Endocytosis
(a) FLAG-tagged μ opioid receptors (μORs) remained predominantly in the plasma membrane in cells incubated in the absence of agonist (NT). μORs in cells incubated with 5 μM of the agonists DAMGO, methadone (MD), or etorphine (ET) were endocytosed, as indicated by redistribution of antibody-labeled receptors from the plasma membrane to numerous endocytic vesicles. Morphine (MS) or the mixed agonist/antagonist buprenorphine (Bup) failed to induce detectable endocytosis of the μOR.
(b) Internalization of receptors in response to each agonist examined in (a) was confirmed biochemically using cell surface biotinylation and protection, where internalized receptors are protected from cleavage by membrane-impermeant reducing agent.
(c) The relative activity of the agonists DAMGO, morphine (MS), and methadone (MD) for causing rapid internalization of the μOR was quantitated by immunofluorescence flow cytometry.
opioid withdrawal symptoms
Physical, characterised by abstinence syndrome (LC)
Sweating, gooseflesh (cold turkey), irritability, aggression, restlessness, insomnia, bone pain, diarrhoea, vomiting, involuntary leg movement
Psychological, craving to avoid withdrawal effects
What is the mechanism of dependence is opioids- homeostatic compensatory neuroadaptation
Chronic drug administration – homeostatic adaptive changes to oppose the drug action. Withdrawal of the drug can cause a rebound effect
continuous activation of mu opioid receptor causes desnsitization
withdrawal-
usually activation of opioid receptor reduces AC and cAMP so withdrawal will increase cAMP
happens in many different neurones including noradrenergic neurones that project from the locus coerelus to different regions in the brain- inducing noradrenaline in the brain
these cause physiical withdrawal symptoms- these are homeostatic compensiations of the opioid pharmacolgical effects
Dopamine D2 images of Drug Addiction
this is associated with increased craving and relapse of the drug
dependence reduces frontal cortex activity
What is the mechanism of dependence in inhibiting the HPA axis?
Heroin addicts have Hyporesponsivity HPA axis Cocaine addicts have Hyperesponsivity HPA CRF in extended Amygdala increased in withdrawal CRF antagonist block withdrawal in animal models CRF desregulation long lasting (tolerance kicks in)
How opioids work in pain modulation
Opioids excite the PAG neurons and neurons in the nucleus reticularis paragigantocellularis (NRPG) stimulating neurons in the nucleus raphe magnus (NRM)
The 5-HT and enkephalin neurons of the NRM run to the dorsal horn and inhibit transmission
Opioids can act directly on the dorsal horn (and on peripheral terminals)
endogenous opioids can also act on PAG where opioid receptors are found
endogenous opioids also knownn to be released from interneurones in dorsal horn of spinal cord
release of endogenous peptides from neurones that can act on brain, spinal cord or periphery
How dynorphin causes aversion
Stress activates dynorphin through a CREB (cyclic AMP (cAMP)-responsive-element-binding protein) pathway
Dynorphin in then release in the Nacb
Dynorphin binds to KOPr in Nacb and inhibits release of dopamine in the Nacb
Dynorphin binds to KOPr in VTA and inhibits dopaminergic neuron activation
This induces aversion
What is the role of endogenous opioids in addiction
Drugs of abuse including nicotine, alcohol, cocaine induce the release of enkephalins and endorphins
Release of endogenous opioid mediate the pleasurable effect of some drugs of abuse (as well as palatable food)
Cocaine induced the release of dynorphins in the striatum thus mediating the aversive effects of cocaine
Cocaine induces MOP and KOP upregulation
What is the effect of cocaine on the KOP system
From all these studies we can conclude that both MOP and KOP system play a central opposing regulatory roles in the motivational effect effects of cocaine. Cocaine activates the MOP system which is involved in the positive reinforcement, hedonic effect of cocaine by enhancing dopamine release in the nucleus accumbens which is evident in non dependent individuals. Cocaine also activates the dynorphin/KOP system which has the opposite effect of MOP. It reduces,opposes the +reinforcement, the rewarding and sensitisation effect of cocaine by inhibiting the release of dopamine in the nucleus accumbens. That could trigger the transition from an active +positive reinforcement effect of cocaine in non dependent individuals to a supressed positive reinforcement effect of the drug which occurs during dependence. In addition the activation of the KOP system by cocaine also activates the negative reinforcement by inducing dysphoria, stress and anxiety which will be driving the SA of the drug. In other words all these studies suggest that activation of the KOP system by cocaine plays a central role for the transition from +reinforcement, impulsive to a -reinforcement compulsive cocaine SA. This implies that mixed MOP/KOP antagonist could be beneficial for the treatment of cocaine dependence and the prevention of relapse. Indeed a recent clinical trial using a buprenorphine (Kappa antagonist) and naltrexone (non slelective opioid antagonist) improved compliance compared to naltrexone alone in cocaine addicts supporting the therapeutic potential of KOP andtagonists.
Describe the genetics for opioid addiction
certain polymorphisms of the opioid receptor show that A118G
responsible for greater addiction to opioids
functional polymorphism where aspartin is changed to aspartate
this single amino acid change increaseds severity of addiction
