Modulation of Slow Desensitization (Tachyphylaxis) of Acid-Sensing Ion Channel (ASIC)1a.

Among the proton-activated channels of the ASIC family, ASIC1a exhibits a specific tachyphylaxis phenomenon in the form of a progressive decrease in the response amplitude during a series of activations. This process is well known, but its mechanism is poorly understood. Here, we demonstrated a partial reversibility of this effect using long-term whole-cell recording of CHO cells transfected with rASIC1a cDNA. Thus, tachyphylaxis represents a slow desensitization of ASIC1a. Prolonged acidifications provided the same recovery from slow desensitization as short acidifications of the same frequency. Slow desensitization and steady-state desensitization are independent processes although the latter attenuates the development of the former. We found that drugs which facilitate ASIC1a activation (e.g., amitriptyline) cause an enhancement of slow desensitization, while inhibition of ASIC1a by 9-aminoacridine attenuates this process. Overall, for a broad variety of exposures, including increased calcium concentration, different pH conditions, and modulating drugs, we found a correlation between their effects on ASIC1a response amplitude and the development of slow desensitization. Thus, our results demonstrate that slow desensitization occurs only when ASIC1a is in the open state.

The short third intracellular loop and cytoplasmic tail of bitter taste receptors provide functionally relevant GRK phosphorylation sites in TAS2R14.

For most G protein-coupled receptors, the third intracellular loop (IL3) and carboxy-terminal tail (CT) are sites for G protein-coupled receptor kinase (GRK)-mediated phosphorylation, leading to ß-arrestin binding and agonist-specific desensitization. These regions of bitter taste receptors (TAS2Rs) are extremely short compared with the superfamily, and their function in desensitization is unknown. TAS2R14 expressed on human airway smooth muscle cells relax the cell, suggesting a novel target for bronchodilators. To assess IL3 and CT in agonist-promoted TAS2R14 desensitization (tachyphylaxis), we generated fusion proteins of both the WT sequence and Ala substituted for Ser/Thr in the IL3 and CT sequences. In vitro, activated GRK2 phosphorylated WT IL3 and WT CT proteins but not Ala-substituted forms. TAS2R14s with mutations in IL3 (IL-5A), CT (CT-5A), and in both regions (IL/CT-10A) were expressed in human embryonic kidney 293T cells. IL/CT-10A and CT-5A failed to undergo desensitization of the intracellular calcium response compared with WT, indicating that functional desensitization by GRK phosphorylation is at residues in the CT. Desensitization of TAS2R14 was blocked by GRK2 knockdown in human airway smooth muscle cells. Receptor:ß-arrestin binding was absent in IL/CT-10A and CT-5A and reduced in IL-5A, indicating a role for IL3 phosphorylation in the ß-arrestin interaction for this function. Agonist-promoted internalization of IL-5A and CT-5A receptors was impaired, and they failed to colocalize with early endosomes. Thus, agonist-promoted functional desensitization of TAS2R14 occurs by GRK phosphorylation of CT residues and ß-arrestin binding. However, ß-arrestin function in the internalization and trafficking of the receptor also requires GRK phosphorylation of IL3 residues.

Attenuation of Response to Repeated Drug Administration: A Proposal for Differentiating Tachyphylaxis and Tolerance.

Attenuation of drug response with repeated administration is referred to as tachyphylaxis or tolerance, though the distinction between these two is obscured through both their usage in the literature and imprecise definitions in common pharmacology texts. In this perspective, I propose that these terms be distinguished by the mechanisms underlying the attenuation of drug response. Specifically, tachyphylaxis should be reserved for attenuation that occurs in response to cellular depletion, whereas tolerance should be used to describe attenuation that arises from cellular adaptations. A framework for understanding behavioral tolerance, physiologic tolerance, and dispositional tolerance as distinct phenomena is also discussed. Using this framework, a classification of drugs exhibiting attenuation of drug response with repeated administration is presented. SIGNIFICANCE STATEMENT: Distinction between tachyphylaxis and tolerance is unclear in the literature. Nonetheless, a mechanistic basis for distinguishing these important terms has practical implications for managing or preventing attenuation of drug response with repeated administration.

Brolucizumab for pre-treated patients with choroidal neovascularization and signs of tachyphylaxis to aflibercept and bevacizumab.

PURPOSE: Treatment of choroidal neovascularization due to age-related macular degeneration is a challenging topic since an increasing number of patients show reduced morphological response to conventional treatment with intravitreal injections. The present study tested the hypothesis that the newly introduced anti-VEGF antibody brolucizumab does not only show promising results in pre-treated patients but is also a viable option in cases of tachyphylaxis to aflibercept or bevacizumab. METHODS: Thirty-six eyes of 34 patients with a history of at least 10 anti-VEGF injections as well as persistent retinal fluid following the past 5 monthly injections with aflibercept and bevacizumab prior to first treatment with brolucizumab were included in the study. Morphological and functional treatment response was compared before and after switching to brolucizumab. RESULTS: Mean best-corrected visual acuity did not significantly change after treatment with brolucizumab. In contrast, central retinal thickness significantly decreased 4 weeks after treatment with brolucizumab from 340.36 to 282.22 µm (p < 0.001) as well as pigment epithelial detachment from 346.73 to 280.47 µm (p < 0.001). In 24 eyes (66.67%), complete resolution of intra-and subretinal fluid was observed after a single dose of brolucizumab. No serious adverse events, such as intraocular inflammation and retinal vasculitis, were reported after a single injection of brolucizumab. CONCLUSION: Brolucizumab is not only effective in treatment-naïve patients as shown in the pivotal HAWK and Harrier trials, but also in pre-treated patients as seen in the present study. Our data also suggest that brolucizumab is potent in patients with signs of tachyphylaxis to other anti-VEGF agents and thus a viable treatment option.

Recovery from tachyphylaxis of TRPV1 coincides with recycling to the surface membrane.

The transient receptor potential vanilloid-1 (TRPV1) ion channel is essential for sensation of thermal and chemical pain. TRPV1 activation is accompanied by Ca2+-dependent desensitization; acute desensitization reflects rapid reduction in channel activity during stimulation, whereas tachyphylaxis denotes the diminution in TRPV1 responses to repetitive stimulation. Acute desensitization has been attributed to conformational changes of the TRPV1 channel; however, the mechanisms underlying the establishment of tachyphylaxis remain to be defined. Here, we report that the degree of whole-cell TRPV1 tachyphylaxis is regulated by the strength of inducing stimulation. Using light-sheet microscopy and pH-sensitive sensor pHluorin to follow TRPV1 endocytosis and exocytosis trafficking, we provide real-time information that tachyphylaxis of different degrees concurs with TRPV1 recycling to the plasma membrane in a proportional manner. This process controls TRPV1 surface expression level thereby the whole-cell nociceptive response. We further show that activity-gated TRPV1 trafficking associates with intracellular Ca2+ signals of distinct kinetics, and recruits recycling routes mediated by synaptotagmin 1 and 7, respectively. These results suggest that activity-dependent TRPV1 recycling contributes to the establishment of tachyphylaxis.

Treprostinil palmitil, an inhaled long-acting pulmonary vasodilator, does not show tachyphylaxis with daily dosing in rats.

BACKGROUND: Treprostinil palmitil (TP) is an inhaled long-acting pulmonary vasodilator prodrug of treprostinil (TRE) that has been formulated for delivery as a suspension (treprostinil palmitil inhalation suspension; TPIS) and as a dry powder (treprostinil palmitil inhalation powder; TPIP). In humans, tachyphylaxis is frequently observed with continuous intravenous (IV) or subcutaneous (SC) infusion of TRE and requires dosage escalation to maintain activity. The aim of the present study was to determine whether tachyphylaxis occurs with repeat daily administration of inhaled TPIS. METHODS: Experiments were performed in male Sprague-Dawley rats prepared with a telemetry probe implanted into the right ventricle to measure the change in right ventricular pulse pressure (ΔRVPP) induced by exposure to a 10% oxygen gas mixture. TPIS (6 mL) at concentrations of 0.25, 0.5, and 1 mM was given by nose-only inhalation using an Aeroneb Pro nebulizer, either as a single administration or daily for 16 or 32 consecutive days. In studies involving consecutive daily administrations of TPIS, the delivered TP dosage was 140.3 µg/kg at 1 mM and ranged from 40.2 to 72.2 µg/kg at 0.5 mM. A separate cohort of telemetered rats received continuous IV infusion of TRE via an Alzet mini-pump at a dosage rate of 250 ng/kg/min for 16 days. Blood and lung tissue samples were obtained, and the concentration of TRE in the plasma and TRE and TP in the lungs were measured approximately 1 h after TPIS administration. RESULTS: Dose-response studies with TPIS administered as a single administration inhibited the hypoxia-induced increase in RVPP in both a concentration-dependent (0.25, 0.5, and 1 mM) and time-dependent (1-24 h) manner. TPIS, given QD or BID at inhaled doses ranging from 40.2 to 140.3 µg/kg for 16 or 32 consecutive days, produced statistically significant (P < .05) inhibition of the increase of RVPP due to hypoxia over the full duration of the dosing periods. By contrast, the inhibition of the hypoxia-induced increase in RVPP observed with IV TRE infusion (250 ng/kg/min) disappeared after 16 days of infusion. The plasma concentrations of TRE were significantly higher after IV TRE (range, 2.85-13.35 ng/mL) compared to inhaled TPIS (range, 0.22-0.73 ng/mL) CONCLUSIONS: There was no evidence of tachyphylaxis with repeat daily dosing of TPIS for a period of up to 32 days. The absence of tachyphylaxis with TPIS is likely related to its local vasodilatory effects within the lungs, combined with an absence of sustained high plasma concentrations of TRE.

Persistent Toll-like receptor 7 stimulation induces behavioral and molecular innate immune tolerance.

Toll-like receptors 7 and 8 (TLR7 and TLR8) are endosomal pattern recognition receptors that detect a variety of single-stranded RNA species. While TLR7/8 agonists have robust therapeutic potential, clinical utility of these agents is limited by sickness responses associated with treatment induction. To understand the kinetics and mechanism of these responses, we characterized the acute and chronic effects of TLR7 stimulation. Single-cell RNA-sequencing studies, RNAscope, and radiolabeled in situ hybridization demonstrate that central nervous system gene expression of TLR7 is exclusive to microglia. In vitro studies demonstrate that microglia are highly sensitive to TLR7 stimulation, and respond in a dose-dependent manner to the imidazoquinoline R848. In vivo, both intraperitoneal (IP) and intracerebroventricular (ICV) R848 induce acute sickness responses including hypophagia, weight loss, and decreased voluntary locomotor activity, associated with increased CNS pro-inflammatory gene expression and changes to glial morphology. However, chronic daily IP R848 resulted in rapid tachyphylaxis of behavioral and molecular manifestations of illness. In microglial in vitro assays, pro-inflammatory transcriptional responses rapidly diminished in the context of repeated R848. In addition to TLR7 desensitization, we found that microglia become partially refractory to lipopolysaccharide (LPS) following R848 pretreatment, associated with induction of negative regulators A20 and Irak3. Similarly, mice pre-treated with R848 demonstrate reduced sickness responses, hypothalamic inflammation, and hepatic inflammation in response to LPS. These data combined demonstrate that TLR7 stimulation induces acute behavioral and molecular evidence of sickness responses. Following prolonged dosing, R848 induces a refractory state to both TLR7 and TLR4 activation, consistent with induced immune tolerance.

Tachyphylaxis during treatment of exudative age-related macular degeneration with aflibercept.

PURPOSE: At present, the standard treatment of neovascular age-related macular degeneration (AMD) is the repeated administration of antivascular endothelial growth factor (VEGF) agents. However, we often encounter patients who develop tachyphylaxis for anti-VEGF agents. In this study, we investigated the characteristics of patients who developed tachyphylaxis on repeated intravitreal aflibercept (IVA) injections for neovascular AMD and the frequency of tachyphylaxis. METHODS: Three hundred thirteen eyes (313 patients) with treatment-naïve AMD who achieved resolution soon after starting IVA and were followed up for ≥ 12 months were enrolled in this retrospective, interventional, consecutive case series. The eyes were investigated for tachyphylaxis to aflibercept. Tachyphylaxis was defined as absence of any improvement (more than 100 µm) in or worsening of CRT within 1 month after more than two repeated monthly IVA injections when the exudative change remained. RESULTS: Twenty-eight (8.9%) of the 313 eyes developed tachyphylaxis (occult with no classic, n = 14; polypoidal choroidal vasculopathy, n = 14) at an annual rate of about 3%. The mean number of IVA injections was 10.5 ± 7.8, and the mean interval until tachyphylaxis was 20.9 ± 14.0 months. There was a significant difference in the AMD subtypes between the group with tachyphylaxis and the group without it (p = 0.0029). Occult with no classic type and polypoidal choroidal vasculopathy were the only AMD subtypes in the eyes with tachyphylaxis. In the analysis of the eyes that had occult with no classic or polypoidal choroidal vasculopathy, only intraretinal edema was significantly less common (p = 0.042). A combination of photodynamic therapy and aflibercept was effective in 13 (87%) of 15 eyes with tachyphylaxis, and switching to intravitreal ranibizumab was effective in 5 (56%) of 9 eyes. CONCLUSIONS: Tachyphylaxis occurs after repeated IVA injections in a minority of patients with AMD for a long term and is more likely to occur in eyes with lesions beneath the retinal pigment epithelium and no intraretinal edema. Treatment of AMD should be performed keeping this fact in mind, while considering the consecutive treatment.

Acidosis counteracts itch tachyphylaxis to consecutive pruritogen exposure dependent on acid-sensing ion channel 3.

Tachyphylaxis of itch refers to a markedly reduced scratching response to consecutive exposures of a pruritogen, a process thought to protect against tissue damage by incessant scratching and to become disrupted in chronic itch. Here, we report that a strong stimulation of the Mas-related G-protein-coupled receptor C11 by its agonist, Ser-Leu-Ile-Gly-Arg-Leu-NH2 (SL-NH2) or bovine adrenal medulla 8-22 peptide, via subcutaneous injection in mice induces tachyphylaxis to the subsequent application of SL-NH2 to the same site. Notably, co-application of acid and SL-NH2 following the initial injection of the pruritogen alone counteracted itch tachyphylaxis by augmenting the scratching behaviors in wild-type but not in acid-sensing ion channel 3-null, animals. Using an activity-dependent silencing strategy, we identified that acid-sensing ion channel 3-mediated itch enhancement mainly occurred via the Mas-related G-protein-coupled receptor C11-responsive sensory neurons. Together, our results indicate that acid-sensing ion channel 3, activated by concomitant acid and certain pruritogens, constitute a novel signaling pathway that counteracts itch tachyphylaxis to successive pruritogenic stimulation, which likely contributes to chronic itch associated with tissue acidosis.

An Allosteric Potentiator of the Dopamine D1 Receptor Increases Locomotor Activity in Human D1 Knock-In Mice without Causing Stereotypy or Tachyphylaxis.

Allosteric potentiators amplify the sensitivity of physiologic control circuits, a mode of action that could provide therapeutic advantages. This hypothesis was tested with the dopamine D1 receptor potentiator DETQ [2-(2,6-dichlorophenyl)-1-((1S,3R)-3-(hydroxymethyl)-5-(2-hydroxypropan-2-yl)-1-methyl-3,4-dihydroisoquinolin-2(1H)-yl)ethan-1-one]. In human embryonic kidney 293 (HEK293) cells expressing the human D1 receptor, DETQ induced a 21-fold leftward shift in the cAMP response to dopamine, with a Kb of 26 nM. The maximum response to DETQ alone was ∼12% of the maximum response to dopamine, suggesting weak allosteric agonist activity. DETQ was ∼30-fold less potent at rat and mouse D1 receptors and was inactive at the human D5 receptor. To enable studies in rodents, an hD1 knock-in mouse was generated. DETQ (3-20 mg/kg orally) caused a robust (∼10-fold) increase in locomotor activity (LMA) in habituated hD1 mice but was inactive in wild-type mice. The LMA response to DETQ was blocked by the D1 antagonist SCH39166 and was dependent on endogenous dopamine. LMA reached a plateau at higher doses (30-240 mg/kg) even though free brain levels of DETQ continued to increase over the entire dose range. In contrast, the D1 agonists SKF 82958, A-77636, and dihydrexidine showed bell-shaped dose-response curves with a profound reduction in LMA at higher doses; video-tracking confirmed that the reduction in LMA caused by SKF 82958 was due to competing stereotyped behaviors. When dosed daily for 4 days, DETQ continued to elicit an increase in LMA, whereas the D1 agonist A-77636 showed complete tachyphylaxis by day 2. These results confirm that allosteric potentiators may have advantages compared with direct-acting agonists.

Targeting VEGF in eye neovascularization: What's new?: A comprehensive review on current therapies and oligonucleotide-based interventions under development.

Roughly ten years ago the FDA approved most of the presently used anti-VEGF drugs for the treatment of neovascular AMD and other eye pathologies characterized by ocular neoangiogenesis. However, the recent findings on the physiologic activities of VEGF isoforms impose to reconsider the inhibitory effects of pan-VEGF antagonists and the concept that to face pathological alterations at ocular level is possible only through the full block of all VEGF isoforms. In fact, although pan-VEGF agents rapidly and effectively contrast ocular neovascularization, vascular leakage, and other pathological changes, in the long-term the inhibition of all VEGF isoforms likely may result in the loss of the physiologic effects exerted by VEGF121 and the anti-angiogenic VEGF165b. Notably, selective inhibitors of VEGF165a, such as pegaptanib, spare these targets. Moreover, preclinical and clinical evidence suggests that also systemic side effects, secondary to intraocular treatment with non-selective anti-VEGF drugs, may be reinterpreted in light of these recent findings, which may be useful to clinicians for the choice of the most appropriate anti-VEGF agent. Another aspect that should be considered is the involvement of VEGF-independent pathways in ocular neovascularization, therefore a combined therapy can represent a more effective pharmacological approach that might help also to counteract tachyphylaxis, an important issue in anti-VEGF treatment. This complex picture and the recent findings on current anti-VEGF drugs should be therefore taken into account to guide the development of novel agents targeting VEGF and/or other key factors involved in the pathogenesis of neovascular ocular diseases along the signaling pathways stimulated by the various isoforms. Accordingly, this review also reports on novel pharmacological molecules targeting VEGF at ocular level and currently under development, with a special attention to oligonucleotide-based interventions.

Tachyphylaxis to local anaesthetics. What is the clinical evidence? A systematic review.

BACKGROUND: Tachyphylaxis or acute tolerance to local anaesthetics has been reported, but the prevalence in clinical analgesia is obscure, and the mechanisms behind this phenomenon remain unclear. We sought to examine the clinical significance of tachyphylaxis from the available literature. METHODS: We performed a systematic review of the literature utilising the databases PubMed and Embase employing the search terms [Tachyphylaxis AND Local Anaesthetics AND Human] AND [Tolerance AND Local Anaesthetics AND Human]. RESULTS: A total of 66 records were identified. Thirty-four articles were assessed in full text for eligibility. Twenty studies were considered relevant for qualitative analyses, but only six studies were randomised controlled trials. Because of the heterogeneity of the randomised controlled trials, it was not possible to conduct a meta-analysis. CONCLUSION: Studies documenting tachyphylaxis with clinical use of local anaesthetics are surprisingly scarce, and the mechanisms behind it remain unclear.

Structural domains underlying the activation of acid-sensing ion channel 2a.

The acid-sensing ion channels (ASICs) are a family of ion channels expressed throughout the mammalian nervous system. The principal activator of ASICs is extracellular protons, and ASICs have been demonstrated to play a significant role in many physiologic and pathophysiologic processes, including synaptic transmission, nociception, and fear. However, not all ASICs are proton-sensitive: ASIC2a is activated by acid, whereas its splice variant ASIC2b is not. We made a series of chimeric ASIC2 proteins, and using whole-cell electrophysiology we have identified the minimal region of the ASIC2a extracellular domain that is required for ASIC2 proton activation: the first 87 amino acids after transmembrane domain 1. We next examined the function of different domains within the ASIC2b N-terminus and identified a region proximal to the first transmembrane domain that confers tachyphylaxis upon ASIC2a. We have thus identified domains of ASIC2 that are crucial to channel function and may be important for the function of other members of the ASIC family.

Adverse effects of topical corticosteroids in paediatric eczema: Australasian consensus statement.

Atopic eczema is a chronic inflammatory disease affecting about 30% of Australian and New Zealand children. Severe eczema costs over AUD 6000/year per child in direct medical, hospital and treatment costs as well as time off work for caregivers and untold distress for the family unit. In addition, it has a negative impact on a child's sleep, education, development and self-esteem. The treatment of atopic eczema is complex and multifaceted but a core component of therapy is to manage the inflammation with topical corticosteroids (TCS). Despite this, TCS are often underutilised by many parents due to corticosteroid phobia and unfounded concerns about their adverse effects. This has led to extended and unnecessary exacerbations of eczema for children. Contrary to popular perceptions, (TCS) use in paediatric eczema does not cause atrophy, hypopigmentation, hypertrichosis, osteoporosis, purpura or telangiectasia when used appropriately as per guidelines. In rare cases, prolonged and excessive use of potent TCS has contributed to striae, short-term hypothalamic-pituitary-adrenal axis alteration and ophthalmological disease. TCS use can also exacerbate periorificial rosacea. TCS are very effective treatments for eczema. When they are used to treat active eczema and stopped once the active inflammation has resolved, adverse effects are minimal. TCS should be the cornerstone treatment of atopic eczema in children.