Intracellular free calcium concentrations in single taste receptor cells in the guinea pig

Calretinin immunoreactivity in taste buds and afferent fibers of the grey mullet Chelon labrosus

The presence of the calcium-binding protein calretinin in taste buds of a teleost, the thick-lipped grey mullet, was investigated using immunohistochemical techniques. Taste bud sensory cells had calretinin immunoreactivity. The nerve fiber plexus innervating taste buds, the ganglia and the viscerosensory roots projecting to the vagal lobe, also showed calretinin immunoreactivity. These results demonstrate for the first time the occurrence of calretinin in the taste buds and the taste afferent system of a teleost.

Bitter taste transduced by PLC-beta(2)-dependent rise in IP(3) and alpha-gustducin-dependent fall in cyclic nucleotides

Current evidence points to the existence of multiple processes for bitter taste transduction. Previous work demonstrated involvement of the polyphosphoinositide system and an alpha-gustducin (Galpha(gust))-mediated stimulation of phosphodiesterase in bitter taste transduction. Additionally, a taste-enriched G protein gamma-subunit, Ggamma(13), colocalizes with Galpha(gust) and mediates the denatonium-stimulated production of inositol 1,4,5-trisphosphate (IP(3)). Using quench-flow techniques, we show here that the bitter stimuli, denatonium and strychnine, induce rapid (50-100 ms) and transient reductions in cAMP and cGMP and increases in IP(3) in murine taste tissue. This decrease of cyclic nucleotides is inhibited by Galpha(gust) antibodies, whereas the increase in IP(3) is not affected by antibodies to Galpha(gust). IP(3) production is inhibited by antibodies specific to phospholipase C-beta(2) (PLC-beta(2)), a PLC isoform known to be activated by Gbetagamma-subunits. Antibodies to PLC-beta(3) or to PLC-beta(4) were without effect. These data suggest a transduction mechanism for bitter taste involving the rapid and transient metabolism of dual second messenger systems, both mediated through a taste cell G protein, likely composed of Galpha(gust)/beta/gamma(13), with both systems being simultaneously activated in the same bitter-sensitive taste receptor cell.

Cellular mechanisms of taste transduction

Taste receptor cells respond to gustatory stimuli using a complex arrangement of receptor molecules, signaling cascades, and ion channels. When stimulated, these cells produce action potentials that result in the release of neurotransmitter onto an afferent nerve fiber that in turn relays the identity and intensity of the gustatory stimuli to the brain. A variety of mechanisms are used in transducing the four primary tastes. Direct interaction of the stimuli with ion channels appears to be of particular importance in transducing stimuli reported as salty or sour, whereas the second messenger systems cyclic AMP and inositol trisphosphate are important in transducing bitter and sweet stimuli. In addition to the four basic tastes, specific mechanisms exist for the amino acid glutamate, which is sometimes termed the fifth primary taste, and for fatty acids, a so-called nonconventional taste stimulus. The emerging picture is that not only do individual taste qualities use more than one mechanism, but multiple pathways are available for individual tastants as well.

Calbindin D28k-like immunoreactivity in the gustatory epithelium in the rat

The distribution of calbindin D28k (CB)-like immunoreactivity (-LI) in the gustatory epithelium was examined in the adult rat. In the circumvallate and foliate papillae, CB-like immunoreactive (-IR) nerve fibers were observed in the subgemmal region, and some of these penetrated the taste buds. Two or three spindle-shaped gustatory cells displayed CB-LI in each taste bud of these lingual papillae; the immunoreactivity was restricted to the cytoplasm. In the fungiform papilla, CB-IR nerve fibers were detected in the subgemmal region, but no CB-IR cells were observed in the taste buds of the fungiform papillae. In the taste buds of the incisive papillae, many CB-IR intragemmal nerve fibers were observed, but no apparent CB-IR cells were detected. In the soft palate, CB-IR nerve fibers associated with the taste buds were also observed, but no CB-IR cells were detected in the taste buds. The present findings indicate that CB-IR gustatory cells were only localized in the taste buds in the posterior lingual papillae (circumvallate and foliate papillae), but not in the taste buds in other gustatory epithelium.

Rapid kinetics of second messenger production in bitter taste

The tasting of bitter compounds may have evolved as a protective mechanism against ingestion of potentially harmful substances. We have identified second messengers involved in bitter taste and show here for the first time that they are rapid and transient. Using a quench-flow system, we have studied bitter taste signal transduction in a pair of mouse strains that differ in their ability to taste the bitter stimulus sucrose octaacetate (SOA); however, both strains taste the bitter agent denatonium. In both strains of mice, denatonium (10 mM) induced a transient and rapid increase in levels of the second messenger inositol 1,4,5-trisphosphate (IP3) with a maximal production near 75-100 ms after stimulation. In contrast, SOA (100 microM) brought about a similar increase in IP3 only in SOA-taster mice. The response to SOA was potentiated in the presence of GTP (1 microM). The GTP-enhanced SOA-response supports a G protein-mediated response for this bitter compound. The rapid kinetics, transient nature, and specificity of the bitter taste stimulus-induced IP3 formation are consistent with the role of IP3 as a second messenger in the chemoelectrical transduction of bitter taste.

Calcium mobilization in isolated cochlear spiral ganglion cells of the guinea pig

Single cochlear spiral ganglion cells (SGCs) were isolated using enzymatic and mechanical techniques. Intracellular free calcium ion concentrations ([Ca2+]i) in the SGCs were measured using a digital imaging microscope and the Ca(2+)-sensitive fluorescence dye fura-2. In the presence of the Ca2+ ionophore ionomycin (1 microM), there was an irreversible increase in [Ca2+]i. Depolarization by high K+ (150 mM) led to an increase in [Ca2+]i in SGCs, and this effect was reversible. The SGCs apparently possess voltage-gated calcium channels.