Receptors, Opioid, muReceptors, OpioidAnalgesics, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesNarcotic AntagonistsNaloxoneNaltrexoneOpioid-Related DisordersEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Dopamine AgonistsEnkephalinsEnkephalin, D-Penicillamine (2,5)-NarcoticsMorphineDynorphinsEndorphinsEnkephalin, LeucineBuprenorphineGABA AgonistsDose-Response Relationship, DrugEnkephalin, Leucine-2-AlanineBenzeneacetamidesPurinergic P1 Receptor AgonistsDiprenorphinePainMuscarinic AgonistsMorphinansAdrenergic alpha-2 Receptor AgonistsNicotinic AgonistsRats, Sprague-DawleyPyrrolidinesSerotonin 5-HT2 Receptor Agonistsbeta-EndorphinAdrenergic AgonistsBenzomorphansFentanylSerotonin 5-HT1 Receptor AgonistsPain MeasurementHistamine AgonistsAnalgesicsRadioligand AssayPurinergic P2 Receptor AgonistsMethadoneAdrenergic alpha-AgonistsAdrenergic beta-AgonistsDrug ToleranceGABA-A Receptor AgonistsHydromorphoneLigandsAdenosine A1 Receptor AgonistsCholinergic AgonistsSubstance Withdrawal SyndromeAnalgesiaPiperidinesExcitatory Amino Acid AgonistsOligopeptidesCannabinoid Receptor AgonistsGuanosine 5'-O-(3-Thiotriphosphate)Adrenergic alpha-1 Receptor AgonistsBenzylidene CompoundsDrug InteractionsRats, WistarDiterpenes, ClerodaneEthylketocyclazocineLevorphanolCHO CellsAdrenergic beta-3 Receptor AgonistsBinding, CompetitiveHeroinCyclic AMPOpiate Substitution TreatmentGuinea PigsAdenosine A3 Receptor AgonistsButorphanolCricetinaeMorphine DependenceCells, CulturedSignal TransductionGABA-B Receptor AgonistsReceptors, Dopamine D2Chronic PainBehavior, AnimalSerotonin 5-HT4 Receptor AgonistsPrescription Drug MisuseCyclazocineAzocinesCannabinoidsMorphine DerivativesCell LineTime FactorsCalciumAnalgesics, Non-NarcoticSerotonin AntagonistsGTP-Binding ProteinsAdenosineHyperalgesiaNeuronsIsoproterenol