Review articlePros and Cons of Clinically Relevant Methods to Assess Pain in Rodents
Introduction
The ultimate goal of pain research is to develop better treatments to reduce suffering and restore function in chronic pain patients. The problem is that current treatments do not adequately manage chronic pain (Borsook et al., 2014). Barriers to effective treatment include inadequate efficacy of current pharmacological therapies, unpleasant or dangerous side effects that limit dosing, the development of tolerance and dependence with repeated administration, fear of addiction, limited knowledge about how pain is coded, and difficulties assessing the social, emotional, and cognitive aspects of pain. The promise of animal research is to improve understanding of the biological mechanisms underlying pain and the endogenous systems that provide pain relief. Advances in our knowledge of how molecular mechanisms and neural circuits differ between different pain conditions (e.g. evoked vs. spontaneous pain; protective vs. pathological pain; deep tissue vs cutaneous pain) will enhance pain treatment by identifying novel targets. Clinically relevant pain assessment techniques will be especially useful in identifying molecular targets and neural circuits that can be used to guide development of more effective treatments. Using appropriate animal models of pain and thoughtful interpretation of the measures will allow the screening of novel compounds for analgesic efficacy. To date, few treatments targeting pain mechanisms in animals have translated to clinical application in chronic pain patients (Mao, 2012; Mogil et al., 2010a).
There are many reasons for the failure of basic research to produce novel treatments. The endpoint for classical stimulus-evoked pain tests that have been used for decades in rodents - von Frey (Frey, 1896), tail flick (D’amour and Smith, 1941), hotplate (O’Callaghan and Holtzman, 1975), and Hargreaves tests (Hargreaves et al., 1988) – is the inhibition of a behavioral response following acute application of a noxious stimulus. In many cases, the stimulus is applied to normal, non-pathological skin. Drugs that inhibit these responses in uninjured animals block normal, protective responses to noxious stimuli. Any treatment that prevents a patient from responding to a hot stove or a blister on the toe puts the patient at risk. An ideal analgesic compound will leave pain-evoked responses intact and reduce pathological pain. The goal of the clinician is to reduce pathological pain so the patient can engage in normal life activities, not to block responses to noxious stimuli. This mismatch between the goal of clinicians to restore function and the aim of preclinical studies to inhibit normal pain-evoked responses has limited drug development.
Pain-evoked tests are also problematic because they can’t distinguish between analgesic and motor or other disruptive side effects. An animal’s lack of response to a noxious stimulus could indicate analgesia, paralysis, sedation, or lack of motivation. Some studies deal with this problem by conducting additional tests to screen for side effects (Morgan et al., 2006), but this is time-consuming and most studies do not include such an analysis. A change in preclinical research from using pain-evoked tests to function based tests would facilitate the identification of drugs that inhibit nociception in the absence of disruptive side effects.
The first step in the alignment of animal research and clinical practice was the development of different chronic pain conditions in laboratory animals (Bennett and Xie, 1988; Hong and Abbott, 1994; Iadarola et al., 1988; Ness and Gebhart, 1988; Schaible and Schmidt, 1985). These studies have advanced animal research from analysis of pain-evoked responses in healthy animals (e.g., hot plate and tail flick tests) to analysis of chronic conditions such as inflammatory and neuropathic pain. The second step in aligning animal research and clinical practice is the use of similar behavioral endpoints. The most common method to assess pain in humans is self-report using visual analog scales, but a number of non-verbal methods that can be used in both humans and animals are also available (Chow et al., 2016; Rabbitts et al., 2014). This review is focused on the pros and cons of behavior measures that can be used to assess clinically relevant pain in both humans and animals.
Our definition of clinical relevance is an animal study that uses a persistent pain condition that occurs in humans and is measured using endpoints that can also be measured in humans. We do not claim, nor do we believe, that only studies that use these criteria are clinically relevant. Clinical relevance depends on the research objective. Pain-evoked tests such as tail withdrawal from hot water may be clinically relevant for studies examining thermal pain mechanisms, but not clinically relevant if the goal is to assess treatments for neuropathic pain. Many pain-evoked tests (e.g., the hot plate & von Frey tests) have been used to assess the analgesic efficacy of drugs to treat pain. These tests have a long history and have contributed much to pain research (Le Bars et al., 2001). However, our more narrow definition of clinical relevance is necessary for the identification of new treatments for chronic pain.
A new generation of clinically relevant pain measures has been developed over the past decade. The tests differ widely, including both pain-evoked and pain-depressed methods. Although some of these tests have been reviewed before (Cobos and Portillo-Salido, 2013; Negus et al., 2006; Tappe-Theodor and Kuner, 2014), our goal is to describe the advantages and disadvantages of these pain measures as a way to facilitate adoption of the best test for the goals of a particular study.
Section snippets
Grimace scale
The Mouse and Rat Grimace Scale have been widely cited as a clinically relevant method to assess nociception. Although grimacing can also be evoked in aggressive or fearful contexts (Defensor et al., 2012), in positive emotional states (Finlayson et al., 2016), and in response to aversive taste experiences (Berridge, 2000), pain grimacing is distinguished by the context and associated behaviors (e.g., orbital tightening, ear position). The method to assess pain grimacing consists of examining
Conclusion
The key to successful research is using tests that match the goals of the study: For example, gait analysis to understand the impact of arthritis on movement, reversal of pain-depressed wheel running for drug development, and CPP to assess the affective component of pain. We do not claim that the tests described in this review are better than other tests, but they do mimic clinical features of pain in unique and important ways. Our goal in describing problems with each test is not to discourage
Acknowledgments
The authors acknowledge funding from the Collaborative Research Center 1158 (SFB1158, Heidelberg Pain Consortium, Germany) from the Deutsche Forschungsgemeinschaft (DFG) to ATT and NIH grant NS095097 to MMM.
References (139)
- et al.
Pharmacological interrogation of a rodent forced ambulation model: leveraging gait impairment as a measure of pain behavior pre-clinically
Osteoarthr. Cartil.
(2016) - et al.
Behavioral and physiologic indicators of pain in nonverbal patients with a traumatic brain injury: an integrative review
Pain Manag. Nurs.
(2014) Sickness and behaviour in animals: a motivational perspective
Neurosci. Biobehav. Rev.
(1999)- et al.
A peripheral mononeuropathy in rat that produces disorders of pain sensation like those seen in man
Pain
(1988) Measuring hedonic impact in animals and infants: microstructure of affective taste reactivity patterns
Neurosci. Biobehav. Rev.
(2000)- et al.
Validation and implementation of a novel high-throughput behavioral phenotyping instrument for mice
J. Neurosci. Methods
(2014) - et al.
Gait analysis in a rat model of osteoarthrosis
Physiol. Behav.
(1997) - et al.
Gait analysis as an objective measure in a chronic pain model
J. Neurosci. Methods
(2002) - et al.
Conditioned place preference reveals tonic pain in an animal model of central pain
J. Pain
(2011) Burrowing: a sensitive behavioural assay, tested in five species of laboratory rodents
Behav. Brain Res.
(2009)