In the present study we have shown for the first time that the IL-6 signal transducer gp130 in Nav1.8 expressing primary afferents has little impact on the induction of mechanical hypersensitivity but is critically involved in the maintenance of nociceptor sensitization to mechanical stimuli in a mouse model of cancer pain. Mice lacking gp130 in nociceptors showed some signs of mechanical hypersensitivity during the first days after induction of experimental malignant soft tissue cancer that were comparable to controls. The mice significantly recovered from hypersensitivity in the later stages of the observation period. In contrast, mechanical hypersensitivity progressively became more severe in gp130 expressing control animals. The delayed recovery of mechanical hypersensitivity suggests a critical role of gp130 dependent signaling not only for the induction but more prominently for the maintenance of long-term mechanical hypersensitivity in cutaneous nociceptors. Cancer pain is considered exceptional and at least partially distinct from neuropathic and inflammatory pain. However, our data show that gp130 expressed in nociceptors is also essential for the development of mechanical hypersensitivity following inflammation and/or nerve injury. Together, the data suggest that IL-6 signal transducer gp130 is an essential prerequisite for long-term mechanical hypersensitivity associated with cancer, inflammation and nerve injury.
In humans all three mentioned conditions are characterized by pronounced mechanical hyperalgesia and/or allodynia and in mouse models corresponding hypersensitivity to noxious and/or innocuous mechanical stimuli is regularly reported. Although the pathologies are complex and specific for the respective disease, they share certain aspects of inflammatory reactions involving components of innate immunity including sequential release of cytokines . In particular, cytokines of the IL-6 family are important regulators of the immune response. There is increasing evidence that IL-6 like cytokines may be causally involved in the etiology of neuropathic pain and mechanical allodynia following malignant tumor, nerve injury or inflammation . The IL-6 like cytokine family includes IL-11, IL-27, leukaemia inhibitory factor (LIF), ciliary neurotrophic factor (CNTF), oncostatin M (OSM), cardiotrophin (CT-1), neuropoietin, cardiotrophin-like cytokine (CLC) and B cell stimulating factor (BSF-3). The question arises which of these members is most critical for the regulation of nociceptor sensitivity. Although not considered a classical proinflammatory cytokine, LIF appears to be an interesting candidate since LIF mRNA is up-regulated in inflammation . LIF receptor is expressed in DRG neurons and up-regulated by nerve injury . However, the role of LIF in nociception is still controversially discussed. Although LIF differentially regulates capsaicin and heat sensitivity in cultured sensory neurons , LIF injection into the mouse paw induces local mechanical, but not thermal hypersensitivity [10, 27]. In the CFA inflammation model LIF has anti-inflammatory and analgesic effects . In addition, OSM has certain roles in inflammation . OSM receptors are expressed in DRG neurons, are associated with nociceptor sensitization in inflammation and form heterodimers with gp130 [30–32].
Although several members of the IL-6 family have been associated with painful conditions, research in the last years has mainly focused on IL-6. Increased IL-6 serum levels have been detected in patients with neuropathy, malignant tumors, musculoskeletal disorders, burn injury or autoimmune and chronic inflammatory conditions like RA [33–38]. IL-6 is up-regulated following experimental peripheral nerve injury and exhibits a growth promoting effect on primary sensory neurons [39–42]. Intraplantar, intracerebroventricular or intrathecal injection of IL-6 induces thermal and mechanical hypersensitivity in rodents [6, 10, 17, 18, 43, 44]. In addition, recordings from nociceptors in vivo and in vitro revealed a role for IL-6 in sensitizing nociceptors to thermal and mechanical stimulation [11, 12, 17]. IL-6-/- mice show a phenotype with reduced thermal hypersensitivity after experimental inflammation or nerve constriction [7, 13, 14]. Antisera neutralizing endogenous IL-6 inhibit inflammatory hyperalgesia  and the orally available, small molecule IL-6 receptor antagonist TB-2-081 reverses pain in a pancreatitis rodent model . Moreover, neutralizing IL-6 strategy has evolved as effective pain therapy in humans .
Most cytokines of the IL-6 family bind to heteromeric complexes composed of ubiquitously expressed gp130 and distinct μ receptor subunits with signal transduction domains (e.g. LIF-R or OSM-R). In contrast, IL-6 signaling entirely depends on the availability of gp130 homomers which are activated by IL-6 bound to the ligand binding IL-6 receptor μ-subunit (IL-6-R) which is present in few cell types only [47, 48]. In most systems including sympathetic neurons, IL-6 effects depend on the presence of the soluble IL-6 receptor (sIL-6R)  which after ligand binding heteromerizes with membrane bound gp130 [47, 50]. Furthermore, IL-6/sIL-6R complex or Hyper-IL-6 (HIL-6), a synthetic fusion protein mimicking the IL-6/sIL-6R complex [51, 52], increase nociceptor responsiveness and induce thermal hypersensitivity [9, 11, 12]. A dual regulation of heat sensitivity by IL-6 and its soluble receptor sIL-6R has been reported . The sensitization involves activation of Janus tyrosine kinase (JAK), adapter proteins Gab1 and Gab2 and ultimately PKC-δ which regulates the heat transducer ion channel TRPV1 [10, 12]. Despite a recent report that IL-6 but not the signal transducer gp130 is up-regulated in neuropathic rats  gp130 seems to play a crucial role in pathological pain since antagonizing sgp130 prevents acute nociceptor sensitization in experimental arthritis . This acute effect of IL-6 on mechanosensitivity in this study rather seems to be partially indirect. We have previously reported that gp130 expressed in nociceptors is required for IL-6 induced regulation of TRPV1 and thermal hypersensitivity . Here we show that mice lacking gp130 in nociceptors (SNS-gp130-/-) develop but recover from mechanical hypersensitivity in mouse models of cancer, inflammatory and neuropathic pain. Our data suggest that gp130 expressed in nociceptors is a critical regulator of the maintenance of mechanical hypersensitivity in nociceptors in particular in the CCI mouse model for neuropathic pain.
At least three possible signaling pathways may be activated following gp130 activation: the classical signal transducer and activator of transcription 3 (STAT3) pathway is activated in primary afferent neurons by peripheral inflammation possibly through OSM receptor . Although STAT3 signaling is beneficial to axonal growth through activating transcription of unidentified genes in DRG neurons , STAT3 is differentially activated by IL-6 cytokines in DRG sensory neurons by CNTF and LIF but not IL-6 . For thermal hypersensitivity IL-6 signals via activation of the adapter proteins Gab1/2, PI3K, PKC-δ and regulation of TRPV1 . In contrast, the sequelae of mechanical hypersensitivity and even mechanical nociceptive transduction remain largely enigmatic to date. Recently, the importance of translation for the regulation of de novo protein synthesis has been discovered for IL-6 induced mechanical nociceptive plasticity which is blocked by inhibitors of general and cap-dependent protein synthesis . Although Mnk1 and ERK have been reported as upstream regulators of the translation factor eIF4F , the nature of possible downstream target proteins accounting for IL-6 mechanical hypersensitivity remain to be elucidated. In Purkinje neurons, chronic IL-6 exposure alters electrophysiological properties and calcium signaling . Such general increases in excitability may account for mechanical nociceptive plasticity. Nonetheless, IL-6 has not been found to enhance excitability in unmyelinated sensory axons in normal and injured peripheral nerve . Therefore more likely, ion channels potentially involved in mechanotransduction may be regulated by gp130 dependent translation or transcription [58, 59]. Although a direct link of IL-6 and such channels is still absent, the up-regulation of IL-6 and the mechanosensitive ion channel TRPA1 in mustard oil colitis  may be indicative for the regulation of TRP and other mechanosensitive ion channels by IL-6/gp130 signaling. Further studies will be required to elucidate the final target of gp130 in mechanonociception.