Opioids Analgesics Flashcards
Describe the 3 different forms of analgesia
Local anaesthetics; analgesia close to the source of the pain
Central analgesia; analgesia through central/ spinal sites
GA; loss of consciousness +/- analgesia
How are the majority of opioid analgesics metabolised?
CYP enzymes p450 metabolic enzymes) in phase 1 of metabolism
Which opioid stands out in terms of metabolism?
Morphine; not metabolised by CYP in phase 1
Alternatively; morphine is metabolised in phase 2 via glucuronidation (kidney) via UGT2B7
This property of a lack of interaction with p450 (liver) enzymes makes it a useful drug as it is unlikely to interact with other drug metabolism
Describe the impact of CYP2D6 polymorphisms in opioid metabolism
Prone to polymorphisms e.g. ultra rapid metabolisers (strong analgesia and possible overdose; 1-2%) Extensive metabolisers (88%) Intermediate metabolises (8%) and poor metabolizers (8% - poor analgesia)
Are codeine and hydrocodone pharmacologically active as its inert state?
No; metabolised by CYP2D6 to morphine where there is binding to opioid receptors
Describe the impact of age on the prescribing of opioids
In general, older patients tend to have a reduced clearance
Describe the effect of sex on the prescribing of opioids
Majority have no effects between sexes except;
Oxycodone; concentrations around 25% higher in women than men
Describe the impact of ethnicity on the prescribing of opioids
Chinese patients tend to have a higher clearance of morphine (influence with enzymes involved with glucoronidation)
CYP2D6 allele variants may alter metabolism of codeine and hydrocodone; more common in Asian or African descent
Describe the impact of hepatic impairment on opioid prescribing
Dose adjustment recommended with morphine, codeine, oxycodone, methadone, hydromorphone
Oxycodone CI in hepatic impairment
Hydrocodone and codeine are often administered in combo with paracetamol; monitor LFTs
Describe the impact of renal impairment on opioid prescribing
Dose adjustment recommended for all
Caution with morphine (renally excreted)
Describe the impact of substrates, inhibitors and inducers of p450 on the metabolism of opiates
Substrates such as statins, BZD, SSRIs and psych drugs also require p450 to be metabolised which therefore individuals who are taking these drugs should be treated with some opioids cautiously as the other drugs may be competing for metabolism
Inhibitors such as chemo agents, anti-fungal, antiretroviral drugs - these drugs bind to p450 and inhibit the enzyme action. Enhanced levels of opioids
Inducers such as statins and anticonvulsant agents act to increase the expression of p450; reduced circulating level of opioids
Describe the 3 opioid receptors
MOPr (mu)
DOPr (delta)
KOPr (kappa)
What is alternative splicing?
Multiple different forms of mRNA can be produced from one single gene via splicing of exons and introns splicing
What is the interest with mu opioid receptor alternative splicing?
Alternative exons can be included in the mRNA
This can give rise to different types of mRNA in different cells
This results in the receptors having different functional properties
Variability between mu receptors in different brain regions
Can opioid receptors comibine?
Yes; wealth of evidence to show that mu and delta receptors can combine to form heterooligomers/ dimers
Describe the basic pharmacology of mu receptors
Gene; OPRM1
Selective agonist; DAMGO
G protein; Gi/o
Effectors; decreased adenylyl cyclase, increased potassium, decreased calcium
Numerous alternative splicing variants reported
Polymorphisms; 118 A-G; decreases morphine and alfentanil effects
Describe the basic pharmacology of delta receptors
Gene; OPRD1
Selective agonist; DPDPE
G protein; Gi/o
Effectors; decreased adenylyl cyclase, increased potassium, decreased calcium
Alternative splicing >1 likely
Polymorphisms; several, involved in naltrexone response in alcoholics
Describe the basic pharmacology of kappa receptors
Gene; OPRK1
Selective agonist; U50
G protein; Gi/o
Effectors; decreased adenylyl cyclase, increased potassium, decreased calcium
Alternative splicing >1
Polymorphisms involved in naltrexone response in alcoholics
Describe the basic pharmacology of nocicpetin receptors
Gene; ORL1
Selective agonist; nociceptin
G protein; Gi/o
Effectors; decreased adenylyl cyclase, increased K+, decreased Ca2+
3 alternative splicing variants reported
Polymorphisms involved with opioid addiction
How can muR polymorphisms affect opioid analgesia?
APRM1 A11G gene variant can decrease morphine’s analgesic responses post op
Carriers of G-allele observed to exhibit higher opioid analgesic requirements
Strongest in Asian patients, morphine users and those receiving surgery to a viscus
Where are opioid receptors expressed in the pain pathways?
Throughout;
Expressed in the primary afferent nociceptors, particularly are presynaptic terminal (to inhibit sub P and glutamate release to prevent excitation of DH neurons)
Thalamus
Cortex
Also involved with descending inhibitory control of pain pathway (disinhibit the neurons in PAG to allow for descending inhibitory influences in pain)
What creates the central pore of a voltage activated calcium channel?
Alpha 1
What subunits support and modify the actions of the calcium subunit?
Alpha 2
Delta
Beta - influences inactivation and voltage dependent activation
Gamma
Alpha 1 = pore conducting, contains pore loops which give selectivity to calcium ions
Are there different types of voltage activated calcium channels?
Yes
Not all are inhibited by opioid binding
Describe VA Ca2+ channels Cav 1.1-1.4
CaV 1.1-1.4; all sensitive to dihydropyridine (specific antagonist) = L type calcium channel (channel supports currents that are long lasting and therefore show less inactivation)
Excitation-contraction coupling of cardiac myocytes
Hormone release; regulates transcription, synaptic regulation, nT release from sensory cells
Describe VA Ca2+ channels Cav 3.1-3.3
No specific antagonist
T type channel; transient, inactivate more rapidly then L type
Pacemaker cells; repetitive firing
Describe VA Ca2+ channel Cav 2.3
R type; resistant to inactivation
Selective antagonist; SNX-482
Dendritic calcium transients
Describe VA Ca2+ channel CaV 2.2
N current; nerve terminal
Found on nerve cell terminals and dendrites
Controls nT release; dendritic Ca2+ transients; hormone release
Describe Va Ca2+ channel CaV 2.1
P/Q current; purkinje cells of cerebellum with a specific splice variant
nT release, dendritic calcium transients, hormonal release
Which calcium channels are most relevant to opioids?
CaV 2.1 and 2.2
Located at nerve terminals where they can affect nT release
These can be inhibited by opioid binding
Describe the different types of calcium channels on their influence of neurons
L-type; cell soma, Influences gene expression
T-type; participate in rising phase of action potential as it travels through axon hillock
N and P/Q type; presynaptic where they control calcium entry into synaptic terminal; instrumental on vesicle docking and nT release
What is the impact of mu opioid receptor coupling to calcium channels via whole cell patch clamp recording?
Beta gamma subunit of G protein binds to calcium channel to exert its effects - inactivation
Presence of DAMGO there is an inhibition of part of the voltage activated calcium current (but not completely) - therefore suggests that there are other types of calcium channel types in the cell (not just N, P/Q) such as resistant L or T type calcium channels
Describe tolerance to opioids
Adaptation that occurs in the neurons that respond to neurons leading to reduced sensitivity and reduced analgesia
However, this is not a reduction in efficacy but instead a reduction in potency - higher doses are required to create the same level of analgesia
What other aspect of opioid analgesic side effect can tolerance create?
Opioid induced hyperalgesia
Pain initially goes down on exposure to morphine, after prolonged exposure there is a paradoxical recovery of pain perception, but individuals can become hyperalgesic
This is due to tolerance to endogenous opioid production - lack of potency of exogenous analgesics but also a decline of innate ability to resist pain via endogenous opioid signalling mechanisms
Stimuli that previously doesn’t cause pain becomes painful (allodynia) resulting in a hypersensitivity to pain (hyperalgesia)
Describe the main side effects of opioid analgesia
Reward = addiction Resp depression Immunosuppression Constipation Hyperalgesia Tolerance/ dependence
Describe the 4 mechanisms that underlie tolerance
Receptor tolerance: decreased surface receptor, decreased receptor coupling, decreased receptor protein, decreased receptor RNA
Cell tolerance and withdrawal: initially decreased cAMP, with then an adaptation. Therefore on removal of the opioid, there is an unmasked hypertrophy of cAMP
System tolerance and withdrawal: feedback circuits adapt
Synaptic plasticity in tolerance and withdrawal: well established at GABAergic synapses. increased AMPAR insertion; increased LTP to drug seeking behaviour
Describe potential targets for peripheral nociception inhibition (novel analgesics)
Selective Nav and Kv ligands
Cav, TRP and P2X antagonists
Biased GPCR signalling
GPRC dimers
Describe potential targets for central (DH) targets for novel analgesics
Opioid receptors Cannabinoid receptors mGluRs GABA AMPA/ NMDA Microglial cells Astrocytes
Is it easy to demonstrate opioid tolerance in the lab?
Yes; tail withdrawal assay
10mg/kg a day; shows tolerance by day 4
Does morphine analgesia require mu receptors?
Yes; MOR +/- mice causes a rightward shift in the dose response to morphine
MOR -/- mice show no analgesic affects in response to morphine analgesia
What do mu receptors mediate in the effects of opioid analgesia?
Analgesia Reward Resp depression Immunosuppression Constipation Tolerance, withdrawal and dependence
What was the effect of specific k/o of mu receptors in nociceptors but not the microglia?
The effect of morphine tolerance was abolished with no disruption of analgesia
Peripheral mu opioid receptor expression on nociceptors is required for full analgesic tolerance
Interesting research to lead to development of novel analgesics which antagonist the peripherally located mu opioid receptors - hypothetically, this would decrease tolerance with no effects on central analgesic effects
How does the mu receptor activation cause tolerance?
Primarily due to beta arrestin protein 2 (bind to GPCRs)
What is the action of barr2 to result in opioid receptor tolerance
B-Arr2 will endocytose mu receptors via formation of clathrin coated pits. This allows for recycling of the receptor to be reinserted to the membrane without the agonist present.
Additionally brings kinases such as MAPK, Src, Akt to the receptor to phosphorylate downstream effectors
Can result in lysosomal degradation of the mu receptor
What are the effects on opioids in b-arr2 k/o mice?
Lack of tolerance development Decreased resp depression Decreased constipation Increased reward Increased basal analgesia mediated by constituently active MOPrs
What happens in +/- MOPs in terms of tolerance to morphine analgesia?
Shift in dose response to analgesia; show a basal level of tolerance
Apply morphine daily; much more profound tolerance to WT
No. of Mu opioid receptors is critical to the rate of analgesic tolerance develops
Are the delta receptors required for tolerance to morphine analgesia?
Delta doesn’t play much of a role in morphine analgesia (-/- DOP show similar dose dependency to morphine at WT)
Morphine tolerance is greatly diminished in -/- and +/- mice
DOP participates in tolerance
How can delta receptors influence tolerance of mu receptors?
Formation of heterodimers
Mu and delta receptors can combine and influence each others properties
This heterodimer has distinctive properties
This can occur in the SC and DH neurons
Is there a link between DOPrs and b-arr2?
DOPr recruits B-arr2 to MOPrs
These heterodimers recruit b-arr2 in the absence of an agonist
TOLERANCE
Do DOPs MOPrs dimers modulate analgesia?
Delta -/- and +/- show resistance to the development of tolerance
Antagonism of DOPs reduced morphine tolerance
Bivalent drugs with MOP and DOP antagonist may be advantageous
The bivalent opioid MDAN-21 analgesic causes little tolerance and dependence in mice
Does morphine internalize MOP receptors?
No; after 30 mins of exposure, morphine fails to cause endocytosis of mu opioid receptor
This analgesia shows tolerance
Mu opioid receptors become phosphorylated, with increased expression of B-arr2 = densitization
Does DAMGO internalize MOP receptors?
Yes; rapid endocytosis of mu opioid receptors
This analgesic mechanism does not cause tolerance
Therefore this result is the complete opposite of what is expected (you would associate tolerance with an internalization of mu opioid receptors)
What is the RAVE hypothesis?
Receptor Activation Versus Endocytosis – endocytosis resensitized receptors
Agonists that activate receptors without inducing endocytosis cause more tolerance because endocytosis allows for re-sensitization of desensitized receptors
This was proposed to explain greater tolerance induced by on-internalizing agonist morphine
What is the proposition to explain why different agonists at mu cause different side effects?
Agonist mediated phosphorylation after its bound the agonist
In order to recruit b-arr the mu receptor must be phosphorylated at the C-terminal. This phosphorylation occurs at theoronine and seronine residues
Describe the barcode hypothesis
Biased agonists bind to opioid receptors
If balanced; agonists activate a range of different types of kinases to lead to G protein signalling and B-arr2 recruitment
However; the agonists can be biased
G-protein biased ligand (endogenous opiate peptides) leads to activation of GRK2/3 –> recruitment of B-arr2 which allows for internalization of receptor (no tolerance)
B-arr biased ligand (alkaloids e.g. morphine) leads to activation of GRK5/6 –> activation of B-arr2 in a conformation that results in downstream signalling and NO internalization of mu (TOLERANCE)
Describe the ability of DAMGO, morphine and PZM21 to activate G1 and to recruit B-rr2
DAMOG; highest levels of G1 activation. Highest levels of B-arr2 recruitment
Morphine; Intermediate levels of G1 activation, much lower B-rr2 recruitment
PZM21; similar G activation to morphine, no b-arr2 recruitment
What is PZM21
Functionally selective mu opioids receptor which produced mu opioid receptor mediated G1 signalling with a similar potency and efficacy as morphine, but much less/no B-arr2 recruitment
Biased novel mu receptor agonist
Describe the difference between the experimental paradigms; hotplate analgesia and tail-flick analgesia
Hotplate analgesia; effective response. Measures the licking of paw after exposure to a hot plate (increased analgesia, reduced licking)
Tail flick is a reflexive response
Describe the impact of PZM-21 to hotplate analgesia and tail flick analgesia
Hotplate analgesia - very similar to morphine
Tail-flick analgesia - very limited effect
Can therefore postulate that PZM-21 has its effects out-with the CNS and modulates the behaviour towards pain
Describe the impact of PZM-21 on constipation and resp depression (linked to b-arr2 recruitment)
PZM-21 causes less constipation than morphine
PZM-21 causes much less resp depression (resp frequency) than morphine
This is relevant to the danger of opioids
However, further studies have demonstrated that it may act more like a partial agonist and puts into question the decreased resp depression assoc with PZM-21
Is there a link between G protein activation and side effect profiles of opioids
Low intrinsic efficacy for Gi can explain an improved side effect profile
Likely partial agonists
Increased therapeutic window as the intrinsic efficacy goes down (inverse correlation)
Improved therapeutic drugs are less to do with bias but more to do with G protein efficacy
Describe the mechanisms of morphine tolerance
G protein signal leads to phosphorylation of opioids receptor
B-arr2 recruitment and arrestin binding leading to endocytosis
What is the cell response of GTP and b-arr2 in opioid binding
Decreased cAMP
Decreased calcium
Increased potassium
Increased MAPK, Scr and Akt
Describe the role of Scr in morphine tolerance
Regulation of the constitutive activity and recycling of mu opioid receptors in the DRG
Describe the impact of dasatinib (Src inhibitor) on morphine
Prevents morphine tolerance in inflammatory hyperalgesia
Tail withdrawal latency; again prevents morphine tolerance quite significantly
Does dasatinib change the dose dependent relationship to morphine?
No; no change in morphine or basal tail withdrawal latencies
**Inhibition of Src Kinase can prevent morphine tolerance
Can dasatinib reverse morphine tolerance?
Yes; morphine alone for 3 days, after 3 days with dasatinib, there was reversal of morphine tolerance
What has C-Src been implicated in?
Neuropathic and inflammatory pain
Phosphorylates NMRAR to enhance the response == hyperalgesia
Hub for NMDA receptor regulation
We know NMDA receptors are involved in pain due to the action of ketamine
NMDA receptors are responsible for the aspects of morphine evoked tolerance
Describe some drugs which can improve opioid analgesia
Naloxone and methylnaltrexone (mu opioid antagonist)
Dasatinib (Scr)
Ketamine (NMDA)
Rapamycin (mTORC1)
Describe the use of opioids in the perioperative period
Could complicate and hinder recovery
Impact on the need for prolonged opioid analgesia
Describe the multimodal analgesia therapies utilized to reduce the reliance on opioids in the perioperative period
Simple analgesia; paracetamol, NSAIDs
Anti-neuropathic drugs and atypical analgesia; gabapentinoids, ketamine, magnesium, alpha 2 adrenergic agonists, steroids, IV lidocaine
Invasive techniques; nerve blocks with LA
Describe the role of Nav1.7 mutation in the sensitivity of pain
Endogenous opioids contribute to insensitivity to pain in humans and mice lacking Na+ channel NaV1.7
Upregulation of endogenous opioid release from nociceptive peptides
Is there dysregulation of pain in fibromyalgia at a molecular level?
Endogenous opioidogenic dysregulation
Reduction in regions in which opioid systems are found
Reduction in PET imaging analysis of mu opioid receptor expression
In some situations in which pain is enhanced; there is evidence for a dysfunctional endogenous opioid system
Can acupuncture help with pain?
Yes; can stimulate endogenous opioid systems
How can the placebo effects related to analgesics be explained?
Placebo effects are mediated by endogenous opioid activity on mu opioid receptors
Describe the role of innate analgesia on inflammatory hyperalgesia
Inflammatory pain decreases even when the inflammation is still present
This is due to an upregulation of endogenous opioids
When naltrexone is administered, the inflammatory hyperalgesia returns. This suggests that there is endogenous opioid activity
Tissue injury produces muR constitutive activity that represses spinal nociceptive signalling for months
How can the fragmented maternal care be quantified?
Dam number of sorties
Much more in restricted bedding paradigm
What is the impact of ELA on sensitivity to pain?
CFA to evoke inflammatory sensitivity
ELA mice show statistically significant enhancement of mechanical withdrawal threshold. Prior to application of the adjuvant, there is an upregulation of the analgesic response in mice
When exposed to adjuvant; both control and ELA mice show heightened mechanical sensitivity
ELA mice tend to be more resistant to acute pain.
However, ELA mice show increased tolerance to upregulated mu constitutive opioid receptor function. Greater degree of chronic pain in ELA mice
What is the impact of ELA on response to noxious heat?
Enhanced resilience to noxious heat in the acute setting (basal tail latency)
However, the tail withdrawal latency declines, similar to inflammatory hyperalgesia paradigm
Increased tolerance in ELA
Is there differences in ELA mice response to morphine?
Dose response relationship to morphine is shifted to the right for ELA mice.
Consistent with increased tolerance; shift of the dose response relation ship to the right
Tolerance develops rapidly and to a much greater extent
Upregulation of endogenous opioid signalling which then primes the receptors to tolerance when exposed to exogenous alkaloid molecules such as morphine
What is epigenetics?
Study of heritable phenotype changes that do not involve alteration in the DNA sequence
External modifications to DNA that turns genes on or off with no modifications of DNA sequence, but instead how these cells read genes.
Epigenetics alter the physical structure of DNA; not the molecular sequence
How does ELA confer epigenetic changes?
ELA gives rise to altered neural signals which influence regulatory genes within the cell - influences acetylation and regulation of pain, and opioid receptors
Epigenetic markers control where and how much protein is made by a gene, effectively turning it on or off, thereby shaping how brains and bodies develop
Is there a change in MOR gene expression in ELA mice?
Thalamus; reduction in mu opioid receptor expression (esp female mice - pain sensitivity between males and females varies)
Is there a change in DOR gene expression in ELA mice?
Upregulation of delta expression in the spinal cord; stat significantly in male and female mice
This upregulation of DOR in the spinal cord gives rise to increased mu-delta heterodimers which show a greater tendency to tolerance development (ability to pre-recruit b-arr2)
Altered pain signalling = enhanced tolerance
Does exposure to ELA increase risk for chronic pain later in life?
Yes; children and adolescents with ACEs had an increased risk for chronic pain, and this association increased in a dose-dependent fashion (strongest associations included financial instability, living with a mentally ill adult or having experienced race discrimination)
