Pain MeasurementPainAnalgesics, OpioidChronic PainPain ManagementPain ThresholdPain, PostoperativePain, IntractableLow Back PainBack PainNarcoticsMorphineOpioid-Related DisordersNeck PainAbdominal PainAcute PainAnalgesicsPelvic PainNaloxonePain PerceptionOxycodoneNarcotic AntagonistsFacial PainOpioid PeptidesReceptors, OpioidEnkephalin, MethionineFentanylPain, ReferredReproducibility of ResultsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-AnalgesiaShoulder Pain