DiprenorphineReceptors, OpioidMorphinansCompetitive BiddingEnkephalin, LeucineNarcotic AntagonistsEtorphineReceptors, Opioid, muReceptors, Opioid, kappaReceptors, Opioid, deltaNaloxoneEnkephalinsEnkephalin, Leucine-2-AlanineEnkephalin, Ala(2)-MePhe(4)-Gly(5)-NarcoticsBuprenorphineCarbon RadioisotopesFentanylNaltrexoneAnalgesics, OpioidMorphineBrainTomography, Emission-Computed