Topical application of a P2X2/P2X3 purine receptor inhibitor suppresses the bitter taste of medicines and other taste qualities.

BACKGROUND AND PURPOSE: Many medications taste intensely bitter. The innate aversion to bitterness affects medical compliance, especially in children. There is a clear need to develop bitter blockers to suppress the bitterness of vital medications. Bitter taste is mediated by TAS2R receptors. Because different pharmaceutical compounds activate distinct sets of TAS2Rs, targeting specific receptors may only suppress bitterness for certain, but not all, bitter-tasting compounds. Alternative strategies are needed to identify universal bitter blockers that will improve the acceptance of every medication. Taste cells in the mouth transmit signals to afferent gustatory nerve fibres through the release of ATP, which activates the gustatory nerve-expressed purine receptors P2X2/P2X3. We hypothesized that blocking gustatory nerve transmission with P2X2/P2X3 inhibitors (e.g. 5-(5-iodo-4-methoxy-2-propan-2-ylphenoxy)pyrimidine-2,4-diamine [AF-353]) would reduce bitterness for all medications and bitter compounds. EXPERIMENTAL APPROACH: Human sensory taste testing and mouse behavioural analyses were performed to determine if oral application of AF-353 blocks perception of bitter taste and other taste qualities but not non-gustatory oral sensations (e.g. tingle). KEY RESULTS: Rinsing the mouth with AF-353 in humans or oral swabbing it in mice suppressed the bitter taste and avoidance behaviours of all compounds tested. We further showed that AF-353 suppressed other taste qualities (i.e. salt, sweet, sour and savoury) but had no effects on other oral or nasal sensations (e.g, astringency and oral tingle). CONCLUSION AND IMPLICATIONS: This is the first time a universal, reversible taste blocker in humans has been reported. Topical application of P2X2/P2X3 inhibitor to suppress bitterness may improve medical compliance.

Hindbrain ghrelin and liver-expressed antimicrobial peptide 2, ligands for growth hormone secretagogue receptor, bidirectionally control food intake.

Hindbrain growth hormone secretagogue receptor (GHSR) agonism increases food intake, yet the underlying neural mechanisms remain unclear. The functional effects of hindbrain GHSR antagonism by its endogenous antagonist liver-expressed antimicrobial peptide 2 (LEAP2) are also yet unexplored. To test the hypothesis that hindbrain GHSR agonism attenuates the food intake inhibitory effect of gastrointestinal (GI) satiation signals, ghrelin (at a feeding subthreshold dose) was administered to the fourth ventricle (4V) or directly to the nucleus tractus solitarius (NTS) before systemic delivery of the GI satiation signal cholecystokinin (CCK). Also examined, was whether hindbrain GHSR agonism attenuated CCK-induced NTS neural activation (c-Fos immunofluorescence). To investigate an alternate hypothesis that hindbrain GHSR agonism enhances feeding motivation and food seeking, intake stimulatory ghrelin doses were administered to the 4V and fixed ratio 5 (FR-5), progressive ratio (PR), and operant reinstatement paradigms for palatable food responding were evaluated. Also assessed were 4V LEAP2 delivery on food intake and body weight (BW) and on ghrelin-stimulated feeding. Both 4V and NTS ghrelin blocked the intake inhibitory effect of CCK and 4V ghrelin blocked CCK-induced NTS neural activation. Although 4V ghrelin increased low-demand FR-5 responding, it did not increase high-demand PR or reinstatement of operant responding. Fourth ventricle LEAP2 reduced chow intake and BW and blocked hindbrain ghrelin-stimulated feeding. Data support a role for hindbrain GHSR in bidirectional control of food intake through mechanisms that include interacting with the NTS neural processing of GI satiation signals but not food motivation and food seeking.

Hypophagia induced by salmon calcitonin, but not by amylin, is partially driven by malaise and is mediated by CGRP neurons.

OBJECTIVE: The behavioral mechanisms and the neuronal pathways mediated by amylin and its long-acting analog sCT (salmon calcitonin) are not fully understood and it is unclear to what extent sCT and amylin engage overlapping or distinct neuronal subpopulations to reduce food intake. We here hypothesize that amylin and sCT recruit different neuronal population to mediate their anorectic effects. METHODS: Viral approaches were used to inhibit calcitonin gene-related peptide (CGRP) lateral parabrachial nucleus (LPBN) neurons and assess their role in amylin's and sCT's ability to decrease food intake in mice. In addition, to test the involvement of LPBN CGRP neuropeptidergic signaling in the mediation of amylin and sCT's effects, a LPBN site-specific knockdown was performed in rats. To deeper investigate whether the greater anorectic effect of sCT compared to amylin is due do the recruitment of additional neuronal pathways related to malaise multiple and distinct animal models tested whether amylin and sCT induce conditioned avoidance, nausea, emesis, and conditioned affective taste aversion. RESULTS: Our results indicate that permanent or transient inhibition of CGRP neurons in LPBN blunts sCT-, but not amylin-induced anorexia and neuronal activation. Importantly, sCT but not amylin induces behaviors indicative of malaise including conditioned affective aversion, nausea, emesis, and conditioned avoidance; the latter mediated by CGRPLPBN neurons. CONCLUSIONS: Together, the present study highlights that although amylin and sCT comparably decrease food intake, sCT is distinctive from amylin in the activation of anorectic neuronal pathways associated with malaise.

GIP Receptor Agonism Attenuates GLP-1 Receptor Agonist-Induced Nausea and Emesis in Preclinical Models.

Glucagon-like peptide 1 receptor (GLP-1R) agonists decrease body weight and improve glycemic control in obesity and diabetes. Patient compliance and maximal efficacy of GLP-1 therapeutics are limited by adverse side effects, including nausea and emesis. In three different species (i.e., mice, rats, and musk shrews), we show that glucose-dependent insulinotropic polypeptide receptor (GIPR) signaling blocks emesis and attenuates illness behaviors elicited by GLP-1R activation, while maintaining reduced food intake, body weight loss, and improved glucose tolerance. The area postrema and nucleus tractus solitarius (AP/NTS) of the hindbrain are required for food intake and body weight suppression by GLP-1R ligands and processing of emetic stimuli. Using single-nuclei RNA sequencing, we identified the cellular phenotypes of AP/NTS cells expressing GIPR and GLP-1R on distinct populations of inhibitory and excitatory neurons, with the greatest expression of GIPR in γ-aminobutyric acid-ergic neurons. This work suggests that combinatorial pharmaceutical targeting of GLP-1R and GIPR will increase efficacy in treating obesity and diabetes by reducing nausea and vomiting.

GDF15 Induces an Aversive Visceral Malaise State that Drives Anorexia and Weight Loss.

The anorectic and weight-suppressive effects of growth differentiation factor-15 (GDF15) are attracting considerable attention for treating obesity. Current experiments in rats investigate whether GDF15 induces an aversive visceral malaise-based state that mediates its acute anorectic effect and, through aversion conditioning, exerts longer-term anorexia. Visceral malaise, conditioned affective food responses (taste reactivity), gastric emptying (GE), food intake, and body weight are evaluated after acute and chronic systemic dosing of GDF15 or long-acting Fc-GDF15. Pica, a marker of visceral malaise, is present at all anorectic GDF15 doses. Moreover, malaise induced by GDF15 does not decline over time, suggesting the lack of an improved tolerance after prolonged exposure. One association between GDF15 and novel food conditions a disgust/aversive response that persists beyond GDF15 acute action. Delayed GE is not a requirement for GDF15-induced anorexia. Clinical studies are required to evaluate whether GDF15's aversive-state-based anorexia will be contraindicated as an obesity treatment.

GDF15 Induces Anorexia through Nausea and Emesis.

Growth differentiation factor 15 (GDF15) is a cytokine that reduces food intake through activation of hindbrain GFRAL-RET receptors and has become a keen target of interest for anti-obesity therapies. Elevated endogenous GDF15 is associated with energy balance disturbances, cancer progression, chemotherapy-induced anorexia, and morning sickness. We hypothesized that GDF15 causes emesis and that its anorectic effects are related to this function. Here, we examined feeding and emesis and/or emetic-like behaviors in three different mammalian laboratory species to help elucidate the role of GDF15 in these behaviors. Data show that GDF15 causes emesis in Suncus murinus (musk shrews) and induces behaviors indicative of nausea/malaise (e.g., anorexia and pica) in non-emetic species, including mice and lean or obese rats. We also present data in mice suggesting that GDF15 contributes to chemotherapy-induced malaise. Together, these results indicate that GDF15 triggers anorexia through the induction of nausea and/or by engaging emetic neurocircuitry.