Measuring pain in the (knockout) mouse: big challenges in a small mammal
Introduction
Pain plays a clearly adaptive role in protecting against, and recuperating from serious injury, but pain that persists is maladaptive and likely a pathology in its own right. The understanding and treatment of pain has thus long been a high priority. Pain research in humans has always been hampered by its subjective nature and obvious ethical impasses. Animal models have thus been greatly relied on, although they themselves are associated with any number of interpretational challenges. An increasing number of studies of pain employ the laboratory mouse, Mus musculus. Advantages of this mammalian species include relatively inexpensive maintenance and ease of breeding, and the availability of a wide variety of genetic models. Compared with the rat, for example, a far larger number of inbred mouse strains—of which each member is genetically identical—are commercially available (see [24], [75]). Inbred strains have proven invaluable for classical and molecular genetic studies including gene mapping efforts. However, the factor most responsible for the recent acceleration of biomedical studies involving the mouse is the development of transgenic ‘knockout’ technology in this species only. This approach has been enthusiastically applied to pain research (see [56], [62]), such that over 20% of published pain research articles in the past 2 years in which the mouse was the subject species presented or reviewed findings from knockout mice.
Unfortunately, the application of behavioral assays of nociception to transgenic mice has been inconsistent and sometimes of poor quality, leading in some cases to unreliable (i.e. non-replicable) and misleading conclusions. This state of affairs likely results from the small number of laboratories with extensive experience performing behavioral assays of nociception in the mouse. Experiments with knockout mice are also affected in significant ways by issues related to genetic background (‘genotype’), another field where expertise is restricted.
Historically, of course, the typical non-human pain research subject has been the laboratory rat. Though both species belong to the Muridae rodent family and (other than the difference in size) appear superficially similar, mice are not small rats, having diverged evolutionarily over 10 million years ago [31]. Accordingly, adapting behavioral assays from the rat to the mouse is a non-trivial task. The specific challenges of performing behavioral assays of nociception in the mouse, and the impact of genotype on these assays, will be the foci of this review. These same issues are explored in greater depth in an upcoming book chapter, including our recommendations as to optimizing testing protocols for this species [53].
Section snippets
General considerations
Mice are genetically, physiologically, and behaviorally distinct from rats, resulting in differential effects of housing, handling and habituation. All three of these ‘h’ factors impact especially on pain research because of the robust inhibitory effect of stress on nociceptive sensitivity. That is, stress-induced analgesia (SIA) (see [34]) can easily confound measurements of ‘basal’ nociceptive sensitivity, as well as interact with exogenously administered analgesics. Such interactions may be
Nociceptive assays
Pain is not a unitary phenomenon, and thus a wide variety of assays have been developed to model different types of pain. Although acute thermal and mechanical assays exhibit more than adequate empirical validity (i.e. predictive power) for many analgesics [81], acute pain is virtually non-existent as a clinical entity. Thus, a number of animal models of ‘chronic’ pain have been developed (most often in the rat, of course), generally involving an injury of inflammatory or neuropathic (i.e.
Strain differences
As if the practical challenges associated with nociceptive testing in the mouse weren't daunting enough, pain researchers working with knockout mice face a host of additional complications related to the fact that they are comparing genotypes. Mice feature prodigious interindividual differences, and this fact has been exploited in the development of inbred strains. A major focus of our laboratory is in fact to document and ultimately explain the genetic basis of variability in nociceptive and
Is the study of behavioral pain genetics reliable?
A recent study by Crabbe and colleagues in two other laboratories [15] documented the sensitivity of eight mouse genotypes (including the 5-HT1B knockout mouse) on a battery of six behavioral assays. What was intriguing about the study is that the data were collected simultaneously in the three laboratories, and the authors went to extraordinary lengths to control for a large number of environmental factors. Although the data probably did not entirely justify it, the pessimistic conclusion
Acknowledgements
This research is supported by USPHS grants DA11394 and DE12735 (JSM). SGW is supported by a National Research Service Award (MH12544).
References (94)
New hot plate tests to quantify antinociceptive and narcotic antagonist actions
Eur. J. Pharmacol.
(1974)- et al.
Exposure to a nonfunctional hot plate as a factor in the assessment of morphine-induced analgesia and analgesic tolerance in rats
Pharmacol. Biochem. Behav.
(1979) - et al.
Involvement of the midbrain reticular formation in self-injurious behavior, stereotyped behavior, and analgesia induced by intranigral microinjection of muscimol
Brain Res.
(1986) Differentiating analgesic and non-analgesic drug activities on rat hot plate: effect of behavioral endpoint
Pain
(1991)- et al.
High-intensity nociceptive stimuli minimize behavioral effects induced by restraining stress during the tail-flick test
J. Pharmacol. Toxicol. Methods
(1992) - et al.
The formalin test: a quantitative study of the analgesic effects of morphine, meperidine, and brain stem stimulation in rats and cats
Pain
(1977) - et al.
Differential effects of weekly and daily exposure to the hot plate on the rat's behavior
Physiol. Behav.
(1994) - et al.
Structure of the rat's behaviour in the hot plate test
Behav. Brain Res.
(1993) - et al.
Changes in nociception after 6-hydroxydopamine lesions of descending catecholaminergic pathways in mice
Pharmacol. Biochem. Behav.
(1986) - et al.
Repeated exposure to sham testing procedures reduces reflex withdrawal and hot-plate latencies: attenuation of tonic descending inhibition?
Neurosci. Lett.
(1989)
Gene-targeting studies of mammalian behavior: is it the mutation or the background genotype?
Trends Neurol. Sci.
Effects of supraspinal orphanin FQ/nociceptin
Peptides
Formalin pain in mice, a useful technique for evaluating mild analgesics
J. Neurosci. Methods
Spinal monoamine and opiate systems partly mediate the antinociceptive effects produced by glutamate at brainstem sites
Brain Res.
Daily rhythms of analgesia in mice: effects of age and photoperiod
Brain Res.
Naloxazone and pain-inhibitory systems: evidence for a collateral inhibition model
Pharmacol. Biochem. Behav.
The influence of a targeted deletion of the IFNγ gene on emotional behaviors
Brain Behav. Immun.
Shortened pain response time following repeated algesiometric tests in rats
Physiol. Behav.
Mice, gene targeting and behaviour: more than just genetic background
Trends Neurol. Sci.
Partial sciatic nerve injury in the mouse as a model of neuropathic pain: behavioral and neuroanatomical correlates
Pain
Role of histamine H1 receptor in pain perception: a study of the receptor gene knockout mice
Eur. J. Pharmacol.
Nociceptive and morphine antinociceptive sensitivity of 129 and C57BL/6 inbred mouse strains: implications for transgenic knock-out studies
Eur. J. Pain
Transgenic studies of pain
Pain
Orphanin FQ is a functional anti-opioid peptide
Neuroscience
Strain-dependent effects of supraspinal orphanin FQ/nociceptin on thermal nociceptive sensitivity in mice
Neurosci. Lett.
Heritability of nociception. I. Responses of eleven inbred mouse strains on twelve measures of nociception
Pain
Heritability of nociception. II. ‘Types’ of nociception revealed by genetic correlation analysis
Pain
Methodological refinements to the mouse paw formalin test. An animal model of tonic pain
J. Pharmacol. Methods
Cyclophosphamide cystitis in mice: behavioural characterisation and correlation with bladder inflammation
Eur. J. Pain
Individual differences in the hotplate test and effects of habituation on sensitivity to morphine
Pain
Immobilization and restraint effects on pain reactions in animals
Pain
Alterations in nociceptive threshold and morphine-induced analgesia produced by intrathecally administered amine antagonists
Brain Res.
Scoring the mouse formalin test: a validation study
Eur. J. Pain
The formalin test: an evaluation of the method
Pain
The tail-flick latency is influenced by skin temperature
APS J.
Regulation of body temperature and nociception induced by non-noxious stressors in rat
Brain Res.
Animal models for pain research
Mol. Med. Today
Nociceptin/orphanin FQ: role in nociceptive information processing
Prog. Neurobiol.
Behavioural characterization and amounts of brain monoamines and their metabolites in mice lacking histamine H1 receptors
Neuroscience
Nociceptive responses to high and low rates of noxious cutaneous heating are mediated by different nociceptors in the rat: behavioral evidence
Pain
Comparison of different animal models of chronic pain
Adv. Pain Res. Ther.
Characterization of stress-induced potentiation of opioid effects in the rat
J. Pharmacol. Exp. Ther.
Inbred strain differences in morphine-induced analgesia with the hot plate assay: a reassessment
Behav. Genet.
Background genotype modulates the effects of g-PKC on the development of rapid tolerance to ethanol-induced hypothermia
Addiction Biol.
Primary afferent tachykinins are required to experience moderate to intense pain
Nature
The capsaicin receptor: a heat-activated ion channel in the pain pathway
Nature
Impaired nociception and pain sensation in mice lacking the capsaicin receptor
Science
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