Receptors, Opioid, muNarcotic AntagonistsReceptors, OpioidAnalgesics, OpioidNaltrexoneReceptors, Opioid, deltaNaloxoneReceptors, Opioid, kappaOpioid PeptidesEndorphinsMorphineNarcoticsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-EnkephalinsOpioid-Related DisordersDynorphinsEnkephalin, D-Penicillamine (2,5)-Enkephalin, LeucineDiprenorphineEnkephalin, Leucine-2-AlanineMorphinansBenzeneacetamidesAnalgesicsbeta-EndorphinBuprenorphineAzocinesDose-Response Relationship, DrugBenzomorphansEnkephalin, MethioninePainPyrrolidinesPain MeasurementSubstance Withdrawal SyndromeFentanylRats, Sprague-DawleyDrug ToleranceMorphine DependenceOxymorphoneLevorphanolHeroinBenzylidene CompoundsAnalgesiaPiperidinesDiterpenes, ClerodaneAnalgesics, Non-NarcoticOligopeptidesDrug InteractionsInjections, SpinalMeptazinolInjections, IntraventricularPruritusEtorphineDrug Inverse AgonismAntipruriticsPeriaqueductal GrayTetrahydroisoquinolinesBehavior, AnimalRats, WistarHormone AntagonistsNociceptorsNeurokinin-1 Receptor AntagonistsGuinea PigsPain ThresholdDopamine AntagonistsExcitatory Amino Acid AntagonistsHyperalgesiaTime FactorsConditioning, OperantMethadoneSpinal CordMotor ActivitySomatostatinEthanolHydromorphoneStereoisomerismLigandsSelf AdministrationMice, Inbred ICRHallucinogensInjections, SubcutaneousBrainHistamine H2 AntagonistsElectric StimulationInterleukin 1 Receptor Antagonist ProteinMuscarinic AntagonistsAnalysis of VarianceGABA AntagonistsRadioligand AssayCapsaicinRats, Inbred StrainsOpiate Substitution TreatmentNeuronsInjections, IntraperitonealHistamine H1 AntagonistsPurinergic P1 Receptor AntagonistsQuaternary Ammonium CompoundsLuteinizing HormoneBinding, CompetitiveChronic PainNicotinic Antagonists