Targeting Chemosensory Ion Channels in Peripheral Swallowing-Related Regions for the Management of Oropharyngeal Dysphagia

Topical ATP Application in the Peripheral Swallowing-Related Regions Facilitates Triggering of the Swallowing Reflex Involving P2X3 Receptors

The swallowing reflex is a critical component of the digestive process, triggered when food or liquids pass from the oral cavity to the oesophagus. Although adenosine triphosphate (ATP) is involved in various physiological processes, its potential to trigger the swallowing reflex has not been fully explored. This study investigated the ability of ATP to induce the swallowing reflex and examined the involvement of the purinoreceptor P2X3 in this process. We observed that the topical application of exogenous ATP to the superior laryngeal nerve (SLN)-innervated swallowing-related regions dose-dependently facilitated the triggering of the swallowing reflex. P2X3 receptors were predominantly localized on nerve fibres within these regions, including intraepithelial and subepithelial nerves and those associated with taste-bud-like structures. In the nodose-petrosal-jugular ganglionic complex, approximately 40% of retrogradely traced SLN-afferent neurons expressed P2X3, with 59% being medium-sized, 30% small, and 11% large. Prior topical application of a P2X3 antagonist in SLN-innervated, swallowing-related regions significantly reduced the number of ATP-induced swallowing reflexes. Furthermore, topical application of a P2X3 receptor agonist more selective than ATP facilitated reflex triggering in a dose-dependent manner. These findings suggest that exogenous ATP facilitates the triggering of the swallowing reflex through the activation of P2X3 receptors. This activation excites afferent neurons that supply peripheral swallowing-related regions, stimulating the swallowing central pattern generator to facilitate the reflex. The current findings suggest the therapeutic potential of ATP or P2X3 agonists for dysphagia treatment and provide valuable physiological insights into the involvement of purinergic signaling in triggering the swallowing reflex.

Oropharyngeal swallowing hydrodynamics of thin and mildly thick liquids in an anatomically accurate throat-epiglottis model

Understanding the mechanisms underlying dysphagia is crucial in devising effective, etiology-centered interventions. However, current clinical assessment and treatment of dysphagia are still more symptom-focused due to our limited understanding of the sophisticated symptom-etiology associations causing swallowing disorders. This study aimed to elucidate the mechanisms giving rise to penetration flows into the laryngeal vestibule that results in aspirations with varying symptoms. Methods: Anatomically accurate, transparent throat models were prepared with a 45° down flapped epiglottis to simulate the instant of laryngeal closure during swallowing. Fluid bolus dynamics were visualized with fluorescent dye from lateral, rear, front, and endoscopic directions to capture key hydrodynamic features leading to aspiration. Three influencing factors, fluid consistency, liquid dispensing site, and dispensing speed, were systemically evaluated on their roles in liquid aspirations. Results: Three aspiration mechanisms were identified, with liquid bolus entering the airway through (a) the interarytenoid notch (notch overflow), (b) cuneiform tubercle recesses (recess overflow), and (c) off-edge flow underneath the epiglottis (off-edge capillary flow). Of the three factors considered, liquid viscosity has the most significant impact on aspiration rate, followed by the liquid dispensing site and the dispensing speed. Water had one order of magnitude higher aspiration risks than 1% w/v methyl cellulose solution, a mildly thick liquid. Anterior dispensing had higher chances for aspiration than posterior oropharyngeal dispensing for both liquids and dispensing speeds considered. The effects of dispending speed varied. A lower speed increased aspiration for anterior-dispensed liquids due to increased off-edge capillary flows, while it significantly reduced aspiration for posterior-dispensed liquids due to reduced notch overflows. Visualizing swallowing hydrodynamics from multiple orientations facilitates detailed site-specific inspections of aspiration mechanisms.

Neuroendocrine cells initiate protective upper airway reflexes

Airway neuroendocrine (NE) cells have been proposed to serve as specialized sensory epithelial cells that modulate respiratory behavior by communicating with nearby nerve endings. However, their functional properties and physiological roles in the healthy lung, trachea, and larynx remain largely unknown. In this work, we show that murine NE cells in these compartments have distinct biophysical properties but share sensitivity to two commonly aspirated noxious stimuli, water and acid. Moreover, we found that tracheal and laryngeal NE cells protect the airways by releasing adenosine 5'-triphosphate (ATP) to activate purinoreceptive sensory neurons that initiate swallowing and expiratory reflexes. Our work uncovers the broad molecular and biophysical diversity of NE cells across the airways and reveals mechanisms by which these specialized excitable cells serve as sentinels for activating protective responses.

Sustained Effects of Capsaicin Infusion into the Oropharynx on Swallowing in Perfused Rats

OBJECTIVES

To examine the sustained effects of oropharyngeal capsaicin stimulation on the regulation of swallowing, we recorded the swallowing-related nerve activities during continuous infusion of capsaicin solution into the oropharynx.

METHODS

In 33 in situ perfused brainstem preparation of rats, we recorded the activities of the vagus, hypoglossal, and phrenic nerves during fictive swallowing. The interburst intervals (IBIs) of the swallowing-related nerves during sequential pharyngeal swallowing (sPSW) elicited by electrical stimulation of the superior laryngeal nerve (SLN) during concurrent capsaicin stimulation of 10, 1, and 0.1 μM (n = 28) were compared with those during oropharyngeal infusion of saline (control) (n = 5).

RESULTS

The IBIs during SLN-induced sPSW were reduced at 5 min after initiation of continuous infusion of 10 and 1 μM capsaicin solution. The IBIs showed significant decreases to -25.8 ± 6.9%, -25.9 ± 5.3, -18.3 ± 3.7, and -12.0 ± 1.6 at 30 min following 1 μM capsaicin stimulation at SLN stimulus conditions at 5 Hz of 1.2 times threshold, 10 Hz of 40 μA, 5 Hz of 60 μA, and 10 Hz of 60 μA, respectively. Continuous capsaicin stimulation of 0.1 μM solution did not show significant sustained effects.

CONCLUSION

Pharmacological stimulation of capsaicin could provide time-dependent effects on the likelihood of swallowing, particularly subserving sustained facilitation of swallowing reflex with appropriate concentration of capsaicin.

LEVEL OF EVIDENCE

NA Laryngoscope, 134:305-314, 2024.

Transient receptor potential channels as an emerging therapeutic target for oropharyngeal dysphagia

Oropharyngeal dysphagia is a serious health concern in older adults and patients with neurological disorders. Current oropharyngeal dysphagia management largely relies on compensatory strategies with limited efficacy. A long-term goal in swallowing/dysphagia-related research is the identification of pharmacological treatment strategies for oropharyngeal dysphagia. In recent decades, several pre-clinical and clinical studies have investigated the use of transient receptor potential (TRP) channels as a therapeutic target to facilitate swallowing. Various TRP channels are present in regions involved in the swallowing process. Animal studies have shown that local activation of these channels by their pharmacological agonists initiates swallowing reflexes; the number of reflexes increases when the dose of the agonist reaches a particular level. Clinical studies, including randomized clinical trials involving patients with oropharyngeal dysphagia, have demonstrated improved swallowing efficacy, safety, and physiology when TRP agonists are mixed with the food bolus. Additionally, there is evidence of plasticity development in swallowing-related neuronal networks in the brain upon TRP channel activation in peripheral swallowing-related regions. Thus, TRP channels have emerged as a promising target for the development of pharmacological treatments for oropharyngeal dysphagia.

Gustatory stimulus interventions for older adults with dysphagia: a scoping review

BACKGROUND

Gustatory stimulus interventions have been shown to improve swallowing function in older adults with dysphagia. However, the optimal intervention strategies as well as their effects and safety remain unclear.

AIMS

To explore current evidence regarding gustatory stimulus interventions for dysphagia in older adults.

METHODS

Nine electronic databases (PubMed, Web of Science, Embase, CINAHL, Cochrane Library, China National Knowledge Infrastructure, Wanfang Database, China Science and Technology Journal Database, and Sinomed) were searched from their inception to August 2022.

RESULTS

This review identified 263 articles, and 15 met the inclusion criteria. The types of gustatory stimulus interventions included spicy (n = 10), sour (n = 3), and mixed (sour-sweet) stimuli (n = 2), with most studies focusing on spicy stimuli. The most frequently reported spicy stimulus was capsaicin. Further, the most commonly reported intervention frequency was thrice a day before meals for 1-4 weeks. The stimuli concentrations and dosages could not be standardized due to the among-study heterogeneity. These studies reported 16 assessment tools and 42 outcomes, which mainly included videofluoroscopy and swallowing response time respectively. More than half of the included studies reported no adverse effects of gustatory stimulus interventions.

CONCLUSION AND DISCUSSIONS

Gustatory stimulus interventions improved swallowing function in older adults with dysphagia. However, assessment tools and outcomes for dysphagia should be standardized in the future, and explore personalized interventions based on different diseases and their stages, to determine the most cost-effective interventions, and to prevent its complications.

Pharmacological activation of transient receptor potential vanilloid 4 promotes triggering of the swallowing reflex in rats

The swallowing reflex is an essential physiological reflex that allows food or liquid to pass into the esophagus from the oral cavity. Delayed triggering of this reflex is a significant health problem in patients with oropharyngeal dysphagia for which no pharmacological treatments exist. Transient receptor potential channels have recently been discovered as potential targets to facilitate triggering of the swallowing reflex. However, the ability of transient receptor potential vanilloid 4 (TRPV4) to trigger the swallowing reflex has not been studied. Here, we demonstrate the involvement of TRPV4 in triggering the swallowing reflex in rats. TRPV4 immunoreactive nerve fibers were observed in the superior laryngeal nerve (SLN)-innervated swallowing-related regions. Retrograde tracing with fluorogold revealed localization of TRPV4 on approximately 25% of SLN-afferent neurons in the nodose-petrosal-jugular ganglionic complex. Among them, approximately 49% were large, 35% medium, and 15% small-sized SLN-afferent neurons. Topical application of a TRPV4 agonist (GSK1016790A) to the SLN-innervated regions dose-dependently facilitated triggering of the swallowing reflex, with the highest number of reflexes triggered at a concentration of 250 μM. The number of agonist-induced swallowing reflexes was significantly reduced by prior topical application of a TRPV4 antagonist. These findings indicate that TRPV4 is expressed on sensory nerves innervating the swallowing-related regions, and that its activation by an agonist can facilitate swallowing. TRPV4 is a potential pharmacological target for the management of oropharyngeal dysphagia.

Excitatory neurons in paraventricular hypothalamus contributed to the mechanism underlying acupuncture regulating the swallowing function

Paraventricular hypothalamus (PVH) is demonstrated to regulate stress, feeding behaviors, and other related homeostatic processes. However, no direct evidence has been investigated for the role of PVH in swallowing function. Acupuncture therapy at Lianquan (CV23) acupoint has been reported to improve the swallowing function in clinical trials, but its underlying mechanism still needs to be uncovered. Thus, we aimed to explore whether PVH involved the acupuncture mediated regulating swallowing function. Chemogenetics, electromyography (EMG) recording, and immunofluorescence staining methods were combined to demonstrate that neurons in PVH could be activated by electroacupuncture (EA) stimulation at CV23, and this neuronal cluster was represented as excitatory neurons. Furthermore, we mapped both the inputs and outputs of PVH neurons using viral tracing. The neurons in PVH projected with the brain regions, including parabrachial nucleus (PBN) and the solitary tract nucleus (NTS), which both participated in the swallowing process. The EA function regulating the swallowing was attenuated after inhibiting the neurons in PVH in the post stroke dysphagia. In conclusion, this study suggested that EA at CV23 could regulate swallowing function involving the excitatory neurons in PVH.

TRPA1s act as chemosensors but not as cold sensors or mechanosensors to trigger the swallowing reflex in rats

We examined the role of TRPA1s in triggering the swallowing reflex. TRPA1s predominantly localized on thin nerve fibers and fibroblast-like cells in swallowing-related regions and on small to medium-sized superior laryngeal nerve-afferents in the nodose–petrosal–jugular ganglionic complex. Topical application of a TRPA1 agonist, allyl isothiocyanate (AITC), dose-dependently triggered swallowing reflexes. Prior topical application of a TRPA1 antagonist significantly attenuated the AITC-induced reflexes. Application of cold AITC (4 °C) very briefly reduced the on-site temperature to < 17 °C (temperature at which TRPA1s can be activated), but had no effect on triggering of the reflex. By contrast, reducing the on-site temperature to < 17 °C for a longer time by continuous flow of cold AITC or by application of iced AITC paradoxically delayed/prevented the triggering of AITC-induced reflexes. Prior application of the TRPA1 antagonist had no effect on the threshold for the punctate mechanical stimuli-induced reflex or the number of low-force or high-force continuous mechanical pressure stimuli-induced reflexes. TRPA1s are functional and act as chemosensors, but not as cold sensors or mechanosensors, for triggering of the swallowing reflex. A brief cold stimulus has no effect on triggering of the reflex. However, a longer cold stimulus delays/prevents triggering of the reflex because of cold anesthesia.

Pharmacological Management of Dysphagia in Patients with Alzheimer's Disease: A Narrative Review

Alzheimer's disease (AD) and dysphagia are important health and socioeconomic problems in the aging population. Currently, the medical treatment of dysphagia in AD patients remains insufficient, and there are significant gaps in the management and clinical needs to postpone tube feeding. Literatures published over the last 30 years were searched in the PubMed and Embase databases. All relevant and promising pharmacological management studies were included. Because of the heterogeneity in design and methodology, only narrative reports were mentioned. Nine studies were included with two case reports, two case series, and two observational and three randomized controlled trials. The key approaches and clinical problems related to dysphagia include onset pattern, dementia stage, review of offending drugs and polypharmacy, and comorbidities (cerebrovascular disease, hypertension, parkinsonism, depression, and anorexia). The corresponding strategies of pharmacological treatments are further proposed and discussed comprehensively, with transient receptor potential channel modulators as promising treatment. With the integration of adequate and potential pharmacomanagement, AD patients with dysphagia can achieve a good prognosis and postpone tube feeding to maintain a better quality of life. More rigorous studies are needed to verify the effectiveness of innovative strategies and develop targets for neurostimulation.

Pivotal role of Transient Receptor Potential Channels in oral physiology

BACKGROUND

Transient Receptor Potential (TRP) Channels constitute a large family of non-selective permeable ion channels involved in the perception of environmental stimuli with a central and continuously expanding role in oral tissue homeostasis. Recent studies indicate the regulatory role of TRPs in pulp physiology, oral mucosa sensation, dental pain nociception and salivary gland secretion. This review provides an update on the diverse functions of TRP channels in the physiology of oral cavity, with emphasis on their cellular location, the underlying molecular mechanisms and clinical significance.

METHODS

A structured search of bibliographic databases (PubMed and MEDLINE) was performed for peer reviewed studies on TRP channels function on oral cavity physiology the last ten years. A qualitative content analysis was performed in screened papers and a critical discussion of main findings is provided.

RESULTS

TRPs expression has been detected in major cell types of the oral cavity, including odontoblasts, periodontal ligament, oral epithelial, salivary gland cells, and chondrocytes of temporomandibular joints, where they mediate signal perception and transduction of mechanical, thermal, and osmotic stimuli. They contribute to pulp physiology through dentin formation, mineralization, and periodontal ligament formation along with alveolar bone remodeling in dental pulp and periodontal ligament cells. TRPs are also involved in oral mucosa sensation, dental pain nociception, saliva secretion, swallowing reflex and temporomandibular joints' development.

CONCLUSION

Various TRP channels regulate oral cavity homeostasis, playing an important role in the transduction of external stimuli to intracellular signals in a cell type-specific manner and presenting promising drug targets for the development of pharmacological strategies to manage oral diseases.

Molecular and Neural Mechanism of Dysphagia Due to Cancer

Cancer is one of the most common causes of death worldwide. Along with the advances in diagnostic technology achieved through industry–academia partnerships, the survival rate of cancer patients has improved dramatically through treatments that include surgery, radiation therapy, and pharmacotherapy. This has increased the population of cancer “survivors” and made cancer survivorship an important part of life for patients. The senses of taste and smell during swallowing and cachexia play important roles in dysphagia associated with nutritional disorders in cancer patients. Cancerous lesions in the brain can cause dysphagia. Taste and smell disorders that contribute to swallowing can worsen or develop because of pharmacotherapy or radiation therapy; metabolic or central nervous system damage due to cachexia, sarcopenia, or inflammation can also cause dysphagia. As the causes of eating disorders in cancer patients are complex and involve multiple factors, cancer patients require a multifaceted and long-term approach by the medical care team.