Postsynaptic P2X3‐containing receptors in gustatory nerve fibres mediate responses to all taste qualities in mice

Abstract

Key pointsr Acute inhibition of purinergic receptors with a selective P2X3 antagonist prevents transmission of information from taste buds to sensory nerves.r The P2X3 antagonist has no effect on taste-evoked release of ATP, confirming the effect is postsynaptic.r The results confirm previous results with P2X2/3 double knockout mice that ATP is required for transmission of all taste qualities, including sour and salty. Previously, ATP was confirmed to be required for bitter, sweet and umami tastes, but was questioned for salty and sour tastes due to pleomorphic deficits in the double knockout mice.r The geniculate ganglion in mouse contains two populations of ganglion cells with different subunit composition of P2X2 and P2X3 receptors making them differently susceptible to pharmacological block and, presumably, desensitization.Abstract Taste buds release ATP to activate ionotropic purinoceptors composed of P2X2 and P2X3 subunits, present on the taste nerves. Mice with genetic deletion of P2X2 and P2X3 receptors (double knockout mice) lack responses to all taste stimuli presumably due to the absence of ATP-gated receptors on the afferent nerves. Recent experiments on the double knockout mice showed, however, that their taste buds fail to release ATP, suggesting the possibility of pleiotropic deficits in these global knockouts. To test further the role of postsynaptic P2X receptors in afferent signalling, we used AF-353, a selective antagonist of P2X3-containing receptors to inhibit the receptors acutely during taste nerve recording and behaviour. The specificity of AF-353 for P2X3-containing receptors was tested by recording Ca 2+ transients to exogenously applied ATP in fura-2 loaded isolated geniculate ganglion neurons from wild-type and P2X3 knockout mice. ATP responses were completely inhibited by 10 μM or 100 μM AF-353, but neither concentration blocked responses in P2X3 single knockout mice wherein the ganglion cells express only P2X2-containing receptors. Furthermore, AF-353 had no effect on taste-evoked ATP release from taste buds. In wild-type mice, I.P. injection of AF-353 or simple application of the drug directly to the tongue, inhibited taste nerve responses to all taste qualities in a dose-dependent fashion. A brief access behavioural assay confirmed the electrophysiological results and showed that preference for a synthetic sweetener, SC-45647, was abolished following I.P. injection of AF-353. These data indicate that activation of P2X3-containing receptors is required for transmission of all taste qualities.

Progress and renewal in gustation: new insights into taste bud development

“…Among the taste cell types, only Type III cells form conventional synapses on sensory nerve fibers (Yang et al, 2000(Yang et al, , 2004; they release GABA and serotonin, which are likely to be neuromodulators rather than neurotransmitters (Chaudhari, 2014). By contrast, ATP released by Type II cells functions as a bona fide neurotransmitter, as it is required for the transmission of all taste information from taste buds to sensory nerves (Finger et al, 2005;Vandenbeuch et al, 2015). The source of the ATP required for Type III cell signaling to afferent fibers is not known.…”

“…Type I cells express membrane-localized NTPDase2 (Entpd2), an ectoATPase that converts ATP to ADP. Type II cells use ATP as a neurotransmitter to signal to sensory nerves (Finger et al, 2005;Vandenbeuch et al, 2015), yet Type II cells lack presynaptic specializations; rather, Type II cells release ATP in a non-vesicular manner (Huang et al, 2007;Romanov et al, 2013Romanov et al, , 2007, probably via CALMH1 ion channels (Taruno et al, 2013). Thus, NTPDase2-expressing Type I cells are likely to clear excess ATP released by Type II cells to ensure efficient neurotransmission (Bartel et al, 2006;Finger et al, 2005;Vandenbeuch et al, 2013).…”

P2X receptors

“…[56]. ATP release by taste buds is the first step in all modalities of taste sensation: the ATP activates P2X2/P2X3-subunit-containing receptors on the gustatory nerve [57]. An analogous signalling role from urothelium to primary afferent nerves has been inferred for sensing bladder distension [58].…”

“…One confounding factor with such trials of P2X3 receptor antagonists will be the difficulty in conducting 'blind' trials. ATP is a transmitter released by taste buds and it activates P2X3-subunit-containing receptors on gustatory nerves [57], and all the patients in the trial who took AF-219 reported disturbance of taste [99].…”

Diversity in cell motility reveals the dynamic nature of the formation of zebrafish taste sensory organs

“…Upon stimulation, taste bud cells, which are dedicated to specific taste qualities, release transmitters (Chaudhari and Roper, 2010;Roper, 2013), including ATP (Finger et al, 2005;Taruno et al, 2013) that bind to their corresponding receptors (Huang et al, 2011b;Kinnamon and Finger, 2013) and transmit information to sensory afferent fibers (Barretto et al, 2015;Vandenbeuch et al, 2015). Taste bud cells communicate with each other [i.e.…”