Page - 65 - in Advances in Neuroimmunology

BrainSci. 2016,6, 16 Despite thenumberof reports inhumanandpreclinicalmodelsdescribing theneurotoxiceffects of alcohol, themechanismof howalcohol produces neurodegeneration is unclear [16]. One such mechanismthathasrecentlygainedattention is the impactofexcessivealcoholconsumptiononthe neuroimmunesystem,andparticularly,microglia [17,18].Analysisof thebrainsofhumanalcoholics suggests thatexcessivealcoholconsumption leads tomicroglialactivation[19–21],butwhether this activation is thecauseorconsequenceofalcohol-inducedneurodegeneration isanactivedebate [22]. Thisdiscussion isdue, inpart, toa lackofunderstandingof theeffectofalcoholonmicrogliacoupled with the recent appreciation of the role ofmicroglia in both neurodegenerative and regenerative processes [22–25].Althoughmicrogliahavehistoricallybeendiscussedasthephagocytesof thecentral nervoussystem(CNS),thesecellsarefarmorecomplex,existinginacontinuumofphenotypesorstages ofactivation[26].Microgliaareconstantlysurveyingtheparenchymainnon-pathological conditions; where inresponse toevenasubtlechange in theirenvironment,microgliaalter theirmorphological and functional characteristics, a process termedmicroglial activation [27]. The nomenclature for thesestagesorphenotypesvary. TermslikeM1andclassicalactivationareappliedwhenmicroglia haveanamoeboidmorphologyandsecretepro-inflammatorycytokines,whereasM2andalternative activationareused todescribemicrogliawithbushier ramifications that secreteanti-inflammatory cytokines [26,28]. Inneurodegenerativediseaseswheremicroglial activationdrivesneuronal loss, microgliaaregenerally fullyorclassicallyactivated(i.e.,M1phenotype), secretingpro-inflammatory factorsandundergoinguncontrolledphagocytosis [25,29].Howalcoholaffectsmicroglia isnotwell describedandappears tovarydependingonthemodel.Most reportsofalcohol-inducedmicroglia activationassumethatallactivatedmicrogliaarepro-inflammatory[19,23,30].However, in theone modelwithalcohol-inducedneurodegeneration, theMajchrowicz four-daybingemodel,onlya low levelofactivationoralternative (M2)phenotypehasbeenobserved[22,24,31]. ThevariabilityofmicroglialphenotypesobservedacrossdifferentAUDmodelsmaybedueto the patternofalcoholexposure, specifically intermittentversus sustained intoxication. Interestingly, the intermittentexposuremodelsshowstrongerevidenceofpro-inflammatorymicrogliaevenwith lower dosesofethanol[22,30]. Thesedisparatefindingsacrossmodels ledustoquestionwhethertheinitialhit ofalcoholexposure“primes”microgliasuchthat intermittentexposure leads toapotentiatedresponse. Primedmicrogliahavesimilarmorphologyandcytokine/growthfactorprofilesas theM2/alternative microglia,butprimedmicroglialactivation ispotentiatedwhensubsequentneuroimmunomodulators areapplied[28,32,33]. Ethanol’sability toprimemicrogliaandexacerbate theneuroimmuneresponse to subsequent neuroimmune stimuli is suggested also by the enhancedmicroglia response toLPS followingalcohol exposure [23,34,35]. However, theabilityof a second“hit”or insult of ethanol to potentiate theneuroimmuneresponse(independentofperipheral immunomodulators)hasnotbeen examined. Therefore, the current studydetermineswhether a secondbinge ethanol exposure can potentiate themicrogliaresponsetobingealcoholexposure. Investigatingwhetherrepeatedethanol exposure differentially affectsmicroglia is important considering that themajority of individuals sufferingfromanAUDdrinkinabingepatternthatproducesperiodsofhighBECsinterspersedwith periodsofwithdrawalandabstinence[36–38]. Specifically, thisstudyexaminesbothfunctionaland morphological indices ofmicroglial activation in thehippocampus andentorhinal cortex, regions consistentlydamagedin thismodel [7,8]. 2.MaterialsandMethods 2.1.AlcoholAdministrationModel Atotalof33adultmaleSprague-Dawleyrats (Table1;CharlesRiverLaboratories;Raleigh,NC, USA)wereused in theseexperiments. Proceduresperformedwereapprovedby theUniversityof KentuckyInstitutionalAnimalCareandUseCommittee (protocol#2008-0321,approved20/6/2008) andconformedto theGuidelines for theCareandUseofLaboratoryAnimals [39].Animalsweighed approximately 275–300 g at arrival and were pair-housed in a University of Kentucky AALAC 65