Hyporesponsivity to mu-opioid receptor agonism in the Wistar-Kyoto rat model of altered nociceptive responding associated with negative affective state

Humanin attenuates metabolic, toxic, and traumatic neuropathic pain in mice by protecting against oxidative stress and increasing inflammatory cytokine

Neuropathic pain is associated with diverse etiologies, including sciatica, diabetes, and the use of chemotherapeutic agents. Despite the varied origins, mitochondrial dysfunction, oxidative stress, and inflammatory cytokines are recognized as key contributing factors in both the initiation and maintenance of neuropathic pain. The effects of the mitochondrial-derived peptide humanin on neuropathic pain, however, remain unclear, despite its demonstrated influence on these mechanisms in numerous disease models. This study aimed to evaluate the effects of humanin on pain behavior in murine models of metabolic (streptozotocin/STZ), toxic (oxaliplatin/OXA), traumatic (sciatic nerve cuffing/cuff), and neuropathic pain. A secondary objective was to assess whether humanin modulates oxidative damage and inflammatory cytokine levels in these neuropathic pain models. Humanin (4 mg/kg) was administered intraperitoneally (i.p.) to BALB/c male mice with induced neuropathic pain over a period of 15 days, with pain thresholds assessed using hot plate, cold plate, and Von Frey tests. Serum levels of antioxidant enzymes, oxidative stress markers, and inflammatory/anti-inflammatory cytokines were measured via enzyme-linked immunosorbent assay (ELISA). In neuropathic pain-induced mice, humanin administration resulted in a statistically significant increase in pain threshold values in the STZ + Humanin, OXA + Humanin, and cuff + Humanin groups compared to their respective control groups (P < 0.05) over 15 days. Furthermore, humanin treatment significantly elevated antioxidant enzyme levels and anti-inflammatory cytokine concentrations, while reducing oxidative stress markers and pro-inflammatory cytokine levels compared to control groups (P < 0.01). These findings suggest that humanin exhibits therapeutic potential in the treatment of neuropathic pain induced by STZ, OXA, and cuff models. The ability of humanin to mitigate neuropathic pain through the suppression of oxidative stress and inflammatory cytokines indicates its promise as a novel therapeutic strategy.

Simultaneous Administration of Hyperbaric Oxygen Therapy and Antioxidant Supplementation with Filipendula ulmaria Extract in the Treatment of Thermal Skin Injuries Alters Nociceptive Signalling and Wound Healing

Background and Objectives: Thermal skin injuries are a prevalent cause of skin damage, potentially leading to severe morbidity and significant mortality. In this study, we intended to estimate the effects of HBO (hyperbaric oxygen treatment) and antioxidant supplementation with Filipendula ulmaria extract, individually and simultaneously, in the treatment of thermal skin injuries. Materials and Methods: As a thermal skin injury experimental model, we used two-month-old male Wistar albino rats. Thermal injuries were made with a solid aluminium bar at a constant temperature of 75 °C for 15 s. Hyperbaric oxygen treatment was performed in a specially constructed hyperbaric chamber for rats (HYB-C 300) for seven consecutive days (100% O2 at 2.5 ATA for 60 min). Antioxidant supplementation was performed with oral administration of Filipendula ulmaria extract dissolved in tap water to reach a final concentration of 100 mg/kg b.w. for seven consecutive days. Results: Simultaneous administration of hyperbaric oxygen therapy and antioxidant supplementation with Filipendula ulmaria extract significantly ameliorated the macroscopic and histopathological characteristics of the wound area and healing. Also, this therapeutic approach decreased the local expression of genes for proinflammatory mediators and increased the expression of the μ-opioid receptor and the MT1 and MT2 receptors in the wound area and spinal cord, with a consequent increase in reaction times in behavioural testing. Conclusions: In conclusion, the presented results of our study allow evidence for the advantages of the simultaneous employment of HBO and antioxidant supplementation in the treatment of thermal skin injuries, with special reference to the attenuation of painful sensations accompanied by this type of trauma.

The Wistar Kyoto Rat: A Model of Depression Traits

There is an ongoing debate about the value of animal research in psychiatry with valid lines of reasoning stating the limits of individual animal models compared to human psychiatric illnesses. Human depression is not a homogenous disorder; therefore, one cannot expect a single animal model to reflect depression heterogeneity. This limited review presents arguments that the Wistar Kyoto (WKY) rats show intrinsic depression traits. The phenotypes of WKY do not completely mirror those of human depression but clearly indicate characteristics that are common with it. WKYs present despair- like behavior, passive coping with stress, comorbid anxiety, and enhanced drug use compared to other routinely used inbred or outbred strains of rats. The commonly used tests identifying these phenotypes reflect exploratory, escape-oriented, and withdrawal-like behaviors. The WKYs consistently choose withdrawal or avoidance in novel environments and freezing behaviors in response to a challenge in these tests. The physiological response to a stressful environment is exaggerated in WKYs. Selective breeding generated two WKY substrains that are nearly isogenic but show clear behavioral differences, including that of depression-like behavior. WKY and its substrains may share characteristics of subgroups of depressed individuals with social withdrawal, low energy, weight loss, sleep disturbances, and specific cognitive dysfunction. The genomes of the WKY and WKY substrains contain variations that impact the function of many genes identified in recent human genetic studies of depression. Thus, these strains of rats share characteristics of human depression at both phenotypic and genetic levels, making them a model of depression traits.

Characterization of pain-, anxiety-, and cognition-related behaviors in the complete Freund's adjuvant model of chronic inflammatory pain in Wistar-Kyoto rats

Introduction

Chronic pain is often associated with comorbid anxiety and cognitive dysfunction, negatively affecting therapeutic outcomes. The influence of genetic background on such interactions is poorly understood. The stress-hyperresponsive Wistar-Kyoto (WKY) rat strain, which models aspects of anxiety and depression, displays enhanced sensitivity to noxious stimuli and impaired cognitive function, compared with Sprague-Dawley (SD) counterparts. However, pain- and anxiety-related behaviors and cognitive impairment following induction of a persistent inflammatory state have not been investigated simultaneously in the WKY rats. Here we compared the effects of complete Freund's adjuvant (CFA)-induced persistent inflammation on pain-, negative affect- and cognition-related behaviors in WKY vs. SD rats.

Methods

Male WKY and SD rats received intra-plantar injection of CFA or needle insertion (control) and, over the subsequent 4 weeks, underwent behavioral tests to assess mechanical and heat hypersensitivity, the aversive component of pain, and anxiety- and cognition-related behaviors.

Results

The CFA-injected WKY rats exhibited greater mechanical but similar heat hypersensitivity compared to SD counterparts. Neither strain displayed CFA-induced pain avoidance or anxiety-related behavior. No CFA-induced impairment was observed in social interaction or spatial memory in WKY or SD rats in the three-chamber sociability and T-maze tests, respectively, although strain differences were apparent. Reduced novel object exploration time was observed in CFA-injected SD, but not WKY, rats. However, CFA injection did not affect object recognition memory in either strain.

Conclusions

These data indicate exacerbated baseline and CFA-induced mechanical hypersensitivity, and impairments in novel object exploration, and social and spatial memory in WKY vs. SD rats.

Role of 5-HT1A-mediated upregulation of brain indoleamine 2,3 dioxygenase 1 in the reduced antidepressant and antihyperalgesic effects of fluoxetine during maintenance treatment

The reduced antidepressant and antihyperalgesic effects of selective serotonin reuptake inhibitors (SSRIs) such as fluoxetine during maintenance treatment has been reported, but little is known about the molecular mechanism of this phenomenon. In three comorbid pain and depression animal models (genetic predisposition, chronic social stress, arthritis), we showed that the fluoxetine's antidepressant and antihyperalgesic effects were reduced during the maintenance treatment. Fluoxetine exposure induced upregulation of the 5-hydroxytryptamine 1A (5-HT1A) auto-receptor and indoleamine 2,3 dioxygenase 1 (IDO1, a rate-limiting enzyme of tryptophan metabolism) in the brainstem dorsal raphe nucleus (DRN), which shifted the tryptophan metabolism away from the 5-HT biosynthesis. Mechanistically, IDO1 upregulation was downstream to fluoxetine-induced 5-HT1A receptor expression because 1) antagonism of the 5-HT1A receptor with WAY100635 or 5-HT1A receptor knockout blocked the IDO1 upregulation, and 2) inhibition of IDO1 activity did not block the 5-HT1A receptor upregulation following fluoxetine exposure. Importantly, inhibition of either the 5-HT1A receptor or IDO1 activity sustained the fluoxetine's antidepressant and antihyperalgesic effects, indicating that 5-HT1A-mediated IDO1 upregulation in the brainstem DRN contributed to the reduced antidepressant and antihyperalgesic effects of fluoxetine. These results suggest a new strategy to improving the therapeutic efficacy of SSRI during maintenance treatment.

Anxiety enhances pain in a model of osteoarthritis and is associated with altered endogenous opioid function and reduced opioid analgesia

Introduction

Negative affect, including anxiety and depression, is prevalent in chronic pain states such as osteoarthritis (OA) and associated with greater use of opioid analgesics, potentially contributing to present and future opioid crises.

Objectives

We tested the hypothesis that the interaction between anxiety, chronic pain, and opioid use results from altered endogenous opioid function.

Methods

A genetic model of negative affect, the Wistar-Kyoto (WKY) rat, was combined with intra-articular injection of monosodium iodoacetate (MIA; 1 mg) to mimic clinical presentation. Effects of systemic morphine (0.5-3.5 mg·kg^-1) on pain behaviour and spinal nociceptive neuronal activity were compared in WKY and normo-anxiety Wistar rats 3 weeks after MIA injection. Endogenous opioid function was probed by the blockade of opioid receptors (0.1-1 mg·kg^-1 systemic naloxone), quantification of plasma β-endorphin, and expression and phosphorylation of spinal mu-opioid receptor (MOR).

Results

Monosodium iodoacetate-treated WKY rats had enhanced OA-like pain, blunted morphine-induced analgesia, and greater mechanical hypersensitivity following systemic naloxone, compared with Wistar rats, and elevated plasma β-endorphin levels compared with saline-treated WKY controls. Increased MOR phosphorylation at the master site (serine residue 375) in the spinal cord dorsal horn of WKY rats with OA-like pain (P = 0.0312) indicated greater MOR desensitization.

Conclusions

Reduced clinical analgesic efficacy of morphine was recapitulated in a model of high anxiety and OA-like pain, in which endogenous opioid tone was altered, and MOR function attenuated, in the absence of previous exogenous opioid ligand exposure. These findings shed new light on the mechanisms underlying the increased opioid analgesic use in high anxiety patients with chronic pain.