Animal models of pain Flashcards
2 types of chronic pain
- inflammatory
- neuropathic
Chronic pain arises due to
diseases (HIV, MS, cancer) or
injury (peripheral nerve, spinal cord)
Chronic pain is often ____ and responds poorly to ______
Often intractable and respond poorly to standard analgesics (NSAIDS, Opioids)
Animal models of pain important b/c
it is essential to understand the pathophysiology of these conditions and to (hopefully) develop new and better agents for pain relief
affect 11-29% of canadians
Pain definition
An unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.
Pain is influenced by _____ and _____ processes
Physiological and cognitive processes
Ex. Emotion, Memory, Culture
Understanding pain in animal research–needs to model…
affect/emotion and culture
Nociception
The perception of real or potential tissue-damaging stimuli
In the lab we typically measure
nociceptive withdrawal reflexes
Noicieptive assays process
a) Establish baseline response
-latency to respond to a stimulus
-duration of response to a stimulus
-quantitative measure i.e force or grams required to elicit response
b) Manipulate
c) Re-assess the response
Change from baseline reflects altered sensory processing
Hyperalgesia
An increased response to a stimulus which is normally painful
Allodynia
Pain due to a stimulus which does not normally evoke pain
Things measured in assays: latency to response
time to respond
Things measured in assays: duration of response
how long the response lasts
Things measured in assays: quantative measures
How much stimulus does it take to elicit a response
Manipulations in assays
can be damage to a peripheral nerve (mimic neuropathic pain) OR induced inflammation
Common sensory modalities tested in assays
- Thermal
- mechanical
- chemical
Thermal tests include
- noxious heat (<49°C), cold (>4°C)
- innocuous warm/cool
Mechanical tests include
- punctate stimulation (von Frey Hairs)
- pressure
Chemical tests include
- capsaicin, mustard oil (both activate ‘heat’)
“Hargreaves test” measures (what modality)
Thermal sensitivity
“Hargreaves test”–how
Radiant heat briefly applied to surface of the hindpaw
Measure latency to withdraw from heat source
Several trials for each paw
Calculate average withdrawal latency
“Hargreaves test”–baseline; manipulation and expected result
Baseline: threshold latency: ~12s
Manipulate: hindpaw inflammation
Re-assess: 3hrs after inflammation
latency to withdraw ~4s. = heat hyperalgesia
Acetone test measures (what modality)
sensitivity to cold/cool stimuli
Acetone test process
A small drop of acetone onto surface of hindpaw (normally innocuous)
Measure duration of response
Allodynia reflected by prolonged lifting and guarding of the paw
Acetone test–baseline; manipulation and expected result
Baseline: threshold duration of response: ~1s
Manipulate: animal model of MS
Re-assess: duration of response increases in mice with disease =Cold Allodynia
Von frey hairs measures
sensitivity to punctate mechanical stimuli
Von frey hairs: How
Hairs are calibrated to deliver specific
amount of force when applied to the surface of the skin
Hairs are applied in ascending order of bending force (to hind paw)
Determine threshold force that elicits nociceptive withdrawal reflexes
Von frey hairs–baseline; manipulation and expected result
Baseline: threshold 14g bending force
Manipulate: hindpaw inflammation
Re-assess: 3hrs after inflammation, force required to elicit nociceptive behaviour is ~3g =Tactile allodynia
Issues with measuring treatments that relieve pain
It could be a true analgesic/anti-allodynic effect OR just sedation OR motor impairment (to explain lack of response)
How to test if it is a true analgesic
Have to run other experiments to control for altered locomotion or sedation (assess motor function)
How do we assess locomotor function as a control for treatments
“Open-field” activity OR Rotorod assay (Assesses gross locomotor ability, co-ordination, balance)
Rotorod assay
put animal on a rotating beam and monitor latency to falling off the beam–compare treatment to control
Inflammatory pain cause
pain that arises as a result of cutaneous tissue injury
How to model inflammatory pain
Inject small volumes of mediators that stimulate immune/inflammatory reactions (ex. carrageenan, CFA)
How long do inflammatory pain models last
Affect sensory function (allodynia, hyperalgesia) for hours to days
Neuropathic pain cause
pain that arises as a result of injury to the nervous system (peripheral or central).
How long do Neuropathic pain model last
Sensory function affected (allodynia, hyperalgesia) for days to weeks
“Seltzer Model” aka
Partial Nerve Injury
Partial Nerve Injury/“Seltzer Model” HOW
Ligate (tie off) 1/3-1/2 of sciatic nerve
Advantages of Partial Nerve Injury/“Seltzer Model”
relative ease of procedure
Diadvanatages of Partial Nerve Injury/“Seltzer Model”
- variability of injury from subject to subject; - variability between experimenters
- nerves are rarely fully severed in real life (more often crushed–this doesn’t match that)
“Bennett Model” aka
Chronic Constriction Injury
Chronic Constriction Injury/ “Bennett Model” Process
4 chromic cat gut sutures loosely ligate sciatic nerve –> Leads to a gradual but progressive axonal injury AND Generates local inflammatory reaction
Chronic Constriction Injury/ “Bennett Model” Advantages
closely resembles human pathology (i.e. crushed nerves)
Chronic Constriction Injury/ “Bennett Model” Disadvantages
- difficult surgery
- high variability between subjects & experimenters (even more variable than the seltzer model)
Chung model aka
Spinal Nerve Ligation Injury
Spinal Nerve Ligation Injury /“Chung Model” Process
Tightly ligate and cut L5 spinal nerve close to DRG (before it emerges and joins the schiatic nerve)–leave L3, L4, L6 uninjured
= mixing of injured and uninjured
Spinal Nerve Ligation Injury /“Chung Model” Advantages
Consistent injury across all subjects (not variable)
Spinal Nerve Ligation Injury /“Chung Model” Disadvantages
Very complicated surgery (have to remove spinal lamina to access spinal nerve–MEGA invasive)
Spared Nerve Injury: process
Fully transect (cut) 2 of 3 terminal branches of the sciatic nerve (tibial & common peroneal) leave sural nerve intact (also mixing of injured and uninjured –> generates chronic and persistent change in pain sensitivity)
Spared Nerve Injury: Advantages
- consistency of injury across subjects
- ease vs. “Chung” model.
Neuropathic Pain-Peripheral Nerve Injury produces
- long term tactile allodynia
- thermal hyperalgesia
- cold allodynia
AS intended then can use this pain to look at treatment modalities
How to asess changes in sensory threshold AND how aversive (emotional aspect) the stimuli is with escape avoidance paradigm
- more true to pain
2 steps:
1) Nerve injury or CFA inflammation + von Frey Hair stimulation (changes in threshold)
2) Run testing in the context of an “Escape/Avoidance” paradigm (affective changes)
“Escape/Avoidance” paradigm
Allowing an animal to choose b/t:
1 compartment that is dark and cozy (preferred) but has von frey hair stim
VS
Other compartment that is bright (scary to rats as they are prey) BUT no tactile stim
“Escape/Avoidance” paradigm expected results
If allodynia occurs they will prefer (i.e. spend more time in) the brightly lit; if not they will prefer the cozy (as the mechanical stim isn’t off putting)
“Escape/Avoidance” paradigm expected results
Measure time spent in “light” side of testing environment
Controls: spend only 30% of the time in light side
Nerve Injury/CFA: spend up to 90% of time in the light side (choose scary side over pain)
“Conditioned Place Preference Paradigm”
Teach animal to associate one area with something (usually rewarding substances) and measuring time spent b/t the areas associated vs. not-associated with this substance
“Conditioned Place Preference Paradigm” in Pain
Associate chamber with drug that removes pain
If there is an ongoing spont pain (as seen in humans) the animal should prefer the side that relieves the pain
“Conditioned Place Preference Paradigm” : Clonidine OR. omega-conotoxin (vs. saline)
Both signal relief
Pre-conditioning–equal time in both
in Sham: equal time in both
with SNL (damage)–prefer drug side over saline (b/c it removes pain)
Adenosine
In humans-spinal adenosine reduces ‘evoked’ pain. Ineffective for ongoing pain
adenosine in animal studies of CPP
adenosine reverses mechanical thresholds BUT
No preference for adenosine over saline after SNL (measure of spont pain)
Open field test measures…
anxiety state (affective component of pain)
Open field test in pain studies
NO injury–most of time spent against wall but curiosity brings them to explore the centre
HIV-neuropathy (PAIN)–large amount of time against the wall, less curious, not exploring center = ANXIETY
How to test pain vs noicieption
Incorporate assays that examine psychological learned associations to get additional insight into ‘qualitative’ aspects of the pain experience in rodents
