Enkephalin, Ala(2)-MePhe(4)-Gly(5)-Receptors, Opioid, muEnkephalin, D-Penicillamine (2,5)-Analgesics, OpioidEnkephalinsNarcotic AntagonistsReceptors, Opioid, deltaReceptors, OpioidReceptors, Opioid, kappaNaloxoneMorphineRats, Sprague-DawleyDopamine AgonistsGABA AgonistsOpioid PeptidesNaltrexoneDose-Response Relationship, DrugNarcoticsBenzeneacetamidesPurinergic P1 Receptor AgonistsMuscarinic AgonistsGuanosine 5'-O-(3-Thiotriphosphate)Nicotinic AgonistsDiprenorphinePyrrolidinesAdrenergic AgonistsAdrenergic alpha-2 Receptor AgonistsSerotonin 5-HT2 Receptor AgonistsEtorphineSerotonin 5-HT1 Receptor AgonistsHistamine AgonistsOligopeptidesPurinergic P2 Receptor AgonistsDrug ToleranceAdrenergic beta-AgonistsGABA-A Receptor AgonistsSomatostatinRadioligand AssayCholinergic AgonistsAdenosine A1 Receptor AgonistsEnkephalin, Leucine-2-AlanineAdrenergic alpha-AgonistsAnalgesicsAdrenergic alpha-1 Receptor AgonistsBaclofenPeriaqueductal GrayMicroinjectionsExcitatory Amino Acid AgonistsBenzomorphansEnkephalin, LeucineGTP-Binding ProteinsDrug InteractionsRats, WistarLigandsCannabinoid Receptor AgonistsNeuronsCyclic AMPAdrenergic beta-3 Receptor AgonistsEnkephalin, MethionineCells, CulturedAdenosine A3 Receptor AgonistsBinding, CompetitiveGuinea PigsCHO CellsPatch-Clamp TechniquesPertussis ToxinElectric StimulationSpinal CordGABA-B Receptor AgonistsSignal TransductionSerotonin 5-HT4 Receptor AgonistsCricetinaeDynorphinsSulfur RadioisotopesCalciumEthylketocyclazocineSubstantia GelatinosaPain MeasurementAdenylate CyclaseNucleus AccumbensReceptors, Dopamine D2Behavior, AnimalLocus CoeruleusCell LineBicucullineElectrophysiologyCarbacholNociceptorsIsoproterenolPurinergic AgonistsMiniature Postsynaptic PotentialsAdenosineInjections, IntraventricularMembrane PotentialsAdrenergic beta-1 Receptor AgonistsPiperidinesAcetylcholineSerotonin AntagonistsMorphine Derivatives