Back-translating the clinical manifestations of human disease burden into animal models is increasingly recognized as an important facet of preclinical drug discovery. We hypothesized that inbred rat strains possessing stress hyper-reactive-, depressive- or anxiety-like phenotypes may possess more translational value than common outbred strains for modeling neuropathic pain. Rats (inbred: LEW, WKY, F344/ICO and F344/DU, outbred: Crl:SD) were exposed to Spared Nerve Injury (SNI) and evaluated routinely for 6 months on behaviours related to pain (von Frey stimulation and CatWalk-gait analysis), anxiety (elevated plus maze, EPM) and depression (sucrose preference test, SPT). Markers of stress reactivity together with spinal/brain opioid receptor expression were also measured. All strains variously developed mechanical allodynia after SNI with the exception of stress-hyporesponsive LEW rats, despite all strains displaying similar functional gait-deficits after injury. However, affective changes reflective of anxiety- and depressive-like behaviour were only observed for F344/DU in the EPM, and for Crl:SD in SPT. Although differences in stress reactivity and opioid receptor expression occurred, overall they were relatively unaffected by SNI. Thus, anxio-depressive behaviours did not develop in all strains after nerve injury, and correlated only modestly with degree of pain sensitivity or with genetic predisposition to stress and/or affective disturbances.

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Background Translating efficacy of analgesic drugs from animal models to humans remains challenging. Reasons are multifaceted, but lack of sufficiently rigorous preclinical study design criteria and phenotypically relevant models may be partly responsible. To begin to address this fundamental issue, we assessed the analgesic efficacy of morphine in three inbred rat strains (selected based on stress reactivity and affective/pain phenotypes), and outbred Sprague Dawley ( SD ) rats supplied from two vendors. Methods Sensitivity to morphine (0.3–6.0 mg/kg, s.c.) was evaluated in the hot plate test of acute thermal nociception, the Complete Freund's Adjuvant ( CFA ) model of inflammatory‐induced mechanical hyperalgesia, and in a locomotor motility assay in male rats from the following strains; Lewis ( LEW ), Fischer (F344), Wistar Kyoto ( WKY ), and SD 's from Envigo and Charles River. Results F344 and SD rats were similarly sensitive to morphine in hot plate and CFA ‐induced inflammatory hyperalgesia (Minimum Effective Dose ( MED ) = 3.0 mg/kg). WKY rats developed a less robust mechanical hypersensitivity after CFA injection, and were less sensitive to morphine in both pain tests ( MED = 6.0 mg/kg). LEW rats were completely insensitive to morphine in the hot plate test, in contrast to the reversal of CFA ‐induced hyperalgesia ( MED = 3.0 mg/kg). All strains exhibited a dose‐dependent reduction in locomotor activity at 3.0–6.0 mg/kg. Conclusion Sensory phenotyping in response to acute thermal and inflammatory‐induced pain, and sensitivity to morphine in various inbred and outbred rat strains indicates that different pathophysiological mechanisms are engaged after injury. This could have profound implications for translating preclinical drug discovery efforts into pain patients. Significance The choice of rat strain used in preclinical pain research can profoundly affect the outcome of experiments in relation to (a) nociceptive threshold responses, and (b) efficacy to analgesic treatment, in assays of acute and tonic inflammatory nociceptive pain.

The sucrose preference test is a popular test for anhedonia in the chronic unpredictable stress model of depression. Yet, the test does not always produce consistent results. Long food and water deprivation before the test, while often implemented, confounds the results by introducing unwanted drives in the form of hunger and thirst. We assessed the reliability of the test when only short or no fasting was used. We searched PubMed, Embase, and Web of Science for studies in rats exposed to chronic unpredictable stress that used no more than 6 hours of food and/or water deprivation before the test. Sweet consumptions, for stressed and control/antidepressant-treated animals, in 132 studies were pooled in a random effects model. We found a decrease in sweet consumption in stressed rats, compared to controls, that was halved when a non-caloric sweetener was used and significantly reduced when sucrose consumption was corrected for body weight. The effect was reversed when the stressed rats were treated with antidepressants. Methodological strategies meant to control for recognized sources of bias when conducting the test were often missing, and so was a clear and complete report of essential study information. Our results indicate that not only is food and water deprivation before the test unnecessary, but not recommended. Even in absence of long fasting, we found evidence of an additional effect on sweet consumption that is unrelated to anhedonia. Without properly controlling for non-hedonic drivers of consumption, the test is unreliable as a proxy measure of anhedonia. Strengthening the methodological rigor and addressing the confounding effect of metabolic factors in the sucrose preference test prevents misleading conclusions that harm the translatability of the associated research and perpetuates the use of animals for little gain.

IntroductionThe induction of neuropathic pain-like behaviors in rodents often requires surgical intervention. This engages acute nociceptive signaling events that contribute to pain and stress post-operatively that from a welfare perspective demands peri-operative analgesic treatment. However, a large number of researchers avoid providing such care based largely on anecdotal opinions that it might interfere with model pathophysiology in the longer term.ObjectivesTo investigate effects of various peri-operative analgesic regimens encapsulating different mechanisms and duration of action, on the development of post-operative stress/welfare and pain-like behaviors in the Spared Nerve Injury (SNI)-model of neuropathic pain.MethodsStarting on the day of surgery, male Sprague-Dawley rats were administered either vehicle (s.c.), carprofen (5.0mg/kg, s.c.), buprenorphine (0.1mg/kg s.c. or 1.0mg/kg p.o. in Nutella®), lidocaine/bupivacaine mixture (local irrigation) or a combination of all analgesics, with coverage from a single administration, and up to 72 hours. Post-operative stress and recovery were assessed using welfare parameters, bodyweight, food-consumption, and fecal corticosterone, and hindpaw mechanical allodynia was tested for assessing development of neuropathic pain for 28 days.ResultsNone of the analgesic regimes compromised the development of mechanical allodynia. Unexpectedly, the combined treatment with 0.1mg/kg s.c. buprenorphine and carprofen for 72 hours and local irrigation with lidocaine/bupivacaine, caused severe adverse effects with peritonitis. This was not observed when the combination included a lower dose of buprenorphine (0.05mg/kg, s.c.), or when buprenorphine was administered alone (0.1mg/kg s.c. or 1.0mg/kg p.o.) for 72 hours. An elevated rate of wound dehiscence was observed especially in the combined treatment groups, underlining the need for balanced analgesia. Repeated buprenorphine injections had positive effects on body weight the first day after surgery, but depressive effects on food intake and body weight later during the first week.ConclusionPost-operative analgesia does not appear to affect established neuropathic hypersensitivity outcome in the SNI model.

The choice of vendor used to source the same strain of rat for use in preclinical pain research can profoundly affect the level of nociceptive hypersensitivity and response to reference analgesics in neuropathic versus inflammatory models.

Background: P2X7 receptor antagonists have potential for treating various central nervous system (CNS) diseases, including neuropathic pain, although none have been approved for clinical use. Reasons may include insufficient understanding of P2X7 receptor signalling in pain, and the lack of a corresponding preclinical mechanistic biomarker.Methods: Lu AF27139 is a highly selective and potent small molecule antagonist at rat, mouse and human forms of the P2X7 receptor, with excellent pharmacokinetic and CNS permeability properties. In the current experiments, we probed the utility of previously characterized and novel signalling cascades exposed to Lu AF27139 using cultured microglia combined with release assays. Subsequently, we assessed the biomarker potential of identified candidate molecules in the rat chronic constriction injury (CCI) model of neuropathic pain; study design limitations precluded their assessment in spared nerve injury (SNI) rats.Results: Lu AF27139 blocked several pain-relevant pathways downstream of P2X7 receptors in vitro. At brain and spinal cord receptor occupancy levels capable of functionally blocking P2X7 receptors, it diminished neuropathic hypersensitivity in SNI rats, and less potently in CCI rats. Although tissue levels of numerous molecules previously linked to neuropathic pain and P2X7 receptor function (e.g. IL-6, IL-1β, cathepsin-S, 2-AG) were unaffected by CCI, Lu AF27139-mediated regulation of spinal PGE2 and miRNA (e.g. rno-miR-93-5p) levels increased by CCI aligned with its ability to diminish neuropathic hypersensitivity. Conclusions:We have identified a pain-relevant P2X7 receptor-regulated mechanism in neuropathic rats, which could hold promise as a translatable biomarker and by association enhance the clinical progression of P2X7 receptor antagonists in neuropathic pain.

Researchers performing experiments on animals should always strive towards the refinement of experiments, minimization of stress and provision of better animal welfare. An adequate analgesic strategy is important to improve post-operative recovery and welfare in laboratory rats and mice. In addition, it is desirable to provide post-operative analgesia using methods that are minimally invasive and stressful. This study investigated the antinociceptive effects of orally administered buprenorphine ingested in Nutella® in comparison with subcutaneous buprenorphine administration. By exposing the animal to a thermal stimulus using a hot plate, significant antinociceptive effects of voluntarily ingested buprenorphine administered in Nutella® were demonstrated. This was evident at doses of 1.0 mg/kg 60 and 120 min post administration ( P < 0.01), although antinociceptive effects were not as marked as with subcutaneous administration, and had a later onset. It is advised to administer the oral formulation of buprenorphine in Nutella® in a 10-fold higher dose, as well as approximately 60 min earlier, than with the more commonly employed subcutaneous route of administration.