Enkephalin, MethionineEnkephalinsEnkephalin, LeucineEndorphinsMorphineNaloxoneEnkephalin, Ala(2)-MePhe(4)-Gly(5)-Narcotic AntagonistsMethionineEnkephalin, Leucine-2-AlanineEnkephalin, D-Penicillamine (2,5)-Morphine DependenceReceptors, Opioid, muAnalgesics, OpioidNarcoticsDrug ToleranceReceptors, OpioidReceptors, Opioid, deltaMorphine DerivativesSubstance Withdrawal SyndromeNaltrexoneLocus CoeruleusEthylketocyclazocineCyclazocineReceptors, Neurokinin-3Periaqueductal GrayOpioid PeptidesLevallorphanRats, Sprague-DawleyD-Ala(2),MePhe(4),Met(0)-ol-enkephalinReceptors, Opioid, kappaDynorphins