Effect of estrogen on morphine- and oxycodone-induced antinociception in a female femur bone cancer pain model

Glial and neuroimmune cell choreography in sexually dimorphic pain signaling

Chronic pain is a major global health issue that affects all populations regardless of sex, age, ethnicity/race, or country of origin, leading to persistent physical and emotional distress and to the loss of patients' autonomy and quality of life. Despite tremendous efforts in the elucidation of the mechanisms contributing to the pathogenesis of chronic pain, the identification of new potential pain targets, and the development of novel analgesics, the pharmacological treatment options available for pain management remain limited, and most novel pain medications have failed to achieve advanced clinical development, leaving many patients with unbearable and undermanaged pain. Sex-specific susceptibility to chronic pain conditions as well as sex differences in pain sensitivity, pain tolerance and analgesic efficacy are increasingly recognized in the literature and have thus prompted scientists to seek mechanistic explanations. Hence, recent findings have highlighted that the signaling mechanisms underlying pain hypersensitivity are sexually dimorphic, which sheds light on the importance of conducting preclinical and clinical pain research on both sexes and of developing sex-specific pain medications. This review thus focuses on the clinical and preclinical evidence supporting the existence of sex differences in pain neurobiology. Attention is drawn to the sexually dimorphic role of glial and immune cells, which are both recognized as key players in neuroglial maladaptive plasticity at the origin of the transition from acute pain to chronic pathological pain. Growing evidence notably attributes to microglial cells a pivotal role in the sexually dimorphic pain phenotype and in the sexually dimorphic analgesic efficacy of opioids. This review also summarizes the recent advances in understanding the pathobiology underpinning the development of pain hypersensitivity in both males and females in different types of pain conditions, with particular emphasis on the mechanistic signaling pathways driving sexually dimorphic pain responses.

Spinal cannabinoid receptor 2 activation reduces hypersensitivity associated with bone cancer pain and improves the integrity of the blood-spinal cord barrier

Background

Disruption of the blood–spinal cord barrier (BSCB) can facilitate inflammation that results in pain hypersensitivity. Proinflammatory cytokines produced by activated microglia and astrocytes damage the BSCB. This study aims to explore whether the BSCB is damaged in the bone cancer pain (BCP) model and to investigate a potential role and mechanism of JWH015 ((2-methyl-1-propyl-1H-indol-3-yl)−1-naphthalenylmethanone), a selective cannabinoid receptor 2 (CB2R) agonist, in preserving the BSCB integrity in the BCP model.

Methods

We used a male mouse model of BCP. Pain hypersensitivity was measured over time. Evans blue dye extravasation, transmission electron microscopy and Western blotting were performed to investigate the permeability and structural integrity of the BSCB. Immunofluorescence staining and western blotting were used to investigate the effect of JWH015 on the activation of glial cells and the levels of proinflammatory cytokines.

Results

A single intrathecal injection of JWH015 ameliorated pain hypersensitivity, the BSCB disruption and microglia and astrocyte activation. Decreases in the expression of ZO-1 and claudin-5 were partially restored by JWH015. The levels of the proinflammatory cytokines interleukin-1β and tumor necrosis factor-α and the enzyme MMP9 were reduced by JWH015. However, all effects were prevented by pretreatment with a CB2R-selective antagonist, AM630 ((6-iodo-2-methyl-1-(2-morpholinoethyl)−1H-indol-3-yl)(4-methoxyphenyl)methanone).

Conclusions

JWH015 alleviates neuroinflammation and maintains the BSCB integrity and permeability in a mouse model of BCP, which is probably mediated by inhibiting glial cells activation. This study reveals the new analgesic mechanism of JWH015 on BCP and provides a perspective to explore novel drugs that target the BSCB to control BCP.

A novel immunocompetent model of metastatic prostate cancer-induced bone pain

BACKGROUND

Over 70% to 85% of men with advanced prostate cancer (PCa) develop bone metastases characterized by severe bone pain and increased likelihood of bone fracture. These clinical features result in decreased quality of life and act as a predictor of higher mortality. Mechanistically, the skeletal pathologies such as osteolytic lesions and abnormal osteoblastic activity drive these symptoms. The role of immune cells in bone cancer pain remains understudied, here we sought to recapitulate this symptomology in a murine model.

METHODS

The prostate cancer bone metastasis-induced pain model (CIBP) was established by transplanting a mouse prostate cancer cell line into the femur of immunocompetent mice. Pain development, gait dynamics, and the changes in emotional activities like depression and anxiety were evaluated. Animal tissues including femurs, dorsal root ganglion (DRG), and spinal cord were collected at killing and microcomputed tomography (μCT), histology/immunohistochemistry, and quantitative immunofluorescent analysis were performed.

RESULTS

Mice receiving prostate cancer cells showed a significantly lower threshold for paw withdrawal responses induced by mechanical stimulation compared with their control counterparts. Zero maze and DigiGait analyses indicated reduced and aberrant movement associated emotional activity compared with sham control at 8-weeks postinjection. The μCT analysis showed osteolytic and osteoblastic changes and a 50% reduction of the trabecular volumes within the prostate cancer group. Neurologically we demonstrated, increased calcitonin gene-related peptide (CGRP) and neuronal p75NTR immune-reactivities in both the projected terminals of the superficial dorsal horn and partial afferent neurons in DRG at L2 to L4 level in tumor-bearing mice. Furthermore, our data show elevated nerve growth factor (NGF) and TrkA immunoreactivities in the same segment of the superficial dorsal horn that were, however, not colocalized with CGRP and p75NTR .

CONCLUSIONS

This study describes a novel immunocompetent model of CIBP and demonstrates the contribution of NGF and p75NTR to chronic pain in bone metastasis.

Endoplasmic Reticulum Stress Contributes to Nociception via Neuroinflammation in a Murine Bone Cancer Pain Model

BACKGROUND

Prolonged endoplasmic reticulum stress has been identified in various diseases. Inflammatory mediators, which have been shown to induce endoplasmic reticulum stress in several studies, have been suggested to serve as the important modulators in pain development. In this study, the authors hypothesized that the endoplasmic reticulum stress triggered by inflammatory mediators contributed to pain development.

METHODS

The authors used a male mouse model of bone cancer pain. The control mice were intrathecally injected with tumor necrosis factor-α (TNF-α) and lipopolysaccharide, the bone cancer pain mice were intrathecally injected with the endoplasmic reticulum stress inhibitors 4-PBA and GSK2606414. The nociceptive behaviors, endoplasmic reticulum stress markers, and inflammatory mediators were assessed.

RESULTS

Increased expression of the p-RNA-dependent protein kinase-like endoplasmic reticulum kinase and p-eukaryotic initiation factor 2α were found in the spinal neurons during bone cancer pain, along with upregulation of inflammatory mediators (TNF-α, interleukin 1β, and interleukin 6). Intrathecal administration of TNF-α or lipopolysaccharide increased the expression of endoplasmic reticulum stress markers in control mice. Inhibition of endoplasmic reticulum stress by intrathecal administration of 4-PBA (baseline vs. 3 h: 0.34 ± 0.16 g vs. 1.65 ± 0.40 g in paw withdrawal mechanical threshold, 8.00 ± 1.20 times per 2 min vs. 0.88 ± 0.64 times per 2 min in number of spontaneous flinches, P < 0.001, n = 8) or GSK2606414 (baseline vs. 3 h: 0.37 ± 0.08 g vs. 1.38 ± 0.11 g in paw withdrawal mechanical threshold, 8.00 ± 0.93 times per 2 min vs. 3.25 ± 1.04 times per 2 min in number of spontaneous flinches, P < 0.001, n = 8) showed time- and dose-dependent antinociception. Meanwhile, decreased expression of inflammatory mediators (TNF-α, interleukin 1β, and interleukin 6), as well as decreased activation of astrocytes in the spinal cord, were found after 4-PBA or GSK2606414 treatment.

CONCLUSIONS

Inhibition of inflammatory mediator-triggered endoplasmic reticulum stress in spinal neurons attenuates bone cancer pain via modulation of neuroinflammation, which suggests new approaches to pain relief.

Effect of chronic pretreatment with 17β-estradiol and/or progesterone on the nociceptive response to uterine cervical distension in a rat model

It is widely known that visceral pain is more prevalent in women than in men, and this phenomenon is interpreted as a consequence of the gonadal hormone modulation of pain perception and transduction. Uterine cervical distension might cause obstetric and gynecologic pain with clinical relevance to visceral pain. In this study, we focused on the roles of 17β-estradiol and progesterone in visceral nociception with the use of a rat model of uterine cervical distension. Female ovariectomized rats were injected with 17β-estradiol (E₂) or progesterone (P₄) for 21 days, after which visceral pain-induced spinal c-fos expression and visceromotor reflex changes were compared between ovariectomized and hormone-substituted groups. We found that uterine cervical distension induced a drastic increase in spinal c-fos expression and visceromotor reflex activity, and ovariectomy inhibited the increase in c-fos expression induced by visceral pain; this inhibition was reversed by estrogen but not progesterone replacement. This study demonstrates that estrogen is involved in uterine cervical nociception, while progesterone plays less of a significant role.

Endogenous Opiates and Behavior: 2016

This paper is the thirty-ninth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2016 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Sex differences in responsiveness to the prescription opioid oxycodone in mice

Over-prescription and increased nonmedical use of oxycodone has become a major concern. Despite its increased use, preclinical data concerning oxycodone's effects are still limited, especially in rodent models. To address this, we examined oxycodone's effects on place preference, locomotor activation, corticosterone levels, and thermal analgesia across a range of doses (between 0.3 and 10mg/kg) in gonadally intact, adult male and female C57BL/6J mice. Males and females showed oxycodone-induced conditioned place preference and did not show significant between-sex differences in their place preference behavior. During both CPP conditioning sessions and open field assay, locomotor activity was increased by 1, 3, and 10mg/kg oxycodone in females and by 3 and 10mg/kg oxycodone in males. Plasma corticosterone levels were higher in females (compared to males) at baseline as well as following acute oxycodone injection and open field testing. The time course of oxycodone-induced analgesia was similar in males and females, however the total antinociceptive effect (AUC0-120min) was larger in males compared to females at the highest dose tested (10mg/kg). Taken together, these data suggest that male and female mice are modestly different in their responses to oxycodone.