MINIREVIEW SERIES FOR THE 50TH
VOLUMEOur recent studies on sensory transduction:from vision to taste
Akimichi Kaneko
Department of Physiology, School of Medicine, Keio University, Tokyo, Japan(Received for publication on November 30, 2000)

Abstract. The transduction in photoreceptors and chemoreceptors has common mechanisms. In
photoreceptors, activation of rhodopsin by light triggers the enzymatic cascade mediated by a G-protein, transducin. In olfactory cells activation of the receptor molecule by odorant triggers the enzymaticcascade mediated by a G-protein, Golf. In taste receptor cells biochemical studies have also suggested ametabotropic transduction hypothesis, but we recently identi®ed a cationic channel that was directly
gated by a bitter taste substance. By reviewing these recent studies carried out in our laboratory in thelast 10 years, the transduction machinery in these sensory receptor cells are summarized. (Keio J Med
50 (1): 13±19, March 2001)
Key words: phototransduction, retina, L-glutamate, chemoreception, olfaction, taste

mission of retinal neurons. Nowadays, the fundamental
retinal circuit for processing the retinal image has been
elucidated.4
The molecular mechanism of phototransduction is
now understood in detail.5 Figure 1 summarizes schematically the phototransduction process. When the incident light is absorbed by a rhodopsin molecule sittingin the disc membrane of the outer segment of a photoreceptor, the conformation of the rhodopsin molecule ischanged and an enzymatic cascade is started. The ®rst
step of this cascade is the activation of transducin, a Gprotein. Transducin will then activate the cyclic GMPphosphodiesterase, which decomposes cyclic GMP and
reduces its cytoplasmic concentration. The plasma
membrane of the photoreceptor outer segment has cyclic GMP-gated cation channels, which are maintainedin the open state in the dark when the concentration of
the cytoplasmic cyclic GMP is high. Reduction of cyclic
GMP concentration results in the closure of the channel
and cell hyperpolarization. L-Glutamate is the transmitter of vertebrate photoreceptors.6 It is released tonically in the dark when the photoreceptor is maintainedin the depolarized state. Light-induced hyperpolarization results in the reduction of the amount of transmitter release.

In commemorating the 50th volume of the Keio
Journal of Medicine, I would like to introduce the research activity of the Sensory Physiology Group of the
Department of Physiology in the recent 10-year period.
The history of the group goes back to 1957, when thelate Professor Tsuneo Tomita started research on thephysiology of the retina. In 1960s, Tomita and his groupdemonstrated, for the ®rst time in the world, the existence in the vertebrate retina of three types of cone
photoreceptors with different spectral sensitivity, andestablished the physiological basis of the trichromaticmechanism of color vision.1 Thus, they gave the ultimate answer to the long-lasted battle between the trichromatic theory and the opponent color theory on themechanism of color vision.
Following this discovery, they elucidated the mechanism on the generation of the hyperpolarizing lightevoked responses of photoreceptors.2,3 The common
knowledge at that time was that neural excitation isaccompanied by depolarization of the cell. Our experiments presented evidence to interpret this contradictory observation. We found that light induced closing,
not opening, of the cation-permeating channel. Theseobservations stimulated, worldwide, the research on themechanism of phototransduction and synaptic trans-

Reprint requests to: Dr. Akimichi Kaneko, Department of Physiology, School of Medicine, Keio University, Shinjuku-ku, Tokyo 160-8582,
Japan, e-mail: kaneko@physiol.med.keio.ac.jp
13

14Fig. 1 A schematic illustration of phototransduction. The incident
light activates rhodopsin (R) residing on the disc membrane. The
activation of rhodopsin triggers a cascade chain reaction via transducin (G), a G-protein, followed by cyclic GMP phosphodiesterase(PDE), a hydrolyzing enzyme of cyclic GMP (cGMP). The plasma
membrane of the outer segment has cationic channels that open whencGMP binds to them. Decomposition of cGMP results in the closureof this channel. The channel passes Na and Ca 2 . Ca 2 entered into
the cytoplasm binds to S-modulin (S-mod) that inhibits rhodopsin
kinase (RK) which is the enzyme to phosphorylate the light-activatedrhodopsin to terminate its activity. Thus, Ca-S-modulin complex prolongs the lifetime of light-activated rhodopsin and enhances the sensitivity of photoreceptors. Depolarization of photoreceptors accelerates the release of L-glutamate, the transmitter of photoreceptors
from the terminal.

One of the recent contributions of our group to theunderstanding of phototransduction is the discovery of
a Ca 2 -binding protein, S-modulin, that controls thesensitivity of photoreceptors during adaptation.7 Under
the high Ca 2 condition S-modulin slows down phosphorylation of the activated rhodopsin, which is themechanism to stop the enzymatic cascade by inactivating the light-activated rhodopsin. In the dark, Ca 2
comes in through the cyclic GMP-gated cation channel,
and binds to S-modulin. The target of Ca 2 -S-modulin
complex is rhodopsin kinase. Rhodopsin kinase halts
the activity of the light-activated rhodopsin by phosphorylation. Thus, the lifetime of rhodopsin is longer in

Kaneko A: Overview of sensory transduction

the dark-adapted state than in the light-adapted state,
and the light sensitivity of photoreceptors is controlled.
The signal from photoreceptors is transmitted tobipolar cells, the second-order neuron of the retina.
There are two types of bipolar cells. The ON-type
bipolar cells are depolarized by light, while the OFFtype bipolar cells are hyperpolarized by light. Bothtypes of light responses are generated by L-glutamate
released by the photoreceptors. Recently, the basis of
the opposite polarity of light-evoked responses in bipolar cells was discovered.8,9 The two types of bipolarcells have different L-glutamate receptors; the metabotropic type in the ON-type bipolar cell and the ionotropic type in the OFF-type bipolar cell. The signaltransduction in the ON-type bipolar cell is very similar
to that in the photoreceptors (Fig. 2A). The ON-type
bipolar cells have cyclic GMP-gated cation channels in
the plasma membrane and a high cytoplasmic cyclic
GMP concentration. Consequently, cyclic GMP generates a ``standing'' inward current. These bipolar cells
have a metabotropic glutamate receptor, mGluR6, inthe dendritic tips.10 Metabotropic glutamate receptors
have 7 membrane-spanning helices, and their activation
initiates an enzymatic cascade via a GTP-binding protein. The response of ON-type bipolar cells is interpreted as follows. L-glutamate binds to mGluR6, which
in turn triggers a cascade of a GTP-binding protein anda cGMP phosphodiesterase (PDE). Activated PDE
decomposes cytoplasmic cGMP and reduces its concentration, thus removing cGMP molecules from thecGMP-gated cation channels. Closure of the channel
causes hyperpolarization. Light illumination reduces
the amount of L-glutamate release, and stops the enzymatic cascade described above. The end result is thecell depolarization. Presence of the GTP-binding protein (Go) has been demonstrated in the dendrites of the
ON-type bipolar cells.11
Response generation in OFF-type bipolar cells is
more straightforward (Fig. 2B). Ionotropic glutamate
receptor channels in the dendrites of OFF-type bipolarcells have been proposed to be of the AMPA type.9
These channels have cation permeabilities of low selectivity, passing Na , K and Ca 2 . The reversal potential of the L-glutamate-induced inward current isnear 0 mV. In the dark, L-glutamate released fromphotoreceptors opens this receptor channel, maintaining the cell in a depolarized state. Light illumination
reduces the amount of L-glutamate release, and hyperpolarizes the OFF-type bipolar cell.
The metabotropic mechanism of sensory transduction is also found in the olfactory receptor cell (Fig.
3).12 The receptor cell in the olfactory epithelium isequipped with cilia emanating from the free surface ofthe receptor cell into the nasal cavity. On the plasma
membrane of the olfactory cilia, the receptor protein

Keio J Med 2001; 50 (1): 13±19

A

15

B

Fig. 2 Schematic diagram illustrating the mechanism of response generation in ON- and OFF-type bipolar cells. A) The ON-type bipolar cell
has a metabotropic glutamate receptor (mGluR) which, when activated by glutamate, triggers an enzymatic cascade consisting of GTP binding
protein and cGMP phosphodiesterase (PDE). The activated PDE decomposes cytoplasmic cGMP and reduces its concentration. The plasma
membrane has cGMP-gated channels through which cations ¯ow continuously. Reduction of cytoplasmic cGMP results in closure of the channels and causes hyperpolarization. Because light illumination reduces the amount of glutamate release, an inverse cascade operates, anddepolarizes the cell. B) The OFF-type bipolar cell has an ionotropic glutamate receptor. At this receptor, glutamate opens a cation channel with
low ionic selectivity. In the dark, glutamate is tonically released from photoreceptors and the channels remain open, keeping the cell depolarized. Illumination reduces glutamate release and results in the closure of channels and hyperpolarization of the cell.

with 7 membrane-spanning helices is incorporated.
Binding of an odorant to this receptor protein activates
the receptor molecule, and triggers the enzymatic cascade mediated by a G-protein, Golf , followed by thesynthesis of cyclic AMP from the cytoplasmic ATP byan adenylate cyclase. The ciliary membrane has cyclic
AMP-gated cation channels. Cytoplasmic cyclic AMP
binds to this channel and opens it, and consequently
the inward current through the channel depolarizes theolfactory receptor cell.
As has been stated, the metabotropic enzymatic cascade is widely used in the sensory transduction systems:
photoreceptors, ON-type retinal bipolar cells and olfactory receptors. These observations drove us to expand our research ®eld into the chemosensory system.
We found the taste transduction as one of the research®elds that needs to be studied with modern technology.
The transduction mechanism of bitter taste is still acontradictory matter among researchers. The major
current hypothesis, based mainly on the biochemical

data, involves metabotropic cascades, either via cytoplasmic cyclic nucleotide monophosphate (cNMP) (Fig.
4) or via IP3 . It has been shown that a taste receptor cell
expresses a speci®c G protein, gustoducin,13 which
activates phosphodiesterase leading to the decomposition of cNMP.14,15 The frog taste receptor cell has acationic channel, which is kept closed at a high cNMP
concentration. Bitter stimuli reduce the cNMP concentration and release the cationic channel from the closed
state.16
The IP3 hypothesis (Fig. 5) proposes that a bitter
substance increases the cytoplasmic IP3 concentrationby activating a G protein and phospholipase C. IP3
triggers Ca 2 release from the endoplasmic reticulum,
which in turn directly or indirectly induces transmitter
release from the taste receptor cells.17±20
We found recently an entirely different mechanism
for the bitter taste transduction,21,22 which will be
described in detail. We excised an outside-out patch
membrane from an isolated taste receptor cell of the

16

Fig. 3 Schematic diagram illustrating the mechanism of response
generation in olfactory receptor cells. Odorant binds to the olfactory
receptor molecule (R) contained in the plasma membrane of the olfactory cilia and activates the molecule. The activated receptor molecule triggers an enzymatic cascade consisting of GTP binding protein
and adenylate cyclase (AC). AC synthesize cyclic AMP (cAMP)
which binds the cationic channel on the plasma membrane. Na andCa 2 ¯ow in through the opened channel and depolarize the receptor
cell. Ca 2 in the cytoplasm opens Ca-dependent Cl channel and theoutward movement of Cl results in an enhanced depolarization of
the receptor cell.

fungiform papillae of the bullfrog. Under our recording
condition, none of the second messenger candidates ortheir precursors (e.g. cyclic nucleotide, IP3 , Ca 2 , ATP
and GTP) was present on the cytoplasmic face of thepatch membrane. Application of a bitter taste substance, 1 mM quinine, to the external surface of thepatch membrane induced channel openings (Fig. 6).
The channel events can be seen more clearly in an
expanded time scale (Fig. 6B). The mean amplitude of
the unitary current was approximately ±1.0 pA. Quinine applied to the cytoplasmic side of the membrane
was ineffective. The channel was activated not only by
quinine but also by other bitter taste substances, denatonium or strychnine (Fig. 7).
The response could be recorded more than 10 min
after the excision of the patch membrane (Fig. 8). Thisobservation strongly indicates that during this period
any soluble substances are expected to be washed away
if it remained on the cytoplasmic side of the patch
membrane. Furthermore, we tested whether there is

Kaneko A: Overview of sensory transduction

Fig. 4 A model of bitter taste transduction based on the G protein ±cNMP cascade hypothesis. A bitter tastant binds to the receptor thatis coupled to a speci®c G protein, gastoducin. Active gastoducin triggers phosphodiesterase activation leading to the decomposition of
cNMP. The cationic channel, kept closed at a high cNMP concentration is now released from the closed state. Through this channel cation ¯ows in and depolarizes the cell.

Fig. 5 A model of bitter taste transduction based on the G protein ±IP3 cascade hypothesis. A bitter substance increases the cytoplasmic
IP3 concentration by activating a G protein and phospholipase C. IP3
triggers Ca 2 release from the endoplasmic reticulum, which in turn
directly or indirectly induces transmitter release from the taste receptor cells.

Keio J Med 2001; 50 (1): 13±1917Fig. 6 Currents induced by bitter substances in the outside-out membrane patches. A) Schematic diagram showing the experimental procedure. After the whole-cell con®guration (upper panel) was established, a patch membrane was excised from an isolated taste receptor cell by apatch pipette ®lled with a solution containing no cNMP, Ca 2 , ATP, or GTP. A bitter substance was applied to the outer surface of the patch
membrane by pressure (lower panel). B) Quinine-induced current response recorded from an outside-out membrane patch held at ±79 mV.
Upward step of the horizontal line on the top indicates the timing of 1 mM quinine application from the puffer pipette with a pressure of 20 kPa.
The outside of the membrane was bathed in the 120 mM NaCl (no Ca 2 added) solution and the patch pipette contained 120 mM CsCl. The
amplitude histogram on the right side of the current trace was made from 2000 points during the 2-s quinine application. The histogram was®tted by a sum of two Gaussian distributions represented by continuous curves. The higher peak represents the closed state (C) and the lower
peak represents the open state of one channel (O). The event labeled with an asterisk has a quadruple amplitude to that of the unitary current.
C) A part of the trace in B (indicated by a short bar below the current trace) reproduced in an expanded time scale. D) Denatonium-induced
current response in another outside-out patch. Denatonium (5 mM) was puff-applied with a pressure of 50 kPa. The bath solution contained 115
mM NaCl and 2.5 mM KCl (no Ca 2 added). Other conditions were identical to those in B. (Reproduced from Tsunenari, et al., J. Physiol. 519,
1999).

Fig. 7 Response of a taste receptor cell isolated from the fungiform
papilla of the bullfrog recorded under the whole cell con®guration
voltage clamped at 54 mV. Records from three different cells. Bitter
substances (5 mM quinine, 5 mM denatonium, 10 mM strychnine)
were applied by pressure ejection. The patch pipette was ®lled witha solution containing 115 mM CsCl, 2 mM MgCl2 , 0.5 mM CaCl2 ,2 mM Na-EGTA, 10 mM Na-HEPES, and 2 mM Na2 ATP (pH 7.2).
The bath solution contained 115 mM NaCl, 2.5 mM KCl, 2 mM
Na-HEPES, and 2 mM glucose (pH 7.2).

18

Fig. 8 Evaluation of whether the cytoplasmic soluble elements contribute to the response generation. A) Response of a membrane patch
to quinine recorded 4, 5, 8 and 14 min after membrane excision. Quinine (5 mM) was ejected from a puffer pipette in the timing indicatedby the bar over each current trace. The holding voltage was 54 mV.
B) Magnitude of the integrated response of A as expressed by theamount of charge generated during quinine application.

Kaneko A: Overview of sensory transduction

any sign indicating the contribution of G protein cascade to the response generation. If G protein activation
precedes the channel opening, addition of GDPbS, ahydrolysis-resistant GTP analog that binds to G protein
without activating it, would block the cascade, and thus
block the channel opening (Fig. 9B). On the other
hand, it is expected that addition of GTPgS, a hydrolysis-resistant activator of G protein, would activate G
protein, if some remained on the patch membrane, andwould activate the channel continuously (Fig. 9C). We
added either GDPbS (1 mM) or GTPgS (1 mM) to thecytoplasmic side of the membrane, but the response
was unaffected (Fig. 9). These data suggest that thebitter taste substances directly gated the channel.
We also studied the properties of the bitter taste
substance-gated channel. The response was dosedependent. The lowest effective concentration of quinine
was 0.1 mM, and the response to quinine saturated at
1 mM. The dose response curve was ®tted in a Hill equation with a K1=2 of 0.52 mM and a coef®cient of 3.8.
The channel had non-selective permeability to cation.
The permeability ratio to Na , K , and Cs wasFig. 9 A) A model illustrating the bitter taste transduction based on the G protein ± cNMP cascade hypothesis and the G protein ± IP3 cascade
hypothesis. B) A current response to quinine (1 mM, ejected from a puffer pipette with pressure) recorded 3.6 min after the membrane excision
with a patch pipette containing 1 mM GDPbS. The holding voltage was ±79 mV. C) A current response (1 mM, ejected from a puffer pipettewith pressure) recorded 4.0 min after the membrane excision with a patch pipette containing 1 mM GTPgS. The holding voltage was 54 mV.
The bath solution (B±C) contained 115 mM NaCl, 2.5 mM KCl (no Ca 2 added) and the patch pipette solution contained 120 mM CsCl.
(Reproduced from Tsunenari T, et al., J. Physiol. 519, 1999)

Keio J Med 2001; 50 (1): 13±19

PNa : PK : PCs  1 : 0:48 : 0:39, and the single channel
conductance was 10 pS. Similar values were obtained in
our previous study on the whole-cell current response21(PNa : PK : PCs  1 : 0:5 : 0:42) and by noise analysis(12 pS).23
The present ®ndings give new evidence to suggestthat bitter taste transduction involves direct channel
opening by the bitter taste substance, but does not necessarily rule out the possibility of the metabotropic
process involving the second-messenger system. In fact,
it has been suggested that the taste transduction contains parallel pathways of different mechanisms.24 It
will now be of interest to ask whether the ionotropic
and the metabotropic processes co-exist in bitter tastetransduction, and if so how they interact.
In this review article I have made a brief summary of
the research achievement of our laboratory in the past
10 years. The sensory organ detects the information of
our surrounding environment. Study of the sensorytransduction mechanism makes a bridge between thepsychological phenomena and the physiological performance of our body structure. It also elucidates themechanisms of various sensors sensitive to various
parameters within the body: body temperature, pH andother chemical components of the body ¯uid, and thestretch. Studies on these biological sensors are indispensable not only to understand our body function, butalso to help designing sensitive and reliable sensors in
the industrial ®elds.
Acknowledgments: The work on the taste receptor cell was supported
in part by Grants-in-Aid for Scienti®c Research from HFSPO,PRESTO (to Takashi Kurahashi) and Ministry of Education, Science,Sports and Culture (to Takashi Tsunenari, Takashi Kurahashi andAkimichi Kaneko), the funds from Keio Health Consulting Center
and Keio Gijuku Academic Development Fund (to Takashi Tsunenari).

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