Endorphinsbeta-EndorphinPainPain ManagementPain MeasurementNaloxonebeta-LipotropinEnkephalinsAnalgesicsPain, PostoperativeAnalgesics, OpioidReceptors, OpioidAdrenocorticotropic HormoneChronic PainTreatment OutcomePain ThresholdLow Back PainBack PainAbdominal PainNeck PainPain, IntractablePelvic PainPain Perception