“Intracerebroventricular administration of the selective mu (mu) opioid receptor agonist, (D-Ala2,N-MePhe4,Gly-ol5)enkephalin DAGO ; 25-50 micrograms), or the selective delta (delta) opioid agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE; 3-50 micrograms), like morphine, produced significant decreases in hepatic glutathione 3 h after administration.”