Molecular signaling and targets from itch: lessons for cough

Association of pruritus and chronic cough: an all of us database study

PURPOSE

Based on a potential shared pathophysiology tied to mast cell activity and neurogenic inflammation that may link pruritus and chronic cough (CC), this study, leveraging the All of Us database, examines the association between the two conditions.

MATERIALS AND METHODS

A nested case-control comparison was used to examine the association, identifying cases with SNOMED codes 418363000 (pruritus) and 68154008 (CC). Matching was performed on a 1:4 ratio by age, sex, and ethnicity using the MatchIt package in R, followed by maximum likelihood method to estimate odds ratios (ORs) and 95% confidence intervals from 2x2 contingency tables.

RESULTS

CC patients (n = 2,388) were more than twice as likely to be diagnosed with pruritus (OR: 2.65) and pruritus patients (n = 22,496) were more than twice as likely to be diagnosed with CC (OR: 2.57), than respective matched controls.

CONCLUSIONS

These results highlight the potential bidirectional relationship between CC and pruritus, suggesting possible shared immune and neural pathways. Treatments like difelikefalin and nalbuphine that modulate these pathways, alongside P2X3 targeting agents, are emerging as potential therapeutic approaches for itch and chronic cough given the possible interconnected pathophysiology. This study's insights into the associations between pruritus and CC may pave the way for targeted therapeutic strategies that address their shared mechanisms.

Clinical Aspects of Chronic Idiopathic Postnasal Drip: An Entity Not to Be Overlooked

BACKGROUND/AIM

Postnasal drip may be related to several diseases, but not all patients are clearly diagnosed. Patients with chronic, idiopathic postnasal drip symptoms are easily overlooked, and their clinical features are yet to be identified. This study aimed to analyze the clinical features and response to first generation antihistamine-decongestant therapy in patients with chronic idiopathic postnasal drip, suggesting it as a distinct entity.

PATIENTS AND METHODS

A retrospective cohort study involving 157 chronic idiopathic postnasal drip patients was conducted, analyzing demographics, symptoms, and treatment response to first-generation antihistamines and nasal decongestants.

RESULTS

Mean age of patients was 55.4±17.0 years old. Median duration of symptom was 36 months (range=12-66 months) and severity in the visual analogue scale was 7 (range=5-8). Throat discomfort was the most frequently associated symptom (73.7%). Cough was recorded in 30.3% of patients. Viscosity of postnasal drip was associated with rhinorrhea and throat discomfort. Of the patients, 71.6% responded positively to 1st generation antihistamine-decongestant medication. However, 25.9% of patients presented symptom re-occurrence. Patients with nasal stiffness or persistent symptoms presented a higher re-occurrence rate compared to others.

CONCLUSION

This study outlines the clinical features of patients with chronic idiopathic postnasal drip and suggests it as a distinctive entity., This proposal aims to enhance diagnostic precision and promote further research in the field.

Current understanding of Bordetella-induced cough

Typical pathogenic bacteria of the genus Bordetella cause respiratory diseases, many of which are characterized by severe coughing in host animals. In human infections with these bacteria, such as whooping cough, coughing imposes a heavy burden on patients. The pathophysiology of this severe coughing had long been uncharacterized because convenient animal models that reproduce Bordetella-induced cough have not been available. However, rat and mouse models were recently shown as useful for understanding, at least partially, the causative factors and the mechanism of Bordetella-induced cough. Many types of coughs are induced under various physiological conditions, and the neurophysiological pathways of coughing are considered to vary among animal species, including humans. However, the neurophysiological mechanisms of the coughs in different animal species have not been entirely understood, and, accordingly, the current understanding of Bordetella-induced cough is still incomplete. Nevertheless, recent research findings may open the way for the development of prophylaxis and therapeutic measures against Bordetella-induced cough.

Evaluation methods and influencing factors of cough sensitivity

Increased cough sensitivity is an important mechanism of chronic cough, and the evaluation of cough sensitivity helps understand the mechanism of cough and explore better methods to reduce cough. Evaluation methods may be direct or indirect. Direct methods include mechanical stimulation and chemical stimulation, and indirect methods include laryngeal reflex test, questionnaires, and brain functional magnetic resonance imaging (fMRI). Chemical stimulation is the most common method, while the capsaicin cough challenge test is proven and widely used. In this article, we will compare evaluation methods and explore influencing factors of cough sensitivity.

A descending pathway emanating from the periaqueductal gray mediates the development of cough-like hypersensitivity

Chronic cough is a common refractory symptom of various respiratory diseases. However, the neural mechanisms that modulate the cough sensitivity and mediate chronic cough remain elusive. Here, we report that GABAergic neurons in the lateral/ventrolateral periaqueductal gray (l/vlPAG) suppress cough processing via a descending pathway. We found that l/vlPAG neurons are activated by coughing-like behaviors and that tussive agent-evoked coughing-like behaviors are impaired after activation of l/vlPAG neurons. In addition, we showed that l/vlPAG neurons form inhibitory synapses with the nucleus of the solitary tract (NTS) neurons. The synaptic strength of these inhibitory projections is weaker in cough hypersensitivity model mice than in naïve mice. Important, activation of l/vlPAG GABAergic neurons projecting to the NTS decreases coughing-like behaviors. In contrast, suppressing these neurons enhances cough sensitivity. These results support the notion that l/vlPAG GABAergic neurons play important roles in cough hypersensitivity and chronic cough through disinhibition of cough processing at the medullary level.

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GABAergic neurons in the l/vlPAG inhibit coughing-like behaviors

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The l/vlPAG sends predominately inhibitory projections to the NTS

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l/vlPAG GABAergic neurons modulate coughing-like behaviors via descending projections

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l/vlPAG-NTS projections mediate cough hypersensitivity via disinhibitory mechanisms

Current and emerging pharmacotherapy for chronic spontaneous Urticaria: a focus on non-biological therapeutics

INTRODUCTION

Chronic spontaneous urticaria (CSU) refers to urticaria (wheals) or angioedema, which occur for a period of six weeks or longer without an apparent cause. The condition may impair the patient's quality of life.

AREAS COVERED

Treatment for CSU is mainly symptomatic. Both AAAAI/ACAAI practice parameters and EAACI/GA2LEN/EDF/WAO guidelines suggest CSU management in a stepwise manner. First-line therapy is with second-generation H₁-antihistamines. Treatment should be stepped up along the algorithm if symptoms are not adequately controlled. Increasing the dosage of second-generation H₁-antihistamines, with the addition of first-generation H₁-antihistamines, H₂ antagonist, omalizumab, ciclosporin A, or short-term corticosteroid may be necessary. New medications are being developed to treat refractory CSU. They include spleen tyrosine kinase inhibitor, Bruton tyrosine kinase inhibitor, prostaglandin D₂ receptor inhibitor, H₄-antihistamine, and other agents. The authors discuss these treatments and provide expert perspectives on the management of CSU.

EXPERT OPINION

Second-generation H₁-antihistamines remain the first-line therapeutic options for the management of CSU. For patients not responding to higher-dose H₁-antihistamines, international guidelines recommend the addition of omalizumab. Efficacy and safety data for newer agents are still pending. Large-scale, well-designed, randomized, double-blind, placebo-controlled trials will further provide evidence on the safety profile and efficacy of these agents in patients with CSU.

Itch and Cough - Similar Role of Sensory Nerves in Their Pathogenesis

Itch is the most common chief complaint in patients visiting dermatology clinics and is analogous to cough and also sneeze of the lower and upper respiratory tract, all three of which are host actions trying to clear noxious stimuli. The pathomechanisms of these symptoms are not completely determined. The itch can originate from a variety of etiologies. Itch originates following the activation of peripheral sensory nerve endings following damage or exposure to inflammatory mediators. More than one sensory nerve subtype is thought to subservepruriceptive itch which includes both unmyelinated C-fibers and thinly myelinated Adelta nerve fibers. There are a lot of mediators capable of stimulating these afferent nerves leading to itch. Cough and itch pathways are mediated by small-diameter sensory fibers. These cough and itch sensory fibers release neuropeptides upon activation, which leads to inflammation of the nerves. The inflammation is involved in the development of chronic conditions of itch and cough. The aim of this review is to point out the role of sensory nerves in the pathogenesis of cough and itching. The common aspects of itch and cough could lead to new thoughts and perspectives in both fields.

Peripheral and Central Mechanisms of Itch

Itch is a unique sensory experience that is encoded by genetically distinguishable neurons both in the peripheral nervous system (PNS) and central nervous system (CNS) to elicit a characteristic behavioral response (scratching). Itch interacts with the other sensory modalities at multiple locations, from its initiation in a particular dermatome to its transmission to the brain where it is finally perceived. In this review, we summarize the current understanding of the molecular and neural mechanisms of itch by starting in the periphery, where itch is initiated, and discussing the circuits involved in itch processing in the CNS.

Antitussive therapy: A role for levodropropizine

Cough is a protective reflex that serves to clear the airways of excessive secretions and foreign matter and which sometimes becomes excessive, and troublesome to patients. Cough is one of the most common reasons why individuals seek medical attention. A range of drugs have been developed in the past with antitussive activity and different mechanisms of action, but there are still very few safe and effective treatments available. The poor tolerability of most available antitussives is closely related to their action on the central nervous system (CNS). An international group of experts specialized in cough met to discuss the need to identify an effective antitussive treatment with a good tolerability profile. The aim of this expert review is to increase the knowledge about the cough mechanism and the activity of levodropropizine, a peripherally acting antitussive drug.

Early intervention by Captopril does not improve wound healing of partial thickness burn wounds in a rat model

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Inhibition of the Renin Angiotensin System does not influence the inflammatory reaction in the burn wounds.

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Inhibition of the Renin Angiotensin System early during burn wound healing does not improve the process.

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Inhibition of the Renin Angiotensin System early during burn wound healing does not reduce scar formation.

The Renin Angiotensin System is involved in fibrotic pathologies in various organs such as heart, kidney and liver. Inhibition of this system by angiotensin converting enzyme antagonists, such as Captopril, has been shown beneficial effects on these pathologies. Captopril reduced the inflammatory reaction but also directly influenced the fibrotic process.

Prolonged and excessive inflammatory response is a major cause of hypertrophic scar formation in burns. We therefore evaluated the effect of Captopril on the healing of partial thickness burn wounds in a rat model.

Partial thickness contact burns were inflicted on the dorsum of the rats. The rats received either systemic or local treatment with Captopril. The inflammatory reaction and wound healing (scar) parameters were investigated and compared to control animals.

In this study we could not detect positive effects of either administration route with Captopril on the inflammatory reaction, nor on wound healing parameters. The local treatment showed reduced wound closure in comparison to the systemic treatment and the control group.

Early Captopril treatment of burn wounds did not show the beneficial effects that were reported for fibrotic disorders in other tissues. To influence the fibrotic response Captopril treatment at a later time point, e.g. during the remodeling phase, might still have beneficial effects.

A novel formulation of inhaled sodium cromoglicate (PA101) in idiopathic pulmonary fibrosis and chronic cough: a randomised, double-blind, proof-of-concept, phase 2 trial

BACKGROUND

Cough can be a debilitating symptom of idiopathic pulmonary fibrosis (IPF) and is difficult to treat. PA101 is a novel formulation of sodium cromoglicate delivered via a high-efficiency eFlow nebuliser that achieves significantly higher drug deposition in the lung compared with the existing formulations. We aimed to test the efficacy and safety of inhaled PA101 in patients with IPF and chronic cough and, to explore the antitussive mechanism of PA101, patients with chronic idiopathic cough (CIC) were also studied.

METHODS

This pilot, proof-of-concept study consisted of a randomised, double-blind, placebo-controlled trial in patients with IPF and chronic cough and a parallel study of similar design in patients with CIC. Participants with IPF and chronic cough recruited from seven centres in the UK and the Netherlands were randomly assigned (1:1, using a computer-generated randomisation schedule) by site staff to receive PA101 (40 mg) or matching placebo three times a day via oral inhalation for 2 weeks, followed by a 2 week washout, and then crossed over to the other arm. Study participants, investigators, study staff, and the sponsor were masked to group assignment until all participants had completed the study. The primary efficacy endpoint was change from baseline in objective daytime cough frequency (from 24 h acoustic recording, Leicester Cough Monitor). The primary efficacy analysis included all participants who received at least one dose of study drug and had at least one post-baseline efficacy measurement. Safety analysis included all those who took at least one dose of study drug. In the second cohort, participants with CIC were randomly assigned in a study across four centres with similar design and endpoints. The study was registered with ClinicalTrials.gov (NCT02412020) and the EU Clinical Trials Register (EudraCT Number 2014-004025-40) and both cohorts are closed to new participants.

FINDINGS

Between Feb 13, 2015, and Feb 2, 2016, 24 participants with IPF were randomly assigned to treatment groups. 28 participants with CIC were enrolled during the same period and 27 received study treatment. In patients with IPF, PA101 reduced daytime cough frequency by 31·1% at day 14 compared with placebo; daytime cough frequency decreased from a mean 55 (SD 55) coughs per h at baseline to 39 (29) coughs per h at day 14 following treatment with PA101, versus 51 (37) coughs per h at baseline to 52 (40) cough per h following placebo treatment (ratio of least-squares [LS] means 0·67, 95% CI 0·48-0·94, p=0·0241). By contrast, no treatment benefit for PA101 was observed in the CIC cohort; mean reduction of daytime cough frequency at day 14 for PA101 adjusted for placebo was 6·2% (ratio of LS means 1·27, 0·78-2·06, p=0·31). PA101 was well tolerated in both cohorts. The incidence of adverse events was similar between PA101 and placebo treatments, most adverse events were mild in severity, and no severe adverse events or serious adverse events were reported.

INTERPRETATION

This study suggests that the mechanism of cough in IPF might be disease specific. Inhaled PA101 could be a treatment option for chronic cough in patients with IPF and warrants further investigation.

FUNDING

Patara Pharma.

Blocking TRPA1 in Respiratory Disorders: Does It Hold a Promise?

Transient Receptor Potential Ankyrin 1 (TRPA1) ion channel is expressed abundantly on the C fibers that innervate almost entire respiratory tract starting from oral cavity and oropharynx, conducting airways in the trachea, bronchi, terminal bronchioles, respiratory bronchioles and upto alveolar ducts and alveoli. Functional presence of TRPA1 on non-neuronal cells got recognized recently. TRPA1 plays a well-recognized role of “chemosensor”, detecting presence of exogenous irritants and endogenous pro-inflammatory mediators that are implicated in airway inflammation and sensory symptoms like chronic cough, asthma, chronic obstructive pulmonary disease (COPD), allergic rhinitis and cystic fibrosis. TRPA1 can remain activated chronically due to elevated levels and continued presence of such endogenous ligands and pro-inflammatory mediators. Several selective TRPA1 antagonists have been tested in animal models of respiratory disease and their performance is very promising. Although there is no TRPA1 antagonist in advanced clinical trials or approved on market yet to treat respiratory diseases, however, limited but promising evidences available so far indicate likelihood that targeting TRPA1 may present a new therapy in treatment of respiratory diseases in near future. This review will focus on in vitro, animal and human evidences that strengthen the proposed role of TRPA1 in modulation of specific airway sensory responses and also on preclinical and clinical progress of selected TRPA1 antagonists.

Are itch and scratching the nausea and vomiting of skin?

The physiologic similarities between itch and nausea may not be evident initially, but they share the role of repelling irritants and toxins from the body by inducting scratching and vomiting, respectively. In addition, itch and nausea frequently occur together in certain conditions such as uraemia. Here we show that the mechanisms underlying itch and nausea overlap and that advances in either field may influence the identification of novel drug targets, particularly for itch.

Neuroimmune interactions in itch: Do chronic itch, chronic pain, and chronic cough share similar mechanisms?

Itch and pain are closely related but also clearly distinct sensations. Pain is known to suppress itch, while analgesics such as morphine can provoke itch. However, in pathological and chronic conditions, pain and itch also have similarities. Dysfunction of the nervous system, as manifested by neural plastic changes in primary sensory neurons of the peripheral nervous system (peripheral sensitization) and spinal cord and brain stem neurons in the central nervous system (central sensitization) will result in chronic pain and itch. Importantly, these diseases also result from immune dysfunction, since inflammatory mediators can directly activate or sensitize nociceptive and pruriceptive neurons in the peripheral and central nervous system, leading to pain and itch hypersensitivity. In this mini-review, I discuss the roles of Toll-like receptors (TLRs), transient receptor potential ankyrin 1 (TRPA1) ion channel, and Nav1.7 sodium channel in regulating itch and inflammation, with special emphasis of neuronal TLR signaling and the interaction of TLR7 and TRPA1. Chronic pain and chronic itch are debilitating diseases and dramatically impact the life quality of patients. Targeting TLRs for the control of inflammation, neuroinflammation (inflammation restricted in the nervous system), and hyperexcitability of nociceptors and pruriceptors will lead to new therapeutics for the relief of chronic pain and chronic itch. Finally, given the shared mechanisms among chronic cough, chronic pain, and chronic itch and the demonstrated efficacy of the neuropathic pain drug gabapentin in treating chronic cough, novel therapeutics targeting TRPA1, Nav1.7, and TLRs may also help to alleviate refractory cough via modulating neuron-immune interaction.

Inhibitory control of the cough reflex by galanin receptors in the caudal nucleus tractus solitarii of the rabbit

The caudal nucleus tractus solitarii (NTS) is the main central station of cough-related afferents and a strategic site for the modulation of the cough reflex. The similarities between the characteristics of central processing of nociceptive and cough-related inputs led us to hypothesize that galanin, a neuropeptide implicated in the control of pain, could also be involved in the regulation of the cough reflex at the level of the NTS, where galanin receptors have been found. We investigated the effects of galanin and galnon, a nonpeptide agonist at galanin receptors, on cough responses to mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. Drugs were microinjected (30–50 nl) into the caudal NTS of pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Galnon antitussive effects on cough responses to the mechanical stimulation of the airway mucosa via a custom-built device were also investigated. Bilateral microinjections of 1 mM galanin markedly decreased cough number, peak abdominal activity, and increased cough-related total cycle duration. Bilateral microinjections of 1 mM galnon induced mild depressant effects on cough, whereas bilateral microinjections of 10 mM galnon caused marked antitussive effects consistent with those produced by galanin. Galnon effects were confirmed by using the cough-inducing device. The results indicate that galanin receptors play a role in the inhibitory control of the cough reflex at the level of the caudal NTS and provide hints for the development of novel antitussive strategies.

Transient receptor potential ankyrin 1 receptor activation in vitro and in vivo by pro-tussive agents: GRC 17536 as a promising anti-tussive therapeutic

Cough is a protective reflex action that helps clear the respiratory tract which is continuously exposed to airborne environmental irritants. However, chronic cough presents itself as a disease in its own right and despite its global occurrence; the molecular mechanisms responsible for cough are not completely understood. Transient receptor potential ankyrin1 (TRPA1) is robustly expressed in the neuronal as well as non-neuronal cells of the respiratory tract and is a sensor of a wide range of environmental irritants. It is fast getting acceptance as a key biological sensor of a variety of pro-tussive agents often implicated in miscellaneous chronic cough conditions. In the present study, we demonstrate in vitro direct functional activation of TRPA1 receptor by citric acid which is routinely used to evoke cough in preclinical and clinical studies. We also show for the first time that a potent and selective TRPA1 antagonist GRC 17536 inhibits citric acid induced cellular Ca⁺² influx in TRPA1 expressing cells and the citric acid induced cough response in guinea pigs. Hence our data provides a mechanistic link between TRPA1 receptor activation in vitro and cough response induced in vivo by citric acid. Furthermore, we also show evidence for TRPA1 activation in vitro by the TLR4, TLR7 and TLR8 ligands which are implicated in bacterial/respiratory virus pathogenesis often resulting in chronic cough. In conclusion, this study highlights the potential utility of TRPA1 antagonist such as GRC 17536 in the treatment of miscellaneous chronic cough conditions arising due to diverse causes but commonly driven via TRPA1.

Suppression of the cough reflex by α 2-adrenergic receptor agonists in the rabbit

The α₂-adrenergic receptor agonist clonidine has been shown to inhibit citric acid-induced cough responses in guinea pigs when administered by aerosol, but not orally. In contrast, oral or inhaled clonidine had no effect on capsaicin-induced cough and reflex bronchoconstriction in humans. In addition, intravenous administration of clonidine has been shown to depress fentanyl-induced cough in humans. We investigated the effects of the α₂-adrenergic receptor agonists, clonidine and tizanidine, on cough responses induced by mechanical and chemical (citric acid) stimulation of the tracheobronchial tree. Drugs were microinjected (30–50 nL) into the caudal nucleus tractus solitarii (cNTS) and the caudal ventral respiratory group (cVRG) as well as administered intravenously in pentobarbital sodium-anesthetized, spontaneously breathing rabbits. Bilateral microinjections of clonidine into the cNTS or the cVRG reduced cough responses at 0.5 mmol/L and abolished the cough reflex at 5 mmol/L. Bilateral microinjections of 0.5 mmol/L tizanidine into the cNTS completely suppressed cough responses, whereas bilateral microinjections of 5 mmol/L into the cVRG only caused mild reductions in them. Depressant effects on the cough reflex of clonidine and tizanidine were completely reverted by microinjections of 10 mmol/L yohimbine. Intravenous administration of clonidine (80–120 μg/kg) or tizanidine (150–300 μg/kg) strongly reduced or completely suppressed cough responses. These effects were reverted by intravenous administration of yohimbine (300 μg/kg). The results demonstrate that activation of α₂-adrenergic receptors in the rabbit exerts potent inhibitory effects on the central mechanism generating the cough motor pattern with a clear action at the level of the cNTS and the cVRG.