Drug ToleranceMorphineReflex, RightingEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, LeucineEnkephalin, MethionineMorphine DependenceMorphine DerivativesEnkephalin, D-Penicillamine (2,5)-Enkephalin, Leucine-2-AlanineAnalgesics, OpioidReceptors, Opioid, muNaloxoneEndorphinsReceptors, OpioidNarcoticsReceptors, Opioid, deltaNarcotic AntagonistsReceptors, Neurokinin-3Immune ToleranceEthylketocyclazocineNeurokinin BCyclazocineReceptors, Opioid, kappaTachykininsReceptors, TachykininInjections, SpinalNaltrexoneAnalgesiaPain MeasurementAnalgesicsD-Ala(2),MePhe(4),Met(0)-ol-enkephalinSubstance Withdrawal SyndromeSubstance PDose-Response Relationship, DrugNeurokinin ACodeineDynorphinsPain, PostoperativePainReceptors, Neurokinin-2Glucose Tolerance TestMorphinansRats, Sprague-DawleyBenzomorphansOpioid Peptidesbeta-EndorphinPeriaqueductal GrayEtorphineFentanylRNA, Transfer, GlyNalorphineGuinea PigsBenzeneacetamidesAnalgesia, Patient-ControlledDiprenorphineTransplantation TolerancePentazocineHeroinHydromorphoneCarboxypeptidase HPain ThresholdOxycodoneInjections, IntraventricularBehavior, AnimalSpinal CordLocus CoeruleusOligopeptidesNalbuphineOxymorphoneNeurokinin-1 Receptor AntagonistsDrug InteractionsPiperidinesMeperidineBrainTime FactorsGlycineMolecular Sequence DataThebaineAmino Acid SequencePyrrolidinesReceptors, Neurokinin-1