MethadoneMorphineEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, LeucineEnkephalin, MethionineNarcoticsMorphine DependenceAnalgesics, OpioidReceptors, Opioid, muEnkephalin, Leucine-2-AlanineDrug ToleranceEnkephalin, D-Penicillamine (2,5)-NaloxoneNarcotic AntagonistsMorphine DerivativesReceptors, OpioidReceptors, Opioid, deltaEndorphinsOpioid-Related DisordersSubstance Withdrawal SyndromeLocus CoeruleusOpiate Substitution TreatmentReceptors, Neurokinin-3NaltrexoneHeroin DependencePeriaqueductal GrayNeurokinin BEthylketocyclazocineCyclazocineRats, Sprague-DawleyTachykininsReceptors, TachykininMethadyl AcetatePain MeasurementDose-Response Relationship, DrugBuprenorphineReceptors, Opioid, kappaHeroinD-Ala(2),MePhe(4),Met(0)-ol-enkephalinOpioid PeptidesEtorphineSubstance PNeurokinin ANeonatal Abstinence SyndromePainDynorphinsReceptors, Neurokinin-2Nucleus AccumbensTegmentum MesencephaliGuinea PigsAnalgesicsPyrrolidinesDrug ImplantsBehavior, AnimalDrug InteractionsInjections, SpinalAnalgesiaNeuronsVentral Tegmental AreaBrainSpinal CordDiprenorphineCodeineRNA, Transfer, GlyDrug Administration ScheduleBenzomorphansTime FactorsSubstance Abuse DetectionOligopeptidesPain ThresholdArrestinsbeta-EndorphinBenzeneacetamidesCarboxypeptidase HAdenylate CyclaseMorphinansGuanosine 5'-O-(3-Thiotriphosphate)Tyrosine 3-MonooxygenaseCocaineHyperalgesiaRaphe NucleiRats, Inbred StrainsOpiumMice, Inbred C57BLReceptors, N-Methyl-D-AspartateG Protein-Coupled Inwardly-Rectifying Potassium ChannelsNeurokinin-1 Receptor AntagonistsPain, PostoperativeLimbic SystemFentanylHydromorphoneDrug OverdoseMotor ActivityCyclic AMPNalorphineElectric StimulationExcitatory Amino Acid AntagonistsExcitatory Postsynaptic PotentialsGlycine