Receptors, TachykininTachykininsMorphineNarcotic AntagonistsDose-Response Relationship, DrugReceptors, Opioid, muNaltrexoneNaloxoneReceptors, OpioidAnalgesics, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesMorphine DependenceNarcoticsEndorphinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Morphine DerivativesEnkephalinsEnkephalin, D-Penicillamine (2,5)-Drug ToleranceAnalgesicsEnkephalin, LeucineDiprenorphineRats, Sprague-DawleyEnkephalin, Leucine-2-AlanineDynorphinsMorphinansBenzeneacetamidesPain MeasurementBenzomorphansSubstance Withdrawal SyndromeEnkephalin, MethionineOpioid-Related DisordersPainAzocinesPyrrolidinesbeta-EndorphinAnalgesiaFentanylPiperidinesBuprenorphineInjections, SpinalHeroinOxymorphoneBenzylidene CompoundsDrug InteractionsLevorphanolRats, WistarDopamine AgonistsInjections, IntraventricularBehavior, AnimalEtorphineOligopeptidesGuinea PigsDiterpenes, ClerodanePeriaqueductal GrayAnalgesics, Non-NarcoticRadioligand AssayPruritusPurinergic P1 Receptor AgonistsNeurokinin-1 Receptor AntagonistsDopamine AntagonistsDrug Inverse AgonismExcitatory Amino Acid AntagonistsSpinal CordMotor ActivityTime FactorsNociceptorsMeptazinolLigandsAntipruriticsElectric StimulationGABA AgonistsPain ThresholdHormone AntagonistsHyperalgesiaTetrahydroisoquinolinesNeurokinin ACapsaicinSomatostatinAdrenergic alpha-AgonistsNeuronsInjections, SubcutaneousPurinergic P1 Receptor AntagonistsConditioning, OperantReceptors, Neurokinin-2BrainSubstance PAdenosineNicotinic AgonistsStereoisomerismReceptors, Prostaglandin EAdrenergic beta-AgonistsReceptors, Purinergic P1Binding, CompetitiveGABA AntagonistsCodeineMuscarinic AgonistsRats, Inbred Strains