lecture 29: neuropathic pain and analgesia II Flashcards
1
Q
What is the history of cannabinoids?
A
- 1964: delta9-tetrahydrocannabinol (THC) identified as active principle of cannabis
- 1988: specific CB receptors identified in brain and peripheral nerves → Cb1R
- CB1R one of most widely expressed GPCR in brain; also found in peripheral tissues including peripheral nerves and non-neuronal tissues e.g. muscle, liver and fat
- few years later CB2R identified through homology cloning - expressed in nonneural tissues, esp. immune cells including microglia
- 1992: CB1R discovery lead search for endogenous mediators → anandamide
2
Q
What are cannabinoids?
A
- originally defined as compounds uniquely produced by cannibis plant, then development of synthetic cannabinoids and discovery of cannabinoids in the body → endocannabinoids
- compounds derived from the plant itself are now called phytocannabinoids
3
Q
What are phytocannabinoid compunds?
A
- GW has extensive phytocannabinoid library through breeding of unique “chemotypes” (plants characterised by their chemical content)
- currently available cannabinoids include:
- delta9-THA (delta9-tetrahydrocannabinol)
- CBD (cannabidiol)
- (only 2 cannabinoids well characterised)
- CBG, A, V
- CBDA/V, CBDVA
- CBC
- CBN,V
- THCA
- THCV
- THCVA
- delta8-THC
4
Q
What are cannabinoid CB1 receptors?
A
- CNS
- dense in hypothalamus, cortex, hippocampus, cerebellum
- low density in brainstem
- lack of serious respiratory or cardiotoxicity
- pain pathways in brain and spinal cord
- periphery
- sensory neurons (primary afferents), vasculature, urogenital tissues, gut, skin
- effects
- analgesia, motor coordination, cardiovascular, memory disruption, anti-emesis, appetite stimulation
5
Q
What is the relationship between cannabinoids and pain pathways?
A
- peripheral nerve transmission (primary sensory afferents - CB1 agonists inhibit
- dorsal horn - CB1 agonists inhibit activity of relay neurons
- negative coupling via G1 protein with N-type Ca2+ channels → decreases Ca2+ entry and release of excitatory transmitters
- hyperpolarisation of postsynaptic neuron due to activation of K+ channels
- descending modulatory (inhibitory) control pathway - CB1 agonists enhance activity via a2-adrenoceptor pathways
6
Q
What are cannabinoid agonists?
A
- delta9-THC and cannabidiol (phytocannabinoids - cannabis sativa)
- anandamide and 2-arachidonylglycerol are endogenous agonists
- CP55, 940 and nabilone (synthetic)
- highly fat soluble - slow residual elimination
7
Q
What is sativex?
A
- oromucosal spray (buccal)
- flexible, individualised dosing, self-titration
- ~8-12 sprays/day
- average dose of THC 22-32 mg/day and cannbidiol 20-30 mg/day
- to meet demands, GW pharmaceuticals increased production of cannabis at its fortified greenhouses to 60 ton/year
- mixture of THC and CBD
- THC: analgesic, muscle relaxant, antiemetic, appetite stimulant and psychoactive
- CBD: analgesic, anticonvulsant, muscle relaxant, anxiolytic, neuroprotective, antioxidant and antipsychotic
- adjunctive treatment for symptomatic relief of pain in multiple sclerosis, neuropathic-related cancer pain and AIDS neuropathy
- approved use depends on individual countries
- now launched in UK, Spain, Italy, Germany, Denmark, Czech Republic, Sweden, NZ and Canada
- satives is approved in NZ and Canada for treatment of spasticity due to MS and also approved and marketed in Canada for the relief of neuropathic pain in MS and cancer pain
- active ingredients
- 27mg/ml THC and 25mg/ml cannabidiol
- therapeutic dose of THC highly variable between patients so self-titration important
- well tolerated and low incidence of side effects
8
Q
What is sativex as a reliever of neuropathic pain in MS?
A
- sativex gave 41% improvement over baseline and 20% over placebo
- observed in patients who were already maintained on stable regimen of analgesic medication → improvements were above the best possible pain relief with available therapies
- decrease in pain (and spasticity) scores maintained at more than 6 months
- also increased quantity and quality of sleep
9
Q
What was the law on medical marijuana as at 16 sept 2014?
A
- victorian government to amend legislation to allow drug trials
- new laws that will allow for medical trials of cannabis will be introcued into the victorian parliament today
- the drug is currently listed as a prohibited substance by the Therapeutic Goods Administration (TGA), but there is growing support to allow it to be used by people suffering from terminal illnesses
- Victorian Health Minister David Davis said the legislation would amend the Drugs, Poisons and Controlled Substances Act to support “properly-structured clinical trials” with ethical approvals in place
- Greens Senator di Natale said there is very strong evidence for the use of medicinal cannabis
- “IT has significant benefits for conditions such as nausea from chemotherapy, from some forms of neogenic pain, muscle spasms for conditions like MS,” he told 774 ABC melbourme.
- “What we have to do is effectively change the scheduling of cannabis. It’s currently scheduled as an illegal drug,” he said
- “we’ve got to make it a special category through the TGA for medicinal cannabis and then it should be available under prescription from a doctor”
10
Q
What are other clinical effects of cannabinoids?
A
- anxiolysis
- sleep improvement
- antispasticity in MS (improved mobility)
- appetite (e.g. AIDS, cancer and wasting syndromes)
- psychotropic effects → abuse potential
11
Q
What are palliative effects of THC in cancer?
A
- inhibition of nausea and emesis (THC, nabilone)
- appetite stimulation (THC)
- analgesia (THC)
- inhibition of muscle weakness (THC, nabilone)
- mood effects (sedation, antidepression, anxiolytic) (THC ± cannabidiol)
12
Q
What is immune-nervous system communication in neuropathic pain?
A
- bone marrow-derived progenitor cells migrate:
- peripheral nervous system → macrophages
- central nervous system → microglia (act similar to macrophages when activated)
- glial cells ~ 70% total cells in brain and cord
- provide for nerves: nutrition, protection, insulation
- microglia 5-10% of glia
- others macroglia - astrocytes and oligodendrocytes
- multiple conditions generate neuropathic pain; common underlying mechanism is inflammation at site of damaged nerve
- cytokines, neurotrophic factors, etc, released at injury site
- local actions and can initiate systemic immune response
- → neuroinflammatory environment can activate microglia and astrocytes located in spinal cord and brain
- cytokines, neurotrophic factors, etc, released at injury site
- a) nerve injury provokes recruitment and activation of immune cells at site of nerve lesion, in DRG, and in ventral and dorsal horns of spinal cord
- b) macrophages, T lymphocutes and mast cells invade lesion site and spread around distal stumps of injured nerve fibres. schwann cells begin to proliferate and dedifferentiate to guide regenerating axons
- DRG: macrophafes and few T lymphocytes reside in DRG before injury. their numbers increase sharply after injury. macrophages also move within sheath that satellite cels form around cell bodies of primary sensory neurons
- spinal cord: one week after nerve injury, massive microglial activation found in dorsal horn, in projection territories of central terminals of injured primary afferent fibres
13
Q
What is the role of glia in development and maintenance of neuropathic pain?
A
- following a peripheral injury, the synaptic projection of a pain-sensing neuron within the spinal cord releases ATP
- microglia in vicinity drawn to source of ATP and morphologically change as they approach source and become activated
- fully activated microglia localise around pain-sensing neuron and interact on molecular level, releasing various neuroinflammatory agents
14
Q
How do activated microglia modulate neuropathic pain signalling in dorsal horn?
A
- ATP binds to P2X4R on microglial surface → increase in intracellular Ca2+; Ca2+ influx → translocation of NFkB to nucleus and induction of p38 MAPK pathway
- initiates transcription of various neuroinflammatory agents including cytokines, neurotrophic factors and neurotransmitters
- release of these neuro-inflammatory agents into synaptic cleft and binding to various R → increase in intracellular ions in the neuron, such as Ca2+ and Cl-, which depolarizes the cell and thereby causes sensation
15
Q
What does spinal nerve ligation induce?
A
- microglial activation in spinal cord - effect at 3 days
- double immunofluorescence indicates p-p38 (p38 MAPK) not colocalised with (c) neuronal marker NeuN or (d) astrocyte marker GFAP, but with (e) microglial marker OX-42
- p38 MAPK plays critical role in microglial signalling in neuropathic pain conditions
16
Q
What does activation of astrocytes result in?
A
- the prolongation of a pain state
- production and subsequent release of proinflammatory cytokines (e.g. IL-1b, IL-6, TNFa) activate neighbouring astrocutes by binding to surface receptors → Ca2+ influx
- upon activation, astrocytes undergo hypertrophy and increased production of neuroinflammatory agents are secreted into the synaptic celft. astrocyte activation in conjuction with microglial activation significantly depolarises the neuron increasing its sensitivity and potentiating the pain state
17
Q
What are Cb1 and Cb2 R expression changes?
A
- neuropathic pain
- CB1R and CB2R upregulated in peripheral and central sensory pathways in animal models of neuropathic pain
- potential role: inhinition of neurotransmitter release decreases hyper-excitability of sensory pathways (CB1R); inhibition of inflammation (CB2R)
- CB1R up-regulation enhances analgesic response to exogenous cannabinoids in animal model of neuropathic pain
- neuroinflammation
- CB2R upregulated in microglia-macrophage like cells of patients with MS and in microglia of mouse model of MS
- CB2R expressed on T cells decreases inflammation in MS
- T Cell CB2R is already highly activated by endocannabinoids in MS; increasing receptor number is thus important for increasing efficacy of CB2R agonist
19
Q
What is the relationship between cannabinoids and glial activation?
A
- cannabinoids (CBs) inhibit entry of immune cells into the brain
- through activation of CB2R, CBs also inhibit generation of proinflammatory mediators, while promoting release of antiinflammatory molecules by glial cells. by interacting with CB1R, CBs prevent exotocity by decreasing glutamate release and signalling
- as antioxidant molecules, CBs decrease the toxicity of ROS through CB R-independent effect
22
Q
What was the synergy between morphine and CP55, 940 (i.t. rat tail flick assay)?
A
- example of synergy
- % maximum possible effect
- morphine in combination with cannabinoid agonist (CP55,940)
- given directly into spinal cord
- dose of agonist given i.t
- given ed 20 doses of the cannabinoid agonist and looked at how it affected morphine
- far more potent even with low dose
23
Q
What is dexmedetomidine?
A
- alpha2 agonist
- intensive care
- sedation
- anxiolysis
- analgesia
- cardiovascular and respiratory effects
24
Q
What are alpha2 adrenoceptor agonists?
A
- weak antinociceptive efficacy in acute pain
- located in dorsal horn of spinal cord
- associated with N-type VGCCs
- role in neuropathic pain
25
Q
What are presynaptic receptor agonists?
A
- µ opioid - morphine
- alpha2 adrenoceptor - dexmedetomidine
- Cb1 cannabinoid - CP55,940 (also Cb2)
- all decrease neuronal activity → antinociception
26
Q
What was seen in CB1/2R agonist mouse tail flick?
A
- time after administration of each bolus to tail flick
- peak effect was achieved with 3.0mg/kg and took 60 minutes to reach this level of analgesia
- have to be done in separate mice
27
Q
What are CP55,940 dose-response curves?
A
- take maximum effect from each dose
- construct dose response curve
- log dose of mg/kg
28
Q
What is effective dose for 50% response (ED50)
A
- did similar experiments with other agonists
- mice need about 20 - 30 x more of these drugs than humans do
- high metabolic rate
29
Q
What are isobolograms?
A
- isobols
- theoretical doses for additive interactions
- types of interactions
- additive
- synergy
- sub-additive /antagonism
- take ed50s of the different drugs
- very simple
- isobol joins the ed50s
- black dot is predictive of merely additive
- response ‘way over here’ is define event of synergy
- ‘way over on the right’ subadditive
30
Q
What is isobolographic analysis?
A
- fixed ratio design, 1:1
- dose ratios to be used
- ED50
- CP55, 940 : Morphine
- 1: 1
- 0.5: 0.5
- 0.25: 0.25
- and so on
31
Q
What is seen when combining CP and morphine?
A
- combination
- ed50 x 1
- ed50 x 0.5 etc
- pick maximum effect for all of those
32
Q
What were combination dose-response curves?
A
- dose responsive curves for the different combinations
- arbitray ed50 values
33
Q
What was seen in the isobolograms?
A
- arbitrary values converted to doses
- CP55,940 + morphine was synergistic
34
Q
What was seen with CP55,940 (CB1/2) + morphine (µ)?
A
- often can’t predict the interaction of drugs even if they are colocalised
- cp55,940 + morphine
- hot plate: synergy
- tail flick: synergy
- tail flick is purely spinal
- hot plate requires supraspinal mechanisms to be involved
35
Q
What was seen with CP55,940 (CB1/2) + dexmedetomidine (a2)?
A
- hot plate: synergy
- tail flick: additivity
36
Q
What was seen with dexmedetomidine (a2) + morphine (µ)?
A
- additive in both
- independent action
37
Q
What were differences between 2 acute pain models in mice?
A
- do need to test properly and experimentally
- can’t really predict
