Endorphinsbeta-EndorphinNarcoticsNaloxonebeta-LipotropinEnkephalinsReceptors, OpioidPainAdrenocorticotropic HormoneNalorphineDrug and Narcotic ControlPain ManagementPain, PostoperativeChronic PainLevorphanolMeperidinePain ThresholdPain MeasurementLevallorphanPain, IntractableAnalgesics, OpioidLow Back PainNarcotic AntagonistsAbdominal PainPentazocineBack PainMorphineAnalgesicsNeck PainOpiumAlphaprodineCyclazocineMorphinansPelvic PainPain Perception