Anesthesia, RectalAnalgesics, OpioidPain, PostoperativeInfusions, IntralesionalAnalgesia, Patient-ControlledAnalgesiaMorphinePain MeasurementAnalgesia, EpiduralReceptors, Opioid, muAnalgesia, ObstetricalReceptors, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesNarcotic AntagonistsNaloxoneFentanylAnalgesicsBupivacaineAnesthetics, LocalPainAcupuncture AnalgesiaNaltrexoneOpioid-Related DisordersMeperidineSufentanilEnkephalinsAnesthesia, EpiduralEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Injections, SpinalNarcoticsLabor PainDrug ToleranceEndorphinsNerve BlockBuprenorphineEnkephalin, D-Penicillamine (2,5)-HydromorphoneTramadolLabor, ObstetricAnalgesics, Non-NarcoticDynorphinsAnesthesia, ObstetricalEnkephalin, LeucinePain ThresholdDouble-Blind MethodAmidesAnesthesia, Spinalbeta-EndorphinEnkephalin, Leucine-2-AlanineButorphanolMorphinansPain ManagementPostoperative Nausea and VomitingCesarean SectionBenzeneacetamidesAlfentanilNalbuphineMorphine DerivativesDose-Response Relationship, DrugInterpleural AnalgesiaMethadoneKetorolacDiprenorphineAnesthesia, GeneralHeroinHyperalgesiaConscious SedationPeriaqueductal GrayPain, IntractableLidocaineKetaminePirinitramideLabor Stage, FirstFemoral NerveElectroacupunctureAnesthesia, CaudalPentazocinePregnancyClonidineBenzomorphansAnesthesia and AnalgesiaRats, Sprague-DawleyPiperidinesSubstance Withdrawal SyndromePruritusPyrrolidinesNociceptorsMorphine DependenceOxymorphoneAnesthetics, CombinedAnesthesia, ConductionHypnotics and SedativesSpinal CordChronic PainKetoprofenCodeineThoracotomyPlacebo Effect