Influence of social interaction on nociceptive-induced changes in locomotor activity in a mouse model of acute inflammatory pain: Use of novel thermal assays

Microbiome depletion prior to repeat mild TBI differentially alters social deficits and prefrontal cortex plasticity in adolescent and adult rats

Although aging, repeat mild traumatic brain injury (RmTBI), and microbiome modifications independently change social behavior, there has been no investigation into their cumulative effects on social behavior and neuroplasticity within the prefrontal cortex. Therefore, we examined how microbiome depletion prior to RmTBI affected social behavior and neuroplasticity in adolescent and adult rats. Play, temperament analysis, elevated plus maze, and the hot/cold plate assessed socio-emotional function. Analyses of perineuronal nets (PNNs) and parvalbumin (PV) interneurons was completed. Social-emotional deficits were more pronounced in adults, with microbiome depletion attenuating social behavior deficits associated with RmTBI in both age groups. Microbiome depletion increased branch length and PNN arborization within the PFC but decreased the overall number of PNNs. Adults and males were more vulnerable to RmTBI. Interestingly, microbiome depletion may have attenuated the changes to neuroplasticity and subsequent social deficits, suggesting that the microbiome is a viable, but age-specific, target for RmTBI therapeutics.

(2R,6R)-hydroxynorketamine (HNK) reverses mechanical hypersensitivity in a model of localized inflammatory pain

The ketamine metabolite (2R,6R)-hydroxynorketamine, or (2R,6R)-HNK, was recently reported to evoke antinociception in response to a noxious thermal stimulus in healthy mice and reverse mechanical hypersensitivity in a murine model of neuropathic pain. This study reports the behavioral effects of (2R,6R)-HNK in male and female C57BL/6J mice exposed to a localized inflammatory pain condition and the broad pharmacological mechanism underlying this effect. Hind paw intraplantar injection of λ-carrageenan (CARR) caused inflammation and mechanical hypersensitivity in mice within 2 hours, lasting at least 48 hours. Intraperitoneal administration of (2R,6R)-HNK (10-30 mg/kg i.p.) 2 hours following CARR injection significantly reversed mechanical hypersensitivity within 1 hour in male and female mice, and the effect persisted for 24 hours following a single dose. The magnitude and timing of the analgesic effect of (2R,6R)-HNK were comparable to the non-steroidal anti-inflammatory drug carprofen. The reversal of hypersensitivity by (2R,6R)-HNK was blocked at 4 and 24 hours after administration by pretreatment with the AMPA receptor antagonist NBQX and was not accompanied by changes in locomotor activity. These findings reinforce the growing evidence supporting (2R,6R)-HNK as a novel analgesic in multiple preclinical pain models and further support an AMPAR-dependent mechanism of action. SIGNIFICANCE: The ketamine metabolite (2R,6R)-HNK reversed mechanical hypersensitivity associated with localized inflammation with onset less than one hour and duration greater than 24 hours in an effect comparable to the NSAID carprofen. Reversal of mechanical hypersensitivity by (2R,6R)-HNK is AMPAR-dependent.

Behavioral, hormonal, and neural alterations induced by social contagion for pain in mice

Neurobiology of social contagion/empathy aims to collaborate with the development of treatments for human disorders characterized by the absence of this response - autism spectrum disorder, schizophrenia, and antisocial personality disorder. Previous studies using sustained aversive stimuli (e.g., neuropathic pain or stress) to induce social contagion behaviors in rodents have demonstrated that these conditions may increase hypernociception, anxiogenic-like effects, and defensive behaviors in cagemates. To amplify the knowledge about behavioral, hormonal, and neural alterations induced by cohabitation with a pair in neuropathic pain, we investigated the effects of this protocol on (i) pain (writhing, formalin, hot plate tests) and depression (sucrose splash test) responses, (ii) the serum levels of corticosterone, testosterone, and oxytocin, (iii) noradrenalin, dopamine and its metabolite (DOPAC and HVA) levels in the amygdaloid complex and insular cortex, (iv) neuronal activation pattern (FosB labeling) in the ventral tegmental area (VTA), paraventricular nucleus of the hypothalamus (PVN) and supraoptic nucleus (SO). One day after weaning, male Swiss mice were housed in pairs for 14 days. Then, they were divided into two groups: sciatic nerve constricted cagemate [CNC; i.e., one animal of each pair was subjected to sciatic nerve constriction (NC)], and cagemate sham (CS; a similar procedure but with no nerve constriction), and housed for further 14 days. After 28 days of cohabiting, four independent groups were subjected to (a) behavioral analyses (Exp. 1) and (b) blood samples collected for Elisa assays of corticosterone, testosterone, and oxytocin (Exp. 2), remotion of brains for the (c) HPLC in the noradrenaline dopamine and metabolites quantification (Exp. 3) or (d) immunoassays analyses for FosB labeling (Exp. 4). Results showed that cohabitation with a conspecific in chronic pain induces hypernociception and antinociception in the writhing and formalin tests, respectively, and anhedonic-like effects in the sucrose splash test. Hormonal results indicated a decrease in plasma corticosterone only in nerve constricted mice, in testosterone (CNC and NC animals), and an increase in oxytocin serum levels. The neurochemical analyses demonstrated that the social contagion for pain protocol increases in dopamine turnover in the amygdala and insula. This assay also revealed an increase in noradrenaline levels and dopamine turnover within the insula of NC mice. In the FosB labeling measure, we observed a rise in the VTA, PVN and SO in the CNC group whereas for the NC group an increase of this activation pattern occurred only in the VTA. Present results suggest the role of hormones (testosterone and oxytocin) and neurotransmitters (dopamine) in the modulation of behavioral changes induced by social contagion in animals cohabitating with a conspecific in pain.

Social Buffering as a Tool for Improving Rodent Welfare

The presence of a conspecific can be calming to some species of animal during stress, a phenomenon known as social buffering. For rodents, social buffering can reduce the perception of and reaction to aversive experiences. With a companion, animals may be less frightened in conditioned fear paradigms, experience faster wound healing, show reduced corticosterone responses to novelty, and become more resilient to everyday stressors like cage-cleaning. Social buffering works in diverse ways across species and life stages. For example, social buffering may rely on specific bonds and interactions between individuals, whereas in other cases, the mere presence of conspecific cues may reduce isolation stress. Social buffering has diverse practical applications for enhancing rodent wellbeing (some of which can be immediately applied, while others need further development via welfare-oriented research). Appropriate social housing will generally increase rodents' abilities to cope with challenges, with affiliative cage mates being the most effective buffers. Thus, when rodents are scheduled to experience distressing research procedures, ensuring that their home lives supply high degrees of affiliative, low stress social contact can be an effective refinement. Furthermore, social buffering research illustrates the stress of acute isolation: stressors experienced outside the cage may thus be less impactful if a companion is present. If a companion cannot be provided for subjects exposed to out-of-cage stressors, odors from unstressed animals can help ameliorate stress, as can proxies such as pieces of synthetic fur. Finally, in cases involving conditioned fear (the learned expectation of harm), newly providing social contact during exposure to negative conditioned stimuli (CS) can modify the CS such that for research rodents repeatedly exposed to aversive stimuli, adding conspecific contact can reduce their conditioned fear. Ultimately, these benefits of social buffering should inspire the use of creative techniques to reduce the impact of stressful procedures on laboratory rodents, so enhancing their welfare.

Characterization of acute pain-induced behavioral passivity in mice: Insights from statistical modeling

Affective-motivational disturbances are highly inconsistent in animal pain models. The reproducibility of the open-field test in assessing anxiety, malaise or disability remains controversial despite its popularity. While traumatic, persistent or multiregional pain models are commonly considered more effective in inducing negative affect or functional impairment, the early psychobehavioral changes before pain chronification are often underexplored. Here, we aimed to clarify the fundamental relationship between hypernociception and passive distress-like behavior using a model of transient inflammatory pain. To minimize latent confounders and increase data consistency, male C57BL/6N mice were habituated to the open-field arena 6 times before receiving the unilateral intraplantar injection of prostaglandin E2 (PGE2) or vehicle. Open-field (40-min exploration) and nociceptive behavior were evaluated repeatedly along the course of hypernociception in both wild-type and transgenic mice with a known pronociceptive phenotype. To reduce subjectivity, multivariate open-field behavioral outcomes were analyzed by statistical modeling based on exploratory factor analyses, which yielded a 2-factor solution. Within 3 hr after PGE2 injection, mice developed significantly reduced center exploration (factor 1) and a marginally significant increase in their habituation tendency (factor 2), which were not apparent in vehicle-injected mice. The behavioral passivity generally improved as hypernociception subsided. Therefore, transient inflammatory irritation is sufficient to suppress mouse open-field exploratory activity. The apparent absence of late affective-motivational changes in some rodents with prolonged hypernociception may not imply a lack of preceding or underlying neuropsychological alterations. Procedural pain after invasive animal experiments, however small, should be assessed and adequately controlled as a potential research confounder.

Pain influences food preference and food-related memory by activating the basolateral amygdala in rats

The amygdala has been demonstrated to contribute to pain-related behavior and food preference. Here, the effect of pain on food preference and food-matched visual-cue memory, in the presence or absence of a basolateral amygdala (BLA) lesion, has been evaluated using a novel innovative apparatus and protocol. Forty adult male Wistar rats were randomly divided into five groups (n = 8) as follows: control, pain, ibuprofen + pain, BLA lesion, BLA lesion + pain groups. Bilateral lesions of the BLA were produced by passing a current of 1.5 mA for 7 s. Pain was induced on the right hind paw of the rats by sub-plantar injection of 50 μl of 2.5% formalin. The animals were encountered with four different meals including wholemeal, wholemeal + sugar, white flour, and biscuit. Each test session consisted of six trials with inter-trial intervals of 15 min. The number of visits to each meal zone and port, the amount of time spent in each food zone and port, traveled distance in each food zone, food consumption per each visit and the total food consumption were recorded. The control group showed a high biscuit preference and low white flour preference. Rats suffering BLA lesion and rats in the BLA lesion + pain group exhibited a shifted preference curve. They had a bias toward eating wholemeal + sugar rather than white flour and biscuit. This group also showed an impaired spatial memory. In conclusion, our findings suggest that the BLA may be involved in pain-induced food preference and food-matched visual-cue memory.

Social housing promotes recovery of wheel running depressed by inflammatory pain and morphine withdrawal in male rats

The increased use of opioids to treat pain has led to a dramatic increase in opioid abuse. Our previous data indicate that pain may facilitate the development of opioid abuse by increasing the magnitude and duration of opioid withdrawal. The present study tested the hypothesis that social housing would facilitate recovery of activity depressed by pain and opioid withdrawal. Male Sprague Dawley rats were housed either in pairs or alone and then moved to a cage with a running wheel for 6 h daily to assess pain- and opioid withdrawal-induced depression of wheel running. Rats were implanted with two morphine (75 mg each) or placebo pellets to induce opioid dependence and were simultaneously injected with Complete Freund's Adjuvant or saline into the right hind paw to induce persistent inflammatory pain. Hind paw inflammation depressed wheel running whether rats were implanted with a morphine or placebo pellet. Pair-housed rats showed greater recovery of wheel running than individually housed rats. Spontaneous morphine withdrawal precipitated by removal of the morphine pellets caused a reduction in wheel running that was greater in rats with hind paw inflammation compared to pain free rats. Social housing facilitated recovery from withdrawal in rats with hind paw inflammation, but slowed recovery in pain free rats. These data suggest that social housing facilitates recovery by reducing pain both before and during opioid withdrawal. Our findings are consistent with previous studies showing that social buffering reduces pain-evoked responses.

An effective and concise device for detecting cold allodynia in mice

Detection of cold allodynia is a very important aspect in the study of pain behavior. An effective and concise device for detecting cold pain has always been the hope of many researchers. Here, an easily produced and operated cold plate device is presented for the assessment of cold allodynia in mice. The device used to detect cold allodynia has two components: a chamber consists of a cylinder for animal experiment and a cube box around the chamber for holding ice to keep temperature stable. In the testing chamber, a mouse was placed on the circular plexiglass plate steady at 4 °C above ice for five minutes. The tested mouse will lift its paw when exposed to the cold plate. The number of lifts will present animal's response to the degree of cold stimulation. To evaluate this approach, three commonly used pain models of mice were tested: formalin test, bone cancer pain (BCP), and chronic constriction injury (CCI). As is reported in other literatures, these three pain mice models showed increased sensitivity to cold stimulation. The new device is indeed suitable for detecting cold allodynia behavior in mice. Comparisons with existing devices of detecting cold allodynia, such as the cold plate in the market (UGO, Panlab, Columbus, etc.), the new device has the advantages of low cost, simple operation and easy popularization and can detect cold allodynia behavior of mice very well. This is a very practical and economical device to detect cold allodynia behavior.

Pharmacological validation of voluntary gait and mechanical sensitivity assays associated with inflammatory and neuropathic pain in mice

The urgent need for more effective analgesic treatment options has prompted a re-evaluation of the behavioral tests used to assess pain in pre-clinical research, with an emphasis on inclusion of more voluntary, un-evoked behavioral assessments of pain. In order to validate voluntary gait analysis and a voluntary mechanical conflict-avoidance assay, we tested mouse models of neuropathy (spared nerve injury) and inflammation (complete Freund's adjuvant) alongside reflexive measures of mechanical and thermal hypersensitivity. To establish whether the observed changes in behavioral responses were pain-related, known analgesics (buprenorphine, gabapentin, carprofen) were also administered. Spared nerve injury persistently altered several gait indices, whereas complete Freund's adjuvant caused only transient changes. Furthermore, known analgesics could not reverse these gait changes, despite demonstrating their previously established efficacy in reflexive measures of mechanical and thermal hypersensitivity. In contrast, the mechanical conflict-avoidance assay demonstrated aversion in mice with neuropathy and inflammation-induced hypersensitivity, which could both be reversed by analgesics. We conclude that voluntary gait changes in rodent neuropathic and inflammatory pain models are not necessarily indicative of pain-related adaptations. On the other hand, mechanical conflict-avoidance represents a valid operant assay for quantifying pain-related behaviors in mice that can be reversed by known analgesics.