Neuroendocrine mechanisms in oxaliplatin-induced hyperalgesic priming

Hyperalgesic Priming in the Transition From Acute to Chronic Pain: Focus on Different Models and the Molecular Mechanisms Involved

Poorly treated acute pain can develop into chronic pain, resulting in significant impairment of patients' quality of life. The hyperalgesic priming model is commonly used to study how acute pain transforms into chronic pain. Inflammatory factors, small molecules, opioid receptor agonists, chemotherapy drugs, and stress serve as initiating factors in the hyperalgesic priming model. Various signaling pathways such as PKCε, MOR and ephrin-B2 pathways, and sexual differences also contribute to the transformation process of chronic pain. In this review, we examine various hyperalgesic priming models and their underlying molecular mechanisms. By thoroughly investigating these molecular mechanisms, researchers can more precisely identify the critical nodes involved in pain transformation, thereby developing more targeted treatment strategies.

The Nociceptor Primary Cilium Contributes to Mechanical Nociceptive Threshold and Inflammatory and Neuropathic Pain

The primary cilium, a single microtubule-based organelle protruding from the cell surface and critical for neural development, also functions in adult neurons. While some dorsal root ganglion neurons elaborate a primary cilium, whether it is expressed by and functional in nociceptors is unknown. Recent studies have shown the role of Hedgehog, whose canonical signaling is primary cilium dependent, in nociceptor sensitization. We establish the presence of primary cilia in soma of rat nociceptors, where they contribute to mechanical threshold, prostaglandin E2 (PGE2)-induced hyperalgesia, and chemotherapy-induced neuropathic pain (CIPN). Intrathecal administration of siRNA targeting Ift88, a primary cilium-specific intraflagellar transport (IFT) protein required for ciliary integrity, resulted in attenuation of Ift88 mRNA and nociceptor primary cilia. Attenuation of primary cilia was associated with an increase in mechanical nociceptive threshold in vivo and decrease in nociceptor excitability in vitro, abrogation of hyperalgesia, and nociceptor sensitization induced by both a prototypical pronociceptive inflammatory mediator PGE2 and paclitaxel CIPN, in a sex-specific fashion. siRNA targeting Ift52, another IFT protein, and knockdown of NompB, the Drosophila Ift88 ortholog, also abrogated CIPN and reduced baseline mechanosensitivity, respectively, providing independent confirmation for primary cilia control of nociceptor function. Hedgehog-induced hyperalgesia is attenuated by Ift88 siRNA, supporting the role for primary cilia in Hedgehog-induced hyperalgesia. Attenuation of CIPN by cyclopamine (intradermal and intraganglion), which inhibits Hedgehog signaling, supports the role of Hedgehog in CIPN. Our findings support the role of the nociceptor primary cilium in control of mechanical nociceptive threshold and inflammatory and neuropathic pain, the latter Hedgehog-dependent.

5HT1AR agonist alleviates presurgical prolonged sleep deprivationinduced postsurgical pain in adolescent mice

OBJECTIVES

Sleep deprivation (SD) is a risk factor for the development of chronic pain in adolescents, significantly affecting pain management and prognosis; however, the mechanisms by which SD influences postoperative pain outcomes remain unclear. This study aims to investigate the molecular mechanism through which the spinal 5-hydroxytryptamine 1A receptor (5-HT1AR) regulates the excitation/inhibition (E/I) balance in the dorsal horn to modulate postoperative chronic pain induced by SD in adolescent mice.

METHODS

A pain model combining 4.5 days of SD and a plantar incision was established in adolescent C57BL/6J mice, which were randomly assigned to 4 groups: control (C), SD, hind toe incision (I), and SD combined with hind toe incision (SI). The effects of a single intrathecal injection of the 5-HT1AR agonist 8-hydroxy-2-dipropylamino-tetralin (8-OH-DPAT) and both single and continuous intrathecal injections of the extracellular signal-regulated kinase (ERK) inhibitor U0126 on SD-induced postoperative chronic in mice (SI+8-OH-DPAT group, SI+U0126 group, and SI+Vehicle group) were observed. Paw withdrawal mechanical threshold (PWMT) was measured on the 1st, 3rd, 5th, 7th, 10th, and 14th day before and after model induction. On the 7th day after surgery, immunofluorescence was used to assess 5-HT1AR expression in the spinal dorsal horn, and Western blotting was employed to measure protein expression levels of 5-HT1AR, postsynaptic density protein-95 (PSD95), N-methyl-D-aspartic acid receptor 1 (NR1), phosphorylated NR1 (p-NR1), vesicular glutamate transporter 1 (VGLUT1), gephyrin, glutamic acid decarboxylase 67 (GAD67), vesicular gamma-aminobutyric acid transporter (VGAT), ERK, p-ERK, calmodulin-dependent protein kinase II (CaMKⅡ), phosphorylated CaMKⅡ (p-CaMKⅡ), cAMP- response element-binding protein (CREB), and phosphorylated CREB (p-CREB) in the dorsal horn.

RESULTS

1) Behavioral pain analysis. Compared with group C, the PWMT in groups I and SD decreased significantly on the 1st, 3rd, 5th and 7th day after surgery (all P<0.05), and returned to baseline on the 10th day. Compared with group I, the PWMT in group SI was significantly lower on the 5th, 7th and 10th day (all P<0.05). The SI+8-OH-DPAT group exhibited higher PWMT values at the 1st, 2nd, and 4th hour postoperatively, as well as on the 3rd, 5th, 7th,10th and 14th day compared to the solvent control group (all P<0.05). Similarly, both single and continuous intrathecal injections of U0126 resulted in higher PWMT values at 1, 2, and 4 hours after surgery postoperatively and on the 3rd, 5th and 7th day compared to the SI+Vehicle group (all P<0.05). 2) Immunofluorescence. On the 7th day after surgery, group SI showed decreased 5-HT1AR expression in the spinal dorsal horn, while a single intrathecal injection of 8-OH-DPAT upregulated 5-HT1AR expression (all P<0.05). 3) Western blotting. On the 7th day after surgery, group SI exhibited decreased 5-HT1AR expression and increased ratios of p-CaMKⅡ/CaMKⅡ, p-CREB/CREB, and p-ERK/ERK (all P<0.05). Compared with group C, group SI showed increased expression of PSD95, VGLUT1, and the p-NR1/NR1 ratio, and decreased expression of 5-HT1AR, gephyrin, and VGAT in the dorsal horn (all P<0.05). Compared with group I, group SI demonstrated upregulated 5-HT1AR and PSD95 expression and downregulated gephyrin, GAD67, and VGAT expression (all P<0.05). In the SI+8-OH-DPAT group, the levels of PSD95, VGLUT1, p-NR1/NR1, p-CaMKⅡ/CaMKⅡ, p-CREB/CREB, and p-ERK/ERK on the 7th day after surgery were lower than those in the SI+Vehicle group, while the expression levels of 5-HT1AR, GAD67, and VGAT were upregulated (all P<0.05). Additionally, 2 hours after a single intrathecal injection of U0126, the SI+U0126 group exhibited significantly lower levels of p-CaMKⅡ/CaMKⅡ, p-CREB/CREB, and p-ERK/ERK compared with the SI+Vehicle group (all P<0.05).

CONCLUSIONS

The 5-HT1AR agonist alleviates postoperative chronic pain induced by SD in adolescent mice by inhibiting the CaMKⅡ-ERK-CREB signaling pathway, thereby correcting the E/I imbalance in the spinal dorsal horn.

Mu-Opioid Receptor (MOR) Dependence of Pain in Chemotherapy-Induced Peripheral Neuropathy

We recently demonstrated that transient attenuation of Toll-like receptor 4 (TLR4) in dorsal root ganglion (DRG) neurons, can both prevent and reverse pain associated with chemotherapy-induced peripheral neuropathy (CIPN), a severe side effect of cancer chemotherapy, for which treatment options are limited. Given the reduced efficacy of opioid analgesics to treat neuropathic, compared with inflammatory pain, the cross talk between nociceptor TLR4 and mu-opioid receptors (MORs), and that MOR and TLR4 agonists induce hyperalgesic priming (priming), which also occurs in CIPN, we determined, using male rats, whether (1) antisense knockdown of nociceptor MOR attenuates CIPN, (2) and attenuates the priming associated with CIPN, and (3) CIPN also produces opioid-induced hyperalgesia (OIH). We found that intrathecal MOR antisense prevents and reverses hyperalgesia induced by oxaliplatin and paclitaxel, two common clinical chemotherapy agents. Oxaliplatin-induced priming was also markedly attenuated by MOR antisense. Additionally, intradermal morphine, at a dose that does not affect nociceptive threshold in controls, exacerbates mechanical hyperalgesia (OIH) in rats with CIPN, suggesting the presence of OIH. This OIH associated with CIPN is inhibited by interventions that reverse Type II priming [the combination of an inhibitor of Src and mitogen-activated protein kinase (MAPK)], an MOR antagonist, as well as a TLR4 antagonist. Our findings support a role of nociceptor MOR in oxaliplatin-induced pain and priming. We propose that priming and OIH are central to the symptom burden in CIPN, contributing to its chronicity and the limited efficacy of opioid analgesics to treat neuropathic pain.

Exploring microRNA patterns as biomarkers of FOLFOX chemotherapy-induced peripheral neuropathy in patients with colorectal cancer

FOLFOX is a combination of chemotherapeutic agents (5-fluorouracil, leucovorin, and oxaliplatin) and is used to treat advanced colorectal cancer (CRC) but induces various side effects. Chemotherapy-induced peripheral neuropathy (CIPN) is one of the most critical side effects that compromise the quality of life of patients with CRC undergoing FOLFOX chemotherapy. This study aimed to evaluate circulating miRNA, cortisol and catecholamine as potential biomarkers that can predict FOLFOX-CIPN symptoms. High-throughput microRNA (miRNA) sequencing was performed on the RNA circulating in the plasma of eight patients with CRC who underwent FOLFOX chemotherapy. miRNA expression profiles were evaluated according to two groups: those who underwent ≤3 cycles and those who underwent ≥6 cycles of FOLFOX chemotherapy. The identified miRNAs were validated in 27 patients with CRC who underwent FOLFOX chemotherapy using quantitative reverse transcription polymerase chain reaction. Target genes were predicted using bioinformatics and functional analyses. Cortisol and catecholamine concentrations in peripheral plasma were measured using an enzyme-linked immunosorbent assay. miR-3184-5p was differentially expressed when miRNA expression was compared between the groups that underwent ≤3 and ≥6 cycles of FOLFOX chemotherapy. Cortisol levels were significantly higher in the group that underwent ≥6 cycles of FOLFOX chemotherapy than in the group that underwent ≤3 cycles. This study suggests that miR-3184-5p may be a potential marker for predicting CIPN.

Electrical stimulation enhances mitochondrial trafficking as a neuroprotective mechanism against chemotherapy-induced peripheral neuropathy

Electrical stimulation (ESTIM) has shown to be an effective symptomatic treatment to treat pain associated with peripheral nerve damage. However, the neuroprotective mechanism of ESTIM on peripheral neuropathies is still unknown. In this study, we identified that ESTIM has the ability to enhance mitochondrial trafficking as a neuroprotective mechanism against chemotherapy-induced peripheral neuropathies (CIPNs). CIPN is a debilitating and painful sequalae of anti-cancer chemotherapy treatment which results in degeneration of peripheral nerves. Mitochondrial dynamics were analyzed within axons in response to two different antineoplastic mechanisms by chemotherapy drug treatments paclitaxel and oxaliplatin in vitro. Mitochondrial trafficking response to chemotherapy drug treatment was observed to decrease in conjunction with degeneration of distal axons. Using low-frequency ESTIM, we observed enhanced mitochondrial trafficking to be a neuroprotective mechanism against CIPN. This study confirms ESTIM enhances regeneration of peripheral nerves by increased mitochondrial trafficking.

Role of pattern recognition receptors in chemotherapy-induced neuropathic pain

Progress in the development of effective chemotherapy is producing a growing population of patients with acute and chronic painful chemotherapy-induced peripheral neuropathy (CIPN), a serious treatment-limiting side effect for which there is currently no US Food and Drug Administration-approved treatment. CIPNs induced by diverse classes of chemotherapy drugs have remarkably similar clinical presentations, leading to the suggestion they share underlying mechanisms. Sensory neurons share with immune cells the ability to detect damage associated molecular patterns (DAMPs), molecules produced by diverse cell types in response to cellular stress and injury, including by chemotherapy drugs. DAMPs, in turn, are ligands for pattern recognition receptors (PRRs), several of which are found on sensory neurons, as well as satellite cells, and cells of the immune system. In the present experiments, we evaluated the role of two PRRs, TLR4 and RAGE, present in dorsal root ganglion (DRG), in CIPN. Antisense (AS)-oligodeoxynucleotides (ODN) against TLR4 and RAGE mRNA were administered intrathecally before ('prevention protocol') or 3 days after ('reversal protocol') the last administration of each of three chemotherapy drugs that treat cancer by different mechanisms (oxaliplatin, paclitaxel and bortezomib). TLR4 and RAGE AS-ODN prevented the development of CIPN induced by all three chemotherapy drugs. In the reversal protocol, however, while TLR4 AS-ODN completely reversed oxaliplatin- and paclitaxel-induced CIPN, in rats with bortezomib-induced CIPN it only produced a temporary attenuation. RAGE AS-ODN, in contrast, reversed CIPN induced by all three chemotherapy drugs. When a TLR4 antagonist was administered intradermally to the peripheral nociceptor terminal, it did not affect CIPN induced by any of the chemotherapy drugs. However, when administered intrathecally, to the central terminal, it attenuated hyperalgesia induced by all three chemotherapy drugs, compatible with a role of TLR4 in neurotransmission at the central terminal but not sensory transduction at the peripheral terminal. Finally, since it has been established that cultured DRG neurons can be used to study direct effects of chemotherapy on nociceptors, we also evaluated the role of TLR4 in CIPN at the cellular level, using patch-clamp electrophysiology in DRG neurons cultured from control and chemotherapy-treated rats. We found that increased excitability of small-diameter DRG neurons induced by in vivo and in vitro exposure to oxaliplatin is TLR4-dependent. Our findings suggest that in addition to the established contribution of PRR-dependent neuroimmune mechanisms, PRRs in DRG cells also have an important role in CIPN.

The Primary Cilium and its Hedgehog Signaling in Nociceptors Contribute to Inflammatory and Neuropathic Pain

The primary cilium, a 1-3 μm long hair-like structure protruding from the surface of almost all cells in the vertebrate body, is critical for neuronal development and also functions in the adult. As the migratory neural crest settles into dorsal root ganglia (DRG) sensory neurons elaborate a single primary cilium at their soma that is maintained into adult stages. While it is not known if primary cilia are expressed in nociceptors, or their potential function in the mature DRG neuron, recent studies have shown a role for Hedgehog, whose signaling demonstrates a dependence on primary cilia, in nociceptor sensitization. Here we report the expression of primary cilia in rat and mouse nociceptors, where they modulate mechanical nociceptive threshold, and contribute to inflammatory and neuropathic pain. When siRNA targeting Ift88, a primary cilium-specific intraflagellar transport (IFT) protein required for ciliary integrity, was administered by intrathecal injection, in the rat, it resulted in loss of Ift88 mRNA in DRG, and primary cilia in neuronal cell bodies, which was associated with an increase in mechanical nociceptive threshold, and abrogation of hyperalgesia induced by the pronociceptive inflammatory mediator, prostaglandin E2, and painful peripheral neuropathy induced by a neurotoxic chemotherapy drug, paclitaxel. To provide further support for the role of the primary cilium in nociceptor function we also administered siRNA for another IFT protein, Ift52. Ift52 siRNA results in loss of Ift52 in DRG and abrogates paclitaxel-induced painful peripheral neuropathy. Attenuation of Hedgehog-induced hyperalgesia by Ift88 knockdown supports a role for the primary cilium in the hyperalgesia induced by Hedgehog, and attenuation of paclitaxel chemotherapy-induced neuropathy (CIPN) by cyclopamine, which attenuates Hedgehog signaling, suggests a role of Hedgehog in CIPN. Our findings support a role of nociceptor primary cilia in the control of mechanical nociceptive threshold and in inflammatory and neuropathic pain, the latter, at least in part, Hedgehog dependent.

The Primary Cilium and its Hedgehog Signaling in Nociceptors Contribute to Inflammatory and Neuropathic Pain

The primary cilium, a 1-3 μm long hair-like structure protruding from the surface of almost all cells in the vertebrate body, is critical for neuronal development and also functions in the adult. As the migratory neural crest settles into dorsal root ganglia (DRG) sensory neurons elaborate a single primary cilium at their soma that is maintained into adult stages. While it is not known if primary cilia are expressed in nociceptors, or their potential function in the mature DRG neuron, recent studies have shown a role for Hedgehog, whose signaling demonstrates a dependence on primary cilia, in nociceptor sensitization. Here we report the expression of primary cilia in rat and mouse nociceptors, where they modulate mechanical nociceptive threshold, and contribute to inflammatory and neuropathic pain. When siRNA targeting Ift88 , a primary cilium-specific intra-flagellar transport (IFT) protein required for ciliary integrity, was administered by intrathecal injection, in the rat, it resulted in loss of Ift88 mRNA in DRG, and primary cilia in neuronal cell bodies, which was associated with an increase in mechanical nociceptive threshold, and abrogation of hyperalgesia induced by the pronociceptive inflammatory mediator, prostaglandin E 2 , and painful peripheral neuropathy induced by a neurotoxic chemotherapy drug, paclitaxel. To provide further support for the role of the primary cilium in nociceptor function we also administered siRNA for another IFT protein, Ift 52. Ift 52 siRNA results in loss of Ift 52 in DRG and abrogates paclitaxel-induced painful peripheral neuropathy. Attenuation of Hedgehog-induced hyperalgesia by Ift88 knockdown supports a role for the primary cilium in the hyperalgesia induced by Hedgehog, and attenuation of paclitaxel chemotherapy-induced neuropathy (CIPN) by cyclopamine, which attenuates Hedgehog signaling, suggests a role of Hedgehog in CIPN. Our findings support a role of nociceptor primary cilia in the control of mechanical nociceptive threshold and in inflammatory and neuropathic pain, the latter, at least in part, Hedgehog dependent.