Opioid PeptidesReceptors, OpioidReceptors, Opioid, muNarcotic AntagonistsReceptors, Opioid, kappaReceptors, Opioid, deltaNaltrexoneNaloxoneAnalgesics, OpioidDynorphinsEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-MorphineEnkephalin, D-Penicillamine (2,5)-MorphinansEnkephalin, LeucineBenzomorphansBenzylidene CompoundsAnalgesicsNarcoticsBenzeneacetamidesRats, Sprague-DawleyDose-Response Relationship, DrugPyrrolidinesbeta-EndorphinEthylketocyclazocineEnkephalin, MethionineEndorphinsMorphine DerivativesCyclazocinePiperidinesPain MeasurementInterleukin 1 Receptor Antagonist ProteinPainDiprenorphineEnkephalin, Leucine-2-AlanineNeurokinin-1 Receptor AntagonistsOligopeptidesInjections, IntraventricularPeriaqueductal GrayRats, WistarDrug ToleranceInjections, SpinalAdrenergic alpha-2 Receptor AntagonistsFentanylElectroacupunctureHeroinLoperamideBehavior, AnimalSomatostatinSubstance Withdrawal SyndromeHyperalgesiaDrug InteractionsGuinea PigsPain ThresholdMicroinjectionsPurinergic P1 Receptor AntagonistsNociceptorsEtorphineExcitatory Amino Acid AntagonistsHistamine H2 AntagonistsDopamine AntagonistsSerotonin 5-HT3 Receptor AntagonistsAnalgesics, Non-NarcoticSpinal CordDiterpenes, ClerodaneConditioning, OperantMice, Inbred ICRAngiotensin Receptor AntagonistsOpioid-Related DisordersCricetulusStereoisomerismRadioligand AssayFormaldehydeElectric StimulationSerotonin 5-HT2 Receptor AntagonistsDisease Models, AnimalAdenosine A2 Receptor AntagonistsSelf AdministrationInjections, SubcutaneousHormone AntagonistsLevorphanolEthanolMotor ActivityAdenosine A1 Receptor AntagonistsNeuronsTime FactorsBrainCentral Nervous System DepressantsPurinergic P2 Receptor AntagonistsHistamine H1 AntagonistsBuprenorphineGABA AntagonistsReceptors, EndothelinAcetic AcidBinding, CompetitiveSerotonin AntagonistsAnalgesiaMuscarinic AntagonistsDizocilpine Maleate