Morphine DerivativesMorphineMorphine DependenceAnalgesics, OpioidDrug ToleranceNarcoticsNaloxoneReceptors, Opioid, muNarcotic AntagonistsAnalgesiaPain MeasurementInjections, SpinalCodeineSubstance Withdrawal SyndromePain, PostoperativeAnalgesicsDose-Response Relationship, DrugPainReceptors, OpioidNaltrexoneMolecular StructureAnalgesia, Patient-ControlledNalorphineStructure-Activity RelationshipRats, Sprague-DawleyHeroinHydromorphoneFentanylMorphinansEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Periaqueductal GrayOxycodonePentazocineEnkephalinsReceptors, Opioid, deltaNalbuphine