Narcotic AntagonistsReceptors, OpioidNaltrexoneReceptors, Opioid, muReceptors, Opioid, deltaNaloxoneReceptors, Opioid, kappaEnkephalin, D-Penicillamine (2,5)-Opioid PeptidesEnkephalin, Ala(2)-MePhe(4)-Gly(5)-EnkephalinsBenzeneacetamidesAnalgesics, OpioidDynorphinsEnkephalin, Leucine-2-AlaninePyrrolidinesEndorphinsMorphineNarcoticsEnkephalin, LeucineDiprenorphineEnkephalin, MethionineAzocinesMorphinansOpioid-Related DisordersAnalgesicsDose-Response Relationship, Drugbeta-EndorphinBenzomorphansBuprenorphineSubstance Withdrawal SyndromePain MeasurementRats, Sprague-DawleyPainMorphine DependenceFentanylOxymorphoneLevorphanolDrug ToleranceHeroinBenzylidene CompoundsPiperidinesEtorphineAnalgesiaDiterpenes, ClerodaneMeptazinolDrug InteractionsDrug Inverse AgonismAnalgesics, Non-NarcoticPruritusInjections, IntraventricularAntipruriticsOligopeptidesInjections, SpinalTetrahydroisoquinolinesHormone AntagonistsBehavior, AnimalRats, WistarPeriaqueductal GrayNeurokinin-1 Receptor AntagonistsDopamine AntagonistsExcitatory Amino Acid AntagonistsNociceptorsGuinea PigsConditioning, OperantHistamine H2 AntagonistsMotor ActivityEthanolTime FactorsHallucinogensPain ThresholdSpinal CordInterleukin 1 Receptor Antagonist ProteinInjections, SubcutaneousHyperalgesiaSomatostatinStereoisomerismMuscarinic AntagonistsMice, Inbred ICRSelf AdministrationElectric StimulationGABA AntagonistsLigandsBrainCapsaicinHistamine H1 AntagonistsPurinergic P1 Receptor AntagonistsAnalysis of VarianceRats, Inbred StrainsInjections, IntraperitonealNicotinic AntagonistsHistamine AntagonistsQuaternary Ammonium CompoundsLuteinizing HormoneAdenosine A2 Receptor AntagonistsAdrenergic alpha-1 Receptor AntagonistsNeuronsReflexPurinergic P2 Receptor AntagonistsRadioligand Assay