Comparison of Gustatory Transduction Mechanisms in Vertebrate Taste Cells

14 Pages·1994·5.4 MB·English

by  Toshihide Sato

ZOOLOGICAL SCIENCE 11: 767-780 (1994) 1994Zoological SocietyofJapan
REVIEW
Comparison of Gustatory Transduction Mechanisms
in Vertebrate Taste Cells
Toshihide Sato, Takenori Miyamoto and Yukio Okada
Department ofPhysiology, Nagasaki University School ofDentistry,
Sakamoto, Nagasaki 852, Japan
INTRODUCTION
-20
Animalsinvariousclasses ofvertebrates live indifferent mV — 20sec
H I-
environments, such as the water, the underground, the ImM 10mM 0-5M Deionized
Aceticocid,Q-HCI,NaCl, water
surface oflands or the sky, and eat different types offoods.
Therefore, it is supposed that various vertebrates have
i B
different sensitivities to a variety of chemicals.
Investigations of gustatory transduction mechanisms in
taste cells have been carried out with varying vertebrate
species, such ascatfish, frog, mudpuppy, salamander, mouse,
rat, gerbil and hamster. Comparison of gustatory research
dataobtainedindifferentspeciesofvertebratesmustbedone Fig. 1. Intracellular receptor potentials of a frog taste cell in
dciafrfeefrullfyrboemcaeuascehthoetihrerli.vingWhenevniroannmeinnttserapnredtfatoioodncuosftotmhse iarzenesddporwneascteoerrt.doBacReeftcircoormadciadA,taqissutiefnricoenmlel-aHofCtlastth(eeQ-cpHerlClo1x)oif,matNhlaeCralepgiaicnoandl droeefgiiotohnne-
experimental data obtained from the taste nerve in some tongue. The vertical deflection atthe left shows a penetration
animal is given on the basis of taste cell functions, the taste oftastecellandthatattherightawithdrawalofthecell. From
cell data from the same or similar species should be used. [92].
Someconfusionmayhappenwhenthepropertiesofgustatory
neural responses in one species are explained by the prop-
erties of the taste cell responses in a quite different species.
Some researchers confuse an understanding of gustatory
nerve and cell data because ofciting unadequate references.
In this reviewwe attempted to compare gustatory trans-
duction mechanisms obtained in various mammalian taste
cells (rat, mouse, hamster) and amphibian taste cells (frog,
salamander, mudpuppy), which were mostly studied with
microelectrode techniques and patch electrode techniques.
Although there are many review articles which mentioned
gustatorytransduction mechanisms [9, 30, 41, 43-45, 47, 84],
fewhavecarefullycomparedthoseindifferentvertebrates [9,
45].
10"s 10-1 10"3 10"2 10-1
Concentration(m)
GUSTATORY TRANSDUCTION IN FROG TASTE CELLS
Fig. 2. Relationships between taste stimulus concentration and
1. Characteristics oftaste cell responses response magnitude, obtained from the frog taste cell (upper
graph) andfromthefrogglossopharyngealnerve (lowergraph).
Several species of frogs and toads have been used for Stimuliwereindicatedbydifferentsymbols. Eachpointrepre-
investigation of taste mechanisms. Figure 1 illustrates re- sents the mean value of the maximum magnitude of receptor
ceptorpotentials in frog taste cellsinduced taste stimuli [92]. potentials andgustatoryneuralresponsesobtainedfromseveral
Figure2 shows relationships between stimulus concentration experiments, the number of which is indicated by a numeral
insideaparenthesisaftereachstimulus. Inalltheexperiments
taste receptors were preadapted to 0.01M-NaCl before each
Received October26, 1994 stimulation. From [4].
768 T. Sato, T. Miyamoto and Y. Okada
20
oNaCI (11) 2. Salt taste
1- a•HKCCII ((91)1) The taste cell membrane can be divided into the apical
aQuinine(10) receptive membrane exposed to the oral cavity and the
-cog 1-6-- DCaCli(15) basolateral membrane. The former is usually bathed in the
u 1.4.- superficial fluid (SF) and the latter in the interstitial fluid
1 1-2 (ISF). The amplitudes of the receptor potentials in frog
taste cells induced by salt stimuli are greatly decreased when
interstitial Na+ and Ca2+ are replaced with choline"1",
-5 Rec5eptor potential10(mV) 9t0e,tr9a3]m.ethAydldaimtmioonniuomf+5,mteMtraCeot2h+ylaanmdmo3nifuMm.+te[5t5r,od5o9t,ox8i7n,
Fig. 3. Relationshipsbetweenthereceptorpotentialamplitudeand (TTX) to ISF does not affect the receptor potentials. This
the relative conductance magnitude in frog taste cells. The indicates that TTX-insensitive cation channels in the baso-
latter represents the ratio of the electrotonic potential magni- lateral membrane play an important role in generation ofthe
tudesbetweenrestedandstimulatedstate. Numeralinsidethe receptor potentials [55].
parenthesis is number oftaste cells sampled. From [4]. Afterthe normal ionic composition ofSF and ISFofthe
frog tongue is changed with low-concentration Na+ saline,
and response magnitude in gustatory cells (upper) and in the relationships between membrane potentials and receptor
gustatory nerves (lower) [4]. Salt, bitter and acid stimuli potentials in a frog taste cell evoked by various concentra-
elicit large responses in both gustatory cells and nerves, but tions of NaCl and various types of salts can be analyzed to
sweet stimuli elicit small responses. Intracellular receptor examine the permeability ofthe taste-receptive membrane to
potentials in response to four basic taste stimuli and water cations and anions (Fig. 4). In this situation, the mean
stimulus are depolarizingor hyperpolarizing. However, de- reversalpotentialsfordepolarizingpotentialsofatastecellin
M
polarizations are dominant for salt, acid and bitter stimuli. responseto0.05, 0.2, and0.5 NaClare-40.0, 6.4, and28.8
Depolarization in response to waterisfoundin the taste cells mV, respectively [59]. When adding an anion channel
in the proximalregion ofthe tongue [92] (Fig. 1). Conduct- blocker, SITS (4-acetamide-4'-isothiocyanostilbene-2,2'-
ancechangeduringgenerationofreceptorpotentialsisshown disulfonicacid), to aNaClstimulus, the reversalpotentialfor
in Figure3 [4]. Conductance is increased for salt, acid and receptor potential with NaCl plus SITS becomes about twice
sucrose but reduced slightly for quinine-HCl (Q-HC1) [4, 53, larger than that with NaCl alone [59]. This result indicates
69, 87, 97]. The conductance change during water stimula- that Na+ and Cl~ of the NaCl stimulus permeate the apical
tion shows two types [72]: reduction or increment depending receptive membrane. Previously Akaike and Sato [3] sug-
oncelltypes. Frogtastecellscanproducespikepotentialsin gested that cation and anion ofsalt stimuli directly permeate
response to electrical stimulation [5, 54, 56]. the receptive membrane in frog taste cells.
M
Frog and toad taste organs on the dorsal surface of the Reversal potentials for 0.2 NaCl, LiCl, KCI, and
tongue are located on the top of the fungiform papillae. NaSCNin frogtaste cells are 6.4, 25.4, -1.0, and -7.8mV,
Each fungiform has a large disc-shaped structure of 100-300 respectively, indicating that permeability of the apical taste
fim in diameterwhich is termed the taste discrather than the receptive membrane to cations ofthe CI- salts is ofthe order
taste bud. There are several types oftaste disccells. Clas- ofLi+>Na+>K+ and that the permeability to anions ofthe
sification and nomenclature ofthe disc cells are controversial Na+ salts is SCN">C1~ [59]. These results indicate that
[31, 36, 80, 116]. Usually two types of taste cells are
distinguished depending on their structure. All the taste 10mMNa+,Ca2+-free(sucrose)
cells are innervated by the glossopharyngeal nerves. There 1 mMNa+,Ca2+-free (sucrose)
are gap junctions between supporting cell and taste cell and
between taste cells in the taste disc [86].
In frog taste cell membrane, voltage-gated ion channels
(several types of K+ channels, Na+ channel, Ca2+ channel)
and ligand-gated ion channels (Na+ channel, K+ channel,
-
non-selective cation channel, CI channel) are found.
However, their physiological functions in gustatory transduc-
tions are unclear and underinvestigation [5-8, 25-27, 54-58,
60, 75].
Recently, most studies on gustatory transduction
mechanisms in frog taste cells are carried out in our labora-
tory with microelectrode and patch pipette techniques. Membranepotential(mV)
Therefore, studies on anuran amphibians are focused on our Fig. 4. Relationship between membrane potentials and receptor
potentials induced by NaCl stimuli in a frog taste cell. Super-
experimental data. ficial fluid (SF) and interstitial fluid (ISF) used are shown above
the graph. From [59].
p r
Gustatory Transduction Mechanisms 769
the NaCl stimulus-induced receptor potential in a frog taste 3. Sour taste
cell results from an inflow ofNa+ and Cl~ across cation and The removal of Na+, Ca2+, and Cl~ from the normal
anion channels on the taste-receptive membrane, as well as ISF does not affect the receptor potential in a frog taste cell
an inflow of interstitial Na+ across cation channels on the induced by acid stimuli such as acetic acid and HC1 [53].
basolateral membrane. Salt-induced receptor currents in Interstitial 100mM K+ saline also does not affect the acid
frog taste cells are recorded with single microelectrode or response [53]. The receptor potential is reduced greatly
patch pipette voltage clamping method [58, 73]. Recently, whenCa2+ isremovedfromthesuperficialnormal saline, but
Miyamotoetal. found salt stimulus-gated K+ channels in the is increased when the Ca2+ concentration is elevated [53, 95]
frog receptive membrane which show a high permeability to (Fig. 6). Similar responses are seen in the frog gustatory
Na+ [57, 58, 60]. Fig. 5 illustrates a tentative diagram of nerve [67]. The removal of superficial Cl~ does not affect
NaCl signal transduction in a frog taste cell [55, 59, 60]. thereceptorpotential. Thereceptorpotentialelicited by an
acid stimulus under superficial Ca2+-free saline is partly
NaCl stimulation ctuatuessedfobryNNaa++ i[n53]p.rodLuic+in,gK+t,heNrHec4e+p,toorr cphootleintniea+l.subTsthie-
mm
® receptor potential is unaffected by superficial TTX, but is
Superficial fluid blocked by superficial Ca2+ antagonists such as Co2+ and
Ti r^^ANa b ci- Cd2+. Sr2+ and Ba2+ substitute for Ca2+ in generating the
receptor potential [53]. The receptor potentials observed
under various concentrations of superficial Ca2+ becomes
smallerwhen Na+ is present in the SF, indicating a competi-
tion between Ca2+ and Na+ passing through a Ca2+-
Tightjunction
permeable conductance in the apical receptive membrane
Secondmessenger? [53].
These findings indicate that a large portion ofthe recep-
Na+
> 50 r- T24
TASTE CELL rE 40 —" _L T34
CD
_L
2 30
o? 20 —28
o _1_ 20
Interstitial fluid —
£u• 10 i
Fig.5. Schematic drawing oftransduction ofa NaCl stimulus into cc
receptorpotentialinafrogtastecell, a,cationchannel;b, K+
channel; c, anion channel; d, cation channel. Voltage- no
dependent channels, such as Na+-, K+- and Ca2+-channels,
100
which are related to generation ofspike potentials ofthe taste
cell are removedinthis and otherfigures. 90
80
-
70
Patch pipette studies with excised patch membranes
K+ 60
indicate that there are channels, nonselective cation
channels, and Cl~ channels of various conductances in the
apical receptive membrane of frog taste cells [25-27].
However, contribution ofthese channels to salt signal trans- QCD C5O
duction has not yet been clarified.
Amiloride-blockableNa+ channelsexistinthefrogtaste Superficial saline solutions
cell membrane [8, 55, 66]. Miyamoto etal. with insitutaste Fig.t6.he aRmepllaittiuodneboeftwienpeunttrheesiasmtpalncietuidneduocferdecbeypt1omrMp-otHeCntliailnsfarnodg
pceoltlesntciaolulfdolnloowtinfgin5d0mainchaadnagpetatiinonNaoCflt-hiendreucceepdtivreecmeeptmo-r tnaosrtmealc,ellC.a2T+h-efreetosnagluienessuorlfuatcieonsisanaddapdetieodnitzoed2w0amteMr-.CaT2h+e,
mM
brane to 0.1 amiloride [55]. Therefore, it is likely that resistance is expressed as a percentage of the control in the
earmaillomreimdeb-rbalnocekaobflferoNgata+stcehcaenlnlse.ls TmhaeyNeaxi+stcihnantnheelsbadsoolnaot-t ui5nn4s+tthi5emMuulnQastteiwdmiutslhtaattneeo.drmsatTalhtee(i1as.b6s82oml+Mut6-eCMavQa2l+uw)eitsohafl2itn0hee,mMi5n3-p+Cuta7r2Me+sQissatlwaiinntcehe,
contribute directly to salt signal transduction in frog taste Ca2+-free saline and 64+7MQ with deionized water. No
cells. significant differences are found in any pairs of these figures.
From [53].
:
770 T. Sato, T. Miyamoto and Y. Okada
tor potential induced by acid stimuli is concerned with generated by current carried through a DCCD-sensitive
proton-gatedCa2+ channelsonthetaste-receptivemembrane proton transporter of the receptive membrane [53, 74] (Fig.
[53]. Both divalent (Ca2+, Sr24") and monovalent (Na+, V).
Li+, K+,NH4+,choline"1")cationscanpassthroughtheCa2+
channel. Even after the tongue surface is adapted to pure 4. Bitter taste
water, the amplitude of acid-induced response in a taste cell The ionic mechanism ofthe receptor potential in a frog
remains as large as 35% of the control (Fig. 6). After 0.1 taste cell elicited by quinine-HCl (Q-HC1) has been studied.
mMDCCD (N,N'-dicyclohexylcarbodiimide),aprotonpump The frog taste cells whose receptive membranes are adapted
inhibitor, isaddedtoSF, the acidresponseisgreatlysuppres- to normal saline and deionized water generate depolarizing
sed, indicating a contribution of proton transporter on the receptor potentials at Q-HC1 concentrations higher than 2
receptive membrane to the acid-induced receptor potential and 0.01 mM, respectively [69]. The input resistance ofthe
[74]. taste cell during Q-HC1 stimulation increases slightly [4, 69,
The receptorcurrentfrom a dissociated frog whole taste 97]. The receptor potential does not change even when the
mM
cell can be recorded with a patch pipette filled with 100 membrane potential level is greatly changed. The magni-
mM
CsCl. Applicationof0.1 aceticacidstimuluscontaining tude of the receptor potential is increased by reducing the
mM
80 BaCk to the cell initiates an inward current ofabout concentration ofsuperficial Cl~ on the taste-receptive mem-
—50pA at theholdingpotentialof -40mV [75]. Afterthe brane (Fig. 8), but is independent of the concentration of
taste-receptive membrane alone is damaged, the receptor superficial Na+ [69, 97].
current induced by acetic acid stimulus containing the BaCl2
greatly decreases, indicating that the inward receptor current
12
is induced by Ba2+ passing across proton-gated Ca2+ chan-
nelsontheapicalreceptivemembrane. Cationpermeability 10
of the proton-gated Ca2+ channel is: PCa Pbh'Psi'^W Pes .CO 8
=
1.87:1.17:0.73:0.99:1.00 [75]. Therefore, this channel c
6
should be called rather a proton-gated nonselective cation o
channel than the proton-gated Ca2+ channel. Q. 4
It is concluded that mostofthe acid-induced response in o 2
afrog taste cell is generated by a current carried through the Q.
COD
proton-gated cation channel of the apical receptive mem-
<x>
brane, and that the remainingportion ofthe acid response is CC -2
Acid stimulation
Superficial [CI"] (M)
irieesp
Superficial fluid Fig. 8. Relationship between concentration ofsuperficial Cl~ and
Ca2+ Na+ amplitude of Q-HCl-induced responses in frog taste cells.
,fr^n Numerals in parentheses are numbers of taste cells sampled.
a
From
[69].
InjectionofCl~ intoafrogtastecellgreatlyincreasesthe
receptor potential [69]. The magnitude of the receptor
Tightjunction potential is greatly decreased by removing interstitial Na+ or
-
CI or both, surrounding the basolateral membrane of the
,
taste cell. Furosemide (1 mM) added to the ISF decreases
the receptor potential to 15%, while interstitial ouabain (0.1
mM) and superficial SITS (0.1 mM) do not influence it [69,
97]. Fromthese results,wecanconclude [69,94,96, 98]: (1)
an electroneural Na+/Cl~ cotransport occurs through the
TASTE CELL basolateral membrane ofa frog taste cell in the resting state,
so that Cl~ accumulates inside the cell. (2) Q-HC1 stimula-
tion induces the active secretion of Cl~ across the taste
receptive membrane, resulting in a depolarizing receptor
Interstitial fluid potential (Fig. 9).
Bns©lateral m®inriilbff,iiiii<i
5. Sweet taste
Fig. 7. Schematic drawing of transduction of an acid stimulus into
receptor potential in a frog taste cell, a, proton-gated non- The frog taste cell generates a depolarizing receptor
selective cation channel; b, H ^-transporter. potential accompanying a remarkable reduction of input
Gustatory Transduction Mechanisms 771
Bitter stimulation the ISF largely abolishes it [71]. Interstitial 0.1 mM DCCD
completely inhibits the receptor potential, and interstitial 0.1
i pt5v@ mambrum® mM N-ethylmaleimide decreases the potential to 40% ofthe
Superficial fluid
Bittersubstance control value [71]. Lowering the pH of ISF from 7.2 to 6.3
a l| greatlydecreasesthereceptorpotential. Itisconcludedthat
part of the receptor potential in frog taste cells induced by
cr~*-i sugar stimuli may be produced by an inflow of H+ through
thetaste-receptive membrane [71] (Fig. 11). Theintracellu-
Tightjunction larpH ofthe taste cell may be regulated by a CP-dependent
H+ pump in the basolateral membrane [71].
b Sugar stimulation
is
m®m
::a
Superficialfluid
TASTE CELL Galactose /
2ndmessenger?
Interstitial fUl iasalnteral mambrum©
Tightjunction
Fig.9. Schematic drawingoftransduction ofa bitter stimulus into
receptor potential in a frog taste cell, a, Cl~ pump; b, elec-
troneutral Na+/Cl~ cotransporter.
60 iTcr
TASTE CELL
<§. 40
c
QO) Interstitial fluid ias®lnteral mambmirn®
Q. 20
lWo
i— Fig. 11. Schematicdrawingoftransductionofasugarstimulusinto
> receptor potential in a frog taste cell, a, H+ channel; b, Cl~
D0C) -dependent H+ pump.
-20 L 6. Water taste
The frogtaste cell locatedin the proximal portionofthe
pH of 1 M galactose solution tongue gmenVerates a depolarizing receptor potential that aver-
Fig. 10. Relationship between pH of 1M galactose and reversal a[7g2e]s. 10Water-isnenrseistpiovnesetasttoescteillmsulaarteioclnaswsiitfhieddeiinotonitzweodtwyaptees:r
potential for receptor potentials in frog taste cells. Points are _
means from three orfourtaste cells; bars are SE. From [71]. Cl -dependent and CI"-independent. In Cl~-dependent
cells whose input resistance is decreased or unchanged by
deionized water, the magnitude of the water-induced de-
resistance in response to stimulation with galactose and polarization decreases with an increase in concentration of
sucrose [71]. The magnitude of the receptor potential in superficial Cl~ in contact with the receptive membrane and
mM
response to a galactose solution increases linearly with de- with addition of blockers of anion channels (0.1 SITS
creasingpHin thepHrange6-8, butremainsconstantabove and 0.1 mM DIDS) to deionized water [72]. The reversal
pH8[71]. Thereversalpotentialisincreasedbyonly29mV potentialforthedepolarizationinthistypeshiftsaccordingto
H+ _
bya 10-foldincreaseinthe concentrationofthestimulus, the concentration of superficial Cl [72]. These properties
suggestingthat there arepH-dependent andpH-independent ofthe responses areconsistentwiththoseofthe glossophary-
componentsinthemechanismgeneratingthereceptorpoten- ngeal nerve, which innervates the taste disc. In Cl~-
tial [71] (Fig. 10). Superficially added blockers of anion independent cells whose input resistance is increased by
mM
channels (0.1 SITS) have no effect on the receptor deionizedwater,thereversalpotentialisapproximatelyequal
potential. Na+-free, Ca2+-free, and K+-free ISF do not to the equilibrium potential for K+ at the basolateral mem-
affectthereceptorpotential, buttheeliminationofCl~ from brane [72]. The water-induced response of the glossopha-
772 T. Sato, T. Miyamoto and Y. Okada
Water stimulation
Type A Type B
m®m
Superficial fluid W<§©<§p Superficial fluid B<§©§pt§W§ birami®
lonO
A lonO
1
Chemical
messfinnfir?
Tightjunction Tightjunction
cr
V b
X L- i
r-
i
TASTE CELL TASTE CELL
^^^j
Interstitialfluid Bas®lat©rnl im©innilbiriiini® Interstitialfluid mtm^riOT©
Fig. 12. Schematic drawing of transduction of a water stimulus into receptor potential in frog taste cells. The type A is Cl~-dependent,
water-sensitivetastecell,andthetypeBisCI"-independent,water-sensitivetastecell, a,Cl~ channel;b,K+ channel. Xmeansblockof
channel.
ryngeal nerve is decreased to about 60% ofthe control value 2. Types ofionic channels in taste cells
by addition of interstitial 2mM Ba2+ K+ channels of Taste cells of the mudpuppy, Necturus, are electrically
.
approximately 40pS are found in the frog taste cell mem- excitable and generate action potential in response to taste
brane [6, 26, 27]. The activities of these channels are stimuli [38, 81]. Some basal cells also possess the action
blocked by cAMP in the presence of ATP and cAMP- potential [13]. Usingthe patch-clamptechnique, ithasbeen
dependentprotein kinase [6, 27]. The frog taste cellsinsitu confirmed that taste cells in tailed amphibians possess a
depolarized by intracellular injection of cAMP and cGMP variety of voltage-dependent currents, such as a TTX-
have been found [68]. Probably, the K+ channels are sensitiveNa+ current, aL-type Ca2+ current and several K+
related to a depolarization of CI"-independent, water- currents [39, 51, 106]. The role of the action potential is
sensitive frog taste cells, which is accompanied with increase unclear. The action potential may be necessary to activate
ofthe membrane resistance. It is concluded that the water- the Ca2+ current underlying a neurotransmitter release.
-
induced receptor potential is produced by CI secretion The microelectrode study in mudpuppy taste cells [48] iden-
through the taste-receptive membrane in about 70% of Cl~ tified a Ca2+-dependent chloride conductance which might
A
-dependent, water-sensitive frog taste cells (type in Fig. terminate the depolarizing responses elicited by taste stimuli
12), while it is generated by an inhibition of the resting K+ [108]. Electrical coupling has been observed between a
conductance of the basolateral membrane in the remaining group of taste cells in mudpuppy taste buds [117]. It is
30% ofCl~-independent, water sensitive taste cells [72] (the thought that such groups may form an organization unit
type B in Fig. 12). within taste buds. The taste cell-basal cell synapse also has
been identified in a lingual slice preparation [22].
GUSTATORY TRANSDUCTION IN TAILED
AMPHIBIAN TASTE CELLS 3. Salty taste
In whole-cell recordings from taste cells in the tiger
/. Cellular organization oftaste buds salamander, Ambystoma, it has been shown that Na+ influx
In tailed amphibians, taste buds are found over the through amiloride-sensitive Na+ channels mediates transduc-
whole dorsal surface of the tongue [16]. The glossopharyn- tion of Na+ salts into receptor potentials [107] (Fig. 13).
geal nerve innervates the taste buds. Taste buds contain Amiloride reduces a sustained Na+ current in isolated sala-
three types of cells: dark, light and basal cells [82]. The mander taste cells (Fig. 14) and inhibits a NaCl-induced
basal cellscan be furtherdivided into two types: undifferenti- neural response inthe animals, suggestingthatthesechannels
ated stem cell and Merkel-like cell [19]. The dark and light are located in the apical receptive membrane. The drug,
cells have an elongated, bipolarstructure and are regarded as however, does not block the neural response elicited byNaCl
taste receptor cells which extend apical processes to the taste in the mudpuppy [50]. Alternatively, an apically-located
pore. K+ channels mediate transduction of K+ salts in the mud-
Gustatory Transduction Mechanisms 773
Salt stimulation with the microelectrode technique [115].
l@©P 4. Bitter andsour tastes
Superficial fluid In the mudpuppy, block of the apically-located K+
Na+ K+
DH channelsmaymediatethetransductionofseveraltastestimu-
li, including sour, bitter and CaCl2 stimuli (Fig. 15). Patch-
clamp and microelectrode studies have shown that these
u ir stimuli all reduce the voltage-dependent K+ current in the
Tightjunction mudpuppy taste cells [12, 39, 40]. Since the voltage-
dependentK+ currentisrestrictedtothe apicalmembraneof
K+
the taste cells [83], the channels are directly exposed to
taste stimuli. Recent investigation with single channel re-
cordingalsohasindicatedthatthechannelsareallblockedby
citricacidandquinineappliedtooutersurfaceofthechannels
[17] (Fig. 16). Since these channels exhibit a significant
TASTE CELL open probability at rest, block of the channels can produce
depolarization in taste cells. Similar results have been
obtained in the tiger salamander taste cells [107]. The taste
cells in the animals, however, generate the inward current
Interstitial fluid Busointeral m©mbrao© accompanied by conductance increase in response to sour
stimuli. The blocking mechanism may result in a lack of
Fig. 13. In tigersalamander, Na+ ionsdirectlypassthrough apical discrimination amongthose tastestimuli [14], althoughsingle
pamuiplpoyr,idKe-+senisointsivaelsoNadi+reccthlaynpnaeslsst(har).ougAhltaeprincaatliTvEelAy-,seinnsimtuidv-e unit analysis of the glossopharyngeal nerve response has
K+ channels (b). suggested the discrimination between the sour and bitter
tastes in the mudpuppy [85]. The cross-adaptation analysis
of the glossopharyngeal nerve between taste stimuli and K+
puppy taste cells [40, 42]. It is likely that K+ flux through channel blockers still has not been made.
the channel can depolarize taste cells. The dominant sensi-
tivityofK+ saltsinthe mudpuppyhas alreadybeenreported
D
0.3M NaCI
0.3MNaCI 10mV BO mV
B
on
off
Wwft'^^wHW*H^ww>w^<i^w-o.3 M NaCI +
\\W,.^yfv^v,^N%^\%wV^,vv-'*^,*iV»vw^,VT^M','^^v"^^"HT^1r'"*TAHr^v,,*r^'|M,|'1K«vSMwf)f<jjJJ-011.up3JMvMTTNaXCI
c
T
^fe^r/MVl*##/*JWW
T^^WiMfW*Wf^W b^|-0.5MNaCU
mM
"%.n. 0.25 Amiloride
-250
-20 pA
I
1S
M
Fig. 14. NaCl-inducedtastecellresponsesintigersalamander. Applicationof0.3 NaCIinducesadepolarizationinacurrent-clampedtaste
cell(A). 0.3MNaCIinducesasustainedinwardcurrentinavoltage-clampedtastecell. TheinwardcurrentsarepartiallyblockedbyTTX
(B) andamiloride(C). Effectofholdingpotentialon0.3MNaCl-inducedinwardcurrentsinfourtastecells(D). Thedataareobtained
bythe whole cell recordings from isolated taste cells. From [107].
774 T. Sato, T. Miyamoto and Y. Okada
Acid, bitter or Ca2+ stimulation GUSTATORY TRANSDUCTION IN MAMMALIAN
TASTE CELLS
Superficial fluid Bl©<sp>tw<S m®m§mn®
H+ i, 1. Generalproperties ofgustatory responses
Quinine *~/\ Thetastecellsaretightlypackedinthetastebud. Taste
3
Ca2+
1 1 buds in the tongue exist in three types of lingual papillae:
fungiform, foliate and vallate. The fungiform papillae are
located on the anterior two thirds of the tongue surface and
K+
Tightjunction areinnervatedbythechordatympaninerve,whichhashigher
sensitivity to salty and sweet tastes. The foliate and vallate
papillae are located on the posterior and lateral surfaces of
the tongue, respectively and are innervated by the glosso-
pharyngeal nerve, showing higher sensitivity to sour and
bitter tastes [79] (Fig. 17).
Ultrastructural observation indicates that four types of
TASTE CELL taste bud cells, dark (type I), light (type II), intermediate
(type III) and basal cells, exist in a taste bud. It is believed
that only type III cells which have synaptic contact with the
gustatorynervearegustatorycells, andbasalcellssituatingat
Interstitia Ul SusQlutemi WKsrobran© the bottom of taste bud are a stem cell of taste cells.
Usually, type I, IIandIIIcellsexceptingbasalcellscannotbe
Fig. 15. Schematic drawing of mechanism oftransduction of acid, distinguished from one another by light microscopic figures.
bitterandCaCl2stimuliintoreceptorpotentialsintailedamphi- In electrophysiological studies with intracellular microelec-
bdeipaonltaarsitzeectelalsst.e ceIlnlstabiyleddiraemcpthibbloicaknso,faacpiidcsa,lqKui+nicnheanannedlsCa(a2)+. trodes, taste bud cells are clarified into taste cells and
This diagram comes from [12, 39, 107]. non-taste cells by responsivity to taste stimuli [78]. Taste
A. Rat glossopharyngea * QHCI
O HCl
• NaCl
Controlrvrw^jirnrr d Sucrose
1 mM citricacid in pipette, 2s
.00
cp
a
Wash, 3 min E
KT^YTmiU
q
200
B. Rot chorda tympani o
.14 ieo
1 mM quinine inpipette, 2 min 160 / .
140 / J
120
r /
I
IOO
1
80 /
/ /
tastant*^All4 K* 60 ///
y>
40
10 pA I 20 ms - -79mV C/A 114K+ bath 20 T--*r-+r---grjP^_ns^r
r
Fig. 16. Effect ofcitric acid and quinine, applied via pipette perfu- -6 -5 -4 -3 -2 -I
sion, on apical K+ channels in a cell-attached patch of a
mudpuppytastecell. Citricacidandquinineblockthechannels Logarithm of molar concentration o< stimulus
directly. The recording pipette is put on the apical membrane Fig 17. Comparison of relative response magnitudes for rat
andstimuliarealsoappliedviathepipetteperfusion (diagramat glossopharyngealandchordatympaninerves. Theresponsesin
the bottom). From [17], the two nerves are equated at 1 M NaCl. From [79].
Gustatory Transduction Mechanisms 775
NaCI Q-HCI HCI Sucrose outwardly-rectifying K+ channel, L- and T-type Ca2+ chan-
nels, and a ligand-dependent channel, amiloride-sensitive
Na+ channel are involved in the rat taste cell membrane [2,
11, 29, 102]. 4-aminopyridine-sensitive, tetraethylammo-
nium-sensitive and cyclic-nucleotide-blockable channels are
included in a group of K+ channels [2, 11, 102].
2. Salty taste
Microelectrode studies suggest that depolarizations in
50sec response to salt stimuli are concerned with activation of
Fig. 18. Intracellular^recordedreceptorpotentialsofrattastecells cation channels accompanied with a decrease of membrane
M
in responseMto four basic tMaste stimuli. TaMste stimuli: 0.5 resistance [77, 89, 109]. Schiffman etal. [99] first suggested
NaCI, 0.02 Q-HCI, 0.01 HCI, and 0.5 sucrose. Hori- that the amiloride-sensitive Na+ channels contribute to salty
zontal bars under the records show the duration of stimulus
application. The records A and B are obtainedfrom two taste taste transduction in humans. This hypothesis has been
cells adapted to water. From [91]. supported by many neurophysiological experiments. Ami-
loride greatly suppressesthe chorda tympani nerve responses
cellresponsesconsistofadepolarization, ahyperpolarization to NaCI and LiCl, but does not the response to KC1 [15, 20,
and an intermediate response [89, 91, 110, 112] (Fig. 18). 32, 65]. Recently, localization of amiloride-sensitive Na+
The resting potential and the input resistance of taste channel at the apical membrane of taste cells is clarified by
cells obtained by an intracellular recording method are —30 noninvasivelyrecording action potentials andcurrentsfroma
to —50mV and 10 to 300M£2, respectively when the cells in fungiform papilla [10, 28]. Therefore, it is primarily
ratandhamsterareadaptedtowater[88]. Themeanresting accepted that salt taste transduction occurs through amilor-
potential becomes small when adapted to saline solutions ide-sensitive Na+ channels in mammalian taste cells of the
[89]. The whole-cell clamp experiments show that isolated fungiform papillae (Fig. 19). Amiloride-sensitive Na+ cur-
rat taste cells have a zero-current potential of —50 to —70 rent is confirmed in isolated taste cells of hamster with
mV, an input resistance of 1-3 GQ and a membrane capaci- whole-cell recording (Fig. 20) [29].
tance of 3-5 pF [11]. During gustatory stimulation with Sinceamiloridecannotsuppressthewholesaltresponse,
basic substances changes in input resistance ofrat taste cells the residual salt response is possibly mediated by different
are shown in Table 1. The input resistance increases for all
stimuli excepting salt stimuli [89, 94]. NaCI stimulation
Table1. Change in input resistance of rat taste cells at the
!©®P
peaksofreceptorpotentialselicitedbyfourbasictastestimuli
Superficial
Taste stimuli Res(i%s)ta*nce Recept(omrVp)ottential NKto- of. cel,l,s fluid Amiloride
0.5M NaCI 62+4 25+2 44 NaCI
0.1M CaCl2 73+8 24+5 11
0.02MQ-HC1 178+16 9±2 42
0.01M HCI 148+8 11+2 34
0.5M sucrose 138+12 3.4+2.5 26 Tightjunction
* The values (mean+SE) are expressed as percent of control
mM
input resistance under 41.4 NaCI.
+ The values are mean+SE under 41.4mM NaCI adaptaion.
From [89].
TASTE CELL
Spontaneous andtastant-induced firingsofaction poten-
tials in mammalian taste cells are observed with the patch
clamp method [11], but are not with the intracellularrecord-
ingmethod [77, 88, 89, 94, 109, 110]. This discrepancymay
be partially derived from inactivation of voltage-dependent Interstitial fluid
channelsbyadamage-induceddepolarizationbyamicroelec-
trode [11]. The role ofthe action potentials is unclear, but Fig 19. Schematic drawing of salt signal transduction in mamma-
tuhnedseermaalyowbedennescietsysaorfyCfao2r+incihtaiantnienlgsaintrmanasmmmiattleiranrelteaasstee lsieannsittiavseteNace+llsc.hanNnael+(ad)i.rectBloythpaNsase+s atnhdroCulg~h aarne caomnisliodreirdee-d
topassthroughtightjunctionfrommucosalside toserosalside,
cells [11]. At least five kinds of voltage-dependent ionic resulting in a transepithelial potential change (dotted arrow).
channels: TTX-sensitiveNa+ channel, transientK+ channel, From [118].
776 T. Sato, T. Miyamoto and Y. Okada
Al Bl
On
A 30s
P
-400J
—
—
I amiloride -//{AVP}# 1 amiloride" I amiloride -//-|cAMFt//-| amiloride
A2 B2
control *^ control
+amiloride
postArg'-VP post(81-3Bmri-nc)AMP,
(lOmin) +amiloride
^<p 2.0s
postArg'-VP**r
(15min)
1.0s
U Arg*-VP
8-Br-cAMP
amiloride-sensitive H*current
Na*current
Fig. 20. Suppressiveeffectofamilorideonresponseoftastecellsisolatedfromhamsterfungiformpapillaetocitricacidanditsenhancementby
(arAgViPni)ne(-1v0asmoUpr/emslsinfor(A1r5gm8i-nV)P)(Aaln)dorcA0.M2P5.mMCu8r-rberntosmob-lcoAckMePd(bcyAM30P/o/rM8-amBirl-ocrAidMeP)(I(NBal)).areCuernrheanntcerdespaoftnesrestrtoeactimtreinctacwiidthstiAmrugl8at-iVoPn
(1H+) arealsoenhancedfollowingtreatmentwithArg8-VP(A2)or0.25mM8-Br-cAMP(B2). (C)MeanenhancementofINaandIH+ by
Arg8-VP and 8-Br-cAMP. From [29].
mechanisms. Amiloride sensitivity of several Na+ salts is sponseby4-APis40%. Thus, the mechanismmediatingthe
dependent on the size ofthe anions [23]. However, several residual response may be attributed to adsorption ofcations
anion channel blockers do not affectthe salt responses ofthe andsurfacepotentialchange onthetaste cellmembrane after
chorda tympani nerve [21]. Therefore, Ye etal. [118] have 4-AP suppression [63, 64].
proposedahypothesisthatfieldpotentialsgeneratedbyanion
permeabilitythroughthepericellularpathwayinthetastebud 3. Sour Taste
influence salt signal transduction (Fig. 19). Forthe transductionmechanismofsourtaste, noconclu-
Amiloride-sensitivity of salt responses in the rat chorda sive model has been proposed. It is postulated that in
tympani changes during development or after Na+- hamstertaste cellsH+ includedin acidstimulipassesthrough
deprivation [34]. Currents through amiloride-sensitive Na+ amiloride-sensitive Na+ channels, resulting in a depolariza-
channels in isolated hamster taste cells are enhanced by tion of taste cells [28, 29] (Fig. 19). This hypothesis is
arginine-vasopressin and cAMP (Fig. 19) [29]. Similar plas- consistent with the previous observation in rats [77].
ticityofNaClresponseisknowninfroggustatorysystem [70]. However, amiloride blocks the response to both NaCl and
These resultssuggest agreatplasticityindensityofamiloride- HC1 in hamster [33], but does not in monkey [32] and human
sensitive Na+ channels in mammalian taste cells as in other [99]. Amiloride blocks HC1 response in only sodium-
Na+-absorbing epithelia [114]. selective nerve fiber carrying primarily the information for
The glossopharyngeal nerve in mammals shows a low salty taste in hamster [33] and in rat [65]. Other pathways
sensitivity to salts, and the salt response in glossopharyngeal underlying the transduction mechanism for sour taste stimuli
nerve isneveraffected byamiloride [24]. Thisindicatesthat should be pursued in future even if amiloride-sensitive Na+
no amiloride-sensitive Na+ channels are expressed in the channel pathway may play some role in sour taste transduc-
taste cells of foliate and circumvallate papillae. The trans- tion in mammals.
duction of salt stimuli other than Na+ salts has not been The decrease of membrane conductance during acid
examined well so far with exception of suppression of KC1 stimuli in rat taste cellshasbeenobserved with the intracellu-
response by4-AP, a potassium channel blocker, in ratchorda lar recording method [89]. This might happen if the high
tympani nerve [37]. However, other potassium channel density of K+ channels is localized at the apical membrane
blockers such as tetraethylammonium, BaC^ and quinidine [17, 27]. In addition, it should be noted that proton induces
do not reduce KC1 response. The suppression of KC1 re- an increaseofanionicconductance inlingualepithelia, result-