EndorphinsChronic Painbeta-EndorphinNaloxonebeta-LipotropinEnkephalinsPainReceptors, OpioidPain ManagementAdrenocorticotropic HormonePain MeasurementPain, IntractablePain ThresholdChronic DiseasePain, PostoperativePain PerceptionBack PainAcute PainLow Back PainAnalgesics, OpioidNeck PainAnalgesicsNeuralgiaPain ClinicsAbdominal PainFacial PainPelvic PainMusculoskeletal PainHyperalgesia