MorphineEnkephalinsReceptors, Opioid, muEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Enkephalin, LeucineEnkephalin, MethionineEnkephalin, D-Penicillamine (2,5)-Analgesics, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaNaloxoneNarcotic AntagonistsEnkephalin, Leucine-2-AlanineNaltrexoneEndorphinsReceptors, OpioidMorphine DependenceMorphine DerivativesPain MeasurementDose-Response Relationship, DrugReceptors, Neurokinin-3EthylketocyclazocineNeurokinin BNarcoticsCyclazocineTachykininsReceptors, TachykininDrug ToleranceD-Ala(2),MePhe(4),Met(0)-ol-enkephalinSubstance PNeurokinin AReceptors, Neurokinin-2DynorphinsBenzomorphansAnalgesicsDopamine AgonistsInjections, SpinalBenzeneacetamidesRats, Sprague-DawleyOpioid PeptidesGuinea PigsAnalgesiaRNA, Transfer, GlyMorphinansbeta-EndorphinEtorphineDiprenorphineSubstance Withdrawal SyndromeCarboxypeptidase HOligopeptidesPainPiperidinesNeurokinin-1 Receptor AntagonistsGlycineCodeineGABA AgonistsPyrrolidinesReceptors, Neurokinin-1Amino Acid SequencePain, PostoperativePeriaqueductal GrayFentanylInjections, IntraventricularMolecular Sequence DataPurinergic P1 Receptor AgonistsBrainKineticsBehavior, AnimalDrug InteractionsSpinal CordThiorphanIleumPeptide FragmentsProtein PrecursorsBinding, CompetitiveMuscarinic AgonistsNeuronsRadioligand AssayAdrenergic alpha-2 Receptor AgonistsLocus CoeruleusPentazocineNicotinic AgonistsRats, Inbred StrainsVas DeferensSerotonin 5-HT2 Receptor AgonistsSomatostatinBenzylidene CompoundsAdrenergic AgonistsStructure-Activity RelationshipRats, WistarNalorphineGuanosine 5'-O-(3-Thiotriphosphate)NeprilysinSerotonin 5-HT1 Receptor AgonistsAdrenal MedullaAlanineCyclic AMPPain ThresholdAnalgesia, Patient-ControlledTime Factors