Abstracts - Association for Chemoreception Sciences

#P204 POSTER SESSION IV: CHEMOSENSORY
TRANSDUCTION AND SIGNALING
Expression of GABA receptor subunits and Cl - transporters
of taste buds in mice
Toshiaki Yasuo 1 , Ryusuke Yoshida 1 , Noriatsu Shigemura 1 ,
Robert F. Margolskee 2 , Yuzo Ninomiya 1
1
Section of Oral Neuroscience, Graduate School of Dental Science,
Kyushu University Fukuoka, Japan, 2 Monell Chemical Senses
Center Philadelphia, PA, USA
Taste bud cells (TBCs) communicate with sensory afferent fibers
and may also exchange information with adjacent cells. Recently,
g-aminobutyric acid (GABA) has been proposed as a candidate
neurotransmitter or neuromodulator in mammalian taste buds.
However the precise role for GABA in the taste buds remains
unclear. In this study, we examined possible expression of GABA
receptor subunits by using RT-PCR and potential effect of
basolateral GABA application to single fungiform TBCs on their
electrical activities. The results indicated that TBCs expressed
GABA type A receptor subunits, a1, a2, a5, b2, b3, g2, g3, d, π,
and GABA type B receptor R1, R2, but not expressed GABA
type A receptor subunits a3, a4, a6, b1, g1, e and q. Application
of GABA to TBCs produced both facilitation and inhibition of
the spontaneous firing rates of TBCs; facilitation in some cells,
and inhibition in the other cells. Moreover, firing rates in response
to sweet taste stimuli were increased by the GABA application in
some of sweet-responsive cells, while those in response to bitter
taste stimuli were decreased by GABA in some of bitterresponsive
cells. Furthermore, Cl - transporters (NKCC1 and
KCC2) that mediated GABA actions were detected in TBCs by
using RT-PCR. These results suggest that GABA may participate
in modulation of spontaneous firing rates and gustatory signaling
in taste bud. The Cl - cotransporters may be involved in the
GABAergic signaling.
#P205 POSTER SESSION IV: CHEMOSENSORY
TRANSDUCTION AND SIGNALING
GABA Inhibition in Mouse Taste Buds
Yijen A. Huang 1 , Stephen D. Roper 1,2
1
Department of Physiology and Biophysics, University of Miami
Miller School of Medicine Miami, FL, USA, 2 Program in
Neuroscience, University of Miami Miller School of Medicine
Miami, FL, USA
Cell-to-cell communication in taste buds is an important
component of signal processing in taste buds. For example, ATP
secreted from Receptor (Type II) cells during gustatory
stimulation activates adjacent Presynaptic (Type III) cells to
release serotonin (5-HT). Subsequently, 5-HT feedback inhibits
ATP secretion from Receptor cells (Huang et al., J Neurosci,
2009). g-Aminobutyric acid (GABA) has been proposed as a
candidate neurotransmitter or paracrine hormone in taste
buds. For instance, taste cells synthesize GABA and express
GABAA and GABAB receptors. Here, we tested the actions of
GABA in taste buds during gustatory stimulation. Specifically,
using cellular biosensor techniques (Huang et al., PNAS, 2007),
we studied whether GABA affects taste-evoked ATP secretion
from taste buds and taste cells. As previously shown, taste buds
isolated from mouse vallate papillae secrete ATP in response to
stimulation with a mixture of bitter and sweet compounds
(cycloheximide, 10 µM; saccharin, 2 mM; denatonium, 1 mM; and
SC45647, 0.1 mM). Bath-applied GABA (10 µM) reduced tasteevoked
ATP secretion from isolated taste buds. Muscimol (1 µM)
and baclofen (1 µM), GABAA and GABAB receptor agonists,
respectively, similarly inhibited taste-evoked ATP secretion.
Bicuculline (10 µM), a GABAA receptor antagonist, and
CGP55845 (10 µM), a GABAB receptor antagonist, restored ATP
secretion inhibited by muscimol and baclofen, respectively. In
sum, these findings suggest that during gustatory stimulation,
GABA inhibits Receptor cells by activating GABAA and GABAB
receptors and thereby decreases ATP secretion. Experiments are
underway to identify the source of GABA during gustatory
stimulation and to determine whether GABA affects 5-HT release
from Presynaptic cells. Acknowledgements: Supported by
NIH/NIDCD grant 5R01DC007630 (SDR).
#P206 POSTER SESSION IV: CHEMOSENSORY
TRANSDUCTION AND SIGNALING
Intracellular Ca 2+ and TRPM5-mediated membrane
depolarization are required for taste cells to secrete ATP
Yijen A. Huang 1 , Stephen D. Roper 1,2
1
Miller School of Medicine, University of Miami Miami, FL, USA,
2
Program in Neuroscience, University of Miami Miami, FL, USA
ATP is a transmitter secreted from taste bud Receptor (Type II)
cells through ATP-permeable pannexin 1 gap junction
hemichannels. The elevation of intracellular Ca 2+ and membrane
depolarization are both believed to be involved in transmitter
secretion from Receptor cells. However, the specific roles for Ca 2+
and membrane potential have not been fully elucidated. In the
present study, we show that taste-evoked ATP secretion from
mouse vallate Receptor cells is evoked by the combined actions of
intracellular Ca 2+ release and membrane depolarization.
Unexpectedly, ATP secretion is not blocked by tetrodotoxin, a
voltage-gated Na + channel blocker. This indicates that action
potentials, although likely acting to augment ATP secretion
(Murata et al 2008), are not necessary for transmitter release from
taste Receptor cells. Taste-evoked ATP secretion is absent in
Receptor cells from TRPM5 knockout mice or in Receptor cells
from wild type mice where current through TRPM5 channels has
been blocked. However, ATP secretion can be rescued in these
circumstances by depolarizing Receptor cells with KCl. These
findings suggest that membrane voltage initiated by TRPM5
channels is required for ATP secretion during gustatory reception.
The findings indicate that taste-evoked elevation of intracellular
Ca 2+ has a dual role: (1) to open TRPM5 channels and thereby
depolarize Receptor cells and (2) to act in combination with
membrane depolarization to open ATP-permeable gap junction
hemichannels. Acknowledgements: These studies were funded by
NIH/IDCD grant 5R01DC007630 (SDR)
P O S T E R S
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