Narcotic AntagonistsNaltrexoneReceptors, OpioidReceptors, Opioid, muNaloxoneAnalgesics, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesEndorphinsNarcoticsMorphineOpioid-Related DisordersEnkephalin, Ala(2)-MePhe(4)-Gly(5)-EnkephalinsBehavior, AddictiveDynorphinsEnkephalin, LeucineDiprenorphineEnkephalin, D-Penicillamine (2,5)-Substance Withdrawal SyndromeEnkephalin, Leucine-2-AlanineDose-Response Relationship, DrugEnkephalin, MethionineAzocinesMorphinansBuprenorphineBenzeneacetamidesAnalgesicsbeta-EndorphinBenzomorphansRats, Sprague-DawleyMorphine DependencePyrrolidinesPainHeroinPain MeasurementDrug ToleranceSubstance-Related DisordersFentanylOxymorphoneLevorphanolPiperidinesBenzylidene CompoundsSelf AdministrationBehavior, AnimalAnalgesiaDrug InteractionsEtorphineConditioning, OperantDiterpenes, ClerodaneMeptazinolAnalgesics, Non-NarcoticDrug Inverse AgonismPruritusDopamine AntagonistsInjections, IntraventricularAntipruriticsOligopeptidesMethadoneInjections, SpinalTetrahydroisoquinolinesRats, WistarHormone AntagonistsCocaineMotor ActivityHeroin DependenceExcitatory Amino Acid AntagonistsBrainEthanolPeriaqueductal GrayNeurokinin-1 Receptor AntagonistsTime FactorsNociceptorsGuinea PigsHallucinogensRewardHistamine H2 AntagonistsAnalysis of VarianceHyperalgesiaPain ThresholdSpinal CordInterleukin 1 Receptor Antagonist ProteinStereoisomerismInjections, SubcutaneousElectric StimulationSomatostatinMuscarinic AntagonistsGABA AntagonistsMice, Inbred ICRTobacco Use DisorderSubstance Abuse Treatment CentersNeuronsLigandsNicotinic AntagonistsDopamineNucleus AccumbensOpiate Substitution TreatmentCapsaicinDisease Models, Animal