It is broadly accepted that, as part of the humane care and use of animals in research, the pain experienced by animals should be minimized to the extent possible, consistent with the goals of the research. In some cases, pain may be the subject under study, whereas in other cases, the use of some types of analgesics may interfere with the experimental objectives of the work. This issue of Comparative Medicine provides reviews related to the recognition and treatment of pain, the interaction of pain and pain relief on experimental outcomes, and ethical perspectives on the need to reduce pain in research rodents, whenever possible.

Scientists have ethical and regulatory commitments to minimize pain and distress during their use of sentient laboratory animals. Here I discuss pain as a special form of distress and the long history of ethical and regulatory standards calling on scientists to prevent, minimize, treat or terminate animal pain. Scientists, veterinarians, and IACUC face 2 challenges: knowledge of effective analgesic doses and regimens for all sexes, ages and genotypes of rodent is incomplete, and concerns regarding the effects of analgesic drugs on research outcomes push scientists to request approval to withhold analgesics and leave animal pain unalleviated. IACUC thus conduct what I call an 'ethics of uncertainty,' in which they factor in the limits of available ethically relevant information on the amount of expected animal suffering, the usefulness of analgesics to mitigate this suffering, and the eventual benefits that come from the research. IACUC must factor in current limitations in severity assessments of various experimental manipulations in various strains, inaccurate pain diagnosis, in known effective analgesic and other refinements, and on effects of pain medications and untreated pain on data outcomes, when deciding to allow potentially painful experiments and animal care practices. This article focuses on 3 areas of concern: the limits of veterinary "professional judgment" when the animal model's degree of pain and the efficacy of pain medications are not yet known; the review of proposals with known, unalleviated significant pain and distress (that is, Category E experiments); and the attempt to review the balance between animal welfare harms and scientific objectives. I propose no new regulations, standards, or ethical norms herein but rather explore some of the implications when existing ethical principles are applied to evolving scientific knowledge (and vice versa). I conclude that applying current animal pain management knowledge to prevailing ethical principles will shift IACUC toward greater caution in allowing potentially painful animal experiments, with heightened caution regarding the ability of analgesics to mitigate the animals' pain.

Ensuring that laboratory rodent pain is well managed underpins the ethical acceptability of working with these animals in research. Appropriate treatment of pain in laboratory rodents requires accurate assessments of the presence or absence of pain to the extent possible. This can be challenging some situations because laboratory rodents are prey species that may show subtle signs of pain. Although a number of standard algesiometry assays have been used to assess evoked pain responses in rodents for many decades, these methods likely represent an oversimplification of pain assessment and many require animal handling during testing, which can result in stress-induced analgesia. More recent pain assessment methods, such as the use of ethograms, facial grimace scoring, burrowing, and nest-building, focus on evaluating changes in spontaneous behaviors or activities of rodents in their home environments. Many of these assessment methods are time-consuming to conduct. While many of these newer tests show promise for providing a more accurate assessment of pain, most require more study to determine their reliability and sensitivity across a broad range of experimental conditions, as well as between species and strains of animals. Regular observation of laboratory rodents before and after painful procedures with consistent use of 2 or more assessment methods is likely to improve pain detection and lead to improved treatment and care—a primary goal for improving overall animal welfare.

The use of effective regimens for mitigating pain remain underutilized in research rodents despite the general acceptance of both the ethical imperative and regulatory requirements intended to maximize animal welfare. Factors contributing to this gap between the need for and the actual use of analgesia include lack of sufficient evidence-based data on effective regimens, under-dosing due to labor required to dose analgesics at appropriate intervals, concerns that the use of analgesics may impact study outcomes, and beliefs that rodents recover quickly from invasive procedures and as such do not need analgesics. Fundamentally, any discussion of clinical management of pain in rodents must recognize that nociceptive pathways and pain signaling mechanisms are highly conserved across mammalian species, and that central processing of pain is largely equivalent in rodents and other larger research species such as dogs, cats, or primates. Other obstacles to effective pain management in rodents have been the lack of objective, science-driven data on pain assessment, and the availability of appropriate pharmacological tools for pain mitigation. To address this deficit, we have reviewed and summarized the available publications on pain management in rats, mice and guinea pigs. Different drug classes and specific pharmacokinetic profiles, recommended dosages, and routes of administration are discussed, and updated recommendations are provided. Nonpharmacologic tools for increasing the comfort and wellbeing of research animals are also discussed. The potential adverse effects of analgesics are also reviewed. While gaps still exist in our understanding of clinical pain management in rodents, effective pharmacologic and nonpharmacologic strategies are available that can and should be used to provide analgesia while minimizing adverse effects. The key to effective clinical management of pain is thoughtful planning that incorporates study needs and veterinary guidance, knowledge of the pharmacokinetics and mechanisms of action of drugs being considered, careful attention to individual differences, and establishing an institutional culture that commits to pain management for all species as a central component of animal welfare.

Pain and its alleviation are currently a highly studied issue in human health. Research on pain and response to analgesia has evolved to include the effects of genetics, heritability, and sex as important components in both humans and animals. The laboratory mouse is the major animal studied in the field of pain and analgesia. Studying the inbred mouse to understand how genetic heritable traits and/or sex influence pain and analgesia has added valuable information to the complex nature of pain as a human disease. In the context of biomedical research, identifying pain and ensuring its control through analgesia in research animals remains one of the hallmark responsibilities of the research community. Advancements in both human and mouse genomic research shed light not only on the need to understand how both strain and sex affect the mouse pain response but also on how these research achievements can be used to improve the humane use of all research animal species. A better understanding of how strain and sex affect the response to pain may allow researchers to improve study design and thereby the reproducibility of animal research studies. The need to use both sexes, along with an improved understanding of how genetic heritability affects nociception and analgesic sensitivity, remains a key priority for pain researchers working with mice. This review summarizes the current literature on how strain and sex alter the response to pain and analgesia in the modern research mouse, and highlights the importance of both strain and sex selection in pain research.

Mice and rats are valuable and commonly used as models for the study of cancer. The models and methods of experimentation have the potential to cause pain to some degree, and all charged with ensuring animal welfare must determine how to manage it. A commonly posed question, especially from investigators and IACUC, is whether the provision of analgesic agents will render the model invalid. Left untreated, pain is a stressor and has negative consequences, most notably immune system perturbations. In addition, analgesic agents in the opioid and NSAID drug classes exhibit immunomodulatory activity and influence processes such as cell proliferation, apoptosis, and angiogenesis that are important in cancer formation. Therefore, both pain and the agents used to alleviate it have the potential to act as confounding factors in a study. This review article presents data from both human medicine and work with animal models in an attempt to help inform discussions about the withholding of analgesic agents from animals used in cancer studies.

Neurologic conditions such as stroke and traumatic brain injury are challenging conditions to study in humans. Animal models are necessary to uncover disease processes and develop novel therapies. When attempting to model these or other neurologic diseases, the accompanying anesthesia and analgesia create variables that are not part of the onset of the clinical disease in the human population but are critical components of the postinjury care both in humans and animals. To maximize model validity, researchers must consider whether the disease process or a novel therapy is being studied. Damage to the neurons of the brain or the spinal cord is not painful at the neural tissue itself, but alterations to nociceptive signaling along the pain pathway can induce chronic pain. In addition, trauma or surgery leading to the event is associated with damage to peripheral tissue. Inflammation is inextricably associated with tissue injury. Inflammation is known to evoke nociception in the periphery and drive long-term changes to neurons in the CNS. Analgesics and anesthetics alter these responses yet are required as part of humane animal care. Careful planning for effective drug administration consistent with the standard of care for humans and equivalent animal care is required.

One of the most significant challenges facing investigators, laboratory animal veterinarians, and IACUCs, is how to balance appropriate analgesic use, animal welfare, and analgesic impact on experimental results. This is particularly true for in vivo studies on immune system function and inflammatory disease. Often times the effects of analgesic drugs on a particular immune function or model are incomplete or don't exist. Further complicating the picture is evidence of the very tight integration and bidirectional functionality between the immune system and branches of the nervous system involved in nociception and pain. These relationships have advanced the concept of understanding pain as a protective neuroimmune function and recognizing pathologic pain as a neuroimmune disease. This review strives to summarize extant literature on the effects of pain and analgesia on immune system function and inflammation in the context of preclinical in vivo studies. The authors hope this work will help to guide selection of analgesics for preclinical studies of inflammatory disease and immune system function.

The surgical stress response and resulting physiologic changes can lead to postoperative complications and negatively impact animal welfare. Although appropriate pain management is crucial to reduce the pain and stress response to surgery, analgesic choice can significantly affect bone and wound healing. This review aims to summarize data from rat and mouse studies and to provide recommendations for integrating analgesia into orthopedic and wound healing models in these species. Data from other species, such as humans, rabbits and other rodents, is included, where available. From these data, we conclude that for orthopedic surgical models, opioids, local anesthetics and dissociative agents have minimal impact on fracture healing; cyclooxygenase 2 (COX2) selective nonsteroidal antiinflammatory drugs (NSAID) may be used in the shortterm; and steroids should be avoided. For wound healing models, short-term systemic or topical opioids have negligible impact on wound healing; NSAID or local anesthetics may be used short-term; and systemic steroids should be avoided. Alternative analgesics such as tramadol, gabapentin, ketamine, and acetaminophen warrant consideration and further evaluation for both orthopedic and wound healing models. In all cases, researchers and veterinarians should work together to determine the appropriate analgesic plan to minimize pain, as well as to minimize unwanted effects on the orthopedic and wound healing models themselves.

Sepsis is a multifaceted host response to infection that dramatically affects patient outcomes and the cost of health care. Animal models are necessary to replicate the complexity and heterogeneity of clinical sepsis. However, these models entail a high risk of pain and distress due to tissue trauma, inflammation, endotoxin-mediated hyperalgesia, and other mechanisms. Several recent studies and initiatives address the need to improve the welfare of animals through analgesics and standardize the models used in preclinical sepsis research. Ultimately, the goal is to provide high-fidelity, humane animal models that better replicate the clinical course of sepsis, to provide more effective translation and advance therapeutic discovery. The purpose of this review is to discuss the current understanding of the roles of pain and analgesia in rodent models of sepsis. The current definitions of sepsis along with an overview of pain in human sepsis are described. Finally, welfare concerns associated with animal models of sepsis and the most recent considerations for relief of pain and distress are reviewed.

Pain is a clinical syndrome arising from a variety of etiologies in a heterogeneous population, which makes successfully treating the individual patient difficult. Organizations and governments recognize the need for tailored and specific therapies, which drives pain research. This review summarizes the different types of pain assessments currently being used and the various rodent models that have been developed to recapitulate the human pain condition.

This overview provides a brief summary of the complex interactions that link sleep, pain, and analgesic medications. Sleep scientists and clinicians are well aware of these relationships and understand that maintaining healthy pain-free subjects in a stable environment is essential to generating interpretable data and valid conclusions. However, these concepts and the data that support bidirectional interactions between sleep and pain may be less known to those who are not sleep scientists yet need such information to protect and advance both animal wellbeing and research validity (for example, veterinarians, IACUC members). Abundant human evidence supports the disruptive effect of pain and the modulatory effects of analgesic drugs on sleep; however, analgesic drugs can alter both sleep and the electroencephalogram, which is the primary objective measure for identifying sleep and evaluating sleep properties in both humans and animals. Consideration of the modulatory and interactive relationships of sleep, pain, and analgesic medications is essential to designing and conducting valid and reproducible sleep research using animal subjects.