Influence of innervation on the levels of noradrenaline and serotonin in the circumvallate papilla of the rat

Characterization of the expression pattern of adrenergic receptors in rat taste buds

Taste buds signal the presence of chemical stimuli in the oral cavity to the central nervous system using both early transduction mechanisms, which allow single cells to be depolarized via receptor-mediated signaling pathways, and late transduction mechanisms, which involve extensive cell-to-cell communication among the cells in the bud. The latter mechanisms, which involve a large number of neurotransmitters and neuropeptides, are less well understood. Among neurotransmitters, multiple lines of evidence suggest that norepinephrine plays a yet unknown role in the taste bud. This study investigated the expression pattern of adrenergic receptors in the rat posterior taste bud. Expression of alpha1A, alpha1B, alpha1D, alpha2A, alpha2B, alpha2C, beta1, and the beta2 adrenoceptor subtypes was observed in taste buds using RT-PCR and immunocytochemical techniques. Taste buds also expressed the biosynthetic enzyme for norepinephrine, dopamine beta-hydroxylase (DbetaH), as well as the norepinephrine transporter. Further, expression of the epinephrine synthetic enzyme, phenylethanolamine N-methyltransferase (PNMT), was observed suggesting a possible role for this transmitter in the bud. Phenotyping adrenoceptor expression patterns with double labeling experiments to gustducin, synaptosomal-associated protein 25 (SNAP-25), and neural cell adhesion molecule (NCAM) suggests they are prominently expressed in subsets of cells known to express taste receptor molecules but segregated from cells known to have synapses with the afferent nerve fiber. Alpha and beta adrenoceptors co-express with one another in unique patterns as observed with immunocytochemistry and single cell reverse transcription polymerase chain reaction (RT-PCR). These data suggest that single cells express multiple adrenergic receptors and that adrenergic signaling may be particularly important in bitter, sweet, and umami taste qualities. In summary, adrenergic signaling in the taste bud occurs through complex pathways that include presynaptic and postsynaptic receptors and likely play modulatory roles in processing of gustatory information similar to other peripheral sensory systems such as the retina, cochlea, and olfactory bulb.

Adrenergic signalling between rat taste receptor cells

In taste buds, synaptic transmission is traditionally thought to occur from taste receptor cells to the afferent nerve. This communication reports the novel observation that taste receptor cells respond to adrenergic stimulation. Noradrenaline application inhibited outward potassium currents in a dose-dependent manner. This inhibition was mimicked by the beta agonist isoproterenol and blocked by the beta antagonist propranolol. The alpha agonists clonidine and phenylephrine both inhibited the potassium currents and elevated intracellular calcium levels. Inwardly rectifying potassium currents were unaffected by adrenergic stimulation. Experiments using the RT-PCR technique demonstrate that lingual epithelium expresses multiple alpha (alpha1a, alpha1b, alpha1c, alpha1d, alpha2a, alpha2b, alpha2c) and beta (beta1, beta2) subtypes of adrenergic receptors, and immunocytochemistry localized noradrenaline to a subset of taste receptor cells. Collectively, these data imply strongly that adrenergic transmission within the taste bud may play a paracrine role in taste physiology.

The taste of ethanol in a primate model. II. Glossopharyngeal nerve response in Macaca mulatta

The glossopharyngeal nerve (NG) mediates taste from the posterior part of the tongue. Here, we studied the effects of ethanol on the tongue in recordings from both the whole NG and individual taste fibers of the rhesus monkey, Macaca mulatta. The results show that the nerve activity increased at 0.7 M ethanol, reaching half maximum at around 4 M alcohol. Previously, we identified three types of taste fibers in the rhesus monkey NG: S fibers predominantly responding to sweeteners, Q fibers responding to bitter, such as quinine hydrochloride (QHCl), and M fibers responding best to monosodium glutamate, NaCl and acids [Hellekant, G., Danilova, V., & Ninomiya, Y. (1997). Primate sense of taste: behavioral and single chorda tympani and glossopharyngeal nerve fiber recordings in the rhesus monkey, Macaca mulatta. J Neurophysiol 77, 978-993]. Here, this fiber classification was used to elucidate the oral effects of ethanol and ethanol mixtures with NaCl, sucrose, citric acid and QHCl. One and three molar concentrations of ethanol stimulated all fiber types. Mixtures of ethanol with QHCl elicited a smaller response in Q fibers than did QHCl alone. In S fibers, mixtures of ethanol with sucrose gave a larger response than did sucrose alone. The variability of M fibers was too large to allow a conclusion about the effect of ethanol. These results suggest that ethanol suppresses the taste of QHCl. Similarly, the taste of sucrose might be enhanced by adding ethanol to sucrose. These effects and conclusions corroborate an earlier ethanol study of the chorda tympani (CT) nerve [Hellekant, G., Danilova, V., Roberts, T., & Ninomiya, Y. (1997). The taste of ethanol in a primate model: I. Chorda tympani nerve response in Macaca mulatta. Alcohol 14, 473-484].