Abstract. Despite Chronic pain affects up to 40% of the general population and consequently has a major socio-economic impact on our society (1, 2). Despite affecting millions of people, chronic pain is generally insufficiently treated, leaving a large number of patients with inadequate pain-relief (2, 3). The low success rate for development of new analgesic therapy has been linked to a poor translation from preclinical findings to clinical effects (4, 5), which have led to extensive criticism of the predictive value of the animal models of chronic pain (5-7). However, animal models are currently the best available tool for testing the analgesic potential of new drugs. A way to increase the probability of successful translation could, therefore, be to rethink the way models are used. There is currently an extensive mismatch between preclinical and clinical assessment of pain, and it likely reduces the chances of translation from preclinical findings to clinical efficacy.Most preclinical research relies on detection of responses to mechanical or thermal stimuli, hence the evaluation of evoked pain. In contrast, clinical research aims at quantifying the subjective pain experience, mainly evaluated by self-reporting systems, such as the numeric or visual analogue scales (8), and hence quantifying ongoing pain. Consequently, preclinical research is focused on stimulus-based evoked responses, mainly mediated by spinal processes, whereas clinical studies are evaluating the pain experience, including perception at supraspinal levels. Rodents are still the primary choice for in vivo models of various pain conditions, and although tradition has favoured quantification of evoked responses, rodents do display aversive motivational states that are similar to aspects of human pain perception (9, 10). Aversive affective functions in animals are likely adaptive survival mechanisms, and are based on the ability to alter behavioural decisions or suspend normal behaviour in order to avoid injury or permit faster healing of a damaged body part. Development of behavioural tests designed to quantify suspension of normal behaviour is, therefore, likely a more relevant readout for human pain assessment, and validation and implementation of such tests might ease the translation from in vivo model to patient.In this study we tested if sparing of the affected leg during grid climbing behaviour can be used as a non-stimuli-evoked behavioural test to quantify neuropathic-spared nerve injury (SNI) and cancer-induced bone (CIBP) pain. The test is based on suspension of the animal's normal behaviour, and hence reflects how the ongoing pain affects that animal's natural behaviour. In addition, the tests are based on objective measures, which reduces the variability that can be associated with application of a given stimuli and interpretation of responses by the experimenter.
Materials and MethodsAnimals. Six-week-old male and female C3H/HeNRj mice (Taconic, Tornbjerg, Denmark) were group-housed in Type III IVC cages (Scanbur A/S, Karlslunde...