Analgesia, Patient-ControlledEnkephalinsMorphineEnkephalin, Ala(2)-MePhe(4)-Gly(5)-AnalgesiaEnkephalin, LeucineEnkephalin, MethionineAnalgesia, EpiduralEnkephalin, D-Penicillamine (2,5)-Enkephalin, Leucine-2-AlanineAnalgesia, ObstetricalAnalgesics, OpioidReceptors, Opioid, muEndorphinsReceptors, OpioidNaloxoneMorphine DerivativesMorphine DependenceReceptors, Opioid, deltaReceptors, Neurokinin-3Pain, PostoperativeNarcotic AntagonistsPain MeasurementDrug ToleranceNarcoticsAnalgesicsEthylketocyclazocineNeurokinin BCyclazocineTachykininsReceptors, Opioid, kappaReceptors, TachykininFentanylInjections, SpinalBupivacaineD-Ala(2),MePhe(4),Met(0)-ol-enkephalinPainAnesthetics, LocalSubstance PNaltrexoneNeurokinin AAcupuncture AnalgesiaMeperidineReceptors, Neurokinin-2DynorphinsOpioid PeptidesDose-Response Relationship, DrugSufentanilAnesthesia, Epiduralbeta-EndorphinBenzomorphansMorphinansRNA, Transfer, GlyLabor PainBenzeneacetamidesPain ThresholdRats, Sprague-DawleyCodeineNerve BlockSubstance Withdrawal SyndromePeriaqueductal GrayEtorphineHydromorphoneAnesthesia, ObstetricalCarboxypeptidase HAnalgesics, Non-NarcoticDouble-Blind MethodTramadolGuinea PigsAnesthesia, SpinalPiperidinesDiprenorphinePentazocineLabor, ObstetricAmidesSpinal CordNalbuphinePostoperative Nausea and VomitingOligopeptidesNeurokinin-1 Receptor AntagonistsInjections, IntraventricularOxycodoneKetorolacHeroinCesarean SectionReceptors, Neurokinin-1HyperalgesiaBehavior, AnimalClonidineGlycineBrainOxymorphonePyrrolidinesKetamineProtein PrecursorsNociceptorsThiorphanLocus CoeruleusAmino Acid SequenceAnesthesia, General