Immunocytochemical analysis of P2X2 in rat circumvallate taste buds

Abstract

BackgroundOur laboratory has shown that classical synapses and synaptic proteins are associated with Type III cells. Yet it is generally accepted that Type II cells transduce bitter, sweet and umami stimuli. No classical synapses, however, have been found associated with Type II cells. Recent studies indicate that the ionotropic purinergic receptors P2X2/P2X3 are present in rodent taste buds. Taste nerve processes express the ionotropic purinergic receptors (P2X2/P2X3). P2X2/P2X3Dbl−/− mice are not responsive to sweet, umami and bitter stimuli, and it has been proposed that ATP acts as a neurotransmitter in taste buds. The goal of the present study is to learn more about the nature of purinergic contacts in rat circumvallate taste buds by examining immunoreactivity to antisera directed against the purinergic receptor P2X2.ResultsP2X2-like immunoreactivity is present in intragemmal nerve processes in rat circumvallate taste buds. Intense immunoreactivity can also be seen in the subgemmal nerve plexuses located below the basal lamina. The P2X2 immunoreactive nerve processes also display syntaxin-1-LIR. The immunoreactive nerves are in close contact with the IP3R3-LIR Type II cells and syntaxin-1-LIR and/or 5-HT-LIR Type III cells. Taste cell synapses are observed only from Type III taste cells onto P2X2-LIR nerve processes. Unusually large, “atypical” mitochondria in the Type II taste cells are found only at close appositions with P2X2-LIR nerve processes. P2X2 immunogold particles are concentrated at the membranes of nerve processes at close appositions with taste cells.ConclusionsBased on our immunofluorescence and immunoelectron microscopical studies we believe that both perigemmal and most all intragemmal nerve processes display P2X2-LIR. Moreover, colloidal gold immunoelectron microscopy indicates that P2X2-LIR in nerve processes is concentrated at sites of close apposition with Type II cells. This supports the hypothesis that ATP may be a key neurotransmitter in taste transduction and that Type II cells release ATP, activating P2X2 receptors in nerve processes.

Three-dimensional architectures of P2X2-/P2X3-immunoreactive afferent nerve terminals in the rat carotid body as revealed by confocal laser scanning microscopy

“…P2X2-/P2X3-immunoreactive terminals may be effective structures for intercellular regulation between adjacent type I cells. The hederiform appearance of P2X2-/P2X3-immunoreactive nerve endings in the CB morphologically resembled those of other sensory cell-nerve complexes, such as P2X3-immunoreactive nerve endings around small intensely fluorescent cells in the superior cervical ganglion (Takaki et al 2015), ramified nerve endings with P2X2 and/or P2X3 immunoreactivities in the neuroepithelial body (Brouns et al 2009), P2X2-/P2X3-immunoreactive ramified nerve endings associated with taste cells in the lingual taste buds (Kataoka et al 2006;Yang et al 2012), and P2X3-immunoreactive nerve endings within laryngeal chemosensory cell clusters (Takahashi et al 2016). The branching pattern of sensory nerve endings with terminal swelling may be common characteristics in sensory cell-nerve complexes.…”

Taste buds: cells, signals and synapses

“…Even in the absence of synaptic clefts between type II cells and afferent fibres, ATP secreted into the confined extracellular spaces reaches sufficient concentrations to stimulate P2X receptors on nearby afferent fibres 100 . Interestingly, large ‘atypical’ mitochondria are located directly below the plasma membrane of type II cells near sites where innervating afferent fibres pass in close apposition 111 . These mitochondria are optimally positioned as potential sources of presynaptic ATP for afferent transmission.…”

A review of the associations between single nucleotide polymorphisms in taste receptors, eating behaviors, and health

“…As mentioned, Type II cells do not exhibit typical synapses. Instead, Type II cells respond to stimuli by releasing adenosine triphosphate (ATP) and acetylcholine through hemichannels Finger et al, 2005;Huang et al, 2007;Romanov et al, 2007) which, in turn, activate purinergic receptors on cranial nerve fibers (Bo et al, 1999;Dando & Roper, 2012;Ogura, 2002;Yang, Montoya, Bond, Walton, & Kinnamon, 2012). Type III cells, however, respond to stimulation by releasing serotonin, norepinephrine and L-aminobutyric acid (Cao, Zhao, Kolli, Hivley, & Herness, 2009;Dvoryanchikov, Huang, Barro-Soria, Chaudhari, & Roper, 2011;Huang, Dando, & Roper, 2009;Huang, Maruyama, Stimac, & Roper, 2008;Huang, Pereira, & Roper, 2011;Obata, Shimada, Sakai, & Saito, 1997).…”