Receptors, Opioid, muDose-Response Relationship, DrugRats, Sprague-DawleyReceptors, OpioidAnalgesics, OpioidReceptors, Opioid, deltaReceptors, Opioid, kappaOpioid PeptidesNarcotic AntagonistsNaloxoneBacteriophage muOpioid-Related DisordersNaltrexoneEnkephalinsEnkephalin, Ala(2)-MePhe(4)-Gly(5)-MorphineNarcoticsEnkephalin, D-Penicillamine (2,5)-EndorphinsDynorphinsBuprenorphineEnkephalin, LeucineEnkephalin, Leucine-2-AlanineDiprenorphinePainbeta-EndorphinBenzeneacetamidesAnalgesicsMorphinansFentanylDrug ToleranceMethadonePain MeasurementHydromorphoneBenzomorphansPyrrolidinesAnalgesiaSubstance Withdrawal Syndrome