Contribution of α-Gustducin to Taste-guided Licking Responses of Mice
Advance Access publication March
Contribution of a-Gustducin to Taste-guided Licking Responses of Mice
John I. Glendinning 2 3
Lauren D. Bloom 2 3
Maika Onishi 2 3
Kun Hao Zheng 2 3
Sami Damak 1 2
Robert F. Margolskee 0 1 2
Alan C. Spector 2 4
0 Howard Hughes Medical Institute
1 Department of Physiology and Biophysics, Mount Sinai College of Medicine
2 3009 Broadway , New York, NY 10027 , USA
3 Department of Biological Sciences, Barnard College, Columbia University
4 Department of Psychology, University of Florida , USA
We examined the necessity of a-gustducin, a G protein a-subunit expressed in taste cells, to taste-mediated licking responses of mice to sapid stimuli. To this end, we measured licking responses of a-gustducin knock-out (Gus / ) mice and heterozygotic littermate controls (Gus+/ ) to a variety of bitter , umami , sweet , salty and sour taste stimuli. All previous studies of how Gus / mice ingest taste stimuli have used long-term (i.e. 48 h) preference tests, which may be confounded by post-ingestive and/or experiential effects of the taste stimuli. We minimized these confounds by using a brief-access taste test, which quantifies immediate lick responses to extremely small volumes of sapid solutions. We found that deleting a-gustducin (i) dramatically reduced the aversiveness of a diverse range of bitter taste stimuli; (ii) moderately decreased appetitive licking to low and intermediate concentrations of an umami taste stimulus (monosodium glutamate in the presence of 100 lM amiloride), but virtually eliminated the normal aversion to high concentrations of the same taste stimulus; (iii) slightly decreased appetitive licking to sweet taste stimuli; and (iv) modestly reduced the aversiveness of high, but not low or intermediate, concentrations of NaCl. There was no significant effect of deleting a-gustducin on licking responses to NH4Cl or HCl.
taste; a-gustducin; brief-access taste test; knock-out mice
Introduction
In mammals, the ingestive response to sweeteners, amino
acids and many bitter compounds is initiated by the
interaction of chemical compounds with G-protein-coupled
receptors (GPCRs) on the apical membrane of taste cells
(Gilbertson et al., 2000; Glendinning et al., 2000)
. Two
families of GPCRs are known to mediate this detection
process—the T2Rs and T1Rs. A few T2Rs have been shown
to respond selectively to compounds that elicit bitter taste
sensations in humans
(Adler et al., 2000; Chandrashekar
et al., 2000; Bufe et al., 2002)
. The T1Rs, which appear to
function predominantly as heterodimers, are activated by
amino acids and sweeteners (natural and artificial). The
heterodimer of T1R2+T1R3 responds selectively to compounds
that elicit sweet taste sensations in humans, whereas that of
T1R1+T1R3 responds selectively to compounds that elicit
umami taste sensations
(Nelson et al., 2001; Zhao et al.,
2003)
. Once these taste receptors are activated by their
respective ligands, they activate one or more G-proteins
(gustducin, rod-transducin or Gi2), which in turn stimulate one or
more effector systems
(adenylyl cyclase, phosphodiesterase
or phospholipase C b2; Huang et al., 1999; Clapp et al.,
2001; Yan et al., 2001; Ogura et al., 2002)
. These early
transduction steps initiate a cascade of downstream events,
including the release of neurotransmitter from the taste
cell, generation of action potentials in the postsynaptic
afferent neuron, propagation of the action potentials up the
gustatory neuraxis, and, ultimately, ingestion or rejection
Glendinning et al., 2000; Spector, 2000).
There is compelling evidence that the a-subunit of
gustducin (a-gustducin) contributes significantly to the
transduction of bitter , sweet and umami taste stimuli
(Wong
et al., 1996; Ruiz-Avila et al., 2001; He et al., 2002, 2004;
Ruiz et al., 2003)
. Gustducin is co-expressed with T1Rs or
T2Rs in some taste cells
(Adler et al., 2000; Max et al.,
2001; Li et al., 2002; Kim et al., 2003)
and its subunits have
been shown to activate specific components of known
gustatory effector systems (e.g. PLC-b2, PDE and Ca2+
store-operated channels) in signaling pathways responsive
to sweet and bitter compounds
(Huang et al., 1999; Clapp
et al., 2001; Yan et al., 2001; Ogura et al., 2002)
. a-Gustducin
knockout mice (i.e. Gus / mice) exhibit diminished
behavioral and/or gustatory nerve responsiveness to natural and
artificial sweeteners
(Wong et al., 1996; He et al., 2002)
,
bitter compounds
(Ruiz-Avila et al., 2001; Wong et al., 1996;
He et al., 2002; Caicedo et al., 2003; Ruiz et al., 2003)
and
amino acids
(Ruiz et al., 2003; He et al., 2004)
. Transgenic
expression of wild-type a-gustducin in Gus / mice restores
normal behavioral responsiveness to these taste stimuli
(Ruiz-Avila et al., 2001).
While there is no doubt that a-gustducin contributes to
the transduction of sweet , bitter and umami taste stimuli,
the following observations indicate that other G proteins
are also involved in thes (...truncated)