Opioid Addiction Flashcards
What opioids are classified as opium alkaloids?
Morphine
Codeine
Thebaine
Oripavine
What is heroin?
Refined opium; diamorphine
Does heroin bind directly to the mu receptor?
No; it must cross through the BBB to be modified by enzymes to morphine
Rapid absorption and rapid decay
Describe endogenous pepties?
Endorphins; beta endorphin
Enkephalins e.g. met-enkephalin
Dynorphin
Endomorphins
What are the 3 different opioid receptors?
Mu
Delta
Kappa
7 membrane spanning regions. Agonist binds to extracellular part of transmembrane protein resulting in a conformational change that is transduced across the membrane
Which opioid receptor is linked to addictive behaviour?
Mu
Which gene codes for mu receptors?
OPRM1
Which G protein do all opioid receptors activate?
Gi/o
Inhibitory alpha subunit - inhibit adenylyl cyclase activity
Describe the basic cascade when opioid receptors are activated?
Replacement of GDP by GTP - dissociation of alpha subunit from beta/gamma subunit
Decreased cAMP
Decreased Ca2+
Increased K+
MAPK (kinase)
Which additional receptor can opioid receptors recruit?
Beta-arrestin receptors This acts to activate MAPK Increased Src Increased AKT These allow different cellular signalling events to occur
Interact with beta adrenergic receptors
What is the role of beta-arrestin in membrane trafficking?
Endocytoses the mu receptor into the cell
Thought to be involved with opioid tolerance
What are the mechanisms of morphine tolerance?
Receptor tolerance
Cell tolerance and withdrawal
System tolerance and withdrawal
Synaptic plasticity in tolerance and withdrawal
Describe the mechanism of receptor tolerance in opioid tolerance?
Reduction in number of surface receptors (endocytosis using beta-arrestin)
Reduced coupling of receptor to adenylyl cyclase and other effector molecules
Receptor protein is less abundant
Receptor RNA is downregulated
Reduced inward K+ movement
Describe the mechanism of cell tolerance/ withdrawal in opioid tolerance
With tolerance comes withdrawal potential
Initially with opioid use; cAMP levels decrease (Gi)
The cell will begin to adapt/ show tolerance and the cAMP levels will begin to rise
When naloxone is administered/ morphine stopped abruptly the adaptations via the cell will be unmasked resulting in a large jump in cAMP production - hypertrophy of cAMP signalling
Naloxone precipitated withdrawal
Withdrawal; enhanced pain response, increased agitation, increased excitation, aggressive
Describe system tolerance and withdrawal in opioid tolerance
Systems feedback adaptations occur in opioid-sensitive networks to tolerance and withdrawal
New synapses are formed that have opposing effects of those of receptor stimulation
Describe the role of synaptic plasticity in tolerance and withdrawal
Changes in synaptic plasticity driven by altered presynaptic release probability, which is well established as opioid sensitive GABAergic synapses
Mechanisms resembling LTP +/- LDP involve AMPAR insertion into synapses which produces long term changes in synaptic strength
Formation of new memories associated with drug taking activity
Permanent changes in synapses results in life long craving
How long does positive reinforcement last in drug misuse?
Around 10 mins
Returns to a state that is non euphoric but will still be heavily sedated (profound hypotension) - lethargy
There will be a strong incentive to return to the euphoric state
However, the euphoric feeling/ positive reinforcement will reduce in length
Brain will become less responsive to the drug
There will be no pleasure to the drug anymore but withdrawal will occur when it is stopped
Positive reinforcement turns to negative reinforcement
In terms of the behavioural signs; describe the development of drug dependence
Acute drug state; reinforcement, reward
Chronic drug state; tolerance, sensitization, dependence
Short-term abstinence; withdrawal
Long term abstinence; craving, stress-induced relapse - up to 80% of opioid users will relapse
In terms of molecular changes; describe the development of drug dependence
Acute drug state; increase mesolimbic DA and 5HTergic
Chronic drug state; receptor adaptation; increased cAMP (hypertrophy of cAMP), increased CREB (cAMP dependent transcriptional changes), increased FosB
Short term abstinence; increased glutamergic, NAergic, decreased DAergic and 5HTergic, increased CRF
Long term abstinence; synaptic remodelling, increased CRF and glucocorticoids
What areas of the brain are involved in binge/ intoxication?
VTA
Thalamus
Globus pallidus
Substantia nigra
What areas of the brain are involved with withdrawal/ neg effect of drugs?
Brain stem
NA, CRF
Hypothalamus (stress response)
Central nucleus of amygdala
What areas of the brain are involved with preoccupation/ anticipation “craving”?
Hippocampus
PFC
Insula
Basolateral amygdala
What is the allostatic model of drug dependence?
Impulsive reward seeking behaviour (drug experimentation)
Repeated exposure; hedonic effects diminish
Thereafter compulsive drug use to avoid withdrawal becomes dominant
What is allostasis?
The process of achieving stability/ homeostasis through physiological or behavioural change
Carried out by; alteration of HPA axis, ANS, cytokines
An altered hedonic set point - flattened reward system in absence of drugs pushing the homeostatic set point
What is the typical cycle of heroin high to withdrawal?
Around 6 hours
What is the role of methadone maintenance?
Changes patients from heroin to methadone (mu agonist with a more stable pharmacokinetic profile)
Longer half life than heroin - taken once a day
Causes tolerance of receptor; if they do take heroin then they won’t recieve the same high
Prevents a high or withdrawal to enable individual to return to a more normal environment
What are the therapies for opioid use ``disorder?
Psychosocial therapies; CBT
Methadone (mu receptor agonist)
Buprenorphine (mu receptor partial agonist and kappa receptor antagonist). Combined with naloxone in suboxone with reduce potential for diversion
Naltrexone (opioid antagonist)
Detoxification - symptomatic relief; anti-inflammatory’s, antiemetics, anti-diarrhoeal
Lofexidine (alpha adrenergic receptor agonist)
What is the action of opioids in the central dopaminergic pathway?
Acts inhibit the presynaptic GABAergic neuron onto the DA neuron; increasing DA release in N. Acc
Patch clamp studies show GABA mediated IPSCs are inhibited by morphine in the post synaptic DA neurons == inhibition of the inhibitory neuron equals an increase in DA release
What is the evidence that inhibitors of opioid receptor activity can reduce addictive behaviour?
Attenuation of cocaine and heroin seeking behaviour by mu receptor antagonism
Alcohol seeking behaviour = naltrexone
Reduction in binge eating behaviour
Only drug for tx of gambling addiction is naltrexone
Opioid receptors are involved in the reward pathway in all addictive behaviour
What evidence is there to show that mu receptors are required for morphine reinforcement?
Knockout mu receptor mice have no significant difference after conditioning in the amount of time the mouse spends in the morphine chamber
Describe condition place preference
Day 1; habituate on either side of box
Day 2/3/4; conditioning (AM saline, PM morphine 10mg/kg s/c) - saline in one chamber, morphine in other chamber
Day 5 test; see what side the mouse moves to
Condition place preference
What is the evidence that mu receptor number affects the potency of morphine’s condition place preference?
In comparison to WT mice; MOP +/- (heterozygous; contains 50% of mu receptor potential) at the same dose (10mg/kg s/c) have no significant difference CPP
HOWEVER; if the dose of morphine is increased to 10 mg/kg s/c; there is a significant difference and a change in CPP
More morphine is required in MOP +/- to cause CPP; morphine has become less potent but has the same efficacy
Downregulation of mu receptor = reduction in potency of mu opioid receptor agonists. Consistent with tolerance; where individuals require more opioid to initiate same DA release
In summary; what are mu receptors required for?
Analgesia Resp depression Immunosuppression Constipation Tolerance, withdrawal, dependence Reward/ reinforcement
What is the role of mu receptors and beta arrestin?
They recruit it
Allows for endocytosis of mu receptors and cascade signalling
Evidence to show that beta arrestin is involved in resp depression, constipation and addictive qualities of opioids
What evidence is there to show that the absence of beta-arrestin reduces morphine tolerance (conversely; beta arrestin is responsible in part for tolerance in opioid use disorders)
Knockout beta arrestin mice respond differently to WT
Mouse tail dipped in hot water; amount of time that it takes for mouse to remove tail is an indicator of pain threshold
Morphine causes a prolongation of tail withdrawal due to analgesic properties - as dose of morphine increases, the tail withdrawal is prolonged
In WT; there is a dose dependent curve to tail withdrawal, On day one; morphine is potent. However the animals develop tolerance; (lose potency but efficacy remains)
In beta arrestin knockout mice; there is no tolerance development. Tail withdrawal latency is longer
What is the evidence to show that there is basal mu analgesia in the absence of beta arrestin 2?
There is an increased latency in tail withdrawal in untreated beta arrestin knockout mice
This effect is diminished by the introduction of naloxone
Suggests constitutively active mu opioid receptors in beta arrestin knockout mice
Are receptors always inactive until binding of an agonist?
No; receptors fluctuate between an active and inactive state
Drugs can either bind selectively to and stabilize the active state = agonist
Drugs can bind selectively to and stabilize the inactive state = inverse agonist
Drug that doesn’t selectively bind to an active or inactive state but binds to both equally in a competitive manner = competitive antagonist. Neutral efficacy
Describe the full agonist, competitive antagonists and full inverse agonists in terms of opioid receptors
Full agonist; DAMGO, morphine, fentanyl
Full inverse agonist; naltrexone, naloxone, nalmefene
Competitive antagonist; CTAP, alpha naloxol, beta naloxol, beta naltrexol
What is the evidence to show that b-arr2 knockout mice have basal analgesia by mu constitutive activity (not increase in endogenous peptides such as enkephalins)
Only full inverse agonists have a reduction in tail withdrawal latency
If there was to be an increase in endogenous peptides; both full inverse agonists and competitive antagonists would increase tail withdrawal latency
Do b-arr2 knockout mice show any difference in morphine reinforcement?
At 10 mg/kg s/c; there is no difference between WT and b-arr2 -/- mice in morphine reinforcement
However
At 3mg; in WT mice; there is no significant difference in the preference score, BUT in b-arr2 -/- mice there is an increase in morphine preference
This suggests that morphine becomes more potent in b-arr2 -/- in terms of rewarding/ hedonic effects
In b-arr2 -/- mice; is there an enhanced reward without stimulation of opioids (in a similar manner to an increase in basal analgesia due to constitutively active mu receptors) ?
No; there is no difference in WT and b-arr2 -/- mice in their aversion to naloxone
If the constitutively active mu receptors were to play a role in the euphoria associated with opioids; an aversion to naloxone would be expected
What is the summary of the role (or lack thereof) b-arr2 on opioid actions?
Based on b-arr2 -/- mouse
Increased basal analgesia mediated by constitutively active mu receptors
Decreased morphine tolerance/ dependence
Decreased constipation
Decreased locomotion
Increased reward with lower concentrations of morphine
What nT does the locomotor and reward actions of opioids rely on?
Dopaminergic pathways
Suggested that the action of b-arr2 is mediated at DA receptors instead of mu receptors
What are the effects of morphine on locomotion in mice?
Increased locomotion
Does the locomotion seen in morphine injected mice require mu receptors?
Yes; when WT. mu -/- and b-arr2 -/- mice were compared;
WT and b-arr2 -/- mice showed a significant increase in locomotion after morphine
However; mu -/- mice did not show a significant increase
What is sensitization?
WT mice:
Day 1; distance travelled is substantial
Day 2; distance travelled is higher
Day 3; distance travelled even higher
Injection of morphine shows an enhanced effect; similar to craving (unlike analgesia or euphoria; the effects of craving go up with repeated administration of the drug)
Describe the relationship between mu receptors and sensitization
In mu +/- mice; there is no significant difference between distance travelled on days 1, 2, and 3 = no sensitization
Even at higher doses of morphine (30 mg/kg s/c); there is slight sensitization but it is not as high in WT mice. This suggests that mu receptor downregulation has a role in efficacy of morphine.
Describe the role between b-arr2 and sensitization
In b-arr2 -/- mice; at 3 mg/kg there is reduced locomotor
However; at 10 mg/kg there is a significant rise (albeit smaller than WT) but characteristic sensitization is observed
Full locomotor response is dependent on b-arr2 - a lack of b-arr2 will reduce morphine stimulated locomotion = reduce craving in humans
What dopaminergic pathway does the locomotor effect of morphine utilise?
Substantia nigra
Projects to the caudate and putamen
Contains D1 and D2 projections
What does dopamien regulate?
Locomotion Cognition Reward Emotion Endocrine function Highly expressed in striatum
Describe the dopamine dependent disorders
Schizophrenia BAD Depression PD Drug abuse and dependence
How is DA synthesized?
Tyrosine hydroxylase
What pathways utilise DA?
Nigrostriatal Mesolimbic Neocortical Tuberoinfundibular Tuberohypophysial
Describe the nigrostriatal DA pathway
DA neurons originate in substantia nigra and VTA
Project to caudate, putamen and nucleus accumbens
How many dopamine receptors are present?
D1-5
D1 like; D1 and D5
D2 like; D2, D3 and D4
Describe the D1 like signalling
Activate adenylyl cyclase - couple to Gs
Activated by dopamine
Apomorphine and bromocriptine are partial agonists
Less action via anti-psychotics
Describe the D2 like signalling
Inhibit adenylyl cyclase; Gi/o coupling
Activated by dopamine, apomorphine and bromocriptine
Majority of anti-psychotics will target D2 like receptors (D2, 3, 4)
Are D2 receptors required for morphine CPP (self-administration paradigm)?
Yes
At 3 mg:
+/+ D2 receptors; robust CPP
-/- D2; no stimulation of CPP
10mg;
+/+ D2; CPP
-/- D2; no CPP
Was there a difference in CPP when food reward was looked at?
Yes; knockout mice still showed food preference - therefore D2 receptors are involved in opioid CPP but not food CPP
Did tyrosine hydroxylase (TH) -/- mice show locomotion after morphine?
No; TH is required for the synthesis of DA
DA deficient mice showed no morphine locomotion
Morphine locomotion was restored by an L-dopa injection
Do TH -/- mice exhibit morphine reward?
Yes; morphine CPP is present without dopamine
What receptors is morphine mediated mouse locomotion dependent on?
D1/2 receptors
B-arr2
Stimulation of mouse locomotion is dependent on D1 activation/ phosphorylation of ERK
Morphine stimulated b-arr2 pERK association, D1 receptors are required
Morphine recruits b-arr2 in the presence of pERK
D1 receptors are not required for morphine CPP
D1 mediates barr2 dependent locomotor effects of morphine. This signalling cascade from D1 involves recruitment of barr2 and phosphorlyation of ERK. Signalling cascade that is mediated through D1
Summarise the roles of barr2 at D1 an D2
D1 receptor; ERK allowing dopamine to phosphorylate ERK = locomotion
D2; recruitment of other kinases
B-arr2 = desensitization, tolerance, signalling role (locomotion)
Summarise b-arr2 on behavioural effects of opioid
B-arr2 -/- mice: Increased basal analgesia Decreased morphine tolerance/ dependence Decreased resp depression Decreased constipation Decreased locomotion (D1) Increased reward
Is there any evidence to suggest that ELA modifies addiction behaviour?
Increases opioid relapse behaviour
Potentiates opioid seeking behaviour
Describe ELA model
ELA paradigm/ fragmented care paradigm
Raised metal grid floor; mother unable to make full nest
Mother leaves pups more frequently
Increased early life stress; less bonding
Behavioural studies performed post-natal day 60
What is extinction behaviour in mice in regards to opioid use?
How long it takes mice to stop self administering opioids when there is no drug available
What is the difference in extinction behaviour in control and ELA mice (opioid) ?
CTL mice stopped pressing the lever around 6 days - realising there is no reward from the level
ELA mice; take longer (10 days) to stop pressing the lever
CTL mice reached extinction criteria earlier than ELA mice
Describe the difference in cue induced reinstatement in CTL and ELA mice
No cue; very few active lever pressing after extinction
Cue reinstated; much more lever pressed. This is greater in ELA mice
In heroin primed reinstatement; the active pressing behaviour is even more pronounced
What is the evidence that adverse childhood experiences increase the likelihood of addiction?
4 or more ACEs; much higher rate of smoking, IVDU, suicide attempts, early sexual behaviour, depression and liver disease (alcoholism)
ACEs are the single greatest unaddressed public health threat facing our nation toady
67% of the population have at least on ACE
What are the different types of ACEs?
Abuse; physical, emotional, social
Neglect; physical, emotional
Household dysfunction; mental distress, incarcerated relative, mother treated violently, substance abuse, divorce
