Centrally administered mu- and delta-opioid agonists increase operant responding for saccharin.

In previous reports, ICV administration of selective mu- or delta-opioid receptor agonists was found to stimulate the intake of saccharin and salt solutions in nondeprived rats. In the present study, we measured the effects of selective mu-, delta-, and kappa-agonists on operant responding for saccharin. The selective mu-agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO) and the selective delta-agonist [D-Thr2]-leucine enkephalin-Thr (DTLET) increased responding, whereas the kappa-agonist dynorphin A analog kappa ligand (DAKLI) had no significant effect. These results agree with previous studies on saccharin and salt intake and are consistent with the possibility that the effects of opioids on the intake of these fluids are mediated via enhancement of activity in brain reward pathways.