MorphineEnkephalinsInjections, SpinalAnalgesics, OpioidEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, MethionineEnkephalin, LeucineMorphine DependenceDrug ToleranceReceptors, Opioid, muNaloxoneNarcoticsEnkephalin, D-Penicillamine (2,5)-Narcotic AntagonistsEnkephalin, Leucine-2-AlanineMorphine DerivativesReceptors, OpioidPain MeasurementReceptors, Opioid, deltaPain, PostoperativePruritusSubstance Withdrawal SyndromeAnesthesia, SpinalEndorphinsAnalgesiaAnalgesia, ObstetricalInfusion Pumps, ImplantableRats, Sprague-DawleyPainDose-Response Relationship, DrugAnalgesicsSpinal CordNaltrexoneNalbuphinePain, IntractableMorphinansHyperalgesiaLocus CoeruleusReceptors, Neurokinin-3PirinitramideAnalgesia, Patient-ControlledAnesthesia, ObstetricalBupivacaineLevallorphanLabor PainPeriaqueductal GrayAntipruriticsClonidinePostoperative Nausea and VomitingPreanesthetic MedicationOpioid PeptidesCesarean SectionNeurokinin BEthylketocyclazocineFentanylCyclazocineTachykininsReceptors, TachykininSpinal NervesReceptors, Opioid, kappaTime FactorsDextromethorphanSubstance PD-Ala(2),MePhe(4),Met(0)-ol-enkephalinBehavior, AnimalNucleus AccumbensDrug Administration ScheduleMuscle SpasticityUrinary RetentionDouble-Blind MethodDynorphinsNeurokinin AReceptors, N-Methyl-D-AspartateReceptors, Neurokinin-2Hot TemperatureBrainInjections, SubcutaneousNeuronsAdenylate CyclaseOpioid-Related DisordersMuscle Relaxants, CentralDrug SynergismMotor ActivitySufentanilHalothaneArrestinsRNA, Transfer, GlyG Protein-Coupled Inwardly-Rectifying Potassium ChannelsBenzomorphansAnesthesia, Generalbeta-EndorphinPain ThresholdCyclic AMP-Dependent Protein KinasesRats, WistarVomitingCyclic AMPRespirationMice, Inbred C57BLBenzeneacetamidesCarboxypeptidase H