Changes in saccharin preference behavior as a primary outcome to evaluate pain and analgesia in acetic acid-induced visceral pain in mice

Resilience to Pain-Related Depression in σ1 Receptor Knockout Mice Is Associated with the Reversal of Pain-Induced Brain Changes in Affect-Related Genes

Mice lacking the σ1 receptor chaperone (σ1R-/-) are resilient to depressive-like behaviors secondary to neuropathic pain. Examining the resilience's brain mechanisms could help develop conceptually novel therapeutic strategies. We explored the diminished motivation for a natural reinforcer (white chocolate) in the partial sciatic nerve ligation (PSNL) model in wild-type (WT) and σ1R-/- mice. In the same mice, we performed a comprehensive reverse transcription quantitative PCR (qPCR) analysis across ten brain regions of seven genes implicated in pain regulation and associated affective disorders, such as anxiety and depression. PSNL induced anhedonic-like behavior in WT but not in σ1R-/- mice. In WT mice, PSNL up-regulated dopamine transporter (DAT) and its rate-limiting enzyme, tyrosine hydroxylase (Th), in the ventral tegmental area (VTA) and periaqueductal gray (PAG) as well as the serotonin transporters (SERT) and its rate-limiting enzyme tryptophan hydroxylase 2 (Tph2) in VTA. In addition, μ-opioid receptor (MOR) and σ1R were up-regulated in PAG, and MOR was also elevated in the somatosensory cortex (SS) but down-regulated in the striatum (STR). Finally, increased BDNF was found in the medial prefrontal cortex (mPFC) and hypothalamus (HPT). Sham surgery also produced PSNL-like expression changes in VTA, HPT, and STR. Genetic deletion of the σ1R in mice submitted to PSNL or sham surgery prevented changes in the expression of most of these genes. σ1R is critically involved in the supraspinal gene expression changes produced by PSNL and sham surgery. The changes in gene expression observed in WT mice may be related to pain-related depression, and the absence of these changes observed in σ1R-/- mice may be related to resilience.

Comprehensive Preclinical Assessment of Sensory, Functional, Motivational-Affective, and Neurochemical Outcomes in Neuropathic Pain: The Case of the Sigma-1 Receptor

Chronic pain remains a major health problem and is currently facing slow drug innovation. New drug treatments should address not only the sensory-discriminative but also functional and motivational-affective components of chronic pain. In a mouse model of neuropathic pain induced by partial sciatic nerve ligation (PSNL), we analyzed sensory and functional-like outcomes by hindpaw mechanical stimulation and automated gait analysis (CatWalk). We characterized over time a reward-seeking task based on diminished motivation for natural reinforcers (anhedonic-like behavior). To differentiate the appetitive ("wanting") and consummatory ("liking") aspects of motivational behavior, we quantified the latency and number of approaches to eat white chocolate, as well as the eating duration and amount consumed. We explored a putative chronic pain-induced dysregulation of monoamine function by measuring monoamine levels in the nucleus accumbens (NAc), a well-known brain reward area. Finally, we investigated the role of sigma-1 receptor (σ₁R) modulation, a nonopioid target, in these multiple dimensions by genetic deletion and pharmacological dose-response studies. After 6 weeks, PSNL increased the approach latency and reduced the consumption of white chocolate in 20-25% of the mice, while around 50-60% had one or the other parameter affected independently. After 10 weeks, sham-operated mice also displayed anhedonic-like behavior. PSNL was associated with reduced extracellular baseline dopamine and increased norepinephrine in the NAc and with a suppression of increased dopamine and serotonin efflux in response to the rewarding stimulus. Genetic and pharmacological blockade of σ₁R relieved these multiple alterations in nerve-injured mice. We comprehensively describe sensory, functional, and depression-like impairment of key components of motivated behavior associated with nerve injury. We provide a neurochemical substrate for the depressed mesocorticolimbic reward processing in chronic pain, with a potentially increased translational value. Our results also highlight σ₁R for the therapeutic intervention of neuropathic pain.

Ibuprofen Protection Against Restrained Chronic Stress-induced Depression in Male Rats

Introduction:

Stress predisposes organisms to depression and cognitive impairments, and seems to interact with metabolic homeostasis. The inflammatory response and the upregulation of proinflammatory cytokines are some of the consequences related to chronic stress. In this study, we investigated the preventive effect of chronic administration of ibuprofen, as an inhibitor of cyclooxygenases, on the cognitive and behavioral alterations and the weight gain reduction induced by simultaneous chronic restraint stress in rats.

Materials and Methods:

Male Wistar rats were subjected to chronic restraint stress and injected daily with the variable doses of ibuprofen or vehicle, for 21 consecutive days. Then, all animals were tested with the forced swim test and passive avoidance conditioning. Also, the weight of the animals was recorded before and after the interventions. Ultimately, plasma interleukin 6 (IL-6) levels were measured.

Results:

Chronic stress increased depressive-like behaviors, impaired learning, and disrupted the normal weight gain. However, the animals that received the highest dose of ibuprofen showed less depressive-like behaviors, a better avoidance memory, and a higher weight gain. However, the level of plasma IL-6 did not differ significantly between the study groups.

Conclusion:

The administration of ibuprofen prevents the cognitive and behavioral consequences of chronic stress. During the recovery, the plasma levels of IL-6 were not elevated by stress, and the IL-6 levels did not predict the behavioral performance of the stressed animals. The exact mechanisms of the protective effects of ibuprofen against chronic stress need to be further investigated.

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.

Concentration-dependent Toxicity after Subcutaneous Administration of Meloxicam to C57BL/6N Mice (Mus musculus)

Studies using the Mouse Grimace Scale have shown that for many NSAID, including meloxicam, minimal doses of at least 20 mg/kg may be necessary to achieve adequate peri- and post-operative analgesia in mice. However, more data are needed to determine whether such NSAID doses exceed the threshold for gastrointestinal ulceration or induce other relevant pathology. We administered equal volumes of saline or injectable meloxicam (1 or 5 mg/mL) at a dose of 20 mg/kg SC to 20 young adult male and female C57BL/6N mice daily for 6 d and performed necropsies on all mice on the seventh day. Mice given 5 mg/mL meloxicam subcutaneously developed significantly more severe pathology at the injection site than saline controls. Pathology was characterized by full-thickness epidermal necrosis; cavitary lesions within subcutis, muscle, or fat; steatitis; and myositis. Mice that received 1 mg/mL meloxicam subcutaneously developed lesions that were qualitatively similar but far less severe than those after 5 mg/mL. However, no pathologic lesions typically associated with NSAID toxicity, such as gastric ulceration and liver and kidney lesions, were seen. These results demonstrate that although meloxicam injected subcutaneously causes concentration-dependent skin pathology at the injection site, a dose of 20 mg/kg can be safely administered subcutaneously at a concentration of 1 mg/mL for as long as 6 d.

Molecular, cellular and behavioral changes associated with pathological pain signaling occur after dental pulp injury

Persistent pain can occur after routine dental treatments in which the dental pulp is injured. To better understand pain chronicity after pulp injury, we assessed whether dental pulp injury in mice causes changes to the sensory nervous system associated with pathological pain. In some experiments, we compared findings after dental pulp injury to a model of orofacial neuropathic pain, in which the mental nerve is injured. After unilateral dental pulp injury, we observed increased expression of activating transcription factor 3 (ATF3) and neuropeptide Y (NPY) mRNA and decreased tachykinin precursor 1 gene expression, in the ipsilateral trigeminal ganglion. We also observed an ipsilateral increase in the number of trigeminal neurons expressing immunoreactivity for ATF3, a decrease in substance P (SP) immunoreactive cells, and no change in the number of cells labeled with IB4. Mice with dental pulp injury transiently exhibit hindpaw mechanical allodynia, out to 12 days, while mice with mental nerve injury have persistent hindpaw allodynia. Mice with dental pulp injury increased spontaneous consumption of a sucrose solution for 17 days while mental nerve injury mice did not. Finally, after dental pulp injury, an increase in expression of the glial markers Iba1 and glial fibrillary acidic protein occurs in the transition zone between nucleus caudalis and interpolaris, ipsilateral to the injury. Collectively these studies suggest that dental pulp injury is associated with significant neuroplasticity that could contribute to persistent pain after of dental pulp injury.

Pharmacological sensitivity of reflexive and nonreflexive outcomes as a correlate of the sensory and affective responses to visceral pain in mice

Pain encompasses both sensory and affective dimensions which can be differentially modulated by drugs. Here, we compare the pharmacological sensitivity of the sensory and affective responses using acetic acid-induced abdominal writhings (sensory-reflexive outcome) and acetic acid-induced depression of reward seeking behaviour (RSB, affective-nonreflexive outcome) to a highly palatable food in mice. We found that the expression of RSB critically depends on factors such as sex and previous knowledge and type of the food stimulus. Intraperitoneal administration of acetic acid (iAA) produced a long-lasting (beyond the resolution of writhing behaviour) and concentration-dependent decrease on both appetitive-approach and consummatory dimensions of RSB. Ibuprofen and diclofenac were much more potent in reversing AA-induced changes in RSB: latency to eat (ED₅₀ = 2 and 0.005 mg/kg, intraperinoneally, respectively) and amount consumed (ED₅₀ = 11 and 0.1 mg/kg) than in AA-induced writhing (ED₅₀ = 123 and 60 mg/kg). Morphine and duloxetine inhibited the writhing response (ED₅₀ = 0.8 and 6 mg/kg, respectively) but not the AA-induced changes in RSB. Caffeine was ineffective in both AA-induced writhing and RSB changes. Overall, this study characterized a preclinical mouse model of hedonic deficits induced by pain that can be used to assess affective responses as well as complementary classic reflexive approaches in the evaluation of candidate analgesics.

Antinociceptive Effects of a Novel α2/α3-Subtype Selective GABAA Receptor Positive Allosteric Modulator

Pain remains a challenging clinical condition and spinal GABA_A receptors are crucial modulators of pain processing. α2/α3-subtype GABA_A receptors mediate the analgesic actions of benzodiazepines. Positive allosteric modulators (PAMs) at α2/α3-subtype GABA_A receptors may have analgesic potential. Here we report a new selective α2/α3-subtype GABA_A receptor PAM in in vitro and in vivo pain assays. KRM-II-81 demonstrated similar efficacy at α1/α2/α3 GABA_A receptors and negligible efficacy at α4/α5/α6 GABA_A receptors, with α2 and α3-subtypes being 17- and 28-fold more potent than α1 subtypes in HEK-293T cells expressing GABA_A receptors with different α subunits. In contrast, KRM-II-18B showed significant efficacy at α1/α2/α3/ α5 subtypes, with similar potency at α1/α2/α3 subtypes. Both PAMs and morphine dose-dependently decreased 0.6% acetic acid- and 0.32% lactic acid-induced writhing. The effects of both PAMs were reversed by the benzodiazepine receptor antagonist flumazenil, confirming their action at the benzodiazepine binding site of GABA_A receptors. Both PAMS and morphine all dose-dependently reversed 0.32% lactic acid (but not 0.6% acetic acid) induced suppression of nesting behavior. Acetaminophen, but not the PAMs, reversed acid-depressed locomotor activity. Combined, these findings suggest that KRM-II-81 is a selective α2/α3 subtype GABA_A PAM with significant antinociceptive effects in chemical stimulation-induced pain in mice.

Methods for measuring pain in laboratory animals

Pain is one of the most important variables that members of the lab animal science community try to control and minimize, and it is a critical topic of research in clinical and basic sciences. Objective measurements of pain severity, especially in non-verbal animals that are used as disease models, can be difficult to obtain, but several developments in behavioral neuroscience are making the measurement of pain more consistent, automated and accurate.