Opioid peptides alleviated while naloxone potentiated methamphetamine-induced striatal dopamine depletion in mice

Delta opioid peptide [D-Ala²,D-Leu⁵] enkephalin (DADLE) can partially reverse long-term loss of striatal dopamine transporters induced by multiple doses of methamphetamine via an unknown mechanism. This study was designed to examine the modulating effects of three opioid ligands, DADLE, Leucine enkephalin (L-enk), and naloxone, on the long-lasting dopamine depletion produced by 4 cumulative doses of methamphetamine. Both DADLE (at a dose of 18mg/kg) and L-enk (100μg/kg × 2) effectively attenuated methamphetamine-induced dopamine depletion in the striatum while their protective effects were not blocked by coadministration of naloxone. In contrast, naloxone (10mg/kg × 2) alone potentiated the long-lasting dopamine depletion produced by methamphetamine. Moreover, none of the treatments with DADLE (18mg/kg), L-enk (100μg/kg), or naloxone (10mg/kg) alone affected body temperature. These results suggest that the opioid ligands may, directly or indirectly, modulate this methamphetamine-induced dopamine neurotoxicity in the nigrostriatal system via a temperature-independent mechanism.