Spinal dopaminergic D1 and D5 receptors contribute to reserpine-induced fibromyalgia-like pain in rats

Compounds with dual glutaminase inhibition and Nrf2 activation activities enhance morphine analgesia and reduce pain sensitization in chemotherapy-induced peripheral neuropathy mouse model

Chemotherapy-induced peripheral neuropathy (CIPN) affects millions of patients, substantially impacting their quality of life. However, there are limited effective treatments available for alleviating CIPN. Therefore, discovering new approaches to manage neuropathic pain is in high clinical demand. Compound 968 (C968) and CU1015, both identified as glutaminase inhibitors with applications in cancer therapy, have shown promise in enhancing the anticancer activities of chemotherapy drugs in previous studies. However, their potential impact on CIPN has not been fully elucidated. This study aims to determine effects of C968 and CU1015 on pain and morphine-mediated analgesia in the CIPN mouse model. The CD-1 male and female mice received 4 doses of paclitaxel (intraperitoneal injection) to induce CIPN. Following CIPN development, mice were treated with C968 or CU1015 (intrathecal injection) 24 hours prior to morphine administration. To assess the impact of C968 or CU1015 on CIPN development, mice were treated with these compounds while concurrently receiving paclitaxel injections. The mechanical threshold was measured using the von Frey filaments. We found that C968 or CU1015 enhanced morphine analgesia in CIPN mice. C968 or CU1015 also attenuated the development of CIPN in male, but not in female mice, at the dose tested. This potential sex difference may be linked to the activation of pain-related signal transduction pathways involving ERK and AKT in the spinal cord. These findings suggest that compounds with dual glutaminase inhibition and Nrf2 activation activities could be a novel approach for treating CIPN. SIGNIFICANCE STATEMENT: This study demonstrates that compounds with dual glutaminase inhibition and Nrf2 activation activities could be a promising therapeutic target for managing neuropathic pain.

Systemic quinpirole enhances tramadol analgesia in inflammatory pain, but not in neuropathic pain in male rats

Pain is a morbidity or comorbidity with a high incidence that significantly impacts the well-being of patients. In this study, we evaluated the effects of systemic administration of tramadol, a weak mu-opioid receptor (MOR) agonist, plus quinpirole (a D2-like receptor agonist). The study was performed in naïve rats and in rats with induced inflammatory and neuropathic pain. To measure the antinociceptive effect of the drugs, thermonociceptive and mechanonociceptive stimuli were applied and the emotional aspects of pain were evaluated using conditional place preference (CPP) experiments. Systemic quinpirole produced an antinociceptive effect only in naïve male rats. In naïve female animals, a small but significant pronociceptive effect was observed following quinpirole application. Tramadol plus quinpirole in male animals reversed allodynia and hyperalgesia induced by inflammatory and neuropathic insults, which were not alleviated by either drug alone. CPP experiments revealed that systemic quinpirole plus tramadol treatment was effective only in an inflammatory pain model. To evaluate whether tolerance to the antinociceptive effect was prevented by the combination of the drugs, a repeated administration five-day trial of tramadol plus quinpirole was evaluated under inflammatory pain conditions; quinpirole only slightly prevented the antinociceptive tolerance of MOR agonists. D2-like agonists are effective adjuvants for treating painful conditions in combination with a low dose of MOR agonists. Our results could lead to an investigation of whether these dopaminergic drugs in combination with opioids might reduce the MOR agonist dose while obtaining a higher analgesic effect with fewer side effects in humans.

Amitriptyline and duloxetine attenuate activities of superficial dorsal horn neurons in a rat reserpine-induced fibromyalgia model

Fibromyalgia (FM) is an intractable disease with a chief complaint of chronic widespread pain. Amitriptyline (AMI) and duloxetine (DLX), which are antidepressant drugs, have been reported to ameliorate pain in patients with FM and pain-related behaviors in several rodent models of FM. However, the mechanisms of action of AMI and DLX are not yet fully understood. Here, we examined the effects of these drugs on the responsiveness of superficial dorsal horn (SDH) neurons in the spinal cord, using a rat FM model developed by injecting a biogenic amine depleter (reserpine). Extracellular recordings of SDH neurons in vivo demonstrated that bath application of AMI and DLX at concentrations of 0.1-1.0 mM on the dorsal surface of the spinal cord markedly suppressed spontaneous discharge and von Frey filament-evoked mechanical firing in SDH neurons. The suppression induced by the drugs was noted in a concentration-dependent manner and the suppressive effects resolved after washing the spinal cord surface. These results show that SDH neurons are the site of action for AMI and DLX in a rat reserpine-induced FM model. Spinal mechanisms may underlie the therapeutic effects of these drugs in patients with FM.

Nociplastic pain mechanisms and toll-like receptors as promising targets for its management

ABSTRACT

Nociplastic pain, characterized by abnormal pain processing without an identifiable organic cause, affects a significant portion of the global population. Unfortunately, current pharmacological treatments for this condition often prove ineffective, prompting the need to explore new potential targets for inducing analgesic effects in patients with nociplastic pain. In this context, toll-like receptors (TLRs), known for their role in the immune response to infections, represent promising opportunities for pharmacological intervention because they play a relevant role in both the development and maintenance of pain. Although TLRs have been extensively studied in neuropathic and inflammatory pain, their specific contributions to nociplastic pain remain less clear, demanding further investigation. This review consolidates current evidence on the connection between TLRs and nociplastic pain, with a specific focus on prevalent conditions like fibromyalgia, stress-induced pain, sleep deprivation-related pain, and irritable bowel syndrome. In addition, we explore the association between nociplastic pain and psychiatric comorbidities, proposing that modulating TLRs can potentially alleviate both pain syndromes and related psychiatric disorders. Finally, we discuss the potential sex differences in TLR signaling, considering the higher prevalence of nociplastic pain among women. Altogether, this review aims to shed light on nociplastic pain, its underlying mechanisms, and its intriguing relationship with TLR signaling pathways, ultimately framing the potential therapeutic role of TLRs in addressing this challenging condition.