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Abstract

To qualify as a “basic” taste quality or modality, defined as a group of chemicals that tastealike, three empirical benchmarks have commonly been used. The first is that a candidate

group of tastants must have a dedicated transduction mechanism in the peripheral nervoussystem. The second is that the tastants evoke physiological responses in dedicated afferenttaste nerves innervating the oropharyngeal cavity. Last, the taste stimuli evoke activity incentral gustatory neurons, some of which may respond only to that group of tastants. Here weargue that water may also be an independent taste modality. This argument is based on theidentification of a water dedicated transduction mechanism in the peripheral nervous system,water responsive fibers of the peripheral taste nerves and the observation of water responsiveneurons in all gustatory regions within the central nervous system. e have describedelectrophysiological responses from single neurons in nucleus of the solitary tract !"T#$ and

parabrachial nucleus of the pons, respectively the first two central relay nuclei in the rodent brainstem, to water presented as a taste stimulus in anestheti%ed rats. &esponses to water werein some cases as robust as responses to other taste qualities and sometimes occurred in theabsence of responses to other tastants. 'oth e(citatory and inhibitory responses wereobserved. )lso, the temporal features of the water response resembled those of other tasteresponses. e argue that water may constitute an independent taste modality that is processed

by dedicated neural channels at all levels of the gustatory neura(is. ater*dedicated neuronsin the brainstem may constitute key elements in the regulatory system for fluid in the body,i.e., thirst, and as part of the swallowing refle( circuitry.

Keywords: taste, gustatory, water, nucleus of the solitary tract, parabrachial nucleus of the pons

Introduction

Taste is a vital sensory process that facilitates the ingestion of nutritive substances and the

avoidance of to(ins. +t is not surprising, then, that the perception of taste stimuli is highly

informed by the homeostatic state of the organism ! acobs et al., - // 0 1orton et al., 2334 $

as well as prior e(periences with appetitive and aversive stimuli !5hang and #cott, - /6 0

1c5aughey et al., - 7 $. 8hysiological processing of taste stimuli begins with transduction

at the level of taste receptor cells distributed throughout the oropharyngeal cavity and e(tends

across a network of central neural structures. The sensory domain of the taste system can be

divided into a finite number of qualities or modalities, defined as a group of chemicals that

produce similar taste sensations and which are psychophysically independent of other taste

qualities. Historically, psychophysicists defined four basic taste qualities9 sweet, sour, bitter,

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and salty. &ecently, however, the designation of a fifth quality, umami !savory$, has been

supported by genetic and physiological data from peripheral taste cells !e.g., :hao et

al., 233; $. The designation of discrete taste modalities rests on physiological evidence from

every level of the nervous system and bears important functional implications for the

homeostatic and hedonic mediation of nutrient intake and to(in avoidance.

Though it is well known that water !or hypo*osmolarity$ evokes a unique taste sensation in

insects !<vans and 1ellon r., - 42 0 +noshita and Tanimura, 2334 $, it is perhaps surprising

that water has not heretofore been shown to evoke a distinct taste quality in mammals

!'artoshuk, - 76 $. This, despite substantial physiological evidence showing that there are

neurons that respond to water at every level of the gustatory neura(is. 1any investigators

who have described these water responses have assumed that they are due to somatosensoryor thermal stimulation0 however, here we show responses to water that cannot be accounted

for by these mechanisms. e will therefore argue that water constitutes an independent taste

modality, whose perception is critical for life.

The ob=ectives of the current investigation were to9 !-$ describe the neural responsivity to

water presented as a taste stimulus in cells of the nucleus of the solitary tract !"T#$ and

parabrachial nucleus of the pons !8b"$ and !2$ describe converging evidence that water is an

independent taste modality. e characteri%ed four different water*responsive cell types in the

"T# and 8b" of the rat. 1ost notably, we observed “water best” cells that responded more

vigorously to water than to any other taste stimulus. e argue that the classification of water

as an independent taste modality is supported by physiological evidence and has important

implications for taste*mediated regulation of ingestive behavior.

>o to9

Materials and Methods

Subjects

?ata from -3/ male #prague*?awley rats !;@3A6@3 g$ were included. &ats were given

unrestricted access to food and were paired housed with a -2*h lightAdark schedule. )nimal

care was in accord with the requirements of the +nstitutional )nimal 5are and Bse 5ommittee

of 'inghamton Bniversity. ?ata from "T# cells were obtained from two previously

published investigations of taste processing !&oussin et al., 233/ 0 &osen and ?iLoren%o, 233 $0 data from the 8b" are new.






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Surgery

8rior to surgery, rats were anestheti%ed with urethane !-.@ gCkg, i.p., administered in two

doses, 23 min apart$. #ome animals were given a third intraperitoneal in=ection of

pentobarbital !"embutal, 2@ mgCkg$. 'ody temperature was maintained at ;@A;7D5 with arectal thermistor probe connected to a heating pad !EH5, +nc., 'owdoinham, 1<, B#)$.

)nimals were tracheostomi%ed to facilitate breathing during stimulus delivery. The head was

mounted in a stereota(ic instrument with the upper incisor bar positioned @ mm below the

interaural line. #kin and fascia were removed and a non*traumatic head holder was secured to

the skull with stainless steel screws and dental cement. The occipital bone and meninges were

removed. +n the "T# surgeries, the posterior cerebellum was gently aspirated to e(pose the

underlying medulla. +n the 8b" surgeries, only the portion of the cerebellum overlying the

obe( was aspirated.

Taste stimuli and stimulus delivery

Taste stimuli consisted of 3.- 1 "a5l, 3.3- 1 H5l, 3.3- 1 quinine, and 3.@ 1 sucrose.

5oncentrations have been shown to elicit half*ma(imal potentials in the 5T nerve of the rat

!>anchrow and <rickson, - 73 0 Fgawa et al., - 76 $. Taste stimuli were made from reagent*

grade chemicals dissolved in distilled water and were delivered at room temperature. ater

was presented both before and after taste stimuli and responses to it were analy%ed separately

as described below.

The stimulus delivery system consisted of stimulus*filled reservoirs pressuri%ed with

compressed air and connected via polyethylene tubing to perforated stainless steel tubes

placed in the mouth. Eluid delivery was controlled by computer activation of a solenoid valve

interposed between the reservoir and the tongue. #timuli were delivered at a flow rate of @

mlCs. The taste solution bathed the whole mouth0 this was verified by application of

methylene blue through the system. <ach stimulus trial consisted of -3 s spontaneous activity,

-3 s of distilled water, @ s of tastant, @ s pause, and 23 s of a distilled water rinse. #pontaneous

activity was defined as the firing rate during the initial @ s of the trial when no stimulus

!tastant or water$ was present in the mouth. The inter*trial interval was 2 min. #timuli were

presented in repeated trials for as long as the cell remained well isolated. Eor any given

stimulus, all other stimuli were presented before it was repeated.





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Electrophysiological recording

<lectrophysiological recordings were conducted with etched tungsten microelectrodes !-/A

23 1G, - at - kH%0 EH5, +nc., 'owdoinham, 1<, B#)$. Eor "T# recordings, the

electrode was lowered into the caudal medulla above the rostral "T# located 2.7 mm anterior and -./ mm lateral the obe( and I-.3 mm below the dorsal surface of the brainstem. Eor 8b"

recordings, the electrode was lowered through the cerebellum above the pons located @.6 mm

anterior and -./ mm lateral to the obe( and @A4 mm below the cerebellar surface.

<lectrophysiological activity was digiti%ed with an analog*to*digital interface !1odel -63-,

5ambridge <lectronic ?esigns, 5ambridge, BJ$ and was processed with #pike2 software

!5ambridge <lectronic ?esigns, 5ambridge, BJ$. The signal was amplified !1odel 8@--,

>rass Technologies, est arwick, &+, B#)$ and monitored online with a speaker,oscilloscope and #pike2 software. #ingle cells were identified by periodically delivering a

3.-*1 "a5l solution followed by a water rinse as the electrode was slowly lowered through

the brain. 5ell isolation was based on the consistency of the waveform shape using template

matching and principal component analysis. ) signal*to*noise ratio of ;9- was required for

cell isolation. +solated cells were tested with the e(emplars of the four basic taste qualities

yielding the “response profile” of the cell, defined as the relative response rates across

tastants. ater was presented before tastant delivery and @ s after tastant delivery. Thisallowed the assessment of tastant*mediated alteration of the water response. The cell was

tested for as long as it remained isolated allowing for multiple presentations of the same

stimulus. The precise timing of each spike !- ms precision$ was calculated with respect to the

onset of each stimulus delivery, including water.

Data analysis

The magnitude of response to a given tastant was calculated as the mean firing rate !spike per

second0 sps$ during the first 2 s of tastant delivery minus the average firing rate !sps$ during

the initial @ s of spontaneous activity at the beginning of each trial. 'ecause water was

occasionally found to produce a sustained response in a subset of cells, the response to water

was not used as a baseline. ) taste response was considered to be significant if it was 2.@

standard deviations greater than the average spontaneous firing rate. )ll cells were classified

by their “best stimulus”, defined as the tastant that elicits the highest response magnitude.

8re* and post*tastant water responses were calculated by subtracting the mean spontaneous

firing rate from the firing rate during the initial 2 s of the pre* and post*tastant water delivery,

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respectively. The response magnitude to water was calculated before and after delivery of

each of the other taste stimuli.

The breadth of tuning of taste*responsive cells was calculated with both the traditional

Bncertainty measure !#mith and Travers, - 7 $ and the #electivity measure !&osen and ?iLoren%o, 233 $, a metric designed to capture both breadth of tuning and magnitude of

response. The formula for Bncertainty was H K k M P i!log P i$, where k !scaling factor$ K -.44

for four stimuli and P i is the proportion of response to stimulus irelative to the summed

responses to all four taste stimuli. alues ranged from 3 to -.3 with 3 corresponding to a cell

responsive to only one stimulus and -.3 corresponding to a cell equally responsive to all four

stimuli. #electivity was defined as the difference in response magnitude !in sps$ between the

sum of the two strongest responses and the sum of the two weakest responses !&osen and ?iLoren%o, 233 $.

>o to9

Results

eneral response characteristics

The responses to water and taste stimuli were recorded from -;@ cells ! - "T# cells0 66 8b"cells$, most with several stimulus trial repetitions9 +n the "T# there were -A-7 stimulus

repetitions, median K /0 in the 8b" there were -A24 stimulus repetitions, median K /. Thirty

of - "T# cells !;;N$ and -7 of 66 8b" cells !; N$ responded to water, either preceding or

following a taste stimulus. There were four types of water responses. Three were categori%ed

as water*responsive and one categori%ed as somatosensory. The four response types !shown

in Eigure Eigure-$ - $ were9 !-$ “ ater*e(citatory” These cells showed water response

magnitudes that were equal or greater than the magnitudes of response to all four prototypicaltaste stimuli ! n K -2$. Ten of these cells showed a larger response to water than to other taste

stimuli. These were called “water best” cells. Ff these, four responded only to water and were

called “water specialist” cells. Ff the remaining two cells that showed e(citatory water

responses, one was "a5l best and the other was quinine best. !2$ “ ater*inhibitory” These

cells showed a significant decrease in spontaneous firing rate in response to water ! n K 2$. !;$

“5onditional water” These cells responded to water, either with e(citation or inhibition but

showed significantly larger responses to water delivered after a subset of taste stimuli ! n K

22$. !6$ “#omatosensory water” These cells showed e(citation or inhibition in response to





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water and an equivalent response to at least some taste stimuli ! n K --$. e classified these

responses as somatosensory because it could be argued that the responses to water could be

interpreted as tactile.

Eigure -

Types o! water responses . "#$ ater*e(citatory. "%$ ater*inhibitory. "&$ 5onditional

water. "D$ &apidly adapting !upper$ and slowly*adapting !lower$ somatosensory cells.

'ater(responsive cell types

The distribution of cells showing each type of water response in the "T# and 8b" is shown

in Table Table-. - . #everal aspects of this table are noteworthy. Eor e(ample, water*inhibitory

cells were observed only in the "T#, but the prevalence of conditional water cells in the "T#

!-@ of ;30 @3N$ and 8b" !7 of -70 6-N$ was similar. +mportantly, there was only one

somatosensory cell in 8b" !of -7, 4N$ compared with 23 in "T# !of ;30 47N$. Table

Table- - shows response magnitudes for all stimuli and all cell types.

Table -.

Mean responses )SEM !or water and all taste stimuli in water(response cell types .

ater best cells ! n K -3$ constitute our strongest evidence that the taste of water is processed

by a separate coding channel. )mong these, there were four cells that were water specialists

in that they responded only to water and not to any taste stimulus. Eigure Eigure2 2 shows the

responses to water and the four prototypical taste stimuli in a water best cell recorded in the

8b". +t can be seen that water presented both before and after a tastant, evoked a large

e(citatory response with a phasicAtonic time course. 8resentation of H5l also evoked an

e(citatory response, but it was much smaller than that to water. +n contrast, presentation of

sucrose, "a5l or quinine evoked a return to spontaneous firing rates.

Eigure 2


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*eristimulus time histograms showing the responses o! a *b+ water best cell to water

"solid red arrows$ and sucrose, +a&l, -&l, and .uinine "blue arrows$ . #hown are eight

stimulus presentations superimposed for each histogram.

Eigure Eigure; ; shows the average response magnitudes of "T# and 8b" cells to water,sucrose, "a5l, H5l, and quinine for all cells. The responses to water in the 8b" cells were

significantly higher than responses to water in "T# O t !6;$ K ;.2, p P 3.3-Q. There was no

significant difference in the breadth of tuning of "T# and 8b" cells as assessed by both the

Bncertainty and #electivity measures. The mean H values were 3.7 R 3.3@ and 3.77 R 3.36

for "T# cells and 8b" cells, respectively. The mean #electivity values were -@.; R ;.4 and

- .2 R ;./ for "T# cells and 8b" cells, respectively. The mean spontaneous activity of cells

in the "T# !;.7 R 3./$ and 8b" !;. R -.2$ also did not significantly differ.

Eigure ;

#verage responses to water ")SEM$ and the !our prototypical taste stimuli in cells o! the

+TS "#$ and *b+ "%$ . Bnits were aligned in descending order of magnitude of their

response to water.

The responses to water before and after each of the four prototypical taste qualities are shownin Eigure Eigure6. 6. Tastant delivery altered water responsivity in a stimulus specific manner

in all 22 cells that showed conditional water responses. +t can be seen that water responses

following a taste stimulus could either increase or decrease with respect to the water response

=ust preceding that tastant. Eurthermore, conditional water responses were found for every

taste stimulus in some subset of cells and were equally common in "T# and 8b".

Eigure 6

#verage responses to water delivered be!ore "red$ and a!ter "blue$ each o! the !our

prototypical taste stimuli in the +TS "#$ and *b+ "%$ . Eor each stimulus, cells are aligned

according to their pre*tastant water response.

Table Table2 2 compares the water*responsive cell types in "T# and 8b" with respect to

which taste stimulus elicited the best response. "otably, most conditional water responses

were found in "a5l or H5l best cells in "T# and in "a5l best cells in 8b".














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Table 2.

*revalence o! the /best stimulus0 !or each cell type in the +TS and *b+ .

The water responses of conditional water cells were specific to the taste stimulus that

immediately preceded water delivery. The particular taste stimuli that resulted in conditional

water responses differed between the "T# and 8b" !see Table Table;$. ; $. +t can be seen that

any taste stimulus could be followed by a conditional water response. +n the "T#, the largest

increase in water responsivity was observed when water followed H5l but in the 8b", "a5l,

and quinine produced the largest increases in water response.

Table ;.

Incidence o! conditional response !ollowing each taste stimulus .

ater*inhibitory cells, found only in the "T#, showed significant decreases in spontaneous

activity when water was presented both before and after the delivery of another tastant. These

two cells both responded best to "a5l. Eigure Eigure@ @ shows a response to sucrose and to

"a5l in the two inhibitory*water cells. +n Eigure Eigure@), @), water significantly reduced

spontaneous activity to near %ero while sucrose evoked a return to spontaneous firing rate. +n

this cell, "a5l evoked a brief response followed by a return to spontaneous firing rates

!Eigure !Eigure@ @'$.

Eigure @

Inhibitory responses to water be!ore and a!ter taste delivery in one cell . This cell is

inhibited by water and does not respond to sucrose, as indicated by a return to spontaneous

firing rate "#$ , but shows a brief response to "a5l "%$. 1ean spontaneous 111

>o to9

Discussion

ater delivered to the oropharyngeal cavity evoked activity in a diverse group of neurons in

the "T# and 8b", the first and second central gustatory relays. )bout a third of cells in each

structure !;3 of -, ;;N in the "T#0 -7 of 66, ; N in the 8b"$ responded to water either






















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channels that facilitate neurotransmitter release !>ilbertson et al., 2334 $. Thus a mechanism

e(ists in mammals for transduction of water, apart from other taste stimuli.

ater is also an effective stimulus in peripheral nerves that respond to more traditional taste

qualities. #pecifically, water responses have been observed in the chorda tympani nerve !a branch of the facial nerve innervating taste buds on the rostral 2C; of the tongue$ of the rat,

cat, and dog !8faffmann and 'are, - @3 0 Lil=estrand and :otterman, - @6 0 :otterman, - @4 $,

superior laryngeal nerve !a branch of the vagus nerve innervating taste buds on the palate$ of

the rat !#hinghai, - /3 0 Hanamori, 233- $ and glossopharyngeal nerve !innervating taste buds

on the caudal -C; of the tongue$ of the frog, hamster, and rat !:otterman, - 6 0 Hanamori et

al., - // 0 Erank, - - $. ater specialist fibers have been observed in the superior laryngeal

nerve !#hinghai, - /3 $, a nerve that plays an important role in swallowing !Jitagawa etal., 233 $. +nterestingly, water applied to the posterior tongueClaryn( in humans was shown to

be particularly effective at evoking a swallowing refle( as compared to other taste stimuli

!#hinghai et al., - / $. +n addition, many neurons in the intermediate "T# that mediate

swallowing have been shown to receive convergent input from the superior laryngeal and

glossopharyngeal nerves !Fotani et al., - @ $. +n sum, water*responsive fibers have been

described in several gustatory nerves and may play a role in the swallowing refle(.

ater*specific responses have also been observed in many gustatory processing regions of

the brain including the "T# !"akamura and "orgren, - - $, 8b" !"ishi=o and

"orgren, - 3 $, thalamus ! erhagen et al., 233; $, and gustatory corte( !de )rau=o et

al., 233; $. These responses, though widely observed, are seldom described in detail. +n the

current investigation, we e(tend these observations by describing responses to water elicited

in cells of the "T# and 8b". Though some of these responses may be due to tactile or

thermal stimulation, we detail some responses for which these e(planations are inadequate.

'ased on this evidence, we argue that the neural representation of water parallels that of moretraditional taste qualities.

+n addition to the role of water in evoking a swallowing refle(, the perceptual consequences

of water taste may play a critical role in regulating fluid intake !thirst$ and the maintenance of

hydration. The responses described here were recorded with passive stimulus delivery in a

non*deprived state and in the absence of post*ingestional effects. +t is possible, however, that

homeostatic variables such as thirst may modulate the responses to water. Eor e(ample, in an

imaging study of thirst and water taste processing in the human gustatory corte(, the primary






































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gustatory corte( !anterior insula and frontal operculum$ was activated by water regardless of

thirst but the secondary gustatory corte( !caudal orbitofrontal corte($ only showed water*

evoked activation in a water*deprived state !de )rau=o et al., 233; $. The most likely source of

this water*selective cortical activation is taste*related structures that receive input from the

oropharyngeal area. The ability of cortical neurons to distinguish between water and other

tastes supports the idea that water may be processed as an independent taste quality.

+n addition to oral sensitivity, water may also be detected by chemoreceptors located in the

gut !&o%engurt and #ternini, 2337 $. 5hemical transduction in the enteric nervous system

plays an important role in gut motility, the regulation of nutrient absorption, gastric emptying

and acid secretion !'aggio and ?rucker, 2337 $. The vago*vagal refle(, essential for digestives

processes such as gastric emptying, involves vagal pro=ections to "T# and from there to thedorsal motor nucleus of the vagus which e(tends efferents that mediate gastric function

!1c5ann and &ogers, - 2 0 Jonturek et al., 2336 $. The gastric and intestinal mucosa is

sensitive to both mechanical and chemical stimulation !5ottrell and +ggo, - /6 $. #tudies have

characteri%ed modality specific enteroreceptors for sugars, acids, fats, amino acids as well as

water !+ggo, - @7 0 1ei, - 7/ 0 eanningros, - /2 0 1ei and >arnier, - /4 $. <ndocrine cells of

the luminal tissue transduce mechanical and chemical stimuli via paracrine activation of

vagal afferents which synapse in the caudal "T# !:hu et al., 233- 0 oung et al., 233/ $ andspinal afferents which synapse in the dorsal horn of the spinal cord !Jonturek et al., 2336 $.

#timulation of the small intestine of the cat with various chemical solutions has shown that

water evokes large responses in the nodose ganglion containing cells of the vagus nerve !1ei

and >arnier, - /4 $.

'ater(responsive and somatosensory cell types

The idea that the taste of water is independent from that of other taste qualities represents a

substantial departure from the conventional supposition that water responses are entirely

somatosensory. 8resent data support a clear distinction between water*responsive and

somatosensory cell types. #omatosensory cells appeared to respond only to the mechanical

andCor thermal components of a fluidic stimulus. That is, their responses to water and tastants

were indistinguishable. )s can be seen in Eigure Eigure-, - , some of these responses were

phasic, suggesting rapid adaptation, and others were more tonic, suggesting slow adaptation.

The presence of somatosensory cells is not surprising given the fact that most taste*

responsive cells in both "T# and 8b" receive convergent mechanosensory input !Fgawa and




































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Jaisaku, - /2 0 Fgawa et al., - /2 , - /6 0 Fgawa and Hayama, - /6 $. +n contrast, cells

identified as water*responsive in the present study showed responses to water that could not

be accounted for by the mechanosensory aspects of the stimulus. Eor e(ample, water best

cells responded more vigorously to water than to any of the four other taste stimuli even

though all stimuli would be e(pected to evoke equivalent tactile and thermal sensations. +t is

possible, perhaps likely, that responses in these cells reflect the osmolarity of the solution,

with water being the most hypo*osmotic. +n effect, these cells might respond to taste stimuli

as a mi(ture of water and taste stimulus, with water being the effective stimulus that drives

neural activity. ater*inhibitory and conditional water cells showed water*specific inhibitory

responses that could be predicted by the taste stimulus that preceded water presentation. +n

this case, an interaction of tastant and water, likely peripheral in nature !'artoshuk, - 77 $,

might be at play in these cells. +n fact, it is well known that water presented after some taste

stimuli, usually acid, induce tastes of their own !e.g., Fakley, - /@ $.

&omparison o! +TS and *b+ water responsivity

+n general, differences between the "T# and 8b" suggest that water sensibility may serve

different functions in each structure. +n the "T#, the relatively large proportion of

somatosensory responses to water may be part of neural circuits e(tending from the caudal

"T# that produce ingestive refle(es such as swallowing !Lang, 233 $. +n contrast, the

significantly higher water*evoked firing rates in the 8b" suggest that the 8b" may be part of

the chemosensory pathway for the perception of water along with other taste stimuli.

The sensitivity of gustatory "T# cells to water may play an essential role in the initiation of

swallowing. The superior laryngeal nerve of the rat includes water*selective fibers

!#hinghai, - /3 $ and has been shown to converge with the glossopharyngeal nerve onto

neurons that mediate swallowing in the intermediate "T# !Fotani et al., - @ $. "eurons in

the rostral and ventral "T# pro=ect caudally to the intermediate "T# !#treefland and

ansen, - $ which contains premotor neurons that initiate swallowing when water is

applied to the pharyn( !Lang, 233 $. 8hysiological and behavioral evidence suggests that the

chemosensory properties of water may be important in eliciting the swallowing refle(. Eor

e(ample, water has been shown to be an e(ceptionally effective stimulus for eliciting a

swallowing refle( when applied to the pharyn( and laryn( !#torey, - 4/ 0 #hinghai et

al., - / $. The e(tensive connectivity of the rostral "T# with the intermediate "T#, reticular

formation and various oromotor nuclei may facilitate swallowing and other taste and





























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somatosensory*mediated ingestive refle(es. Fnly about a third of "T# cells pro=ect directly

to the 8b" in rat !Fgawa and Jaisaku, - /2 0 Fgawa and Hayama, - /6 0 1onroe and ?i

Loren%o, - @ $, therefore at least some 8b" activity may reflect taste processing that is

independent of "T# input !see ?i Loren%o et al., 233 $. >ustatory and hepatic*vagal afferents

have been shown to converge to a greater degree in the postero*medial 8b" as compared to

the "T# !Hermann et al., - /; $. agal fibers innervating luminal tissue respond robustly to

water !1ei and >arnier, - /4 $ and may therefore contribute to the enhanced water

responsivity observed in the 8b".

'ater taste and /a!ter(tastes0

5onditional water responses were more prevalent in the "T# than the 8b". This observation

suggests that the "T# responses may be more closely tied to peripheral nerve input than the

8b". 8sychophysical studies over the last 63 years have emphasi%ed the interdependence of

water responses and sensitivity to other taste stimuli. +n - 76, 'artoshuk showed that water*

evoked a taste but that it was highly dependent on adaptation to a preceding taste stimulus

even inclusive of saliva. +n addition, recordings from afferent taste nerves have shown that

some water responses were only observed after pre*e(posure to a particular taste stimulus

!'artoshuk and 8faffmann, - 4@ 0 'artoshuk et al., - 7- $. These responses were similar to

those of the conditional cells reported in the current study, suggesting that responses in these

cells directly reflect peripheral processing. The fact that water that follows certain tastes

produces a taste sensation of its own implies that these responses to water access the

information channels of the taste sensations they evoke as the signal ascends through the

central nervous system.

&onclusions and caveats

The present results are not without their limitations. Eor e(ample, some responses to water

may have been affected by the anesthetic under which they were recorded. Brethane

anesthesia delivered intraperitoneally !+8$, as in the present study, has been shown to produce

hyperglycemia, increased hematocrit and decreased blood plasma protein levels ! an ?er

1eer et al., - 7@ $. #ince polydipsia !thirst$ is a symptom of hyperglycemia, it is possible that

urethane anesthesia could modify responses to water and other taste stimuli.

#ampling errors might also have affected present results. That is, in the recordings of taste

responses in both "T# and 8b", most water responses were observed only incidentally. That






















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is, water responses were only noted “after the fact”, when analyses of responses to other

tastants were analy%ed. +t is therefore possible that we missed some cells that might have

been responsive to water. <ven so, our evidence, and those of others who have described

water responses, clearly suggests that the neural representation of water in the chemosensory

pathway is weaker than that of other taste qualities. Fn the other hand, the consistent

presence of water responses in almost every electrophysiological study of taste, including the

present one, implies that sensibility to water is clearly present and may serve an important

purpose.

+n conclusion, we have described responsivity to water in two brainstem nuclei, the "T# and

8b", in the anestheti%ed rat. ) novel finding of the current study is the observation of water

best neurons and water specialist cells in both "T# and 8b". +mportantly, these dataunderscore the literature showing that water is processed by cells in gustatory nuclei. This

implies that water evokes a gustatory sensation that is supported by an independent neural

representation, different from that of other taste qualities.

&on!lict o! Interest Statement

The authors declare that the research was conducted in the absence of any commercial or

financial relationships that could be construed as a potential conflict of interest.

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