Lesion of dopamine mesolimbic neurons blocks behavioral effects induced by the endogenous enkephalins but not by a mu-opioid receptor agonist.

Lesioning of dopamine neurons by injection of 6-hydroxydopamine into the nucleus accumbens blocked the increased rearing activity measured in the open-field and induced by injection into the ventral tegmental area of: [R)-3-(N-hydroxylcarboxamido-2-benzyl-propanoyl)-L-alanine), kelatorphan (complete inhibitor of enkephalin catabolism) or by (Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr(OtBu)): BUBU (selective delta agonist) but not the hypolocomotion evoked by the mu agonist (Tyr-D-Ala-Gly-N(Me)-Phe-Glyol): DAMGO. This suggests the involvement of different neuronal pathways in mu and delta effects.