Narcotic AntagonistsNaltrexoneReceptors, Opioid, muReceptors, OpioidNaloxoneReceptors, Opioid, deltaAnalgesics, OpioidReceptors, Opioid, kappaOpioid PeptidesEndorphinsNarcoticsMorphineEnkephalin, Ala(2)-MePhe(4)-Gly(5)-EnkephalinsDynorphinsDose-Response Relationship, DrugEnkephalin, D-Penicillamine (2,5)-Enkephalin, LeucineDiprenorphineEnkephalin, Leucine-2-AlanineAzocinesMorphinansBenzeneacetamidesEnkephalin, MethionineOpioid-Related DisordersBenzomorphansAnalgesicsbeta-EndorphinRats, Sprague-DawleyPyrrolidinesBuprenorphineSubstance Withdrawal SyndromePain MeasurementPainFentanylMorphine DependencePiperidinesOxymorphoneLevorphanolBenzylidene CompoundsHeroinDopamine AgonistsDrug ToleranceDrug InteractionsEtorphineDrug Inverse AgonismDiterpenes, ClerodaneOligopeptidesAnalgesiaInjections, IntraventricularAnalgesics, Non-NarcoticRats, WistarMeptazinolPruritusAntipruriticsBehavior, AnimalDopamine AntagonistsHormone AntagonistsInjections, SpinalTetrahydroisoquinolinesGuinea PigsExcitatory Amino Acid AntagonistsNeurokinin-1 Receptor AntagonistsPeriaqueductal GrayLigandsTime FactorsGABA AgonistsNociceptorsMotor ActivityConditioning, OperantRadioligand AssayMuscarinic AntagonistsSpinal CordHistamine H2 AntagonistsPurinergic P1 Receptor AgonistsElectric StimulationHallucinogensEthanolStereoisomerismSomatostatinHyperalgesiaGABA AntagonistsPain ThresholdInjections, SubcutaneousMice, Inbred ICRSelf AdministrationPurinergic P1 Receptor AntagonistsInterleukin 1 Receptor Antagonist ProteinCapsaicinBrainMuscarinic AgonistsNeuronsRats, Inbred StrainsNicotinic AgonistsAnalysis of VarianceAdrenergic alpha-2 Receptor AgonistsNicotinic AntagonistsBinding, CompetitiveHistamine AntagonistsHistamine H1 Antagonists