Peripheral neuropathy is dose limiting in paclitaxel cancer chemotherapy and can result in both acute pain during treatment and chronic persistent pain in cancer survivors. The hypothesis tested was that paclitaxel produces these adverse effects at least in part by sensitizing transient receptor potential vanilloid subtype 1 (TRPV1) through Toll-like receptor 4 (TLR4) signaling. The data show that paclitaxelinduced behavioral hypersensitivity is prevented and reversed by spinal administration of a TRPV1 antagonist. The number of TRPV1 ϩ neurons is increased in the dorsal root ganglia (DRG) in paclitaxel-treated rats and is colocalized with TLR4 in rat and human DRG neurons. Cotreatment of rats with lipopolysaccharide from the photosynthetic bacterium Rhodobacter sphaeroides (LPS-RS), a TLR4 inhibitor, prevents the increase in numbers of TRPV1 ϩ neurons by paclitaxel treatment. Perfusion of paclitaxel or the archetypal TLR4 agonist LPS activated both rat DRG and spinal neurons directly and produced acute sensitization of TRPV1 in both groups of cells via a TLR4-mediated mechanism. Paclitaxel and LPS sensitize TRPV1 in HEK293 cells stably expressing human TLR4 and transiently expressing human TRPV1. These physiological effects also are prevented by LPS-RS. Finally, paclitaxel activates and sensitizes TRPV1 responses directly in dissociated human DRG neurons. In summary, TLR4 was activated by paclitaxel and led to sensitization of TRPV1. This mechanism could contribute to paclitaxel-induced acute pain and chronic painful neuropathy.

This paper tests the contribution of the toll-like receptors (TLRs), TLR4 in particular, in the initiation and maintenance of paclitaxel-related chemotherapy-induced peripheral neuropathy (CIPN). TLR4 and its immediate down-stream signaling molecules MyD88 and TRIF were increased in dorsal root ganglion (DRG) by western blot by day 7 of paclitaxel treatment. The behavioral phenotype, the increase of both TLR4 and MyD88 was blocked by co-treatment with the TLR4 antagonist LPS-RS during chemotherapy. A similar, but less robust behavioral effect was observed using intrathecal treatment of MyD88 homodimerization inhibitory peptide. DRG levels of TLR4 and MyD88 reduced over the next two weeks, whereas these levels remained increased in spinal cord through day 21 following chemotherapy. Immunohistochemical analysis revealed TLR4 expression in both CGRP- and IB4-positive small DRG neurons. MyD88 was only found in CGRP-positive neurons and TRIF was found both in CGRP- and IB4-positive small DRG neurons as well as in medium and large size DRG neurons. In spinal cord TLR4 was only found co-localized to astrocytes but not with either microglia or neurons. Intrathecal treatment with the TLR4 antagonist lipopolysaccharide-RS (LPS-RS) transiently reversed pre-established CIPN mechanical hypersensitivity. These results strongly implicate TLR4 signaling in DRG and spinal cord in the induction and maintenance of paclitaxel related CIPN.

Here it is shown that paclitaxel induced neuropathy is associated with the development of spontaneous activity (SA) and hyperexcitability in DRG neurons that is paralleled by increased expression of low-voltage-activated calcium channels (T-type; Ca v 3.2). The percentage of DRG neurons showing SA and the overall mean rate of SA were significantly higher at day 7 of paclitaxel treatment than in rats receiving vehicle. Ca v 3.2 expression was increased in L4-6 DRG and spinal cord segments in paclitaxel-treated rats, localized to small calcitonin gene-related peptide expressing and isolectin B4 expressing DRG neurons and to glial fibrillary acidic proteinpositive spinal cord cells. Ca v 3.2 expression was also co-localized with toll-like receptor 4 (TLR4) in both the DRG and dorsal horn. T-type current amplitudes and density were increased at day 7 after paclitaxel treatment. Perfusion of the TLR4 agonist lipopolysaccharide (LPS) directly activated DRG neurons, whereas this was prevented by pretreatment with the specific T-type calcium channel inhibitor ML218 hydrochloride. Paclitaxel-induced behavioral hypersensitivity to mechanical stimuli in rats was prevented but not reversed by spinal administration of ML218 hydrochloride or intravenous (i.v.) injection of the TLR4 antagonist TAK242. Paclitaxel induced inward current and action potential discharges in cultured human DRGs neurons and this was blocked by ML218 hydrochloride pretreatment. Furthermore, ML218 hydrochloride decreased

The use of paclitaxel (Taxol®), a microtubule stabilizer, for cancer treatment is often limited by its associated peripheral neuropathy (chemotherapy-induced peripheral neuropathy, CIPN) which predominantly results in sensory dysfunction including chronic pain. Here we show that paclitaxel CIPN was associated with an induction of chemokine monocyte chemoattractant protein-1 (MCP-1) and its cognate receptor CCR2 in primary sensory neurons of dorsal root ganglia (DRG). Immunostaining revealed that MCP-1 was mainly expressed in small nociceptive neurons while CCR2 was expressed in large and medium-sized myelinated neurons. Direct application of MCP-1 consistently induced intracellular calcium increases in DRG large and medium-sized but not small neurons mainly dissociated from paclitaxel- but not vehicle-treated animals. Paclitaxel also induced increased expression of MCP-1 in spinal astrocytes but no CCR2 signal was detected in spinal cord. Local blockade of MCP-1/CCR2 signaling by anti-MCP-1 antibody or CCR2 antisense oligodeoxynucleotides significantly attenuated paclitaxel CIPN phenotypes including mechanical hypersensitivity and loss of intraepidermal nerve fibers (IENFs) in hindpaw glabrous skin. These results suggest that activation of paracrine MCP-1/CCR2 signaling between DRG neurons plays a critical role in the development of paclitaxel CIPN and targeting MCP-1/CCR2 signaling could be a novel therapeutic approach.

Toll-like receptor 4 (TLR4) has been implicated as a locus for initiation of paclitaxel related chemotherapy induced peripheral neuropathy (CIPN). This project explores the involvement of the immediate down-stream signal molecules in inducing paclitaxel CIPN. Mitogen-activated protein kinases (MAPK) and nuclear factor-κB (NFκB) were measured in dorsal root ganglia (DRG) and the spinal cord over time using Western blot and immunohistochemistry in a rat model of paclitaxel CIPN. The effects of MAPK inhibitors in preventing and reversing behavioral signs of CIPN were also measured (group sizes 4–9). Extracellular signal related kinase (ERK1/2) and p38 but not c-Jun N terminal kinase (JNK) or PI3K-Akt signaling expression was increased in DRG. Phospho-ERK1/2 staining was co-localized to small CGRP-positive DRG neurons in cell profiles surrounding large DRG neurons consistent with satellite glial cells. The expression of phospho-P38 was co-localized to small IB4-positive and CGRP-positive DRG neurons. The TLR4 antagonist LPS derived from R. sphaeroides (LPS-RS) inhibited paclitaxel-induced phosphorylation of ERK1/2 and P38. The MAPK inhibitors PD98059 (MEK1/2), U0126 (MEK1/2) and SB203580 (P38) prevented but did not reverse paclitaxel-induced behavioral hypersensitivity. Paclitaxel treatment resulted in phosphorylation of Inhibitor α of NFκB (IκBα) in DRG resulting in an apparent release of NFκB from the IκBα-NFκB complex as increased expression of nuclear NFκB was also observed. LPS-RS inhibited paclitaxel-induced translocation of NFκB in DRG. No change was observed in spinal NFκB. These results implicate TLR4 signaling via MAP kinases and NFκB in the induction and maintenance of paclitaxel-related CIPN.