Action Potential–Enhanced ATP Release From Taste Cells Through Hemichannels

Abstract

Type II taste cells express many taste transduction molecules but lack well-elaborated synapses, bringing into question the functional significance of action potentials in these cells. We examined the dependence of adenosine triphosphate (ATP) transmitter release from taste cells on action potentials. To identify type II taste cells we used mice expressing a green fluorescence protein (GFP) transgene from the ␣-gustducin promoter. Action potentials were recorded by an electrode basolaterally attached to a single GFP-positive taste cell. We monitored ATP release from gustducin-expressing taste cells by collecting the electrode solution immediately after tastant-stimulated action potentials and using a luciferase assay to quantify ATP. Stimulation of gustducin-expressing taste cells with saccharin, quinine, or glutamate on the apical membrane increased ATP levels in the electrode solution; the amount of ATP depended on the firing rate. Increased spontaneous firing rates also induced ATP release from gustducin-expressing taste cells. ATP release from gustducin-expressing taste cells was depressed by tetrodotoxin and inhibited below the detection limit by carbenoxolone. Our data support the hypothesis that action potentials in taste cells responsive to sweet, bitter, or umami tastants enhance ATP release through pannexin 1, not connexin-based hemichannels.

Dispensable ATP permeability of Pannexin 1 channels in a heterologous system and in mammalian taste cells

“…This point of view is strongly supported by the data of Murata et al (Murata et al, 2010), which assayed ATP release from an individual taste cell embedded in a fungiform bud that was still attached to a piece of the lingual epithelium. When applied to a taste pore, tastants stimulated ATP release that was markedly inhibited by TTX, a specific blocker of VG Na + channels (Murata et al, 2010). In experiments with isolated CV taste cells, TTX affected ATP secretion insignificantly (Huang and Roper, 2010).…”

“…Although it is widely accepted that ATP-permeable channels in type II cells are formed by Panx1, no direct evidence has been obtained so far in support of this idea but solely inhibitory effects of CBX on ATP release (Huang et al, 2007;Dando and Roper, 2009;Murata et al, 2010). The existing biophysical and pharmacological data on Panx1 channels were basically obtained in studies of recombinant Panx1 that was expressed mainly in Xenopus oocytes and HEK-293 cells (e.g.…”

“…In the taste bud, Panx1 is expressed in taste cells of all types, including primary chemosensory cells of the type II (Huang et al, 2007;Romanov et al, 2007). Taking into account this finding and strong inhibitory effects of carbenoxolone on ATP release initiated by gustatory stimuli in the taste bud (Huang et al, 2007, Huang et al, 2011Murata et al, 2010), it is believed that Panx1 channels are mainly responsible for ATP secretion. However, this inference has not been confirmed yet with a physiological analysis of Panx1-null mice.…”

Taste perception, associated hormonal modulation, and nutrient intake

“…41,42 Type II cells also express voltage-gated sodium and potassium channels that mediate the secretion of ATP as a function of action potential firing rate. 43 Upon secretion, ATP acts as a neurotransmitter on nearby sensory afferent fibers 44,45 ; ATP also acts as a paracrine/autocrine hormone, binding with receptors expressed on neighboring taste receptor cells. [46][47][48][49][50] Type III cells release serotonin, c-amino butyric acid, and norepinephrine and are most notable for possessing synapses.…”

Gustatory Signaling in the Periphery: Detection, Transmission, and Modulation of Taste Information

“…62) Taste cells release ATP in response to serial depolarization 63) or sweet, bitter, and umami taste stimuli. 64,65) ATP release from taste cells was blocked by a hemichannel blocker, carbenoxolone. 64,65) Therefore the signal transmission from taste cells to gustatory nerve fibers for sweet, bitter, and umami taste may be following: 1) taste cells activated by sweet, bitter, or umami substances increase in [Ca 2ϩ ] i and generate action potentials; 2) Ca 2ϩ and depolarization stimulate hemichannels (possibly the pannexin-1 hemichannel) to open and release ATP; and 3) released ATP activates P2X 2 /P2X 3 receptors on the adjacent gustatory nerve fibers (Fig.…”