University of Groningen

Sex steroid hormones and brain functionMoraga-Amaro, R; van Waarde, A; Doorduin, J; de Vries, E F J

Published in:Journal of Neuroendocrinology

DOI:10.1111/jne.12565

IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.

Document VersionPublisher's PDF, also known as Version of record

Publication date:2018

Link to publication in University of Groningen/UMCG research database

Citation for published version (APA):Moraga-Amaro, R., van Waarde, A., Doorduin, J., & de Vries, E. F. J. (2018). Sex steroid hormones andbrain function: PET imaging as a tool for research. Journal of Neuroendocrinology, 30(2), [e12565].https://doi.org/10.1111/jne.12565

CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).

Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.

Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.

Download date: 20-07-2019

Journal of Neuroendocrinology. 2018;30:e12565. wileyonlinelibrary.com/journal/jne | 1 of 12https://doi.org/10.1111/jne.12565

Received:15June2017  |  Revised:26October2017  |  Accepted:6December2017DOI:10.1111/jne.12565

R E V I E W A R T I C L E

Sex steroid hormones and brain function: PET imaging as a tool for research

R. Moraga-Amaro | A. van Waarde | J. Doorduin | E. F. J. de Vries

DepartmentofNuclearMedicineandMolecularImaging,UniversityMedicalCenterGroningen,UniversityofGroningen,Groningen,TheNetherlands

CorrespondenceErikF.J.deVries,DepartmentofNuclearMedicineandMolecularImaging,UniversityMedicalCenterGroningen,Groningen,TheNetherlands.Email:e.f.j.de.vries@umcg.nl

AbstractSexsteroidhormonesaremajorregulatorsofsexualcharacteristicamongspecies.Thesehormones,however,arealsoproducedinthebrain.Steroidalhormone-mediatedsignallingviathecorrespondinghormonereceptorscaninfluencebrainfunctionatthecellularlevelandthusaffectbehaviourandhigherbrainfunctions.Alteredsteroidhormonesignallinghas been associated with psychiatric disorders, such as anxiety and depression.Neurosteroidsarealsoconsideredtohaveaneuroprotectiveeffectinneurodegenerativediseases.Sofar,theroleofsteroidhormonereceptors inphysiologicalandpathologicalconditionshasmainlybeeninvestigatedpostmortemonanimalorhumanbraintissues.Tostudythedynamicinterplaybetweensexsteroids,theirreceptors,brainfunctionandbe-haviourinpsychiatricandneurologicaldisordersinalongitudinalmanner,however,non-invasive techniques are needed. Positron emission tomography (PET) is a non-invasiveimagingtoolthatisusedtoquantitativelyinvestigateavarietyofphysiologicalandbio-chemicalparametersinvivo.PETusesradiotracersaimedataspecifictarget(eg,receptor,enzyme,transporter)tovisualisetheprocessesofinterest.Inthisreview,wediscussthecurrentstatusoftheuseofPETimagingforstudyingsexsteroidhormonesinthebrain.Sofar,PEThasmainlybeeninvestigatedasatooltomeasure(changesin)sexhormonerecep-torexpression in thebrain, tomeasureakeyenzyme in thesteroidsynthesispathway(aromatase)andtoevaluatetheeffectsofhormonaltreatmentbyimagingspecificdown-streamprocessesinthebrain.Althoughvalidatedradiotracersforanumberoftargetsarestillwarranted,PETcanalreadybeausefultechniqueforsteroidhormoneresearchandfacilitatethetranslationofinterestingfindingsinanimalstudiestoclinicaltrialsinpatients.

K E Y W O R D S

androgenreceptor,neuroimaging,oestrogenreceptor,positronemissiontomography,sexsteroid hormones

1  | INTRODUCTION

Sexsteroidhormonesareafamilyofsteroidalhormonesthatcanbedivided into3classes:oestrogens,progestinsandandrogens.Thesehormonesaremajorregulatorsofsexualfunctions, includingthere-productive cycle, reproductive physiology and the development ofaccessory reproductive organs.1However,ourvisionofthefunction

ofthesehormoneshasbeenexpandedbecausetheynotonlyregu-latesexualbehaviour,butalsoaffectbrain functions, suchasmem-ory,2 anxiety-related behaviour3 and other functions at the cellularlevel.4 Sex steroid hormones are mainly synthesised by the ovaries and testis. The hypothalamic-pituitary-gonadal (HPG) axis is themain systembywhich theproductionand releaseof sexsteroids isregulated.5 Circulating sex hormones can stimulate the release of

ThisisanopenaccessarticleunderthetermsoftheCreativeCommonsAttributionLicense,whichpermitsuse,distributionandreproductioninanymedium,provided the original work is properly cited.©2017TheAuthors.Journal of NeuroendocrinologypublishedbyJohnWiley&SonsLtdonbehalfofBritishSocietyforNeuroendocrinology

2 of 12  |     MORAGA- AMARO et Al.

gonadothropin-releasehormones(GnRH)atthehypothalamus.GnRHinducesthereleaseofluteinisinghormone(LH)andfollicle-stimulatinghormone (FSH) in thepituitary,whichactivate the secretionof ste-roidal sex hormones from the gonads (Figure1A). Peripheral sexhormones are present in the plasma,where they aremainly boundtoplasmaproteinssuchassexhormonebindingglobulin (SHBG)orcorticosteroidbindingglobulin(CBG).6SHBGhashighaffinityforbothoestrogensandandrogens,whereasprogesterone isboundbyCBG.These globulins protect steroid hormones against metabolic degra-dation and, consequently, the fraction of free steroid hormones inplasmaissmall.Yet,thissmallfractionofunboundsteroidhormonescanreadilycrosstheblood-brainbarrierbypassivediffusionasare-sultofthelipophilicnatureofsteroids.However,thereisalsoasig-nificantcontributionofdenovosynthesisedsteroidhormonesinthebrainbecause thebrain itself contains theenzymesneeded for thesynthesisofthesesteroids.7 Sex hormones produced in the brain in-clude 17β-oestradiol,testosteroneandprogesterone,alongwithotherneuroactivesteroidssuchaspregnenolone,dehydroepiandrosteroneand allopregnanolone.8

Inrecentdecades,thespecificreceptorsforsexsteroidhormoneswerefoundtobeexpressedinthebrain.9Currently,mostinformationhasbeenobtainedfromanimalexperiments,whichcannoteasilybetranslatedtohumans,aswellasfrompost-mortemanalysisofhumanbrain tissue.10,11Inmoststudies,westernblottingandinsituhybridi-sationhavebeenusedtoquantifyhormonereceptorsinthebrain.9,12 Suchtechniqueswouldnotallowresearchonthebiologyofsteroidhormonesandtheirreceptorsinthelivinghumanbrain.Oneapproachwithrespecttonon-invasivelyinvestigatingsexhormonereceptorsinthebrainistheuseofpositronemissiontomography(PET)withradio-labelledreceptor ligands.PETallowsthequantificationoffunctionalparameters,suchasreceptordensityandoccupancy.13PETimagingofsteroid receptors is already widely used in oncology to visualise recep-torexpressionandreceptoroccupancyinhormone-sensitivetumourssuch as breast and prostate cancer.14 By contrast, sex hormone re-ceptorimaginginthebrainisstillinitsinfancy.15 Sex steroid receptor imaginginneurosciencesuffersfromsomeadditionalhurdles,suchasthe low receptor expression in some brain regions16 and a poor pene-trationofradioligandsthroughtheblood-brainbarrier.

F IGURE  1 Effectsofsexsteroidsatbothphysiologicalandcellularlevels.(A)Theregulatoryprocessesforthesynthesisofsexsteroidsbythehypothalamic-pituitary-gonadal(HPG)axis.Thehypothalamusregulatestheproductionofluteinisinghormone(LH)andfollicle-stimulatinghormone(FSH)viathereleaseofgonadotrophin-releasinghormone(GnRH).BothLHandFSHstimulatethesynthesisandreleaseofoestrogensandprogesteronefromtheovariesinfemales,aswellastestosteronefromthetestisinmales.Atthesametime,thesesexsteroidscanregulatethereleaseofGnRHfromthehypothalamus,aswellasLHandFSHfromthepituitary.(B)Generalschemeofsexsteroideffectsatcellularlevel.Sexhormonescanbindtoeithercytoplasmaticreceptorsormembrane-associatedreceptors.Whenthemoleculesbindtomembranereceptors,the receptor (coupled to G protein subunits complex: Gα,Gβ and Gγ)activatesphospholipaseC(PLC)toexertarapidnongenomicresponsesviathesecondmessengersinositolphosphate3(IP3+)anddiacylglycerol(DAG).Ontheotherhand,whentheybindtocytoplasmaticreceptors,thecomplexistranslocatedtothenucleus(withthehelpofdifferentco-activators)toexertgenomiceffects

Hypothalamus

(A) (B) Sex steroid Membranesteroidreceptor

PLC

IP3+ DAG

Nongenomicsignaling

Cytoplasmaticsteroidreceptor

Co-activators

Genomicsignalling

Gγ

Gα Gβ– +

– –

–

+

+

+

+

GnRH

Pituitary

LH FSH

Ovaries Testis

TestosteroneOestrogen

Progesterone

     |  3 of 12MORAGA- AMARO et Al.

Inthisreview,wesurveytheavailableliteratureabouttheuseofPETimaginginthefieldofneuroendocrinology,inwhichimagingdataare directly or indirectly correlated with sex steroid hormone (recep-tor)levels.Wediscusstheroleofsexsteroidsinbrainfunctionandbe-haviour,giveanoverviewofthetracersthatarecurrentlyavailableforPETimagingofhormonereceptorsandtheirapplicabilityinbrainre-search,andsummarisetheresultsofPETimagingofthedownstreameffectsofsexsteroidsinthebrain.Basedonthesedata,weproposethatPETisapromisingtechniqueforfuturetranslationalresearchinthisfield.

2  | SEX STEROID HORMONES AND BRAIN FUNCTION

Oestrogens can exert their effects through either intracellular ormembrane-associated oestrogen receptors (ERs); in particular, theintracellular receptors ERα and ERβ, and membrane-associated G-protein regulatormotifs.Uponbindingof oestrogen to theER, theligand-receptorcomplexdimerisesandmigratestothenucleus,wherethedimercanbindtohormoneresponseelements(HRE)inthepro-motorregionofoestrogen-responsivegenes.ActivationoftheHREleadstotheinductionortherepressionofgenetranscription.Inad-dition to this genomic signalling pathway, sex steroids can act vianongenomicsignalling (Figure1B) (fora review,seeKawataetal17).Oestrogensignallingcanaffectvariousaspectsofbrainfunctionandbehaviour.Mostinformationabouttherelationshipbetweenoestro-gensandbraindisorderswasobtainedfromstudiesinfemaleanimalsorwomendemonstratingbehaviouraldifferencesbetweenthediffer-entstagesofthemenstrualcycle.Thereisampleevidenceforaroleofoestrogensinanxietyanddepression,bothfromanimalsandhu-mans.18 Women are vulnerable to depression when the concentration of sexhormones changesmarkedly.This can lead topre-menstrualdysphoric disorder, post-partum depression and perimenopausal orpostmenopausal depression.19 Oestrogens have antidepressant ef-fects when they are administered either alone or in combinationwith antidepressants20,21 and, consequently, oestrogen replacementtherapycanbeusedtopreventthedevelopmentofdepressioninin-dividuals who are at risk.22Oestrogenscanalsohaveneuroprotectiveeffects.Highlevelsofcirculatingoestrogensareassociatedwithlessischaemia-inducedbraininjury.23Asimilareffectisalsoobservedwhenhighlevelsofendogenousoestrogensaresynthesisedinthebrain.24 Oestrogenswerefoundtoplayaroleinneuronalplasticityandspinesynapseformation.25,26Furthermore,manystudieshaveshownposi-tive effects of oestrogens on cognition.27-29 InAlzheimer’s disease,oestrogens have been shown to protect neurones against the toxic-ityofamyloidplaques.30Nevertheless,morestudiesarenecessary31 because investigators from theWomen’s Health InitiativeMemoryStudyfoundthattherapywithacombinationofoestrogenandpro-gestinincreasedtheriskfordementiainpostmenopausalwomenanddidnotimprovetheirperformanceinmildcognitivetasks.32 For this reason,thecontributionofoestrogensandthemoleculardynamicsoftheir interaction with other hormones and neurotransmitters should

bedeterminedtoobtainabetterunderstandingoftheroleofthesesteroidsinbrainfunctionandneuroprotection.

Progestinscanexert theireffects throughboth intracellularpro-gestin receptors (PR-AandPR-B)andmembrane-associatedPRs. Inaddition, these neuroactive steroids can also interact with severalother receptors and ion channels.33Forexample,severalsteroidhor-mones,includingprogesterone,werefoundtobindtosigma-1recep-tors.34Progesteronecanactasasigma-1receptorantagonist.35Underischaemicconditions,progesteroneantagonismofsigma-1receptorscan be neuroprotective because it attenuates the NMDA-inducedinfluxofCa2+via theNMDAreceptor ionchannel.36 Progestins can alsointeractwithoestrogensinthebrain,suchasintheregulationofsynapseformation.37 Progestins are also involved in processes such as maintenanceofthestructuralintegrityofmyelin,38regulationofspino-genesis,synaptogenesis,neuronalsurvivalanddendriticgrowth.39-41 Thereisevidenceindicatingthattheadministrationofexogenouspro-gesterone in animalmodels of traumatic brain injury and ischaemiacan decrease the lesion volume in the brain42 and decrease cognitive deficits.43Likewise,progestinscanexhibitaneuroprotectiveeffectinspinal cord injury.44 Evidence has also been presented suggesting a neuroprotectiveeffectofprogestinsinotherbraindisorders,suchasperipheralnerveinjury,demyelinatingdisease,motoneuronediseases,seizures,depressionandAlzheimer’sdisease.18,45-47

Androgensexerttheireffectsthroughtheandrogenreceptor(AR)subtypesAR-AandAR-B.Androgensareknowntoaffectvariousbrainfunctionsandbehaviour.Themostcommonbehavioural roleofan-drogens is related toaggression.Anexcessofcirculatingandrogensinducesaggressivebehaviourinbothmalesandfemales.48Androgensarealso involved indepressionandanxiety-likedisorders,especiallyafter menopause in women and during hypogonadism in men.49 Alterationsoftestosterone levelswereassociatedwithan increasedriskofmooddisordersandpsychosis.50Anabolicabuseandhyper-orhypoandrogenism are related to mood changes51 and the incidence ofdepression.52Ontheotherhand,androgenscanalsohaveaneu-roprotectiverole.Long-termexposuretoandrogensincreaseshippo-campalneurogenesisandmodulatesthesurvivalofnewneurones.53 Androgensalsoplayaroleinsynapseformationandtheyarecapableof inducing the formationof spine synapses,54 which appears to be mediatedbyNMDAactivity.55

3  | PET TRACERS FOR STEROID HORMONE RECEPTORS IN BRAIN RESEARCH

Despite the increasing knowledge on the roles of sex steroid hor-mones,manyaspectsofthefunctionsandmechanismsofactionofsex steroid hormones in the brain are still incompletely understood andrequirefurtherresearch.Non-invasiveimagingtoolssuchasPETcouldfacilitatesuchresearch.SeveralPETtracersforsteroidhormonereceptorsareavailableandhavebeensuccessfullyusedinoncology,although,todate,thereareonlyfewstudiesinwhichtheyhavebeenusedforbrainresearch.MoststudieswithPETtracersforsteroidhor-mone receptors use either autoradiography or ex vivo tissue counting.

4 of 12  |     MORAGA- AMARO et Al.

Sofar,onlyafewstudieshavemeasuredthe invivodistributionofsteroid receptor ligands in rodents,whereas imaging studies of thehuman brain are lacking.

ThemostfrequentlyusedPETtracerforimagingoestrogenrecep-tors is 16α-[18F] fluoro-17ß-oestradiol ([18F]FES). [18F]FES has been successfully used in both preclinical and clinical studies, mostly inbreast cancer.56 [18F]FESwas the first PET tracer to be applied forquantitativeexvivoassessmentofoestrogenreceptors inthebrain.Thebrainof femaleratswasdissectedandradioactivity indifferentbrain areas was measured ex vivo with a γcounter.Byapplyingdif-ferentdistributiontimes,informationaboutthekineticsofthetracerin the rat brain was obtained.16Specificbindingofthetracerwasob-servedonlyinbrainregionswithhighERdensity,suchasthepituitaryandhypothalamus.Specificbindingcouldbequantifiedbothbyequi-librium and dynamic kinetic analysis.16Twoyearslater,[18F]FESPETwassuccessfullyusedtoidentifyERexpressioninthetumourofsixpatients with brain meningiomas.57Laterstudies, includingourown,haveshownthat[18F]FESPETisabletodetectER-expressioninbrainmetastasesofER-sensitivetumourssuchasbreastcancer.

More than a decade after the experiments of Moresco etal,57 our group investigated whether ER in the rat brain could be quanti-fied invivousing [18F]FESwith adedicated small-animalPET scan-ner.58TheresultsobtainedwereinagreementwiththeexvivodataofMorescoetal.16Specificbindingwasobservedinthepituitaryandhypothalamus,whicharebothbrain regionswithahighERdensity,butnot inotherpartsofthebrain.58Ovariectomyresulted inan in-crease in tracer uptake in the pituitary and hypothalamus,whereas

administrationofexogenousoestradioldecreased[18F]FES uptake in theseregions,indicatingthattraceruptakewassensitivetocirculatingoestrogens competing for thebinding siteof theER.Drivenby thepromisingresultsinrats,weperformedasmall[18F]FESPETstudyinhealthyvolunteers.Inpostmenopausalwomen,[18F]FESPETshowedasignificantlyhigher traceruptake in thepituitarycomparedtoanyotherbrainregion,withonlyslightdifferencesin[18F]FES uptake be-tweenthesebrainregions.Besidesthepituitary,[18F]FES mainly accu-mulatedinwhitematter(Figure2).AdministrationofanexperimentalERantagonist significantly reduced [18F]FES in the pituitary but not in anyotherbrain region. [18F]FES uptake in white matter was also unaffectedbytheantagonist,suggestingthatwhitematteruptakewasdominatedbynonspecificbindingofthelipophilictracer.[18F]FESPETcouldbeappliedtoassessERreceptoroccupancyofthisexperimentaldrug in pituitary59.BycontrasttotheaforementionedofPETstudiesinrats,[18F]FESdidnotshowanyspecificbindinginthehypothalamusinhumans.Thisdiscrepancymightbe related tospeciesdifferencesin receptor expression, nonspecific binding, plasma levels of SHBGorblood-brainbarrierpenetrationofthetracer.However,thereisnoconcreteevidenceavailableyetforanyofthesehypotheses.

Consequently,[18F]FESPETcouldbeusefulforassessingERden-sityinbrainregionswithhighERexpression(pituitary),althoughitdoesnotappeartobesufficientlysensitiveforevaluatingERdensityinotherbrain regions.Therefore, thedevelopmentofnovelPET tracerswithhigheraffinityisurgentlywarrantedtoboosttheresearchinthisarea.

SomePETtracershavebeendevelopedfor imagingofproges-tinreceptorsaswell.21-[18F]Fluoro-16α-ethyl-19-norprogesterone

F IGURE  2  (A)Positronemissiontomography(PET)imagesofthedistributionoftheoestrogenreceptortracer16α-[18F]fluoro-17ß-oestradiol([18F]FES)inthebrainofahealthypostmenopausalwoman.Theimageswereacquired60-90minafterinjectionof200MBqof[18F]FES.TraceruptakeispresentedaskBq/ccimages.Thepituitaryisclearlyvisibleasahotspot(red)inthesagittalimage(topright).Inaddition,theimagesmainlyshowuptakeinwhitematter.(B)PETscanofanaïvefemaleratbrain,60-90minafterinjectionof25MBqof[18F]FES.ThePETscanisco-registeredwithamagneticresonanceimagingtemplateofthebraintoprovideananatomicalreference.Thehighlightedspotrepresentstheactivityofthetracerinthepituitary/hypothalamus

(A) (B)

     |  5 of 12MORAGA- AMARO et Al.

([18F]FENP)wasevaluatedascandidatetracerfortheprogestinre-ceptorand initiallyappearedtoshowspecificuptake intheuterusandintumoursofbothratsandhumans.60However,furtherstud-iesincancerpatientsrevealedthat[18F]FENPcouldnotdetectthePR in a large fraction of positive tumours. [18F]FENP uptake didnot correlate with PR levels and the tumour-to-background ratiowas low.61Moreover, the tracerwas rapidlymetabolised not onlyin the liver and blood, but also by tumour cells.62 Subsequently,tracers with increased metabolic stability have been developed and testedforimagingofPRinhumanbreastcancer,including21-[18F]fluoro-16α,17α-[(R)-(1′-α-furylmethylidene)-dioxy]-19-norpregn-4-ene-3,20-dione ([18F]FFNP)63 and 4-[18F]fluoropropyl-tanaproget([18F]FPTP).64 Both tracers showed specific uptake in the uterus,although,aftertheinitialreports,nofurtherstudieshavebeenpub-lishedandnoneofthetracershasyetbeentestedforimagingofPRin the brain.

SeveralpromisingPETtracersforandrogenreceptorshavebeendeveloped,especiallyfor imagingofprostatecancer.ThefirsttracerforPET imagingof theandrogen receptorswas20-[18F]fluoromibo-lerone ([18F]Fmib),whichwas tested in both rats andbaboonswithpromising results.65,66Moretracershavebeensynthesisedandtestedasmarkersofprostatecancer,67ofwhich16ß-[18F]fluorodihydrotes-tosterone ([18F]FDHT) is themost promising so far. [18F]FDHTwassuccessfullyappliedtoimagetheexpressionofARintumoursnotonlyinpreclinicalstudies,butalsoinpatientswithprostatecancer.68Ourgrouphastested[18F]FDHTforPETimagingofARintheratbrain.69 Ourstudyshowedthat[18F]FDHTismetabolisedveryrapidlyinrats,and its uptake in the brain is very low.69Thisresultsinapoorsignal-to-noiseratio,whichprecludesthedetectionofARintheratbrain.Bycontrast to rats, humans express SHBG,which canprotect steroidssuchas[18F]FDHTfrommetabolicdegradation.70Despitethedisap-pointingresultsobtainedinrats,thestabilisingeffectofSHBGinmenwould stillwarrant investigation of the ability of [18F]FDHTPET tovisualiseARreceptorsinthebrainofhumans.

Inconclusion,itcanbeproposedthatPEThaspotentialasanon-invasivetoolforassessingtheexpressionofsteroidreceptorsinthebrain,providedtracersbecomeavailablethatcanpenetratetheblood-brainbarrierandhavehigheraffinityandmetabolicstability.

4  | PET IMAGING OF AROMATASE AS A BIOMARKER FOR OESTROGEN SYNTHESIS

Aromatase isakeyenzymeinthebiosynthesisofoestrogens; itca-talyses the conversionof testosterone intooestradiol.71Aromataseis expressed in awide variety of tissues, including ovaries, adiposetissue, skin, testicles,muscle, liver and the central nervous system.Aromatase has been suggested as a biomarker for neuroprotectionbecauseitincreasesthelocallevelsofoestrogensininjuredneuronesin the brain.72Aromataseisnotexpressedconstitutivelyinthebrainbut can be induced by testosterone or dihydrotestosterone.73Brainaromatase is involved in, amongst others, the regulation of sexualbehaviour, emotional behaviour, aggression, cognition,memory and

neuroprotection,73making thisenzymean interesting target for thestudyofsexsteroidhormonesinthebrain.

Tracers for aromatase are generally based on enzyme inhibi-tors.Thenonsteroidalaromataseinhibitor[11C]vorozole is the most tested tracer in this field. PET imaging studieswith [11C]vorozole inrhesusmonkeysshowedspecificbindingtoaromataseexvivointhemedialamygdala,bednucleusstriaterminalisandthepre-opticarea,whereasinvivobindingonlyoccurredinthemedialamygdalaandthepre-opticarea.Specifictraceruptakecouldbequantifiedbypharmacokineticmodelling,usingcerebellumasareferencetissue.74 Thesamegroupsubsequentlyappliedthistracerinanonhumanpri-mateandarodentmodelofanabolicsteroidabuse.75PETwith[11C]vorozole demonstrated increased aromatase levels in the bed nu-cleusofthestriaterminalisandthepre-opticareasofratstreatedwithanabolic androgenic steroids, aswell as in thehypothalamusofmacaquemonkeystreatedwiththesesteroids.76,77[11C]vorozole PETimaginginbaboonsshowedthatthemenstruationcyclehadasignificanteffecton tracerbinding in thebrain.78The firsthumanPETstudywith[11C]vorozolewasperformedin2010,79 demonstrat-ing the specificity and kinetics of this tracer in the human brain.Thisstudywasfollowedbyrecentstudiesevaluatingtheradiationdosimetryandbindingkineticsof[11C]vorozole in healthy men and women.80,81

Besides [11C]vorozole, twoothertracers for theassessmentofbrainaromataseweretested.[11C]Letrozolewasinvestigatedinba-boonsanditwasconcludedthatthistracerisnotsuitableforbrainresearchbecauseoftheabsenceofregionalspecific,saturablebind-ing in the brain.82A later study tested the tracer [11C]cetrozole in rhesusmonkeys.Inthissubsequentstudy,[11C]cetrozole displayed betterselectivityandspecificity,aswellasahighersignal-to-noiseratio, than [11C]vorozole. Therefore, it was better suited for thequantitativeanalysisofaromataseexpressionintheamygdala,hy-pothalamus and nucleus accumbens in monkeys.83Sofar,however,nostudieson theuseof [11C]cetrozolePET inhumanshavebeenpublished.

Accordingly,2suitablePETtracersforimagingofaromataseex-pressioninanimalshaverecentlybecomeavailableandoneofthemhasbeensuccessfullyevaluatedinhealthyvolunteers.Furthereval-uation in clinical studies to obtain more insight regarding the value of these tracers is required. If successful, this PET imaging couldprovideanewimpetusforclinicaltrialsontheroleofaromataseinhealth anddisease.The chemical structureof radiotracers for sexhormonereceptorsandaromatasetested inthebrain, isshowninFigure 3.

5  | USE OF PET TRACERS TO STUDY SEX STEROID HORMONE- INDUCED CHANGES IN BRAIN FUNCTION

PETimagingusingradioligandsofreceptorsrelatedtosexsteroidhor-monesignallingmayprovidevaluableinformationabouttheinterac-tionofthesehormoneswithothersignallingsystemsinthebrain,as

6 of 12  |     MORAGA- AMARO et Al.

wellasthepossiblebehaviouraloutcomeofthatinteraction,thusof-feringawiderangeofpossiblestudies.PETimagingmayalsobeusedtostudytheimpactofsteroidhormonesonphysiologicalormetabolicbiomarkers.Below,wediscusssomestudiesinwhichthedownstreameffectsofhormonalchangeswereevaluatedbyPETimaging.

5.1 | Impact of sex steroid hormones in cerebral blood flow

Ageneral approach for studying the impact of steroidhormones istodetectactivationofspecificbrainregionsbymeasurementoftheregionalcerebralblood flow (rCBF).Changes incerebralblood flowcanbedetectedwithPETusingthetracer[15O]H2O.

84,85SuchrCBFchangescanberelatedtospecificchangesinthephysiologicalcon-centrationsofsexhormones.

Onlya fewstudieshaveapplied [15O]H2OPETtostudy the im-pactofhormonalchangesonbrainactivitysofar.[15O]H2OPEThasbeenused to investigate the effect of oestrogen replacement ther-apy on the rCBF. Significant longitudinal differenceswere found inrCBFactivationpatternsduringcognitivetasksbetweencontrolsandwomen on oestrogen replacement therapy.84,85 Inparticular,oestro-gen replacement users showed higher rCBF in the memory circuithippocampus-parahippocampal gyrus-temporal lobe thannon-users.Inanotherstudy,[15O]H2OPETwasusedtoinvestigatetheeffectofendogenous testosteroneon the rCBF in elderlymen.86 Higher en-dogenous testosteroneconcentrationswere foundtocorrelatewithahigherrCBFinbrainregionsassociatedwithmemoryandattention.Recently, [15O]H2OPETwasused to investigate thecorrelationbe-tweenadecreasedproductionofprogesteroneandoestradiolbytheovaries and hippocampalworkingmemory, although no statisticallysignificantchangesinbloodflowwerefound.87

Thus,thelimitednumberofavailablestudiessuggeststhatmea-surement of the rCBF with [15O]H2O PET could be a useful toolfor investigating the impact of sex steroid hormones on cognition.However,adisadvantageof[15O]H2OPETistheexposureofsubjectsto a radioactive substance. For many research questions, magnetic

resonance imaging (MRI) techniques have now replaced [15O]H2OPET. Functional MRI using blood-oxygen-level dependent (BOLD)imaging isa frequentlyused technique formeasuring regionalbrainactivity.AlthoughtheBOLDsignal isdependentonbloodflow, it isnotadirectmeasureoftherCBF.FunctionalMRIusingarterialspinlabelling(ALS),ontheotherhand,measuresthetransitofmagneticallylabelledwaterandthuscanprovideadirectmeasureoftherCBF.Inmanysituations,ALScanthereforeprovideasuitablealternativefor[15O]H2OPET.

5.2 | Brain metabolism and sex hormones: [18F]FDG

2-Deoxy-2-[18F]fluoro-d-glucose ([18F]FDG) is the most frequentlyused radiotracer in PET imaging. Because themetabolic propertiesof [18F]FDGaresimilar to thoseofd-glucose, [18F]FDGPETcanbeusedtodetecttissueswithchangedglucosemetabolism.[18F]FDGismainlyusedforthediagnosisofcancer,infectionsandcardiovasculardiseases.88Glucoseistheprimarysourceofenergyforthebrainand,consequently,[18F]FDGPETcanalsobeusedtoassessbrainglucosemetabolism,whichisoftenusedasasurrogatemarkerforbrainac-tivity.Thus,[18F]FDGPETcanbeusedtoassesschangesincerebralactivityofspecificbrainareasduringthetimecourseofdiseasesandtoevaluatetheeffectoftreatment.

Fewpreclinicalstudiesusing[18F]FDGPEThavebeenperformedto investigate the effect of sex steroid hormones on brain glucosemetabolism.Thefirststudy in thisspecific fieldaimedtodeterminetheneuralcorrelatesofsexualcompetitioninmalerhesusmacaques.The study showed metabolic differences between male monkeysconfrontedwith threats to theirexclusivesexualaccess toa femalemateandcontrols.Thedifferencesinbrainglucosemetabolismwerecorrelated with differences in testosterone levels.89 [18F]FDG PETstudiesinaratmodeloftraumaticbraininjuryaimedtovisualisetheeffectsofhormonetherapy,usingeithersyntheticorendogenousoes-trogens.90,91Thesestudiesdemonstrated that thesteroidhormonesreducedthetrauma-induceddecreaseinglucosemetabolism,suggest-ingabeneficialeffectoncellularsurvival.

F IGURE  3 Chemicalstructureoftestedradiotracersinthebrainforbothsex hormone receptors and oestrogen synthesis. 16α-[18F]fluoro-17ß-oestradiol([18F]FES)istheradioligandusedforoestrogenreceptors(ER).16ß-[18F]fluorodihydrotestosterone([18F]FDHT)corresponds to the tracer tested in the rat brain to visualise androgen receptors (AR).[11C]vorozole,[11C]letrozoleand[11C]cetrozolearealltracersforaromatasequantification,whichistheenzymeresponsibleforoestrogenproductionusingtestosterone as substrate

Aromatase

ER

OH

HO

N

NN

NN

N

NN

CN NC

N N

N

N

11CH3

N11CN

11CH3

CI

OH

OH

18F18F

AR

[18F]FES

[11C] letrozole[11C] vorozole [11C] cetrozole

[18F]FDHT

     |  7 of 12MORAGA- AMARO et Al.

[18F]FDGPETwas also used in several human studies to inves-tigate the effect of steroid hormones on brainmetabolism. Reimanet al92measured[18F]FDGuptakeinspecificbrainregionsoffemalevolunteers aiming to study the effect of circulating oestrogens onbrain glucose metabolism. Different [18F]FDG distribution patternswereobservedduringthedifferentphasesofthemenstrualcycle.Inparticular, a higher [18F]FDGuptakewasobserved in thalamic, pre-frontal,temporoparietalandinferiortemporalregionsduringthemid-follicular phase, whereas the mid-luteal phasewas associatedwithhigher [18F]FDGuptake in superior temporal, anterior temporal, oc-cipital,cerebellar,cingulateandanteriorinsularregions.Otherstudiesinvestigatedtheeffectofhormonaltherapyorhormonaladministra-tiononbrainmetabolism.Ausefulapproachforanassessmentoftheimpactofsteroidhormonesonthemetabolismofthehumanbrainistheuseofpostmenopausalwomenorhypogonadalmalesbecauseofthesignificantdecreaseinbasallevelsofsexualhormonesinthebodyofthesesubjects.Eberlingetal93studiedtheeffectofoestrogenag-onistsandantagonistsinpostmenopausalhealthywomen,orwomenwith breast cancer. Changes in glucose metabolism were mainly ob-served in the frontal lobe and the hippocampus. A small [18F]FDGPETstudyinmenwithhypogonadisminvestigatedtheeffectoftes-tosterone substitution therapy on brain glucose metabolism during a mental rotation test.94In4outof6subjects,testosteronesubstitutionimproved the mental rotation score and only these subjects showed an increase in [18F]FDGuptake inonespecificbrainareacomparedtobaseline.Remarkably,eachoftheseindividualsrevealedadifferentareawithenhancedtraceruptake(rightinferioroccipitalgyrus,right

inferiorfrontalgyrus,rightmiddletemporalgyrus,leftprimaryvisualcortex),whichmakesitdifficulttodrawanyconclusions.

Besides the investigation of regional glucose metabolism, it ispossible to study the connectivity and network changes associ-atedwithsteroidhormone treatmentusing [18F]FDGPET.Ottowitzet al95 investigatedwhether the connectivity of specific brain areaswas associated with systemic hormone levels. They observed thatoestradiol injections inducedsignificantchanges in [18F]FDGuptakeandprefrontal-hippocampalconnectivityinpostmenopausalwomen.Whenpre-andpostmenopausalsubjectswerecompared,changesintheamygdala-corticalnetworkconnectivitywereobservedaswell.96

[18F]FDGPETcanalsobeusedtostudysecondaryeffectsasso-ciated tohormonal therapies. [18F]FDGPETwas applied to investi-gatepossibleneurobiologicalfactorsunderlyingthehotflashesasasecondaryeffectofhormoneadjuvanttreatmentinbreastcancerpa-tients. Reduced glucose metabolism in the hypothalamus and insular cortexwasfoundtobeapredictorofthedevelopmentofhotflashes.97 Otherstudiesinvestigatedtheriskofdevelopingneurocognitivedisor-ders by hormone therapy in menopausal and postmenopausal women. Silverman et al98 investigated the effect of oestrogen-containinghormone therapies on brain glucose metabolism in postmenopausal womenatriskofAlzheimer’sdisease.[18F]FDGPETdemonstratedthatoestrogenshadaneuroprotectiveeffect,whichwasassociatedwithabetterscoreonverbalmemory.Womencontinuingonoestrogen-basedhormonetherapyshowedapreservationofglucosemetabolismintheprecuneus/posteriorcingulatecorticalarea,comprisingabrainregionknowntoshowsignificantdegenerationintheearlystagesof

F IGURE  4  (A)2-Deoxy-2-[18F]fluoro-d-glucose([18F]FDG)positronemissiontomography(PET)imagesofthebrainofahealthywoman.Thesubjecthadfastedfor6hourspriortothescan.StaticPETimageswereacquired30-35minafterinjectionof200MBqof[18F]FDG.[18F]FDGuptakeispresentedasstandardiseduptakevalues(SUV)andisasurrogatemarkerforbrainglucosemetabolism.(B)[18F]FDGPETimagesofthebrainofahealthyfemaleWistarrat.Theimageswerereconstructedfroma30-minstaticscanthatwasacquired45minafteri.v.injectionof17MBqof[18F]FDG.[18F]FDGuptakeispresentedastheSUV

(A) (B)

8 of 12  |     MORAGA- AMARO et Al.

Alzheimer’sdisease.99[18F]FDGPEThasalsobeenusedtostudythecorrelation between brain metabolism and oestradiol brain levels in postmenopausalwomenwithAlzheimer’s disease. In a small study,a direct linear correlationwas found between hippocampal glucosemetabolism and oestradiol levels in the cerebrospinal fluid.100 [18F]FDGPETwas also able to reveal regional changes in brain glucosemetabolism as a result of testosterone replacement therapy in twohypogonadalpatientswithAlzheimer’sdisease.101Furthermore,[18F]FDGPETwasabletodemonstrateacompensatoryeffectoftestoster-one administration on brain hypometabolism in women with anorexia nervosa.102Anexampleof[18F]FDGimaginginthebrainisprovidedin Figure 4.

5.3 | Sex steroid hormones and neurotransmitter activity regulation

Sex steroid hormones are known to participate in many developmen-taland regulatoryprocesses in thebrain.Mostof theseeffectsaremediated by either direct actions on hormone receptors or by indirect modulationofotherneurotransmittersystems.103

Serotonin is an important neurotransmitter that plays a central role inbraindevelopment,stressreactivity,moodandseveralpsychiatricdisorders.104Serotoninsignallingcanbeaffectedbysexsteroidhor-mones.103Serotoninreceptors(5-HTR)arepartofacomplexsignallingpathwayinthebrainandcanbedividedinto7differentfamilies,eachwithdifferentsubtypes.PETtracersareavailable forseveral5-HTRsubtypes.Sofar,onlyfew5-HTRtracershavebeenusedtostudytheinteractions between sex hormones and serotonin neurotransmission.

[11C]-WAY100635canbeusedtomeasuretheexpressionofthe5-HTR1A subtype. Some studies have used this tracer to assess pos-sible correlations between 5HTR1A expression and circulating hor-moneconcentrationsinhumans.PETimagingwith[11C]-WAY100635showed that 5-HTR1A expression in the hippocampus of healthywomenispositivelycorrelatedwithlevelsoftheandrogenandoestro-genprecursordehydroepiandrosterone(DHEA),suggestingarolefortheserotoninergicsystemintheup-regulationofsexsteroids.105PETalso showed that lateralisationof5-HTR1A in language areas (hemi-sphericasymmetry) ispositivelycorrelatedwithplasmalevelsofsexhormones.106OtherPETstudieswith[11C]-WAY100635haveshownthatincreased5HTR1A expression was associated with enhanced pro-gesterone andDHEA levels in pre- andpostmenopausalwomen.107 Sexhormonelevelswerefoundtobecorrelatedwithtestscoresforaggressionand5-HTR1A traceruptake in frontal areas.

108 Serotonin changeshavealsobeenstudied in thebrainofmenopausalwomentreatedwithhormonetherapy,butnosignificantdifferencesin[11C]-WAY100635uptakewerefoundbetweensubjectstreatedwithoes-tradiol alone or oestradiol + progesterone.109

Another receptor of interest is the 5-HTR2A. Longitudinal PETstudies with the tracer [18F]altanserin showed increased 5-HTR2A binding in thewholebrainand in specificbrain regions (eg, thehy-pothalamus and cortex) of postmenopausal women that were firsttreatedwithoestradiol alone, andwere later treatedwith the com-binationofoestradiolwithprogesterone.110,111Another studyusing

the same radiotracer showed a positive correlation between cortical [18F]altanserinbindingandlevelsofendogenousoestradiolinmen.112

A recent study used [11C]SB207145, a specific tracer for im-aging of 5-HTR4. A negative correlation was observed betweenbindingof [11C]SB207145to5-HTR4 receptors in the whole brain and both oestradiol and testosterone levels in healthy men.113 Several studies used the radiolabelled serotonin precursor [11C]-5-hydroxytryptophan ([11C]-5-HTP) and tracers for the serotonintransporter (SERT). [11C]5-HTPand [15O]H2Owereused to inves-tigate thecorrelationbetween regional serotoninsynthesis,bloodflowandthelevelsofsexhormonesandsymptomsofpremenstrualdysphoriainwomen,showinganinversecorrelationbetweenmen-strual phase changes in plasma oestradiol levels and changes in the right-to-left[11C]5-HTPuptakeratiosinthedorsolateralprefrontalcortex.114TwoPETimagingstudieswithdifferenttracersexaminedthe interplaybetween sexhormones and the expressionof SERT,which is known to be related to brain processes affected in psy-chiatric disorders.115 Frokjaer et al116 investigatedthe influenceoftherapy with GnRH agonists on depressive symptoms and SERTavailability using PET with the tracer [11C]DASB. GnRH therapydecreasedoestrogen levels, induceddepressivesymptomsand in-creasedSERTavailabilityinneocortex.Jovanovicetal117used[11C]MADAMPETandshowedadecreaseinSERTexpressionafterlong-termtreatmentofpostmenopausalwomenwithoestrogenaloneoracombinationofoestrogenandtestosterone.

Asmallstudywiththedopaminereceptor ligand[11C]raclopride aimedtovisualisechanges indopamineD2/D3receptoravailabilityover thedifferentphasesof themenstrualcycle inhealthywomen.However,nosignificantchangesin[11C]raclopride binding were ob-served.118Another study evaluated the effect of circulating testos-terone and oestradiol on D2/D3 receptor availability in Göttingenminipigstreatedwithchronicamphetamine ina longitudinaldesign,using[11C]raclopridePET.Thestudydidnotrevealanysignificantcor-relation between the imaging results and the plasma concentrations oftestosteroneoroestradioleither.119Alaterstudyusedadifferenttracer for the same receptor ([18F]fluoroclebopride) in femalecyno-molgus monkeys and demonstrated differences in the distributionvolume ratio in the caudate nucleus and the putamen between the lutealandfollicularphase.120Twoadditionalstudiesonthedopami-nergicsystemhavebeenreportedupon.Kindlundhetal121 used three different tracers tomeasure dopaminergic changes in the rat brainasaresultof treatmentwithanabolic-androgenicsteroids.Changesin the density of dopamine transporterswere assessed using [11C]FE-β-CIT,changesinthedensityofD1receptorswereassessedwith[11C]-(+)-SCH23390andchangesinD2/D3receptorswereassessedwith [11C]raclopride.Treatmentwith the androgennandroloneonlycausedanincreasedin[11C]FE-β-CITbindinginstriatum,indicativeofup-regulationofdopaminetransporters.121Dopaminereceptoravail-abilitywasnotaffected.Anotherstudyassessedtheeffectofsteroidhormonesondopaminemetabolisminovariectomisedfemalerhesusmonkeys,usingthetracer[18F]6-fluoro-l-m-tyrosine.However,signif-icantchangesintheconcentrationsofthedopamineprecursorwerenot detected.122

     |  9 of 12MORAGA- AMARO et Al.

InteractionsofsteroidhormoneswiththeGABAergicneurotans-mitter systems have been studied usingPETwith [18F]flumazenil, atracerforGABAAreceptors.TheeffectofovariectomyandoestradiolreplacementonGABAA receptor expression was investigated in a social subordinationmodeloffemalerhesusmonkeys.Ovariectomycausedan increase in [18F]flumazenil binding in the cortex and other brainareas.Thiseffectwasreversedbyhormonereplacementtherapy.123

Theaforementioned studies show that sex steroidhormones canhaveaneffectonbrainneurotransmittersystemsandalsothat theseeffectscanbemonitorednon-invasivelywithPET.Sofar,onlyfewpubli-cationdescribetheuseofthisapproachinneuroendocrinologystudies,indicatinganareaofresearchthatstillremainsunexplored.Mostofthestudiesshowedinteractionsofsexhormoneswithmajorneurotransmit-ter systems involved in psychiatric disorders such as serotonin and do-pamine,positioningthemaslikelytargetsforfutureresearchinthisfield.

6  | CONCLUSIONS

Thereisongoingresearchontheinfluenceofsexsteroidhormonesonbraindevelopmentandbrainfunction.Althoughtheexpressionofsexsteroid hormone receptors in the brain has been demonstrated9 and somerolesof thesehormones inthebrainhavebeenelucidated,2-4 thereisstillalargegapinourknowledgeofthesehormonesystem.Thiscanbepartlybeascribedtothelackofsuitabletechniquesavail-able forassessing thedynamicsand interplayof thesemolecules inthelivingbrain.Non-invasiveimagingcouldofferagoodopportunitytoinvestigatetheroleofsexsteroidhormonesandtheirreceptorsinthe brain in health and disease.

SpecificradiotracersforPETimagingofoestrogen,progestinandandrogenreceptorshavebeendeveloped,althoughonlyfewofthemhave been tested in the brain. Some successful studies, especiallyusingtheERtracer[18F]FES,havebeenperformed,althoughlowup-takeinbrainareaswithlowreceptordensity,rapidtracermetabolismandunfavourablekineticsofmanytracerslimittheapplicationofthesetracers for thevisualisationandquantificationofchangesofsteroidreceptordensity inspecificbrainareas.Tracerswithhigheraffinitiesandmetabolicstabilityandbetterblood-brainbarrierpenetrationareneededtoexpandthisresearchfield.

PETimagingcanalsobeusedtoquantifytheeffectsofsexsteroidsonbrainperfusionandmetabolism.Hormonetreatmentinconditionssuchasmenopause,hypogonadismandsteroidabuseappearstopro-videusefulparadigmsforstudyingtheeffectofsteroidsignallingonbrain activity or examining the relationship between stress hormone levelsandbiologicaloutcomesinhumans.Studiesofthecorrelationsbetween hormone levels in plasma and regional tracer uptake may alsoprovideuseful informationonthe involvementofspecificbrainregionsandregionalconnectivity.Theuseofanimalmodelsmayalsobeusefulbecausemanyexperimentalmanipulationscanbeappliedinanimals but not in humans.

Furthermore,aplethoraofPETtracersforspecificneurotransmit-terreceptorsandtransportersareavailable.Thesetracersenabletheinvestigationof the interactionbetweensexsteroidhormonesand

various neurotransmitter systems.These studies could help to un-ravelthemechanismsthatareresponsiblefortheimpactofsexste-roidhormonesonbrainfunctionandneuroprotection.Animprovedunderstandingof theseeffects could result in the improvementofexistinghormonetherapies.Studiescouldfocuson,forexample,dis-criminationofspecificreceptorfunctions intermsoffastandsloweffects,sexdifferencesandthemechanismsofactionofsteroidsindiseasesofthebrain.WehavereviewedPETstudiesrelatedtothefunctionofsexhormonesinthebrain.Ifthelimitationsidentifiedcanbeovercome,PETmayprovetobeapromisingnon-invasivetech-nique that can be applied in both experimental animals and human subjects,whichwouldfacilitatethetranslationofinterestingfindingsfromstudiesinexperimentalanimalsintoclinicaltrialsinhumans.

ACKNOWLEDGEMENTS

Wearegrateful toBrunoLimaGiacobboandAnnaSchildtfortheirusefulcomments.

CONFLICT OF INTERESTS

Theauthorsdeclarethattheyhavenoconflictsofinterest.

AUTHOR CONTRIBUTIONS

RM, EdV, AvW and JD co-wrote the manuscript. RM collated thematerial.

ORCID

E. F. J. de Vries http://orcid.org/0000-0002-6915-1590

REFERENCES

1. SimerlyRB.Wiredonhormones:endocrineregulationofhypotha-lamic development. Curr Opin Neurobiol.2005;15:81-85.

2. FrickKM,KimJ,TuscherJJ,FortressAM.Sexsteroidhormonesmat-terfor learningandmemory:estrogenicregulationofhippocampalfunctioninmaleandfemalerodents.Learn Mem.2015;22:472-493.

3. Maeng LY, Milad MR. Sex differences in anxiety disorders: inter-actions between fear, stress, and gonadal hormones.Horm Behav. 2015;76:106-117.

4. Diamanti-Kandarakis E,DattiloM,MacutD, et al.Mechanisms inendocrinology :agingandanti-aging:acombo-endocrinologyover-view. Eur J Endocrinol2017;176:R283-R308.

5. Handa RJ, Weiser MJ. Gonadal steroid hormones and thehypothalamo-pituitary-adrenal axis. Front Neuroendocrinol. 2014;35:197-220.

6. HammondGL.Plasmasteroid-bindingproteins:primarygatekeepersofsteroidhormoneaction.J Endocrinol.2016;230:R13-R25.

7. Shibuya K, Takata N, Hojo Y, et al. Hippocampal cytochromeP450s synthesize brain neurosteroids which are paracrine neuro-modulators of synaptic signal transduction. Biochim Biophys Acta. 2003;1619:301-316.

8. Baulieu EE. Neurosteroids: a novel function of the brain.Psychoneuroendocrinology.1998;23:963-987.

10 of 12  |     MORAGA- AMARO et Al.

9. MitraSW,HoskinE,YudkovitzJ,etal.Immunolocalizationofestro-gen receptor β in the mouse brain: comparison with estrogen recep-tor α. Endocrinology.2003;144:2055-2067.

10. LawsKR,IrvineK,GaleTM.SexdifferencesincognitiveimpairmentinAlzheimer’sdisease.World J Psychiatry.2016;6:54-65.

11. Bao A-M. Colocalization of corticotropin-releasing hormone andoestrogenreceptor-intheparaventricularnucleusofthehypothala-mus in mood disorders. Brain.2005;128:1301-1313.

12. ShughruePJ,LaneMV,MerchenthalerI.Comparativedistributionofestrogenreceptor-alphaand-betamRNAintheratcentralnervoussystem. J Comp Neurol.1997;388:507-525.

13. Takano A. The application of PET technique for the develop-ment and evaluation of novel antipsychotics. Curr Pharm Des. 2010;16:371-377.

14. HospersGAP,HelmondFA,deVriesEGE,DierckxRA,deVriesEFJ.PET imagingof steroid receptorexpression inbreastandprostatecancer. Curr Pharm Des.2008;14:3020-3032.

15. HöferP,LanzenbergerR,KasperS.Testosteroneinthebrain:neuro-imagingfindingsandthepotentialroleforneuropsychopharmacol-ogy. Eur Neuropsychopharmacol.2013;23:79-88.

16. MorescoRM,CasatiR,LucignaniG,etal.Systemicandcerebralki-neticsof16alpha[18F]fluoro-17beta-estradiol:aligandfortheinvivo assessment of estrogen receptor bindingparameters. J Cereb Blood Flow Metab.1995;15:301-311.

17. KawataM,NishiM,MatsudaK,etal.Steroidreceptorsignallinginthebrain-lessonslearnedfrommolecularimaging.J Neuroendocrinol. 2008;20:673-676.

18. WalfAA,FryeCA.Areviewandupdateofmechanismsofestrogeninthehippocampusandamygdalaforanxietyanddepressionbehavior.Neuropsychopharmacology.2006;31:1097-1111.

19. ThorpeL,WhitK,KutcherSP.Clinicalguidelinesforthetreatmentof depressive disorders VI. special populations. Can J Psychiatry. 2001;46:63-76.

20. MontgomeryJC,BrincatM,TappA,etal.Effectofoestrogenandtestosterone implants on psychological disorders in the climacteric. Lancet.1987;329:297-299.

21. Shaukat A, Arain TM, Shahid A, Irfan S, Farrukh S. Estrogen re-placementtherapyfordepressioninperimenopausalwomen.J Coll Physicians Surg Pak.2005;15:597-600.

22. GrigoriadisS,KennedySH.Roleofestrogeninthetreatmentofde-pression. Am J Ther.2002;9:503-509.

23. CarswellHV,DominiczakAF,MacraeIM.Estrogenstatusaffectssen-sitivitytofocalcerebralischemiainstroke-pronespontaneouslyhy-pertensive rats. Am J Physiol Heart Circ Physiol. 2000;278:H290–H294.

24. McCulloughLD,BlizzardK,SimpsonER,OzOK,HurnPD.Aromatasecytochrome P450 and extragonadal estrogen play a role in ischemic neuroprotection. J Neurosci.2003;23:8701-8705.

25. McEwenBS,AkamaKT,Spencer-SegalJL,MilnerTA.Estrogenef-fectsonthebrain:actionsbeyondthehypothalamusvianovelmech-anisms. Behav Neurosci.2012;126:4-16.

26. Herrick SP, Waters EM, Drake CT, McEwen BS, Milner TA.Extranuclear estrogen receptor beta immunoreactivity is on doublecortin-containingcells intheadultandneonatalratdentategyrus. Brain Res.2006;1121:46-58.

27. SheppardPAS,KossWA,FrickKM,CholerisE.Rapidactionsofestro-gens and their receptors on memory acquisition and consolidation in females.J Neuroendocrinol2017;May10.https://doi.org/10.1111/jne.12485.[Epubaheadofprint].

28. VahabaDM,Remage-healeyL,ProgramB.Brainestrogenproduc-tionandtheencodingofrecentexperienceDaniel.Curr Opin Behav Sci.2016;6:148-153.

29. FortressAM,FrickKM.Epigeneticregulationofestrogen-dependentmemory. Front Neuroendocrinol.2014;35:530-549.

30. ThomasT,RhodinJA,SuttonET,BryantMW,PriceJM.Estrogenpro-tectsperipheralandcerebralbloodvesselsfromtoxicityofAlzheimer

peptideamyloid-betaandinflammatoryreaction.J Submicrosc Cytol Pathol.1999;31:571-579.

31. WebberKM,CasadesusG,MarlattMW,etal.Estrogenbowstoanewmaster: the roleofgonadotropins inAlzheimerpathogenesis.Ann NY Acad Sci.2005;1052:201-209.

32. ShumakerSA,LegaultC,RappSR,etal.Estrogenplusprogestinandthe incidenceofdementia andmild cognitive impairment inpost-menopausal women. JAMA.2003;289:2651-2662.

33. Zheng P. Neuroactive steroid regulation of neurotransmitter re-leaseintheCNS:action,mechanismandpossiblesignificance.Prog Neurobiol.2009;89:134-152.

34. Rybczynska AA, Elsinga PH, Sijbesma JW, et al. Steroid hor-mones affect binding of the sigma ligand 11C-SA4503 in tu-mour cells and tumour-bearing rats. Eur J Nucl Med Mol Imaging. 2009;36:1167-1175.

35. MauriceT,GrégoireC,EspallerguesJ.Neuro(active)steroidsactionsat the neuromodulatory sigma1 (σ1)receptor:biochemicalandphys-iological evidences, consequences in neuroprotection. Pharmacol Biochem Behav.2006;84:581-597.

36. CaiW,ZhuY, FuruyaK, LiZ, SokabeM,ChenL.Twodifferentmolecular mechanisms underlying progesterone neuropro-tection against ischemic brain damage. Neuropharmacology. 2008;55:127-138.

37. Woolley CS, McEwen BS. Roles of estradiol and progesterone inregulationofhippocampaldendriticspinedensityduringtheestrouscycle in the rat. J Comp Neurol.1993;336:293-306.

38. SchumacherM,HussainR,GagoN,OudinetJP,MatternC,GhoumariAM.Progesteronesynthesisinthenervoussystem:implicationsformyelination and myelin repair. Front Neurosci2012;6:1-22.

39. McEwenBS.Estradiolandprogesteroneregulateneuronalstructureand synaptic connectivity in adult as well as developing brain. Exp Gerontol.1994;29:479-493.

40. ZhangZ,YangR,ZhouR,LiL,SokabeM,ChenL.Progesteronepro-motesthesurvivalofnewbornneuronsinthedentategyrusofadultmale mice. Hippocampus.2010;20:402-412.

41. MellonSH.Neurosteroidregulationofcentralnervoussystemdevel-opment. Pharmacol Ther.2007;116:107-124.

42. GibsonCL,GrayLJ,BathPMW,MurphySP.Progesterone for thetreatmentofexperimentalbrain injury; a systematic review.Brain. 2008;131:318-328.

43. DjebailiM, GuoQ, Pettus EH, Hoffman SW, Stein DG. The neu-rosteroids progesterone and allopregnanolone reduce cell death,gliosis,andfunctionaldeficitsaftertraumaticbrain injury inrats.J Neurotrauma.2005;22:106-118.

44. Labombarda F, Garcia-Ovejero D. Give progesterone a chance.Neural Regen Res.2014;9:1422-1424.

45. DeutschER,EspinozaTR,AtifF,WoodallE,KaylorJ,WrightDW.Progesterone’s role in neuroprotection, a review of the evidence.Brain Res.2013;1530:82-105.

46. FryeCA,ScaliseTJ.Anti-seizureeffectsofprogesteroneand3α,5α-THP in kainic acid and perforant pathway models of epilepsy.Psychoneuroendocrinology.2000;25:407-420.

47. LiY,RaabyKF,SánchezC,GulinelloM.Serotonergicreceptormech-anisms underlying antidepressant-like action in the progesteronewithdrawalmodelofhormonally induceddepressioninrats.Behav Brain Res.2013;256:520-528.

48. CarréJM,McCormickCM,HaririAR.Thesocialneuroendocrinologyofhumanaggression.Psychoneuroendocrinology.2011;36:935-944.

49. Seidman SN.Normative hypogonadism and depression: does “an-dropause” exist? Int J Impot Res.2006;18:415-422.

50. vanWingenGA,OssewaardeL,BäckströmT,HermansEJ,FernándezG.Gonadalhormoneregulationoftheemotioncircuitryinhumans.Neuroscience.2011;191:38-45.

51. TalihF,FattalO,MaloneD.Anabolicsteroidabuse:psychiatricandphysical costs. Cleve Clin J Med2007;74:341-344.346,349-352.

     |  11 of 12MORAGA- AMARO et Al.

52. McHenry J, Carrier N, Hull E, KabbajM. Sex differences in anxi-ety and depression: role of testosterone. Front Neuroendocrinol. 2014;35:42-57.

53. GaleaLAM,WainwrightSR,RoesMM,Duarte-GutermanP,ChowC,HamsonDK.Sex,hormonesandneurogenesisinthehippocampus:hormonalmodulationofneurogenesisandpotential functional im-plications. J Neuroendocrinol.2013;25:1039-1061.

54. LeranthC,HajszanT,MacLuskyNJ.Androgensincreasespinesyn-apsedensityintheCA1hippocampalsubfieldofovariectomizedfe-male rats. J Neurosci.2004;24:495-499.

55. RomeoRD,McCarthyJB,WangA,MilnerTA,McEwenBS.Sexdif-ferencesinhippocampalestradiol-inducedN-methyl-D-asparticacidbindingandultrastructural localizationofestrogen receptor-alpha.Neuroendocrinology.2005;81:391-399.

56. van Kruchten M, de Vries EGE, Brown M, et al. PET imaging ofoestrogen receptors in patients with breast cancer. Lancet Oncol. 2013;14:e465-e475.

57. MorescoRM,ScheithauerBW,LucignaniG,etal.Oestrogenrecep-tors in meningiomas: a correlative PET and immunohistochemicalstudy. Nucl Med Commun.1997;18:606-615.

58. Khayum MA, de Vries EFJ, Glaudemans AWJM, Dierckx RAJO,DoorduinJ. InVivo imaging of brain estrogen receptors in rats: a16 alpha-F-18-fluoro-17 beta-estradiol PET study. J Nucl Med. 2014;55:481-487.

59. HattersleyG, David F, HarrisA, ClarkinM, Banks K, GlaudemansAWJM,DoorduinJ,KooleM,deVriesEFJ,WilliamsG.Aphase1dose escalation study of RAD1901, an oral selective estrogen re-ceptor degrader, in healthy postmenopausal women. Cancer Res. 2016;76(Suppl4):6-13-02.

60. Pomper MG, Katzenellenbogen JA, Welch MJ, Brodack JW, Mathias CJ. 21-[18F]fluoro-16 alpha-ethyl-19-norprogesterone:synthesisandtargettissueselectiveuptakeofaprogestinreceptorbased radiotracer for positron emission tomography. J Med Chem. 1988;31:1360-1363.

61. DehdashtiF,McGuireAH,VanBrocklinHF,etal.Assessmentof21-[18F]fluoro-16α-ethyl-19-norprogesterone as a positron-emittingradiopharmaceutical for the detection of progestin receptors inhuman breast carcinomas. J Nucl Med.1991;32:1532-1537.

62. VerhagenA,StudenyM,LuurtsemaG,etal.Metabolismofa[18F]fluorine labeled progestin (21-[18F]fluoro-16 alpha-ethyl-19-norprogesterone) in humans: a clue for future investigations.Nucl Med Biol.1994;21:941-952.

63. DehdashtiF, LaforestR,GaoF,etal.Assessmentofprogesteronereceptors inbreastcarcinomabyPETwith21-18F-fluoro-16,17-[(R)-(1′-α-furylmethylidene)dioxy]-19-norpregn-4-Ene-3,20-dione. J Nucl Med.2012;53:363-370.

64. LeeJH,ZhouH,DenceCS,CarlsonKE,WelchMJ,KatzenellenbogenJA.Developmentof[F-18]fluorine-substitutedTanaprogetasapro-gesteronereceptorimagingagentforpositronemissiontomography.Bioconjug Chem.2010;21:1096-1104.

65. Liu AJ, Katzenellenbogen JA, VanBrocklin HF, Mathias CJ,WelchMJ. 20-[18F]fluoromibolerone, a positron-emitting radiotracer forandrogen receptors: synthesis and tissue distribution studies. J Nucl Med.1991;32:81-88.

66. BonaseraTA,O’NeilJP,XuM,etal.Preclinicalevaluationoffluorine-18-labeled androgen receptor ligands in baboons. J Nucl Med. 1996;37:1009-1015.

67. Liu A, Dence CS, Welch MJ, Katzenellenbogen JA. Fluorine-18-labeled androgens: radiochemical synthesis and tissue distribution studies on six fluorine-substituted androgens, potential imagingagentsforprostaticcancer.J Nucl Med.1992;33:724-734.

68. Talbot JN, Gligorov J, Nataf V, et al. Current applications ofPET imaging of sex hormone receptorswith a fluorinated ana-logueofestradiolorof testosterone.Q J Nucl Med Mol Imaging. 2015;59:4-17.

69. KhayumMA,DoorduinJ,AntunesIF,etal.Invivoimagingofbrainandrogenreceptorsinrats:a[18F]FDHTPETstudy.Nucl Med Biol. 2015;42:561-569.

70. TewsonTJ,MankoffDA,PetersonLM,WooI,PetraP.Interactionsof16alpha-[18F]-fluoroestradiol(FES)withsexsteroidbindingprotein(SBP).Nucl Med Biol.1999;26:905-913.

71. Simpson ER, Mahendroo MS, Means GD, et al. Aromatase cyto-chrome P450, the enzyme responsible for estrogen biosynthesis.Endocr Rev.1994;15:342-355.

72. AzcoitiaI,SierraA,VeigaS,HondaS,HaradaN,Garcia-SeguraLM.Brainaromataseisneuroprotective.J Neurobiol.2001;47:318-329.

73. Garcia-SeguraLM.Aromataseinthebrain:notjustforreproductionanymore. J Neuroendocrinol.2008;20:705-712.

74. TakahashiK,BergströmM,FrändbergP,VesströmEL,WatanabeY,LångströmB. Imaging of aromatase distribution in rat and rhesusmonkeybrainswith[11C]vorozole.Nucl Med Biol.2006;33:599-605.

75. BasariaS.Androgenabuseinathletes:detectionandconsequences.J Clin Endocrinol Metab.2010;95:1533-1543.

76. Takahashi K,Onoe K, Doi H, et al. Increase in hypothalamic aro-mataseinmacaquemonkeystreatedwithanabolic-androgenicste-roids:PETstudywith[11C]vorozole.NeuroReport2011;22:326-330.

77. TakahashiK,HallbergM,MagnussonK,etal.Increasein[11C]voro-zole binding to aromatase in the hypothalamus in rats treated with anabolic androgenic steroids. NeuroReport.2007;18:171-174.

78. Pareto D, Biegon A, Alexoff D, et al. In vivo imaging of brainaromatase in female baboons: [11C]vorozole kinetics and ef-fect of the menstrual cycle. Mol Imaging. 2013;12: https://doi.org/10.2310/7290.2013.00068.

79. BiegonA, Kim SW,Alexoff DL, et al. Unique distribution of aro-mataseinthehumanbrain:invivostudieswithPETand[N-methyl-11C]vorozole. Synapse.2010;64:801-807.

80. LoganJ,KimSW,ParetoD,etal.Kineticanalysisof [11C]vorozole binding in the human brain with positron emission tomography. Mol Imaging.2014;13:1-12.

81. BiegonA,AlexoffDL,KimSW, et al.Aromatase imagingwith [N-Methyl-11C]VorozolePETinhealthymenandwomen.J Nucl Med. 2015;56:580-585.

82. KilK-E,BiegonA,DingY-S,etal.SynthesisandPETstudiesof[(11)C-cyano]letrozole (Femara), an aromatase inhibitordrug.Nucl Med Biol.2009;36:215-223.

83. TakahashiK,HosoyaT,OnoeK,etal.11C-Cetrozole:animprovedC-11C-MethylatedPETprobeforaromataseimaginginthebrain.J Nucl Med.2014;55:852-857.

84. ResnickSM,MakiPM,GolskiS,KrautMA,ZondermanAB.EffectsofestrogenreplacementtherapyonPETcerebralbloodflowandneu-ropsychologicalperformance.Horm Behav1998;34:171-182.

85. MakiPM,ResnickSM.LongitudinaleffectsofestrogenreplacementtherapyonPETcerebralbloodflowandcognition.Neurobiol Aging. 2000;21:373-383.

86. Moffat SD, Resnick SM. Long-term measures of free testoster-one predict regional cerebral blood flow patterns in elderly men.Neurobiol Aging.2007;28:914-920.

87. WeiS-M,BallerEB,KohnPD,etal.Brain-derivedneurotrophicfac-torVal66Metgenotypeandovariansteroidsinteractivelymodulateworkingmemory-relatedhippocampalfunctioninwomen:amulti-modal neuroimaging study. Mol Psychiatry.2017;Apr18.https://doi.org/10.1038/mp.2017.72.[Epubaheadofprint].

88. Chierichetti F, Pizzolato G. 18F-FDG-PET/CT. Q J Nucl Med Mol Imaging.2012;56:138-150.

89. Rilling JK, Winslow JT, Kilts CD. The neural correlates of matecompetition in dominant male rhesus macaques. Biol Psychiatry. 2004;56:364-375.

90. Kim H, Cam-Etoz B, Zhai G, HubbardWJ, Zinn KR, Chaudry IH.Salutary effects of estrogen sulfate for traumatic brain injury. J Neurotrauma.2015;32:1210-1216.

12 of 12  |     MORAGA- AMARO et Al.

91. KimH,YuT,Cam-EtozB,vanGroenT,HubbardWJ,Chaudry IH.Treatment of traumatic brain injury with 17α-ethinylestradiol-3-sulfateinaratmodel.J Neurosurg.2017;127:231-319.

92. ReimanEM,ArmstrongSM,MattKS,MattoxJH.Theapplicationofpositronemissiontomographytothestudyofthenormalmenstrualcycle. Hum Reprod.1996;11:2799-2805.

93. Eberling JL,Wu C, Tong-Turnbeaugh R, JagustWJ. Estrogen- andtamoxifen-associated effects on brain structure and function.NeuroImage.2004;21:364-371.

94. ZitzmannM,WeckesserM,SchoberO,NieschlagE.Changesincere-bral glucose metabolism and visuospatial capability in hypogonadal males under testosterone substitution therapy. Exp Clin Endocrinol Diabetes.2001;109:302-304.

95. Ottowitz WE, Derro D, Dougherty DD, Lindquist MA, FischmanAJ,HallJE.FDG-PETanalysisofamygdalar-corticalnetworkcova-rianceduringpre-versuspost-menopausalestrogen levels:poten-tialrelevancetorestingstatenetworks,mood,andcognition.Neuro Endocrinol Lett.2008;29:467-474.

96. OttowitzWE,SiedleckiKL,LindquistMA,DoughertyDD,FischmanAJ,HallJE.Evaluationofprefrontal-hippocampaleffectiveconnec-tivity following24hoursofestrogen infusion: anFDG-PETstudy.Psychoneuroendocrinology.2008;33:1419-1425.

97. JoffeH,DeckersbachT,LinNU,etal.Metabolicactivityintheinsularcortexandhypothalamuspredictshotflashes:anFDG-PETstudy.J Clin Endocrinol Metab.2012;97:3207-3215.

98. Silverman DH, Geist CL, Kenna HA, et al. Differences in re-gional brain metabolism associated with specific formulationsof hormone therapy in postmenopausal women at risk for AD.Psychoneuroendocrinology.2011;36:502-513.

99. RasgonNL,GeistCL,KennaHA,WroolieTE,WilliamsKE,SilvermanDHS.Prospective randomized trial to assess effects of continuinghormonetherapyoncerebralfunctioninpostmenopausalwomenatriskfordementia.PLoS ONE 2014;9:e89095.

100. SchönknechtP,HenzeM,HuntA,KlingaK,HaberkornU,SchröderJ. Hippocampal glucose metabolism is associated with cerebrospi-nalfluidestrogenlevelsinpostmenopausalwomenwithAlzheimer’sdisease. Psychiatry Res.2003;124:125-127.

101. TanRS.TestosteroneeffectonbrainmetabolisminelderlypatientswithAlzheimer’sdisease:comparingtwocasesatdifferentdiseasestages. Aging Clin Exp Res.2013;25:343-347.

102. MillerKK,DeckersbachT,RauchSL,etal.Testosteroneadministra-tion attenuates regional brain hypometabolism in women with an-orexia nervosa. Psychiatry Res.2004;132:197-207.

103. NguyenT-V,DucharmeS,KaramaS.Effectsofsexsteroids in thehuman brain. Mol Neurobiol.2016;54:7507-7519.

104. BrummelteS,McGlanaghyE,BonninA,OberlanderTF.Developmentalchangesinserotoninsignaling: implicationsforearlybrainfunction,behavior and adaptation. Neuroscience.2017;342:212-231.

105. Moser U, Wadsak W, Spindelegger C, et al. Hypothalamic sero-tonin-1A receptor binding measured by PET predicts the plasmalevelofdehydroepiandrosteronesulfateinhealthywomen.Neurosci Lett.2010;476:161-165.

106. FinkM,WadsakW,SavliM,etal.Lateralizationoftheserotonin-1AreceptordistributioninlanguageareasrevealedbyPET.NeuroImage. 2009;45:598-605.

107. SteinP,BaldingerP,KaufmannU,etal.RelationofprogesteroneandDHEASserumlevelsto5-HT1Areceptorbindingpotentialinpre-andpostmenopausal women. Psychoneuroendocrinology.2014;46:52-63.

108. WitteAV,FlöelA,SteinP,etal.Aggressionisrelatedtofrontalse-rotonin-1AreceptordistributionasrevealedbyPETinhealthysub-jects. Hum Brain Mapp.2009;30:2558-2570.

109. Kranz GS, Rami-Mark C, Kaufmann U, et al. Effects of hormonereplacement therapy on cerebral serotonin-1A receptor binding

in postmenopausal women examined with [carbonyl-11C]WAY-100635. Psychoneuroendocrinology.2014;45:1-10.

110. Moses-KolkoEL,BergaSL,GreerPJ,SmithG,MeltzerCC,DrevetsWC.Widespread increases of cortical serotonin type 2A receptoravailability after hormone therapy in euthymic postmenopausalwomen. Fertil Steril.2003;80:554-559.

111. MosesEL,DrevetsWC,SmithG,etal.Effectsofestradiolandpro-gesteroneadministrationonhumanserotonin2Areceptorbinding:aPETstudy.Biol Psychiatry.2000;48:854-860.

112. FrokjaerVG,ErritzoeD,JuulA,etal.Endogenousplasmaestradiolinhealthy men is positively correlated with cerebral cortical serotonin 2Areceptorbinding.Psychoneuroendocrinology.2010;35:1311-1320.

113. PerfalkE,daCunha-BangS,HolstKK,etal.Testosterone levels inhealthy men correlate negatively with serotonin 4 receptor binding. Psychoneuroendocrinology.2017;81:22-28.

114. ErikssonO,WallA,OlssonU,etal.Womenwithpremenstrualdys-phoria lacktheseeminglynormalpremenstrual right-sidedrelativedominanceof5-HTP-derivedserotonergicactivity in thedorsolat-eralprefrontalcortices-apossiblecauseofdisablingmoodsymp-toms. PLoS ONE. 2016;11: e0159538.

115. SugawaraH,BundoM,IshigookaJ, IwamotoK,KatoT.Epigeneticregulationofserotonintransporterinpsychiatricdisorders.J Genet Genomics.2013;40:325-329.

116. FrokjaerVG,PinborgA,HolstKK,etal.Roleofserotonintransporterchangesindepressiveresponsestosex-steroidhormonemanipulation:apositron emission tomography study. Biol Psychiatry.2015;78:534-543.

117. JovanovicH,Kocoska-MarasL,RådestadAF,etal.Effectsofestro-gen and testosterone treatment on serotonin transporter binding in the brain of surgically postmenopausal women - a PET study.NeuroImage.2015;106:47-54.

118. NordströmA-L,OlssonH,HalldinC.APETstudyofD2dopaminere-ceptordensityatdifferentphasesofthemenstrualcycle.Psychiatry Res.1998;83:1-6.

119. LindNM,OlsenAK,MoustgaardA,etal.Mappingtheamphetamine-evokeddopaminereleaseinthebrainoftheGöttingenminipig.Brain Res Bull.2005;65:1-9.

120. Czoty PW, RiddickNV,GageHD, et al. Effect ofmenstrual cyclephaseondopamineD2receptoravailability in femalecynomolgusmonkeys. Neuropsychopharmacology.2009;34:548-554.

121. KindlundhAMS,BergströmM,MonazzamA,etal.Dopaminergicef-fectsafterchronictreatmentwithnandrolonevisualizedinratbrainby positron emission tomography. Prog Neuropsychopharmacology Biol Psychiatry.2002;26:1303-1308.

122. Eberling JL, Roberts JA, Taylor SE, VanBrocklin HF, O’Neil JP,NordahlTE.No effect of age and estrogen on aromatic L- aminoacid decarboxylase activity in rhesus monkey brain. Neurobiol Aging. 2002;23:479-483.

123. MichopoulosV,EmbreeM,RedingK,etal.CRH receptorantago-nismreversestheeffectofsocialsubordinationuponcentralGABAAreceptorbindinginestradiol-treatedovariectomizedfemalerhesusmonkeys. Neuroscience.2013;250:300-308.

How to cite this article:Moraga-AmaroR,vanWaardeA,DoorduinJ,deVriesEFJ.Sexsteroidhormonesandbrainfunction:PETimagingasatoolforresearch.J Neuroendocrinol. 2018;30:e12565. https://doi.org/10.1111/jne.12565