Lecture 1 SOMOGYI.pdf - Faculty of pain medicine

NON NEURONAL OPIOID ACTIONS
Faculty of Pain Medicine: ANZCA
October 16, 2009 (Melbourne)
Andrew Somogyi and Mark Hutchinson
Discipline of Pharmacology (School of Medical Sciences)
Faculty of Health Sciences, Adelaide, Australia
andrew.somogyi@adelaide.edu.au

Background: Opioids
• Major drug class for moderate-severe
– Acute pain
– Malignant pain
• Increased use in nonmalignant persistent pain
• Maintenance treatment for opioid-dependence
– Methadone
– Buprenorphine

Overview: Opioids
• About 30 opioids marketed worldwide
• 2007 Worldwide Consumption
– Codeine 220 tons
– Oxycodone 52 tons
– Morphine 39 tons
– Hydrocodone 38 tons
– Methadone 28 tons
– Pethidine 10 tons
– Hydromorphone 2.2 tons
– Fentanyl 1.3 tons
International Narcotic Control
Board: Report 2008

Target Site: Mu Opioid Receptor (µOR)
Neuronal cell
Agonist Binding:
transmembrane regions II,
III(1 st EC loop),V; N-
terminal domain affects
affinity
G i /G 0 , G s
3 rd Cytoplasmic loop:
Signal transduction,
Desensitization &
Downregulation
• member of G-protein coupled 7 transmembrane spanning receptor
group (GPCR)

Opioid Actions: Intracellular
• G 0 : Presynaptically
– Close Ca 2+ channels fi
– fl transmitter release
• Glutamate, Ach, NA, SP,
5HT
– Stops pain transmission
• G i : Postsynaptically
– Open K + channels fi
DORSAL HORN
– Hyperpolarize & inhibit
neurones
– Makes cell less excitable • descending & ascending inhibitory &
stimulatory pathways

Once Receptor is Activated by Opioid
• Transmission of pain impulse is reduced
• Neuronal cell is no longer in a state of
excitability
• Acute dosing

Mu Opioid Receptor Activation
• Spinal & Supraspinal analgesia
• Respiratory Depression
• Sedation
• Nausea & vomiting
• Miosis
• Euphoria

Opioids Undesirable Effects: Long Term
• Constipation
• Nausea and Vomiting
• Dry Mouth
• HPA Axis Suppression
• Immune Systems Suppression
• Dependence & Tolerance
• Neurotoxicities:
– Allodynia, myoclonus, cognitive failure/delirium, sweating
– Hyperalgesia
• Large variability between patients and between opioids
– Opioid rotation seems to work - WHY?

Traditional View of Opioid
Mechanism of Action
µ
µ
• Is there a
missing link!
ANALGESIA

The CNS Opioid-Immune Story
• Glia
• Toll-like Receptors
• Pro-inflammatory cytokines
• The Duel

What are Glia?
• Immune-like cells: brain & spinal cord
• 90% of the cells in the brain
• No longer considered structural support: passive “mortar”
• Function: support neurons physically (connective tissue)/ metabolically
– Take up & degrade glutamate & GABA for neuronal synthesis
• Bi-directional communication between glia and neurons
Microglia
Astrocytes
Oligodendrocytes

GLIA
• Immunocompetent cells: respond to
– Exogenous signal: stress, subclinical infections, drugs
– Endogenous signal: lipopolysaccharide (LPS)
• Are they important in drug response?
– Analgesia: opioids
– Reward & withdrawal: opioids, alcohol …..

Role of Glia in Pain Neuropathies
• Damaged Neuron, Virus, Diabetes ….
Neuron
Glia
Inflammatory
mediators
Activation signal
Pre synaptic
modulation
Neuronal
Consequence =
Pain
• Neurons release “danger signal” molecules: Heat-shock proteins
• Microglia mop-up molecules via toll-like receptors (TLR2, 4)- signalling
• Constant TLR signalling fi fl (-ve) feedback fi proinflammatory cytokines (IL-1)
• Prolonged proinflammatory pain response “allergy in dorsal horn spinal cord”

Opioids also Dysregulate Glia
• Glia are activated
• Proinflammatory response
• Enhanced Pain - decreased analgesia
• Increased tolerance, dependence, reward, etc
Pain Neuron
Inflammatory
Mediators: IL-1 ..
Gli
a
Opioid agonist
Pre synaptic
modulation
Neuronal
Consequence
Dorsal horn glial activation from any source fi enhanced pain

Where is the Evidence?
Minocycline: An Attenuator of Glial-
Activation
• Precise molecular mechanism(s) unknown
• Not via TLR4

Minocycline Potentiates Morphine-Induced
Analgesia in Rats
• Activation of glia =› pain signals
Hutchinson et al 2008, Brain Behav Immun 22: 1248-1256

Minocycline Attenuates Morphine-Induced
Respiratory Depression in Rats
•Activation of glia = disregulation of respiratory controls
Hutchinson et al Brain Behav Immun 27, 1248, 2008

Minocycline Attenuates Morphine-Induced
Reward in Rats
• Activation of glia =› reward / dependence
Hutchinson et al 2008, Brain Behav Immun 22: 1248-1256

Morphine + Minocycline
Minocycline changes
the Therapeutic Index
of morphine from:
Narrow
to
Wide
In animals

What is the Target on Glia that Binds
to Opioids to cause Activation?
Link:
• Naloxone (µ antagonist) can
modify sepsis via LPS
(lipopolysaccharide) activation
• LPS- very strong glia-immune
activator
• LPS binds to TLRs

Glial Activation: Mechanism- TLRs
Danger signals- Opioids
TLRs

What are these TLRs?
• Toll-Like Receptors
– Family of pattern recognition receptors on Glia
– TLR4 best known for being the endotoxin
receptor
– TLR4 & TLR2 are capable of recognising
endogenous “danger signals”
– Up regulated in neuropathic pain
– if expression is blocked pain does not develop

• Clinically
used Opioids
activate
TLR4 with
different
“potencies”
to µ receptor
•(-)- morphine: mu active
Hutchinson et al 2007

OPIOIDS: Mu Receptor Binding & TLR4
Signal Activation: Different
Mu Receptor
Buprenorphine
M6G
Morphine, methadone
Fentanyl
Oxycodone
Pethidine
M3G
(+)-naloxone/ (+)-naltrexone)
TLR4 Signal
M3G
Oxycodone
Fentanyl
Pethidine
methadone, morphine
Buprenorphine
M6G
(+)-naloxone/ (+)-naltrexone)

Clinically Used Opioids activate TLR4
• All 10 µM
• opioid antagonists «
• no stereoselectivity
mOP active
• M3G potent activator
Hutchinson et al 2007

TLR4 Inactivation Increases Morphine
Analgesia in TLR4 Knockout Mice
• no TLR4 glial activation
– fl pain signalling
– 3x› analgesia with morphine
Hutchinson et al 2009, Brain Behav Immun Epub Aug 14

TLR4 Selective Antagonist (LPS analog):
decreases pain & increases opioid analgesia
• Chronic constriction of rat sciatic nerve model
Neuropathic pain
fl
Opioid analgesia
›
Touch becomes pain
Hutchinson et al 2007

• All 10 µM
mOP active
Opioid
Inactive
Antagonists
Isomers do
not activate
TLR4
BUT
Hutchinson et al 2007

Opioid Inactive Antagonists are
TLR4 antagonists: (+)-naloxone
• TLR4 signalling
• Morphine analgesia
Hutchinson et al 2007, 2009

Opioid Inactive Antagonists are
TLR4 antagonists: (+)-naloxone (2)
• Morphine Tolerance & Hyperalgesia are reduced
Tolerance
Hutchinson et al 2009 In Press

Opioid Analgesia: Cytokines
• Opioids activate glia signalling to upregulate
proinflammatory cytokines (IL-1ß) via TLR4 binding
• Cytokines and analgesia/reward
– Increase extracellular glutamate by down regulating
glutamate transporter GLT-1 (most important mechanism)
– phosphorylate NMDA receptor subunit -> increased activity
– elevate dopamine release in NAc

Cytokines - Reward Mechanisms- Addictive Drugs
Hyman et al Annu Rev Neurosci 29, 565, 2006

A Complicated Molecular Mechanism: TLR4
Binding -> IL-1 release

Opioid-Induced Hyperalgesia (OIH)
• Patients receiving opioids (to treat pain, addiction) may
become more pain sensitive directly due to opioids
• Double-edged sword!
• Initial effects:
– Analgesia, antihyperalgesia
• Chronic effects:
– Hyperalgesia
– Tolerance

OIH in Methadone Maintenance
Subjects: Response to Cold Pressor Pain
Anticipated Response
Actual Response
150
150
Seconds
100
50
Seconds
100
50
0
Placebo Trough Peak
Control
Methadone
patients
0
Placebo Trough Peak
Control
(n=70)
Methadone
patients (n=34)
Major disadvantage: cross sectional study

Chronic Nonmalignant Pain Patients show
Hyperalgesic Responses to Cold Pressor Pain
35
30
Seconds
25
20
15
10
5
0
• Accumulating
Evidence: Long term
opioid exposure per se
causes hyperalgesia
Hay et al J Pain 10, 316, 2009

Opioid-Glial Interactions: Analgesic Tolerance
Analgesia
Morphine
Expected
Pain
Tolerance
Hyperalgesia
IL-1 etc

Summary: Opioid-Glial Interactions
• Opioids cause....
– spinal glial activation via TLR4
– › spinal proinflammatory cytokine (IL-1 ..) synthesis &
release
– Hyperalgesia, respiratory depression, tolerance
• Blockade of opioid-induced glial activation fi
– potentiates acute analgesia
– fl respiratory depression
– fl hyperalgesia, tolerance ….

Non Neuronal Opioid Actions
• Activate Glia –signalling via TLR4
• Proinflammatory response- IL-1 release
• Enhanced Pain - decreased analgesia
• Increased tolerance, dependence, reward, etc
Pain Neuron
Inflammatory
Mediators: IL-1 ..
Gli
a
Opioid agonist
Pre synaptic
modulation
Neuronal
Consequence:
Hyperalgesia
Tolerance
….

Conclusion
• Opioid efficacy & toxicity have a CNS immune
signalling component
• Potential for increasing the therapeutic
window of opioids by decreasing adverse
effects and increasing analgesia
• Need clinical translation studies
• Need newer & more selective TLR4 opioid
binding site inhibitors

Acknowledgements
• USA- Univ of Colorado (Linda Watkins Group)
• NHMRC
• National Institutes of Health (NIDA)
• University of Adelaide
• PhD student: Justin Hay, Liang Liu