NaloxoneNarcotic AntagonistsReversal LearningMorphineReceptors, OpioidEndorphinsEnkephalin, MethionineBuprenorphineReceptors, Opioid, muNarcoticsAnalgesics, OpioidOpioid PeptidesMorphine DependenceEnkephalinsbeta-EndorphinEnkephalin, LeucineOpioid-Related DisordersDrug OverdoseSubstance Withdrawal SyndromeReceptors, Opioid, kappaAnalgesicsNaltrexoneEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Dose-Response Relationship, DrugReceptors, Opioid, deltaDrug ToleranceMorphinansPentazocineEnkephalin, Leucine-2-AlanineLoperamideNalorphineLevallorphanLevorphanolCyclazocineEnkephalin, D-Penicillamine (2,5)-DynorphinsAdministration, SublingualOxycodoneRats, Sprague-DawleyPainInjections, SpinalInjections, IntraventricularPain MeasurementMeptazinolFentanylgamma-CyclodextrinsTime FactorsHeroin DependenceHeroinDiprenorphineEtorphineMeperidineElectric StimulationBehavior, AnimalMorphine DerivativesPyrrolidinesOpiate AlkaloidsNociceptors