Enkephalin, MethionineEnkephalinsMorphineEnkephalin, LeucineEndorphinsNaloxoneEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Morphine DependenceNarcotic AntagonistsMethionineEnkephalin, Leucine-2-AlanineReceptors, Opioid, muEnkephalin, D-Penicillamine (2,5)-Drug ToleranceAnalgesics, OpioidNarcoticsMorphine DerivativesReceptors, OpioidReceptors, Opioid, deltaSubstance Withdrawal SyndromeNaltrexoneLocus CoeruleusEthylketocyclazocinePeriaqueductal GrayCyclazocineRats, Sprague-DawleyAnalgesiaPain MeasurementLevallorphanOpioid PeptidesReceptors, Neurokinin-3Dose-Response Relationship, DrugReceptors, Opioid, kappaDynorphinsD-Ala(2),MePhe(4),Met(0)-ol-enkephalinImmune TolerancePainAnalgesics