The Relevant Neuroimmune Mechanisms Of Pruritus Of Ocular Allergy And Chronic Pain Induced By Peripheral Nerve Injury

Background and ObjectiveItch(also known as pruritus)is a sensation that causes the desire or reflex to scratch.Pruritus is the major symptom of numerous immune-related disorders such as allergic conjunctivitis,atopic dermatitis,contact dermatitis,urticaria and psoriasis.Pruritus is the health challenges worldwide and seriously affects the life quality and prognosis of patients.Allergic conjunctivitis is a common allergic disease with approximaely 20%incidence of in the total population.It has been well known that pruritus mediator released by type ? allergic reaction mediated by immunoglobulin E(IgE)produce allergic ocular itch.A common feature among allergic patients is the elevated level of total IgE and antigen special IgE in the serum and pathological tissues.Currently anti-pruritus treatments focusing on the immune system including anti-histamine or mast cell stabilizer are sometimes ineffective or with annoying side effects.Therefore it is clinically significant to reveal novel mechanisms of allergic ocular itch and explore new treatments and drug targets.Our previous research demonstrated that subpopulation of neurons in the dorsal root ganglion(DRG)and trigeminal ganglion(TG)expressed Fc?RI.We hypothesize that IgE-IC directly activates nociceptive(including pruriceptive)neurons in the TG though neuronal Fc-epsilon receptor ?(Fc?RI),contributing to allergic ocular itch.The purpose of this study is to explore the roles of nociceptive Fc?RI in a mouse model of allergic ocular pruritus.Materials and Method1.Application of predominantly pruritic or painful substance to the eyes of mice and access the itch and pain-related behaviors.2.Allergic mouse model was induced by intraperitoneal injection of oval albumin(OVA,as antigen)and aluminum hydroxide(as immune adjuvant).After successful sensitization,OVA was applied to both eyes of sensitized mice to induce ocular allergy and pruritus.3.IgE immune complex(IgE-IC)was prepared by OVA as antigen and mouse anti-OVA IgE as antibody.Calcium imaging was performed to investigate the effect of IgE-IC for the activation in acutely dissociated TG neurons.4.The expression and cellular distribution of Fc?Rs was investigated in the TG of mice by immunohistochemistry(IHC),Quantitative polymerase chain reaction(qPCR)and Western blotting.Results 1.Topical application predominantly pruritic substance to the eyes of mice mainly induced forelimb-wiping behaviors toward the eyes;while topical application predominantly painful substance mainly induced hind-limb-scratching behaviors toward the eyes.2.Topical application of OVA to the eyes of sensitized mice induced itch-related scratching behaviors which could be largely abolished by topical application of the blocking antibody to Fc?RI,but was only partially alleviated by pretreatment of mast cell stabilizer or histamine I receptor antagonist.3.The ?,? and ?subunits of high-affinity IgE receptor Fc?RI were expressed in TG neurons.4.Fc?RI was up-regulation in TG neurons of sensitized mice,while low-affinity IgE receptor Fc?RII(CD23)was down-regulation in the TG of sensitized mice.5.IgE-IC induces a dose-dependent activation through the neuronal Fc?RI in the dissociated TG neurons.ConclusionOur results showed that:1.Functional Fc?Rs were expressed in TG neurons of mice.2.IgE-IC may induce allergic ocular itch by activating subpopulations of nociceptive TG neurons through neuronal Fc?RI.3.Dysregulation of neuronal Fc?RI and neuronal Fc?RII may involve in the pathogenesis of allergic ocular itch.Clinical implicationsOur research revealed a novel mechanism of allergic pruritus:IgE-IC may directly activate nociceptive sensory neurons via binding to neuronal Fc?RI,and induce allergic ocular pruritus.These findings may provide novel strategies for the treatment of allergic pruritus.Background and ObjectiveMore than 3%of general population suffered from neuropathic pain which has become a major problem of global health.Because of the complicated mechanism of neuropathic pain,patients have been treated insufficiently and ineffectively.Neuropathic pain is associated with sensory abnormalities and altered stimulus-response function of somatic sensation system,such as allodynia,hyperalgesia and spontaneous pain.The International Association for the Study of Pain defined neuropathic pain as“pain caused by lesion or disease of the somatosensory nervous system".Increasingevidences indicate that Toll-like receptor(TLRs),a key receptor in innate immunity,and multiple cytokines such as IL-1?,TNF-a and chemokine regulate the development of neuropathic pain.Myeloid differentiation factor-88(MyD88)is the adaptor of TLRs and IL-1R,mediating the signaling transduction in the activation of these receptors.MyD88 dependent signaling pathway leads to production of many inflammatory cytokines.Recent publications showed that TLRs,IL-1R and MyD88 were found expressed in the dorsal root ganglion(DRG)and spinal dorsal horn(SDH).We hypothesized that suppression of MyD88 adaptor protein in the DRG and SDH could alleviate peripheral nerve injury-induced neuropathic pain.Material and MethodsWe performed chronic constriction injury(CCI)of sciatic nerve in the rats to establish a model of neuropathic pain.Immunofluorescence staining and Western blotting were used to determine the expression of MyD88,TRIF,IBA1 and GFAP in DRG and SDH.Western blotting analyzes the protein levels of IL-1?,HMGB1,phospho-NF-?B p65,NF-?B p65,phosphor-ERK,ERK and TNF-a.We measured the pain-related behaviors(mechanical allodynia and thermal hypersensitivity)of CCI rats after intrathecal injection of MyD88 homodimerization inhibitory peptide(MIP).Results1.Up-regulation of MyD88 in the DRG and SDH was detected after CCI.2.CCI increased the expression of IL-? and HMGB1 in the DRG and SDH.3.CCI activated NF-?B p65and ERK signal in the DRG and SDH.4.Intrathecal MIP injection relieved CCI-induced neuropathic pain.5.Intrathecal MIP injection decreased the expression of MyD88 protein after CCI operation.6.Intrathecal MIP injection suppressed the activation of NF-?B p65and ERK signal after CCI operation.7.Inhibition of MyD88 protein suppressed the activation of gial and production of TNF-?.8.Intrathecal MIP injection did not change the expression and cellular distribution of TRIF in DRG and SDH after CCI.ConclusionsMyD88 protein levels were up-regulated in the DRG and SDH after CCI-induced peripheral nerve injury in the rat.Intrathecal MIP injection alleviated neuropathic pain and relieved the CCI-induced central sensitization by suppressing the function of MyD88 adaptor protein.Therefore,MyD88 dependent signaling pathway may play a role in the development of neuropathic pain.Clinical implicationsOur results suggested that MyD88 dependent signaling pathway may be involved in the pathogenesis of peripheral nerve injury-induced neuroinflammation and neuropathic pain.MyD88 might serve as a potential therapeutic target for neuropathic pain.