Keenan1

DifferencesBetweenMaleandFemaleGustatory

SystemsinRats

ChristopherKeenan

Fall2011

A critical literature review submitted in partial fulfillment of the

requirements for senior research thesis

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Abstract

The importance of taste in survival and pleasure has lead to the evolution of the

gustatory system as the most specific of the five senses. This specificity refers to the keen

ability to discriminate between the five tastants. This study reviews the current literature

regarding the differences between male and female preferences, and abilities of

discrimination.

In terms of the individual tastants, the sex differences in preference are more or

less agreed upon. It has been shown that the males are able to withstand stronger sweet

concentration in the short-term, but females prefer the stronger solutions in the long-term.

Males have also been shown to have a greater ability to discriminate between more dilute

concentrations of a sweet substance. Females have also been shown to be most sensitive

to salt solutions, which gave rise to different theories such as metabolic need and child

rearing. Although not technically a tastant, males also have a greater ability to

discriminate between fats. Females have also been shown to have a greater preference for

MSG. But when LA is added to the MSG, it creates a boosting effect that is greater in the

males.

There have been advancements in research of the corda tympani, which suggest

that the nerve plays a greater roll in the female gustatory system than in the males. The

effects of estrogen and testosterone are still debated due to conflicting research, and

conflicting theories resulting from the data. Overall, advances sex differences are being

made in all areas of the gustatory system. This is with exception of sour, bitter and

umami, which have largely gone unstudied.

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Differences between male and female gustatory systems in rats

Introduction

Thegustatorysystemservesmanyrolesintheregulationhomeostatic

systemsinthebody,aswellasprotectingthosesystemsfromharmfulagents.

Becauseofthisimportance,thegustatorysystemhasevolvedtoservethemost

specificfunctionofallfivesenses.Thetasteofasubstanceallowsanorganismto

discriminatebetweenchemicalsthatarenutritious,andsafetoingest,andthose

thatmaybeharmfulandneedtobespitoutbeforeitdoesharmtothebody(Wolfe

etal,2009).Aflavorcanbemadeupofanycombinationofthefivetastants(sweet,

salty,sour,bitter,andumami)alongwithretronasalolfactorysensations.

Retronasalolfactorysensationsarethosetastantsthatarereleasedintotheairby

chewingandeating,andarepickedupbytheolfactoryepitheliuminthenasal

cavity.Theindividualtastantsarepickedupbyreceptors,whichsitontastebuds,

coveringthetongue.Withinthetongueandmoutharesomatosensoryreceptors

whichallowfoodstobeperceivesbytouchwithinthemouth.Thistooprovidesan

evolutionaryadvantageintheirabilitytoprotectthebodyfromingestingharmful

substances(Wolfeetal,2009).

Tastants

Thespecificityofeachtastanthastheirownevolutionaryadvantageandasa

result,sometastantsareaversiveatbirth,whileothersareappetitiveatbirth.While

wearebornwithaninnatelikeordislikeofcertaintastants,tastepreference

remainsplasticthroughoutthelifespanduetorepeatedexposurestothesame

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flavorswithdifferentresults.(Myers&Sclafani,2006).NaCl,tablesalt,ismost

commonlyusedwhenasaltyflavorisdesiredbecausehumansgenerallyconsiderit

tobethe“saltiest”tastingofallsalts(Wolfeetal,2009).Saltpreference,while

changingthroughoutlife,isgenerallyconsideredtobeappetitivebecauseitis

requiredbythebodyinordertofunctionproperly.However,largequantitiesare

perceivedasaversiveandareharmfultohomeostasis.Sour,thetasteofacids,is

generallyperceivedasmostappetitiveinlowerconcentrations,ashigh

concentrationswilldamagebothinternalandexternalbodilytissues(Wolfeetal,

2009).Themostcommonrepresentationofsourinresearchishydrochloricacid.

Anumberofsubstancesareusedtorepresentthebittertastantsuchas

salicinandquinine.Sinceitisnotpossibletodiscriminatebetweenbittertastants,

allbittertastantsaregenerallyavoidedbecauseoftheevolutionaryassociationof

bitterwithpoison(Wolfetal,2009).Conversely,notallthingsthatcontainsugars

areassociatedwithbeinggood,sothereisagreaterabilitytodiscriminatebetween

differenttypesofsweetness.Examplesofsweetnessareglucose,andfructose

(whichisevensweeterthanglucose)andsucrose(themostcommon).Themajority

ofsweetsareconsideredappetitivebecauseofthebody’snecessityforsweetsfor

theenergytheyprovide.However,thereisstillsomedebateoverhowsweetnessis

detectedsinceallsweetsaredetectedbythesamereceptor.Thiswouldmeanthat

theyalltastethesame.However,artificialsweeteners,suchasaspartame,saccharin,

andsucraloseareeasilydistinguishablefromothernaturalsugars(Wolfeetal,

2009).Savory,alsoknownasumami,isthemostrecenttastantandismostoften

representedbythechemicalmonosodium‐glutamate.

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Whilenotyettechnicallyatastant,emergingresearchsuggeststhatfathasa

tasteandotherorosensorycharacteristics(Stratford,Curtis,&Contreras,2006).

Fatscanbebeneficialasanessentialpartofadietbutcanalsobedetrimentalasitis

directlylinkedtobodyweightandobesity.Forthisreason,itissurprisingthatlittle

researchhasbeenconductedonfatuntilrecently.Ithasbeenfoundthatrats

distinguishbetweendifferenttypesoffattyacids(Larue,1978),evenwhenolfaction

andtextureareminimized(Fukiwatarietal,2003)).Also,manydietaryfatshave

beenfoundtoinhibitdelayedrectifyingpotassiumchannels(Gilbertson,1998;

Gilbertsonetal,1998)andafattyacidtransporterhasbeenfoundinthetastebuds

(Fukiwatarietal,1997).Allofthisevidencepointstofathavingit’sowntaste.

TasteTransduction

Eachreceptor,ofwhichtherearethoughttobetwotypes,isspecifictoone

tastant(Wolfeetal,2009;Yamamoto,2008).Thefirsttypeofreceptoris

metabotropic,whicharealsoknownasGprotein‐coupledreceptors(GPCRs).In

GPCRs,atastantengagesaspecificpartofthereceptorcorrespondingtothespecific

shapeofthetastant.Signalsthatwereceivefromthesetypesofreceptorsare

perceivedassweetorbitter.Theothertypeisionotropicreceptors.Thesereceptors

allowcertainchargedparticlestoenterthereceptorthroughsmallopeningsand,

whentriggered,areperceivedassalty,savory(umami),sour(Wolfeetal,2009;

Yamamoto,2008).

Dependingonwherethereceptorisonthetongue,itthensendsasignalto

oneofthethreecranialnervesinthemouth.Thesearethechordatympani(cranial

nerveVII),theglossopharyngialnerve(cranialnerveIX),andthevagusnerve

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(cranialnerveX)(Wolfeetal,2009;Yamamoto,2008).Theinformationfromthese

nervesthenconvergesintheNucleusofSolitaryTract(NST),andisrelayedagainat

theparabrachialnucleus(PBN)ofthePonsinthebrainstem.Itisthenrelayeda

thirdtimeintheparvocellularpartoftheventralposteromedialthalamicnucleus

(VPM)beforereachingtheprimarycorticalprocessingareafortaste(theinsular

cortex).Fromthere,theinformationisfurtherprocessedinsecondaryareassuchas

thelimbicsystemforemotions,andthehypothalamus,whichregulateshungerand

satiety(Wolfeetal,2009;Yamamoto,2008).

TestingMethods

Oneofthemostwidelyusedmethodsofassessingtastepreferenceisthe

two‐bottletest.Inthismethod,twobottlesofdifferentconcentration,ordifferent

solutionsarepresentedsimultaneouslytothesubject.Afteragivenamountoftime

theamountremainingissubtractedfromthebeginningamount.Thenthetwo

amountsarecompared,andwhicheversolutionisconsumedthemostisdeemedto

bethepreferredsolution.Similarly,itispossibletopresenttwodifferenttypesof

foodwithdifferenttastesandcomparetheamountconsumedtodeterminetaste

preferenceofasolidfood.

AnalternativetothesemethodsisthetastereactivitytestdevelopedbyGrill

andNorgen(1978),inwhichatastestimulusisinfuseddirectlyintothemouthofa

ratthroughanintraoralcannula.Thenthesomaticandorofacialbehaviorselicited

bythestimulusareanalyzedforfrequencyofoccurrence(Clarke&Ossenkop,

1998).Inthistest,therearethreetypesofresponsesthataremeasured.Thefirstis

anactiveresponseinwhichthebehaviorsresultintheconsumptionofthetastant.

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Thenthereistheaversiveresponseinwhichthesubjectengagesinbehaviorsthat

resultintheactiveremovalofthetastantfromtheoralcavity.Andfinally,the

passiveresponseresultsinapassiveremovalofthetastantfromtheoralcavity.

Thistesthelpsresolvemanyoftheproblemsassociatedwithintaketestssuchas

providingabehavioralmeasureofthepalatabilityofthetastant(Clarke&Ossenkop,

1998).

Anothermethod,whichhelpswiththevalidityoftheresearch,istouse

multiplestrainsofratswithinanexperiment,ortouseaspecificstrainofratbread

foraspecificpurpose.InstancesofspecificstrainsareHiSandLoSrats,whichare

breadforhighpreferenceofsaccharinandlowpreferenceofsaccharinrespectively.

Thereareothertechniquesusedtostudysexdifferences,someincluding

surgery.ThefirstisChordaTympani(CT)WholeNerveElectophysiologywherethe

responsesofasinglenervearemeasured.Itisalsohelpfultoovariectomize(OVX)

femaleratssothattheyceasetoproduceestrogen.Withthismethod,the

experimentercaninjectestrogenintoonegroup,andcomparetheeffectstothose

withouttheincreaseestrogen.Anothertechniqueisabilateraltransactionofthe

chordatympani(CTX)(O’Keefeetal,1994).Ifthenerveisseveredandtheratisno

longerabletodiscriminatebetweentastantsor,concentrations,thattheywere

previouslyabletotellapart,thenitcanbeconcludedthattheCTplayedaroleinthe

neuralsignalingofthattastant.

Thispaperexaminesresearchtodateondifferencesintastepreference

betweenmaleandfemalerats.Includingtheeffectofhormonalinfluences,andthe

chordatympani,ontastepreferencesinmaleandfemalerats.

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Sweets,andSugars

Withintheresearchonsweetpreferences,themajorityofresearchhas

lookedatdiscriminationbetweendifferentsugarsandartificialsugars.Ingeneralit

hasbeenfoundthatmalesandfemalesresponddifferentlytosolutionsofglucose

andsaccharin(Valenstienetal,1967).Valenstienetal.(1967),usingatwo‐bottle

preferencetestbetweenwateranda3percentglucosesolutionfoundthatboth

sexespreferredtheglucosetothewater,butthatthefemalesconsumed

significantlymoreoftheglucosesolutionthanthemalesdid.Inasubsequent

preferencetest,theypresentedthesubjectswitha3percentglucosesolutionand

.25percentsaccharinesolution.The25percentsaccharinesolutionwaspreviously

determinedtobethesaccharineconcentrationofmaximumpreferencefortherats.

Forthefirsttwoorthreedays,alloftheratsconsumedmoreofthesaccharine

solutionthantheglucose.However,onthethirdandfourthdays,themales

preferredtheglucosesolution,whilethefemalescontinuedtopreferthesaccharine

solution.Tofollowupontheseresults,theytriedthesametestagain,andathird

time,inwhichthesolutionwasincreasedby1percenteachday.Theresultswere

thesameforallthreetests.Forallofthesolutions,thefemalespreferredthe

saccharinsolutionthewholetime.Eachtime,themalesswitchedtothelesssweet

glucosesolutionreliablyonthethirdorfourthday.Theexperimentswere

conductedwiththreedifferentstrainsofrats,whichsuggestthattheirresultscanbe

generalizedtoallrats.Theirconclusionwasthatthefemaleswereabletoconsume

largequantitiesofaverysweetsolutionforprolongedperiods,whilethemalesdid

notshowthisability(Valenstienetal,1967).

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Othershavealsoconcludedthatdilutesucrosesolutionsarelesspreferred

byfemaleratsthanmalerats.Curtisetal(2004)lookedattheeffectofestradiol‐

benzoate(EB)ontwobottlepreferencetestsbetweena.025Msolutionofsucrose

andwater.Theyfoundthatmaleslickedmoreofthe.025Msolutionthanallofthe

females.Butonlythosefemaleswithraisedestrogen(EBtreated)lickedlessthan

themalestoa.05Msolution.Thissuggeststhatfemalesmaybelesssensitivetothe

tasteofadilutesucrosesolution,oratlowerconcentrations,arelessableto

discriminateamoredilutesolutionfromwaterthenmales(Curtisetal,2004).

TakingCurtisetal,(2004),andValenstienetal.(1967)together,Loneyetal.

(2011),concludedthatpreferenceforsucralosecouldalsobesexuallydimorphic

withmalespreferringconcentrationsclosertothreshold,andfemalesdisplaying

greaterpreferenceathighconcentrations.Basedonthistheory,theyconductedfour

experiments,basedontwo‐bottlepreferencetests.Theyfoundthat,overall,male

andfemaleratshavedifferentpreferencesforsucralose.Basedontheresults,they

separatedbothmalesandfemalesintotwogroupsapiece:thosethatpreferredthe

sucralosesolutions,andthosethatavoided.Againtheyfoundthattheproportionof

prefererstoavoiderswasthesameforbothmalesandfemales.Overall,Loneyetal

(2011)concludedthatthesexdifferencesfoundbyCurtisetal(2004)and

Valenstienetal(1967)didnotextendfromsucrosetosucralose.

Withtheirfindings,Loneyetal(2011)bothconfirmedandopposedseveral

otherpreviousworks.Inatwo‐bottletestof.5g/lsucralosesolutiontowater,

SclafaniandClare(2004)foundthatthemalesdranksignificantlylesssucralose

thanwatersuggestingthatthemaleratsshowedastrongersucraloseavoidance

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thanthefemalerats.Theyalsoshowedthatfemalerats(50%)weremorethan

twiceaslikelytobeclassifiedassucralosepreferersthanmalerats(20%).Refuting

this,Loneyetal(2011)foundthatbothfemaleandmaleratswereequallylikelyto

beclassifiedaspreferers(33%).Dessetal(2009)alsocomparedtheirworkonHiS

andLoSrattoSclafaniandClare(2004).Dessetal(2009)foundthatthemajorityof

LoSmalesandfemaleswerenon‐preferersandthemajorityoftheHiSmalesand

femaleswerepreferersofthesucralosesolution.Thus,Dessetal(2009)agreeswith

SclafaniandClare(2004)thattheoverallnumbersofmaleavoidersandpreferers

arecomparabletothoseofthefemalepreferersandavoiders.Thisgoesforboththe

HiSandLoSratstrains.Similarly,Lonelyetal.(2011)confirmsthelackofsex

differencesinsucralosepreferenceaswelltheabsenceofsexdifferencesintheratio

ofprefererstoavoidersinratsselectivelybredforaspecificsaccharinepreference.

BasedonpreviousresearchbyNissenbaumandClare(1987)thatratstaste

polysaccharidesasdifferentfromsucroseandsaccharine,Sclafanietal(1987)

soughttodetermineiftherewasasexdifferenceintherelativepreferencefor

polysaccharidesandsucrose.Inthefirstofthreeexperiments,theyusedaseriesof

twobottletestsofdifferentconcentrationsofsucrose,maltoseandpolycoseto

determinetastepreference.Theonlysexdifferencesfound,seemedtobedueto

outsidefactors,whichcausedSclafanietal(1987)toconcludethattheirdata

conflictedwithpreviousreportsoffemaleshavingastrongersweetpreferencethan

males(Valenstienetal,1967).Usinganothertwo‐bottlepreferencetestbetween

polycoseandsucroseatvaryingconcentrations,theyfoundthatthemalerats

preferredthepolycosesolutionatlowerconcentrationsthandidthefemales.They

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alsofoundthatthemalespreferredlowerconcentrationsofsucrosethanfemalesas

well.Thus,theyconcludedthatfemaleratshaveastrongersucrosepreferenceand

weakerpolycosepreferencethanmales,whichwassupportedbypreviousresearch

(Valenstienetal1967;Wade,1976).Overall,theyconcludedthatintheshortterm,

30‐minexposures,malesshowedastrongerpolycose,butweakersucrose,

preferencethanfemales.Conversely,femaleratsshowedastrongerpolycose

preferencethanthemalesinthelong‐termtesting(24‐hourexposures).These

findingswithpolycosearesimilartothosefoundwithsucrosebyValenstienetal

(1967).Theirexplanationfortheseresultswasthedifferentnutritional

requirementsofmalesandfemales.

Insummary,ithasbeenfoundthatfemaleswereabletoconsumelarge

quantitiesofasweetsucrosesolutionforprolongedperiods,whilethemalesonly

preferredthesweetsolutionsovershortperiods(Sclafanietal;1987;Valenstienet

al).Then,Curtisetal(2004)foundthatfemaleswerelessabletodiscriminatedilute

sucrosesolutions.Lonelyetal(2011)triedtofollowuponthesestudies,but

ultimatelyfoundthatthefindingsofValenstienetal(1967)andCurtisetal(2004)

didnotextendfromsucrosetosucralose.Ithadalsobeenshownthefemalesshowa

greaterpreferencetosucralosetowaterthanmales(Sclafani&Clare,2004),but

thesefindingsweredisputedbylaterresearch(Lonelyetal,2011).Andtogether,

theresearchconfirmsthelackofsexdifferencesavoidersandpreferersofsucralose

(Lonelyetal,2011;Dess,etal,2009;Scalfani&Clare,2004).Finallyitwas

concludedthatfemaleratshadastrongersucrosepreferenceandaweakerpolycose

preferencethanmalerats(Sclafanietal,1987;Valenstienetal1967;Wade,1976).

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HormonesandtheMenstrualCycle

Hormoneresearchandmenstruationarehighlystudiedfieldswithinthearea

oftastepreferences.Ithasbeenfoundthathighlevelsofestradiol(agonadal

hormonefoundprimarilyinfemales),produceanincreaseinthepreferencefor

palatablesolutions,whereastheoppositeistrueforlowlevelsofestradiol(Clarke&

Ossenkop,1998;Wade&Zucker1970a;Wade,1976).Usingatasteresponsetest

(TRT)ithasbeenfoundthatfemalesnotinestrusproducedlessaversiveresponses

toaversivesolutionsofquinine,andasucrose‐quininemixture,thanbothmalesand

femalesinestrus(Clarke&Ossenkop,1998).Itwasexpectedthatthefemaleswould

findlesstastantstobeaversivebecausetheirbodieswerepreparingforchild

rearing.Buttheirresultsshoweddifferently.Onepossibleexplanationisthatthe

highlevelsofestradiolproducedanavoidanceofthetastantsbasedonfactorsother

thanpreferencesuchasfearoranxiety(Clarke&Ossenkop,1998).However,itis

alsopossiblethatfemalesinestrusaremoresensitivetotastantsignalingin

unhealthyfood,inordertoprotectapotentialpup.

Ithasalsobeensuggestedthattestosteronemayhaveaneffectonthe

developmentoftastepreference(Krecek,1972).Administrationoftestosteroneto

2‐day‐oldfemalesresultedinapatternofintake,ofasalinesolution,resembling

thatofmales.However,thiseffectwasnotpresentwhenthetestosteronewas

administeredto12‐day‐oldfemales.Thusthereisacriticalageatwhichthe

testosteronehasthegreatesteffect.Similarly,ithasbeenshownthatmalesform

strongerconditionedtasteaversions(CTA)thanfemales,andthiseffecthasalso

beenshowntobetheresultoftestosterone(Foy&Foy,2003).CurtisandContreras

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(2006)foundthatmaleshadhigherNaClthresholdsthanfemalesregardlessofEB

treatment,whichraisesthepossibilitythattestosteronemayhaveafurthereffect

onsodiumpreference(Curtis&Contreras,2006;Krecek,1972).

Ithasalsobeenfoundthatestrogenproducesashort‐termincreaseinthe

sucrosedetectionthreshold(Curtisetal,2005).Havingovariectomized(OVX)the

femalerats,halfweregivenEB(estradiolbenzoate)injections,whichsimulatesthe

cyclingfemalerat.Afterbeingexposedto0.2Msucrosesolutions,theratswerethen

immediatelyinjectedwithaLiClsolutiontoconditionatasteaversion.Thenextday,

theratsweregivenatwo‐bottlepreferencetestbetweensucroseandwater,toseeif

thetasteaversiontosucrosegeneralizedtothewater.Withthisexperiment,Curtis

etal(2005)foundthattheratsthathadbeengiventheLiClsolutiondevelopeda

CTAtothe0.2Msucrosesolutionasexpected.TheyalsofoundthattheCTA

generalizedtothe0.075Msucrosesolution.TheCTAalsogeneralizedtothe0.025M

sucrosesolution,butthoseratsgiventheEBtreatmentshowedlessofanaversion,

andalmostequallickingtothe0.025Msucrosesolutionandwater.Theirresults

implythattheratsgiventheEBtreatmentwereunabletodiscriminatethesucrose

solutionfromwater.Afteracoupleofweeks,theytriedmorepairingsoftheLiCl

withsucrosesolutionsandfoundthatalloftheratsshowedaversionstoallofthe

sucroseconcentrations.SincetheEBtreatmenthadwornoff,itsuggeststhatthe

decreaseinaversionsintheEBtreatmentgroupinthefirstexperimentwasdueto

theincreaseinestrogen.Thesefindingsareconsistentwiththeirhypothesisthatthe

estrogenincreasedthedetectionthresholdforsucrose(Curtisetal,2005).

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InoppositiontoCurtisetal(2005),ithasbeenshownthatfemaleshavea

lowerthresholdforNaClduetoanincreasedpreferenceforisotonicandhypotonic

NaClsolutions(Curtis&Contreras,2006).Ithasalsobeensuggestedthatthereisan

evolutionaryadvantagetoanincreasedconsumptionofNaClinthepresenceofhigh

estrogenlevels(Curtis&Contreras,2006).ItisalreadyknownthatNaCl

preferencesincreaseduringlactation(Clarke&Bernstien,2001;Duffyetal,1998)

soitispossiblethattheincreaseofNaClconsumptionduringlactationmay

compensateforincreasedfluidandelectrolytelossduringlactation.Thiswould

makesensebecauseestrogenisknownasasignalthebodytochangehomeostatic

conditionsortoprepareforchildrearing.BecauseanincreaseisNaClisrequired

duringlactationandpregnancy,itwouldmakesensethattheestrogenisasignaler

ofincreasedsodiumpreference.Similarly,OVXratshavebeenshowntoconsume

moreofaNaClsolutionthanmalesoverall(Curtisetal,2004).

Curtisetal(2004)foundthatbothmalesandfemales,incontrolandEB

conditions,lickedatthesamehighratesforhighsucroseconcentrations.Thelackof

sexdifferencesathighconcentrationsbyCurtisetal(2004)isinagreementwiththe

lackofEBeffectonconsumatorybehaviorsandtastereactivity(Clarke&Ossenkop,

1998),orneuralactivity(DiLorenzo&Monroe,1989;DiLorenzo&Monroe,1990).

Curtisetal(2004)foundsimilarresultsinlongtermtestingwhereEBdidnot

decreasethepreferenceforadilutesucrosesolutionoverwater,nordiditalterthe

abilitytoaltertheabilitytodiscriminatethedilutesucrosefromwater.

Insummary,bothestrogenandtestosteronehavebeenshowntoaffectthe

perceivedpalatabilityofatastantinrats.Notonlyhastestosteronebeenshown

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formstrongertasteaversions(Foy&Foy,2003),butithasalsobeenshownto

influencetheperceivedpalatabilityofasubstance(Krecek,1972).Estrogen,onthe

otherhand,hasbeenshowntohavenumerouseffectssuchasproducinga

preferenceincreaseofpalatablesolutionsandadecreaseinpreferencefor

unpalatablesolutions(Clarke&Ossenkop,1998;Wade&Zucker,1970a;Wade,

1976).Estrogenhasalsobeenfoundtoproduceashort‐termincreaseinthesucrose

detectionthreshold(Curtisetal,2005).Theroleofestrogeninchangingthe

palatabilityoftastantsispossiblyexplainedbytheincreaseddemandforcertain

tastantsduetotheirnutritionalvalue.Thesenutritionalneedschangeduring

menstruationandchildrearing,thusestrogenmightchangetastepreferencetohelp

facilitatetheacquisitionoftheseneeds.

TheChordaTympaniandFattyAcids

Recentlyresearchhasbeguntoemergethatsuggestsfathasatasteandother

orosensorycharacteristics(Stratford,Curtis&Contreras,2006).Ithasbeenfound

thatnotonlydoesthechordatympani(CT)playalargeroleinthetasteoffats,butit

playsanevenlargerroleoftastingfatsinfemales(Stratford,CurtisandContreras,

2006).Theratshavingbeengivenalinoleicacid(LA)solution,Stratford,Curtisand

Contreras(2006)injectedthemwithLiClandconditionedatasteaversion.Therats

werethentestedonhowwelltheCTAgeneralizedtolessconcentratedsolutionsof

LA.WhilealloftheratsgeneralizedtheCTAtothehigherconcentrationsofLA,the

femalesgeneralizedthatCTAtolowerconcentrationsthandidthemales.This

suggeststhatmaleratscan’tdiscriminateLAfromwateraswellasfemalesatlower

concentrations.

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Inasubsequentexperiment,Stratford,Curtis,andContreras(2006)

conditionedaCTAtoLAafterCTX.TheCTXimpairedtheabilityofbothmaleand

femaleratstodetectandavoidLAconcentrationsunder22M.Thediscrimination

shiftwasthesameinbothmalesandfemalessuggestingastrongroleoftheCTin

fattransduction.Puttingthetwoexperimentstogether,Stratford,Curtisand

Contreras(2006)concludedthattheCTisinvolvedinfatdiscriminationinboth

malesandfemales,butthelowerdiscriminationthresholdinfemalessuggestthat

theCTplaysagreaterroleinFemales.

Usingthesamemethods,ithasbeenrepeatedlyshownthatCTXimpairsthe

abilityofbothmaleandfemaleratstodetectandavoidbothlinoleicandoleicacids

(Pittmanetal,2007;StratfordCurtis&Contreras,2006).AlongwithStratford,

Curtis,andContreras(2006),Pittmanetal(2007)hasalsoshowedthatfemales

showedgreaterdiscriminationatlowerconcentrationsofLAafterCTX.However,in

thepretrialtests,itwasfemaleswhowerealsomoresensitivetooleicacidthan

malesintwo‐bottletests(Pittmanetal,2007).Thiseffectwasseenagainin

subsequentresearchwherefemalesexhibitedagreaterresponsivenesstofatty

acidsthanmales,butwasextendedtoseveraldifferentstainsofrat(Pittman,2008).

ThefindingsthatfemalesdisplayagreatersensitivitytoLAandoleicacids

(Stratford,Curtis&Contreras,2006;Stratford,Curtis&Contreras,2008;Curtis&

Contreras,2006),wasalsoextendedtoLauricacid(Pittmanetal,2008).

Stratford,Curtis,andContreras(2008),inafollowupstudy,alsousedCTX

andrecordedelectrophysiologicalrecordingswhenMSGandapairingofMSGand

linoleicacid(LA)werepresentedtorats.Overall,theyfoundthatwhentheLAand

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MSGwerepairedtogether,theyproducedneuralsignalsofgreaterintensitythan

justMSGalone.ThustheLAhadanenhancementeffectontheMSG.This

enhancementeffectwasgreatestintheneuronalresponsesofmalestotheMSG/LA

combination.Ontheotherhand,theamplitudeoftheCTresponseswasgreaterin

femalesthanmalesforbothMSGandtheMSG/LAcombination.Overall,maleshad

enhancedresponsestothecombinationofLAandMSGatlowerconcentrationsthan

didfemales(Stratford,Curtis&Contreras,2008).AthirdstudybyCurtisand

Contreras(2006),itwasfoundthatCTresponsestoarangeofNaClsolutions,inCT

wholenerveelectrophysiology,weregreatestinOVX‐EBratsascomparedtomales

andNon‐EBratsshowingthatestrogenmaybeareasonfortheincreasedsensitivity

infemales.

Insummary,fattyacidsalone,althoughnotanofficialtastant,canbe

discriminatedbetween,butbetterinfemalesthanmales(Stratford,Curtis,&

Contreras,2006).Thesesexdifferencesinfattyaciddiscriminationhavebeentested

acrossawidevarietyoffattyacidssuchasLAandoleicacid,(Stratford,Curtis&

Contreras,2006;Stratford,Curtis,&Contreras,2008;Curtis&Contreras,2006)and

lauricacid(Pittmanetal,2008).TheCThasalsobeenshowntohaveeffectsonthe

preferenceanddetectionoffatsandothertastantsingeneral.WhentheCTwas

severedusingCTX,therewasashiftinthediscriminatorythresholdofbothmales

andfemales.Thus,itwasdeterminedthattheCTplayedastrongerrolein

discriminationoffattyacidsinfemalesinmales(Stratford,Curtis&Contreras,

2006).ThisaffectdoesnotseemtogeneralizetoMSGresponsesintheCTbecause

Keenan18

maleshadanenhancedresponsetothecombinationofMSGandLAatlower

concentrationsthandidfemales(Stratford,Curtis&Contreras,2008).

Salty,Bitter,Sour,andUmami

Thesaltytastanthasbeenstudiedinavarietyofways,whichhaddifferences

inpreferencebetweenmalesandfemales(Chowetal,1992;Flynn,Schulkin&

Havens,1993;Kensicki,Dunphy&Ely,2002;Krecek,1973;Kreceketal,1972;

Scheidler,Verbalis&Stricker,1994;Stricker,Thiels,&Verbalis,1991;Wolfe,1982).

Whenpresentedwithasaltysolution,theCTresponsesweregreatestinmalerats

andlowestinOVX‐EBrats(Curtis&Contreras,2006;Curtisetal,2004).Thesex

differencewasattributedtoadecreaseintheCTresponseofthefemalesratherthan

anincreaseinCTresponseofthemales.ThedecreaseinCTresponsebythefemales

wasattributedtothehighlevelsofestrogencomingfromtheEBtreatment.Overall,

thefemaleratswerelesssensitivetothetasteofconcentratedNaClsolutionswhen

estrogenlevelswerehighest,butweremoresensitivetothetasteofdiluteNaCl

solutionsregardlessofestrogenstatus(Curtis&Contreras,2006;Curtisetal,2004).

ItwasalsofoundthatthedetectionofNaClinmaleratswasconsistentlygreater

thaninOVXratsregardlessofEBtreatment,suggestingthattestosteronemayplaya

roleinthegustatoryresponsetoNaCl(Krecek,1972;Curtis&Contreras,2006).

Thesefindingswouldmakesensebecausethedecreasedsensitivityto

sodiuminfemales,whichwouldbeusefulwhenthefemalebodyispreparingfor

lactationandchildrearingwhenthereisanincreaseddemandforsodiumandother

tastants(Clarke&Bernstien,2001;Curtis&Contreras,2006).Theideaofa

biologicalneedwasrefutedbyTardoff,Bachmonov,andReed(2007)whotested

Keenan19

NaClintakein40differentratstrains,andfoundthatonly8strainsshowed

significantdifferencesbetweenmalesandfemales.Theirfindingssuggestedthatif

therewereanincreasedneedforsodiuminfemalerats,thenmorestrainswould

havethisneedfordecreasedsensitivityincommon.

Ithasalsobeensuggestedthatthedifferentintakesofsodiumaredueto

differentmetabolicneeds.Femalesingestednearlytwiceasmuchsalineasmales,

buttheyonlylostabout30%lesssodiumintheirurinethanthemales(Wolfe,

1982).Butintermsofsaltasageneraltastant,concentratedNaClsolutionsareless

aversivetofemaleratsthantomalerats(Curtisetal,2004).Curtisetal(2004)

suggestthatthecontinuedpresenceofestrogenisnotrequiredtocreatethe

developmentalchangesinNaCltastepreferenceproducedbyestrogen.Conversely,

testosteronewasreportedbyChowetal(1992)toreducetheintakeofNaClbyOVX

rats.Asreportedearlier,Krecek(1972)showedthatasingledoseoftestosterone

givenat2daysofagecouldsuppressthesexdifferenceofsaltintakethatoccursin

maturity.However,thissameeffectdidnotoccurwhenthetestosteronewas

injectedat12daysofage.

Verylittlehasbeenwrittenonthetastepreferencesofsour,bitter,and

umami.Mostlythesetastantsarejustmentionedinpassing,orusedasavehicle

towardsanothergoal.Forinstance,thebittertastantquinine,ismostoftenusedasa

waytoconditiontasteaversions.OneinstanceofthisisNance,Gorski,and

Panksepp(1976)whoreportedthatmaleratsdisplayedstrongertasteaversions

thanfemalestoasaccharine–quininesolution.However,WadeandZucker(1970b)

didreportthatquinineinitselfresultedinstrongeraversionsinnormalfemales

Keenan20

thanovariectomizedfemales.Severalstudieshavelookedatquininethresholdsand

theeffectofquinineontheCT(John&Specter,1996)andmechanismsof

transductionpropertiesofthebittertastant(Geran&Travers,2009).However,

Thosestudiesthatdidstudytastepreferenceofthebittertastant(Dahl,Erickson,&

Simon,1997;BoughterJr.etal,2002)didn’tlookatanysexdifferences,anddidn’t

citeanyotherstudiesonbittertastepreferences.Whilethisisastart,moreresearch

needstobedonetounderstandpossibleeffectsofconcentration,andhowsmall

amountsmayinteractwithothertastants.

Similarly,verylittleworkhasbeendoneonsourtastantpreferences,and

thosethathave(Scelara,2004;Amerine,Roessler&Ough,1965;Frank&Blizard,

1999;Pfaffman,1957),didnotlookatsexdifferences.Thusstudiesthatdidusea

sourtastantsuchashydrochloricacidwere,again,concernedwithtastepreferences

butnotthesexdifferencesofthosepreferences.ResearchonUmamihasfalleninto

thesamecategoryassavoryandbitterwherealloftheresearchhaslookedat

preference(Kondohetal,2000)withanemphasisongeneticcomponentsoftaste

(Bachmonov,Tordof,Beauchamp,2000)butnotsexdifferences.However,Stratford,

Curtis,andContreras(2008)studiedthedifferentpreferencesthatmalesand

femaleshadtoMSG.Theyusedelectrophysiologicalrecordingstodeterminethat

malesshowedagreaterresponsetotheMSGthandidthefemales.Theyalsopaired

theMSGwithLAanddeterminedthattheLAhadanenhancingeffectontheMSG.

Overall,maleshadenhancedresponsestothecombinationofLAandMSGatlower

concentrationsthandidfemalessuggestingthatthemaleswerebetterableto

discriminatethetasteofMSGatlowerconcentrations.

Keenan21

Insummary,thegreaterresponsesofmalestoasalinesolution,overfemales,

isthoughttobeproducedbyhigherlevelsofestrogeninfemales(Curtis&

Contreras,2006;Curtisetal,2004).Therehasalsobeenresearchsuggestingthe

roleoftestosteroneinthepreferenceofsalt(Chowetal(1992;Krecek,1972;Curtis

&Contreras,2006).Therehavealsobeenavarietyofexplanationsproposedforthe

sexdifferencesinsodiumpreference(Clarke&Bernstien,2001;Curtis&Contreras,

2006;Tardoff,Bachmonov,andReed,2007;Wolfe,1982)suggestingthatthisis

anotherareathatneedsfurtherexploration.Thepalatabilityofthesour(Scelara,

2004;Amerine,Roessler&Ough,1965;Frank&Blizard,1999,Pfaffman,1957),

bitter(Nance,Gorski,&Panksepp,1976;Wade&Zucker,1970b;John&Specter,

1996;Geran&Travers,2009;Dahl,Erickson,&Simon,1997;BoughterJr.etal,

2002),andumami(Kondohetal,2000;Bachmonov,Tordoff&Beauchamp,2000)

tastantshavebeenresearched,butnotinregardstosexdifferencesinpalatability.

Withtheexceptionofumami(Stratford,Curtis&Contreras2008),theyhavenot

beenexaminedinregardstotheirrelationshiptoothertastantseither.Thisis

clearlyanareathatneedstobeexpandeduponinfutureresearch.

Discussion

Speakinginthemostgeneralterms,itcanfirstbeconcludedthatmore

researchisneededinallareasoftastantpreference.Whilethecurrentstudydidnot

examinetheliteratureofotheranimalsorhumans,manyofthefindingsinratsare

farfromconclusiveandstillindispute.Oneofthebigdisputesisovertheroleof

estrogenandtestosteroneinproducingsexdifferences.Thecurrentliterature

showsevidencethatbothhormonesplayaroleinsexdifferences.Whatexactlythat

Keenan22

roleis,remainsdisputed.Estrogenandtestosteronearealreadyknowntoregulate

manyfunctionsofthebody,throughoutlife,andonaday‐to‐daybasis.Thus,it

wouldbehelpfultogainabetterunderstandingoftheroleofthesehormones,

becauseitcouldleadtomajoradvancesinotherareassuchasreproduction,bodily

development,andhomeostasis.

Itwouldalsobeincrediblyhelpfultoconductmoreresearchonthetastants

umami,sour,andbitter.Somesexdifferenceshavebeennoted,buttheywerenoted

asaside‐note.Itwouldbemosthelpfultodeliberatelylookforsexdifferencesin

tastepreferencesbetweenthesexes.Ifresearchersweretostartwiththisrelatively

simplebaseresearch,thenitcouldleadtofindingsofinteractionsbetweenthe

tastants.ThebestexampleofthisistheenhancementeffectofLAonMSG(Stratford,

Curtis&Contreras,2008).

TheroleoftheCTcanalsobeexaminedmoreinrelationtotaste.Itwas

interestingtoseethattheCThadagreatereffectonfemalesthanitdidmales

(Stratford,CurtisandContreras,2006).Thisfirstraisesthequestionwhetherthe

othertwocranialnervesintheoralcavityhavesimilareffectsonthesameor

differenttastants.Whilethecurrentstudydidnotlookattheliteratureonthe

glassopharyngialandthevagusnerve,itwouldalsobeinterestingtoseeifthereis

anytypeofinteractionbetweenthethreenerves,andthetastantstheyreceive.

Keenan23

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