Narcotic AntagonistsNaltrexoneReceptors, Opioid, muReceptors, OpioidNaloxoneReceptors, Opioid, deltaAnalgesics, OpioidReceptors, Opioid, kappaOpioid PeptidesEndorphinsMorphineNarcoticsDose-Response Relationship, DrugEnkephalin, Ala(2)-MePhe(4)-Gly(5)-EnkephalinsRats, Sprague-DawleyPiperidinesDynorphinsMorphinansAnalgesicsEnkephalin, LeucineDiprenorphineEnkephalin, D-Penicillamine (2,5)-Enkephalin, Leucine-2-AlanineEnkephalin, MethionineAzocinesBenzeneacetamidesPyrrolidinesOpioid-Related DisordersDopamine AntagonistsNeurokinin-1 Receptor AntagonistsExcitatory Amino Acid Antagonistsbeta-EndorphinBenzomorphansHormone AntagonistsPainPain MeasurementMuscarinic AntagonistsSubstance Withdrawal SyndromePurinergic P1 Receptor AntagonistsRats, WistarBuprenorphineDrug InteractionsInjections, IntraventricularAdrenergic alpha-1 Receptor AntagonistsMorphine DependenceNicotinic AntagonistsRadioligand AssayGuinea PigsFentanylHistamine H2 AntagonistsBenzylidene CompoundsOligopeptidesAdenosine A2 Receptor AntagonistsOxymorphoneInterleukin 1 Receptor Antagonist ProteinLevorphanolPurinergic P2 Receptor AntagonistsGABA AntagonistsDrug ToleranceBehavior, AnimalHeroinBinding, CompetitiveInjections, SpinalHistamine AntagonistsHistamine H1 AntagonistsAdrenergic alpha-AntagonistsAnalgesiaEtorphineDrug Inverse AgonismAnalgesics, Non-NarcoticDiterpenes, ClerodaneElectric StimulationReceptors, SerotoninPruritusMeptazinolCapsaicinMotor ActivityTetrahydroisoquinolinesTime FactorsReceptors, EndothelinAntipruriticsLigandsNociceptorsAdrenergic alpha-2 Receptor AntagonistsXanthinesAdenosine A3 Receptor AntagonistsAngiotensin Receptor AntagonistsGABA-A Receptor AntagonistsAdrenergic AntagonistsSerotonin 5-HT3 Receptor AntagonistsNeuronsHyperalgesiaPurinergic P2X Receptor AntagonistsSpinal CordSerotonin 5-HT2 Receptor AntagonistsReceptors, N-Methyl-D-AspartatePirenzepinePyridinesPeriaqueductal Gray