MorphineEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, MethionineEnkephalin, LeucineMorphine DependenceAnalgesics, OpioidReceptors, Opioid, muNarcoticsDrug ToleranceNaloxoneEnkephalin, D-Penicillamine (2,5)-Enkephalin, Leucine-2-AlanineReceptors, OpioidNarcotic AntagonistsReceptors, Opioid, deltaMorphine DerivativesEndorphinsSubstance Withdrawal SyndromeNaltrexoneLocus CoeruleusReceptors, Neurokinin-3Opioid PeptidesLevallorphanPeriaqueductal GrayPain MeasurementRats, Sprague-DawleyAnalgesiaPainReceptors, Opioid, kappaEthylketocyclazocineNeurokinin BOpioid-Related DisordersCyclazocineDose-Response Relationship, DrugTachykininsReceptors, TachykininD-Ala(2),MePhe(4),Met(0)-ol-enkephalinNucleus AccumbensDynorphinsAnalgesicsSubstance PHyperalgesiaNeurokinin ABehavior, AnimalInjections, SpinalReceptors, Neurokinin-2Fentanylbeta-EndorphinSpinal CordTime FactorsNeuronsBrainBenzeneacetamidesOxycodoneAdenylate CyclaseBenzomorphansDrug Administration ScheduleArrestinsEtorphineCodeineDiprenorphineRNA, Transfer, GlyG Protein-Coupled Inwardly-Rectifying Potassium ChannelsMorphinansCyclic AMPCyclic AMP-Dependent Protein KinasesMotor ActivityHydromorphoneMice, Inbred C57BLGuinea PigsRats, WistarPain, PostoperativeCarboxypeptidase HOxymorphoneGTP-Binding Protein beta SubunitsExcitatory Postsynaptic PotentialsSynaptic TransmissionReceptors, N-Methyl-D-AspartateRNA, MessengerPyrrolidinesOligopeptidesMice, KnockoutPiperidinesNeurokinin-1 Receptor AntagonistsInhibitory Postsynaptic PotentialsExcitatory Amino Acid AntagonistsHeroinAnalysis of VarianceMethadonePain, IntractableGlycineAnalgesia, Patient-ControlledThiorphanReceptors, Neurokinin-1Injections, IntraventricularPain ThresholdSufentanilBuprenorphineMeperidine