Bladder afferent hyperexcitability in bladder pain syndrome/interstitial cystitis

Abstract

Bladder pain syndrome/interstitial cystitis is a disease with lower urinary tract symptoms, such as bladder pain and urinary frequency, which results in seriously impaired quality of life of patients. The extreme pain and urinary frequency are often difficult to treat. Although the etiology of bladder pain syndrome/interstitial cystitis is still not known, there is increasing evidence showing that afferent hyperexcitability as a result of neurogenic bladder inflammation and urothelial dysfunction is important to the pathophysiological basis of symptom development. Further investigation of the pathophysiology will lead to the effective treatment of patients with bladder pain syndrome/interstitial cystitis.

Introduction

BPS/IC is a debilitating chronic disease characterized by suprapubic pain related to bladder filling, coupled with additional symptoms, such as increased day- and night-time urinary frequency, without proven urinary infection or other obvious pathology.^1–3 Although the symptoms presented might appear similar to those of a urinary tract infection, urine culture shows no underlying infection and there is no response to antibiotic treatment.^{4, 5} It has been estimated that the prevalence of BPS/IC range from 1.2 per 100 000 population and 4.5 per 10 000 females in Japan.⁶ Although the etiology is unknown, theories explaining the pathology of BPS/IC include altered barrier lining, afferent and/or central nervous system abnormalities, possible contribution of inflammatory or bacterial agents and abnormal urothelial signaling. Figure 1 presents the proposed pathogenesis of BPS/IC in which there is bladder insult and damage to the urothelial layer that, for example, allows substances in urine, such as potassium, to leak into the suburothelium and to prompt a cascade of events as well as cross-talk with other pelvic organs, each contributing to bladder inflammation and pain.¹ In the present review, among these various etiological factors, we focus on the potential mechanisms underlying increased excitability of bladder afferent pathways in BPS/IC.

Fig. 1.

Proposed pathogenesis of BPS/IC.

Plasticity of sensory pathways inducing afferent hyperexcitability

Clinical findings

As a local mechanism inducing afferent sensitization and hyperexcitablity, it has been proposed that urothelial dysfunction that can increase the amount of urothelially-released substances, such as adenosine triphosphate and nitric oxide, and subsequent neurogenic inflammation associated with mast cell activation can lead to changes in the properties of bladder afferent pathways, resulting in increased pain sensation associated with BPS/IC (Fig. 1).¹ Pain is a defining characteristic of BPS/IC. One mechanism by which pain is induced is postulated to involve chronic tissue inflammation that can lead to functional changes in C-fiber afferents.⁷ Hyperactivity and emergence of mechanosensitivity of C-fiber afferents might lead to pain sensation in response to normal non-noxious distension of the bladder. Indirect evidence for this postulate comes from histological analysis of bladders from patients with BPS/IC, which showed marked edema, vasodilation, proliferation of nerve fibers and infiltration of mast cells.⁸ Also, increased afferent activity is evidenced by a statistically significant increase in the number of nerve fibers expressing substance P, which is a major neurotransmitter of C-fiber afferents, and the mRNA levels of substance P receptors (neurokinin-1) in the mucosa of the bladder in patients with IC.⁹ It has also been reported that C-fiber desensitization by means of intravesical application of high-dose capsaicin and resiniferatoxin is effective for treating painful symptoms in BPS/IC patients,^{10, 11} although a prospective, randomized clinical trial using intravesical resiniferatoxin application was not effective in patients with IC.¹² More recently, intravesical application of alkalized lidocaine, a local anesthetic, has been shown to provide sustained amelioration of symptoms of BPS/IC in a placebo-controlled, randomized clinical study.¹³

Overall, sensitization and enhanced excitability of afferent pathways innervating the lower urinary tract is likely to be involved in the pathophysiological basis of BPS/IC (Fig. 2).

Fig. 2.

Summary of the events involved in chronic inflammation of the bladder and hyperexcitability of C-fiber bladder afferent pathways. The events that occur after chronic bladder inflammation (1) are shows in sequential numbers (2–5). Potential therapeutic modalities targeting enhanced synaptic transmission at the spinal cord level or inflammatory changes in the bladder are also shown in the boxes.

Basic research findings

The mechanisms inducing afferent sensitization associated with cystitis are further investigated using various animal models of bladder inflammation, in which increased urinary frequency is initiated by sensitizing mechanosensitive afferents and/or recruitment of afferents normally unresponsive to mechanical stimulation.^14–17 Additionally, pro-inflammatory agents, such as prostaglandin E2, serotonin, histamine and adenosine; as well as neurotrophic factors, such as NGF, can induce functional changes in C-fiber afferents that can lead to these relatively unexcitable afferents becoming hyperexcitable.^18–21 Tissue inflammation in the bladder can also induce changes in the expression of various neurochemical markers in the bladder and bladder afferent pathways. Using an animal model of chronic bladder inflammation induced by cyclophosphamide, it has been reported that the expression of nitric oxide synthase,²² growth-associated protein,²³ pituitary adenylate cyclase-activating polypeptide,²⁴ neuropeptides such as substance P,²⁵ protease activated receptors,²⁶ cyclooxygenase-2 and prostaglandins are increased in afferent neurons in lumbosacral DRG, innervating the bladder in rats with CYP-induced chronic cystitis. Thus, it is likely that chronic bladder inflammation can induce various changes in the expression of inflammation-related proteins/receptors in the bladder and bladder afferent pathways, and that these changes might contribute to afferent neuroplasticity, leading to pain symptoms in BPS/IC (Fig. 2).

Organ cross-talk and BPS/IC

It has been demonstrated that BPS/IC patients often exhibit high comorbidity of other visceral pain syndromes, such as irritable bowel syndrome.²⁷ Recent animal studies have shown that interactions with bidirectional cross-sensitization can occur between the bladder and intestine or female reproductive organs. For example, chemically-induced colitis in rats induces urinary frequency, bladder afferent fiber hyperexcitability and mast cell infiltration in the bladder,^28–30 and release of neuropeptides such as substance P and calcitonin gene-related peptide in the bladder,³¹ and increases excitability of bladder afferent neurons.³² In addition, 17% of rat DRG neurons are double-labeled after dye injection into the bladder and colon wall of the rat, showing the existence of dichotomized afferents innervating multiple visceral organs that could contribute to cross-organ sensitization of visceral organs.³³ In addition, another potential site for organ cross-talk could be found at the spinal cord level, because acute colitis in rats reportedly sensitizes spinal neurons receiving input from the bladder in the dorsal horn of the spinal cord.

Neurotrophic factor and afferent hyperexcitability

Neurotrophic factors, such as NGF, can be synthesized by various inflammatory cells, such as lymphocytes and mast cells, and might function as a prominent factor linking hypersensitivity and development of pain with various inflammatory states, including airway, allergic or neurogenic inflammation including BPS/IC.^{34, 35} In patients with IC, neurotrophins, including NGF, neurotrophin-3 and glial-derived neurotrophic factor, have been detected in the urine.³⁶ Increased expression of NGF is also present in bladder biopsies from women with IC.³⁷ Thus, target organ–neural interactions mediated by an increase of neurotrophins in the bladder and increased transport of neurotrophins to the neuronal cell bodies in afferent pathways might contribute to the emergence of bladder pain in BPS/IC.³⁸ Furthermore, the monoclonal NGF neutralizing antibody, tanezumab, has been included in the clinical testing program, and encouraging results of the phase two efficacy study have been presented. Tanezumab improved the self-reported pain score and urgency episode frequency 6 weeks after a single intravenous injection 200 µg/kg in a cohort of 68 patients with BPS/IC.³⁹ Tanezumab had no significant effect on micturition frequency or mean voided volume per micturition. Among adverse effects, abnormal peripheral sensation symptoms, such as paresthesia and hyperesthesia, were more common in the treatment arm when compared with a placebo.³⁹ Although clinical studies were put on hold after the reports of bone necrosis requiring total joint replacements in clinical trials for ostheoarthritis (http://www.clinicaltrials.gov), proof-of-concept evidence has been provided for the effectiveness of systemic intervention in the NGF system in the treatment of BPS/IC.

In basic research using a rat model of cyclophosphamide-induced chronic cystitis, increased expression of neurotrophic growth factors, such as NGF, brain-derived neutotrophic factor and ciliary neurotrophic factor, in the bladder as well as phosphorylation of tyrosine kinase receptors (TrkA, TrkB) in bladder-innervating afferent neurons has been documented as direct evidence for neurotrophin-mediated signal transduction in chronic bladder inflammation. In addition, the enhanced neurotrophic factor mechanisms were also associated with increased phosphorylated CREB in bladder afferent neurons, and a subpopulation of phosphorylated-CREB-positive cells co-expressed phophorylated-Trk in rats with chronic cystitis.⁴⁰ Furthermore, resiniferatoxin, a C-fiber neurotoxin, reduced CYP-induced upregulation of phosphorylated-CREB in DRG, suggesting that cystitis can be linked with an altered CREB phosphorylation in capsaicin-sensitive C-fiber bladder afferents.⁴⁰ These results suggest that upregulation of phosphorylated-CREB might be mediated by a neurotrophin/Trk signaling pathway, and that CREB phosphorylation could play a role as a transcription factor in lower urinary tract plasticity induced by cystitis. Previous studies also showed that exogenous NGF can induce bladder nociceptive responses and bladder overactivity in rats when applied acutely into the bladder lumen^{15, 41} or chronically to the bladder wall or intrathecal space.⁴² Turpentine oil-induced cystitis instillation exaggerated micturition reflexes and visceral hyperalgesia within hours, resembling symptoms of human BPS/IC, and was associated with increased NGF mRNA level in the bladder.⁴³ Importantly, under these experimental conditions, hyperexcitability of the bladder¹⁵ and viscerosomatic hyperalgesia⁴⁴ can be prevented with recombinant fusion protein of extracellular TrkA domain with the fragment crystallizable portion of immunoglobulin G to neutralize NGF, confirming the causal role of NGF. Along with modulation of NGF bladder levels, upregulation and a shift in spatial expression of both NGF receptors in nerves was reported in CYP cystitis. After the CYP injection, TrkA seems to be upregulated in DRG,⁴⁵ the major pelvic ganglion and bladder smooth muscle, while being downregulated in the urothelium.⁴⁶ Similarly, p75^NTR immunoreactivity was markedly increased in bladder nerve fibers, Schwann cells, the spinal cord and bladder afferent neurons.^{47, 48} Focusing on BPC/IC-related hyperalgesia, rather than bladder hyperactivity, Guerrios et al. found that CYP-induced peripheral hyperalgesia to mechanical stimulation in mice was abolished by NGF antiserum or k252a, a non-specific TrkA antagonist, but did not affect edema in the bladder wall, leukocyte infiltration or hemorrhage.^{49, 50} The role of the low-affinity p75^NTR receptor was also implicated in a study by showing that blockade of p75^NTR with monoclonal antibody, effectively reducing p75 immunoreactivity in the bladder, induces, rather than limits, hyperreflexia both in control and CYP cystitis rats, as reflected by increased frequency of voiding and reduced intercontraction intervals.⁴⁷ Additionally, prevention of NGF binding to p75^NTR with PD90780 increased voiding frequency with decreased voiding volume, intercontraction intervals, increased threshold, baseline and micturition pressure in cystitis rats.

Taken together, there exists some evidence favoring intervention in the NGF system as a viable therapeutic target ameliorating specifically sensory and painful symptoms associated with BPS/IC. As aforementioned, a recent clinical trial showed that systemic administration of monoclonal human NGF antibodies reduces bladder pain symptoms in BPS/IC patients; however, generalized blockade of NGF activity at sites other than the bladder using systemic anti-NGF antibodies was associated with the incidence of paresthesia, hypoesthesia and bone necrosis.³⁹ Thus, the site-specific reduction of NGF would be desirable to reduce the intrinsic toxicity from systemic blockade of NGF. In this regard, we recently reported that treatment with intravesical liposomal antisense suppresses NGF expression in the urothelium, as well as bladder overactivity and chemokine upregulation using rats with acetic acid induced bladder overactivity.⁵¹ Thus, local suppression of NGF in the bladder using intravesical liposome-based delivery techniques could be an attractive approach for BPS/IC treatment, which can avoid systemic side-effects that might be associated with non-specific blockade of NGF expression (Fig. 2).

Potential modalities targeting hyperexcitable bladder afferent pathways for the treatment of BPS/IC

Sensitization of bladder afferent pathways, especially of the C-fiber population, and a subsequent increase in sensory processing in the spinal cord have been proposed as important mechanisms inducing pain/irritative symptoms associated with BPS/IC.⁵² Thus, therapies aiming to reduce sensory processing in the spinal cord, which is increased as a result of afferent hyperexcitability, could be effective for reducing symptoms in BPS/IC patients (Fig. 2). In the following sections, we discuss the potential targets that can suppress bladder afferent activity for the treatment of BPS/IC, based on recent research results including ours.

Opioids

Opioids are the abundantly-expressed inhibitory neurotransmitter. Patients with BPS/IC often have severe and refractory bladder pain that is resistant to non-steroidal anti-inflammatory drugs, requiring long-acting opioids.⁵³ However, the use of systemic opioid therapy, such as morphine or oxycodone, has been limited because of its untoward side-effects, tolerance and dependency. Enkephalins, which are a subfamily of endogenous opioids, are expressed in bladder afferent and efferent pathways to inhibit micturition.⁵⁴ Enkephalinergic mechanisms in the brain and spinal cord also have inhibitory effects on the micturition reflex, and exogenous enkephalins or opiate drugs applied to the sacral spinal cord can suppress micturition.⁵⁵

Replication-deficient HSV vectors encoding preproen-kephalin, one of three genes that encode endogenous opioid peptides, injected into the bladder wall showed reductions in bladder overactivity and nociceptive behavior induced by intravesical application of capsaicin, whereas vector-mediated expression of enkephalin did not affect normal voiding.^{56, 57} These results provide the proof of concept for a new gene therapy approach to enhance endogenous opioid mechanisms and reduce systemic side-effects for the treatment of bladder hypersensitive disorders, such as BPS/IC.

Another study reported the antinociceptive effect of human pro-opiomelanocortin (POMC),⁵⁸ which encodes β-endorphin, another endogenous opioid peptide, which activates µ-opioid receptors on sensory neurons and inhibits pain in the inflamed tissue.^{59, 60} Intravesical instillation of acetic acid during cystometrograms shortened the intercontraction interval in control animals, but not in rats treated with POMC genes delivered by gene gun into the bladder wall. The effects of the POMC gene gun injection were reversed by an intramuscular injection of the opiate receptor antagonist, naloxone, supporting an opioid receptor-mediated analgesic mechanism of action.⁵⁸

Overall, opioid gene therapy targeting the bladder and bladder afferent pathways could be an effective modality for the treatment of intractable pain symptoms of BPS/IC while reducing systemic side-effects.

Adenosine receptors

Adenosine is a neurotransmitter that exerts numerous physiological effects in many organs. Recently, a number of studies showed that activation of A1 receptors produces antinociception in several pain models,⁶¹ and inhibition of A2A receptors attenuates nociceptive responses in mouse models of somatic pain.^{62, 63} Our recent study has also shown that, in rats with irritated bladders induced by intravesical acetic acid application, intravenous or intrathecal ZM24138 (A2A receptor-antagonist) significantly reduces bladder overactivity with significantly greater effects compared with those in untreated rats.⁵² These results show that adenosine A2A receptor-mediated excitatory mechanisms are enhanced in the spinal cord after C-fiber bladder afferent stimulation. Thus, adenosine A2A receptor antagonists might be effective for the treatment of bladder hypersensitive disorders, such as BPS/IC, in which C-fiber afferent function is enhanced (Fig. 2).

GlyT system

Glycine, one of the major inhibitory neurotransmitters in the central nervous system, is well known to have a role in the control of spinal nociceptive pathways⁶⁴ and lower urinary tract function in both physiological and pathological conditions.^65–67 The extracellular concentration of glycine at synapses is regulated by two types of Na⁺/Cl⁻ dependent GlyT: GlyT1 and GlyT2.⁶⁸ GlyT1 is widely distributed in the central nervous system and predominantly expressed in glial cells near both excitatory and inhibitory neurons, whereas GlyT2 is specifically distributed in the spinal cord, cerebellum and brainstem, and localized in the presynaptic terminals of inhibitory glycinergic neurons.^68–70

Recent studies have suggested a therapeutic potential for GlyT inhibitors in the treatment of acute/chronic pain.^{71, 72} Therefore, we have examined the effects of GlyT inhibitors on bladder overactivity and pain behavior in rats.⁷³ Cystometrogram parameters were used to determine the effects of selective GlyT2 inhibitors administered intrathecally to CYP-treated rats under urethane anesthesia. In addition, intravesical administration of resiniferatoxin was used to induce nociceptive responses in a behavioral study in conscious rats. A selective GlyT2 inhibitor (ALX-1393), but not a GlyT1 inhibitor (sarcosine), produced significant increases in inter-contraction interval and micturition pressure threshold in CYP-treated rats. These effects were completely reversed by the glycine receptor antagonist, strychnine. The GlyT2 inhibitor also significantly suppressed nociceptive behaviors in a dose-dependent manner. Compared with GlyT1, mRNA levels of GlyT2 were 23-fold higher in the dorsal spinal cord. Results show that GlyT2 is primarily involved in the clearance of extracellular glycine in the spinal cord, and that GlyT2 inhibition leads to reduction of CYP-induced bladder overactivity and pain behavior.

Therefore, inhibition of GlyT2, but not GlyT1, could be a novel therapeutic modality for the treatment of bladder hypersensitive disorders, such as BPS/IC, without affecting the glycinergic mechanism in the brain (Fig. 2).

Inflammatory and immunogenic mechanisms of BPS/IC

Cytokine and chemokine expression and BPS/IC

Although the etiology of BPS/IC is not fully understood, bladder inflammation associated with the production of inflammatory cytokines/chemokines has been proposed as a potential cause of pathogenesis of the disease to induce afferent hyperexcitability.^{74, 75} In previous studies, cytokines and chemokines, such as IL-2, IL-6, IL-8 and TNF-α, were significantly increased in BPS/IC patients’ bladder tissue and urine than in controls, suggesting that these cytokines might represent specific markers of BPS/IC.^{74, 76, 77} In contrast, IL-4 is a prototypical anti-inflammatory cytokine, which is known to inhibit secretion of inflammatory cytokines, such as IL-1β, TNF-α and IL-6.^78–80 BPS/IC patients have shown lower levels of IL-4 that increased after treatment with a drug (suplatast tosilate) that altered the BPS/IC phenotype and symptoms in these BPS/IC patients,⁸⁰ suggesting that IL-4 could provide a desired anti-inflammatory effect that can alter the overall cytokine profile and subsequent recruitment of T cells and mast cells.

Chemokines also play a pivotal role in the immune response, leading the recruitment of leukocytes to inflammation, tumor growth, angiogenesis and organ sclerosis.⁸¹ CXCR3 and its associated ligands, monokine induced by IFN-γ (MIG/CXCL9), IFN-γ-inducible protein (IP-10/CXCL10), and IFN-γ-inducible T cell α-chemoattractant (I-TAC/CXCL11) are involved in the regulation of autoimmune disorders of endocrine glands. CXCR3 was initially identified on activated T cells and its expression was associated with T helper-mediated immune response.^{82, 83} CXCR3 expresses in epithelial cells, endothelial cells and vascular pericytes.

It has been reported that after IFN-γ stimulation, endocrine epithelial cells secrete CXCL10, which in turn recruits type 1 T helper lymphocytes expressing CXCR3 and secreting IFN-γ, thus perpetuating autoimmune inflammation, suggesting that the chemokines play an important role in endocrine autoimmunity. ⁸¹ Sakthivel et al. showed that serum levels of CXCL9, CXCL10 and CXCL11 were increased in patients with IC.⁸⁴ They also showed that the number of CD4+ T cells, mast cells, natural killer cells, and natural killer T cells were increased at systemic (spleen) and peripheral (urinary bladder and iliac lymph nodes) sites in a mouse model of CYP-induced cystitis. Importantly, CXCL10 blockade attenuated these increases caused by CYP.⁸⁴ We also investigated genes responsible for ulcerative IC among over 40 000 genes using microarray analysis.⁷⁶ We have identified 564 probes that were significantly expressed (P < 0.001) in mRNA by more than fourfold compared with the control group by using the volcano plot analysis. Further ingenuity pathways analysis of these genes showed the top three functions, such as cell-to-cell signaling and interaction, hematological system development and function, and inflammatory disease. In particular, we confirmed the increases in mRNA expression of several genes in the bladder from ulcerative IC, including CXC3-binding chemokines (CXCL9, 10 and 11) and TNFSF14. CXCR3 and its binding chemokines are upregulated in the bladder urothelium of ulcerative BPS/IC patients, which might inhibit the bladder urothelial growth and induce the ulcerative changes in the bladder. In addition, it should be noted that the European Society for the Study of Interstitial Cystitis proposed that Hunner’s “ ‘ulcer’ ” is not a typical chronic ulcer, but rather a distinctive inflammatory lesion presenting a characteristic deep rupture through the mucosa. The term, “Hunner’s ulcer”, should be replaced by “Hunner’s lesion”, and the patients with Hunner’s lesion should be classified into the European Society for the Study of Interstitial Cystitis type 3.⁸⁵ Our recent study using multiplex analysis of 23 cytokines/chemokines with a multiple antigen bead assay also investigated the cytokine/chemokine profile in bladder tissue and urine of BPS/IC patients, and found that vascular cell adhesion molecule-1 and intercellular adhesion molecule-1 were useful for the discrimination of both tissue and urine samples of BPS/IC patients from control patients.⁸⁶

Gene therapy for modulating the cytokine level

TNF-α is a pro-inflammatory mediator that initiates inflammatory reactions of the innate immune system: induction of other cytokine production, activation and expression of adhesion molecules, and stimulation and recruitment of inflammatory cells.⁸⁷ Additionally, it is essential in the development of nociception, not just in inflammatory pain, but also in neuropathic pain.⁸⁸ Many studies have reported that TNF-α influences pain sensation in the peripheral tissue and the spinal cord.⁸⁹ When TNF-α activity is neutralized using anti-TNF-α antibody or TNF-αsR, the development of nociception is suppressed, as has been observed in several rat pain models.^89–91

Recently, gene therapy of TNF-αsR using replication-deficient HSV vectors has been investigated for bladder pain and urinary frequency using a rat model of chemically-induced cystitis.⁹² Although TNF-α mRNA in the bladder was upregulated by the intravesical administration of resiniferatoxin (a TRPV1 receptor agonist), the increase in TNF-α protein levels was suppressed in TNF-αsR-expressing vector-treated rats, showing that HSV vector-delivered TNF-αsR neutralized TNF-α proteins increased by bladder irritation. The suppression of TNF-α protein leads to the downregulation of IL-1β and IL-6, as well as a reduction in myeloperoxidase activity in the resiniferatoxin-treated bladder. Moreover, TNF-α blockade reduced pain sensation and bladder overactivity induced by intravesical instillation of resiniferatoxin.

Another approach investigated for bladder pain is the HSV vector-mediated delivery of IL-4 to the bladder and bladder afferent pathways. IL-4 is a prototypical anti-inflammatory cytokine, known to inhibit the secretion of inflammatory cytokines, such as IL-1β, TNFα and IL-6.^{78, 79} In contrast to TNFα or IL-6, IL-4 is decreased in the urine of BPS/IC patients, and then increases after the treatment of suplatast tosilate, an anti-allergic drug.⁸⁰ In rats, IL-4 expression after HSV vector administration to the plantar foot surface reduced in nociceptive behaviors after painful insult.⁹³ The effect of IL-4 delivered by HSV vector on bladder overactivity and nociceptive behaviors has been evaluated previously.⁹⁴ The IL-4 protein level was increased in the bladder and L6 DRG in IL-4-expressing HSV-injected rats, and myeloperoxidase activity and IL-1β were decreased in the bladder of the rat with chemically-induced cystitis, in which bladder overactivity and nociceptive behaviors were also suppressed.⁹⁴

Overall, gene therapy targeting cytokine production in the bladder could be a new modality for the treatment of bladder hypersensitive disorders, such as BPS/IC, which is associated with increased production of inflammatory cytokines/chemokines (Fig. 2).

Conclusion

Although various etiologies of BPS/IC have been proposed as described in the present review, no one pathological process has been identified in every patient with BPS/IC. Thus, it is likely that the syndrome of BPS/IC could have multiple etiologies, all of which result in similar clinical manifestations (Fig. 1), and that afferent hyperexcitiablity would be an important pathophysiological basis of BPS/IC, for which various therapeutic modalities could be developed (Fig. 2).

Abbreviations & Acronyms

BPS

bladder pain syndrome

CREB

cyclic adenosine monophosphate-responsive element binding protein

CYP

cyclophosphamide

DRG

dorsal root ganglion

FDA

Food and Drug Administration

GlyT

glycine transporters

HSV

herpes simplex virus

IC

interstitial cystitis

IFN-γ

interferon-γ

IL

interleukin

NGF

nerve growth factor

TNF-α

tumor necrosis factor-α

TNF-αsR

tumor necrosis factor-α soluble receptors

Trk

tyrosine kinase receptor

TRP

transient receptor potential