AnalgesiaAnalgesics, OpioidAnalgesia, EpiduralReceptors, Opioid, muReceptors, OpioidAnalgesia, ObstetricalMorphineReceptors, Opioid, deltaAnalgesia, Patient-ControlledOpioid PeptidesReceptors, Opioid, kappaPain MeasurementFentanylPentazocinePainNaloxoneDrug TolerancePain, PostoperativeNarcotic AntagonistsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Pain ThresholdAnalgesicsInjections, Spinalbeta-EndorphinNarcoticsNociceptorsBupivacaineHyperalgesiaAnesthetics, LocalPiperidinesSpinal CordAcupuncture AnalgesiaRats, Sprague-DawleyNaltrexoneOpioid-Related DisordersDose-Response Relationship, DrugMeperidineSufentanilEnkephalinsAnesthesia, EpiduralLabor PainEndorphinsNerve BlockBuprenorphineEnkephalin, D-Penicillamine (2,5)-HydromorphoneTramadolLabor, ObstetricAnalgesics, Non-NarcoticDynorphinsAnesthesia, ObstetricalEnkephalin, LeucineDouble-Blind MethodAmidesAnesthesia, SpinalEnkephalin, Leucine-2-AlanineButorphanolMorphinansPain ManagementPostoperative Nausea and VomitingCesarean SectionBenzeneacetamidesAlfentanilNalbuphineMorphine DerivativesInterpleural AnalgesiaMethadoneKetorolacDiprenorphineAnesthesia, GeneralHeroinConscious SedationPeriaqueductal GrayPain, IntractableLidocaineKetaminePirinitramideLabor Stage, FirstFemoral NerveElectroacupunctureAnesthesia, CaudalPregnancyClonidineBenzomorphansAnesthesia and AnalgesiaSubstance Withdrawal SyndromePruritusPyrrolidinesMorphine DependenceOxymorphoneAnesthetics, CombinedAnesthesia, ConductionHypnotics and SedativesChronic PainKetoprofenCodeineThoracotomyPlacebo EffectLevorphanolNitrous Oxide