Delayed-Action PreparationsMorphineMorphine DependenceMorphine DerivativesAnalgesics, OpioidDrug ToleranceNarcoticsNaloxoneReceptors, Opioid, muNarcotic AntagonistsAnalgesiaInjections, SpinalPain MeasurementHistamine ReleaseDose-Response Relationship, DrugCodeineSubstance Withdrawal SyndromePain, PostoperativeRats, Sprague-DawleyAnalgesicsPainCalciumReceptors, OpioidNaltrexoneNalorphineAnalgesia, Patient-ControlledHeroinHydromorphoneEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Time FactorsFentanylPeriaqueductal GrayEnkephalinsOxycodonePentazocineMorphinans