NaloxoneNarcotic AntagonistsMorphineReceptors, OpioidEndorphinsEnkephalin, MethionineBuprenorphineReceptors, Opioid, muNarcoticsAnalgesics, OpioidOpioid PeptidesMorphine DependenceEnkephalinsbeta-EndorphinEnkephalin, LeucineOpioid-Related DisordersDrug OverdoseReceptors, Opioid, kappaSubstance Withdrawal SyndromeAnalgesicsNaltrexoneEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Receptors, Opioid, deltaMorphinansDrug TolerancePentazocineEnkephalin, Leucine-2-AlanineLoperamideNalorphineLevallorphanLevorphanolCyclazocineDose-Response Relationship, DrugEnkephalin, D-Penicillamine (2,5)-DynorphinsAdministration, SublingualOxycodonePainMeptazinolPain MeasurementInjections, SpinalInjections, IntraventricularFentanylHeroin DependenceHeroinDiprenorphineEtorphineMeperidineOpiate Alkaloids