Involvement of delta-opioid receptors in the effects of morphine on locomotor activity and the mesolimbic dopaminergic system in mice.

Naltrindole (NTI) and naltriben (NTB), a benzofuran derivative of NTI, were recently synthesized as highly selective delta-opioid receptor antagonists. Both NTI and NTB failed to suppress the antinociceptive effect induced by morphine. In contrast, both NTI and NTB significantly suppressed the morphine-induced hyperlocomotion and increase in turnover of dopamine (DA) in the mouse limbic forebrain. These results suggest that delta-opioid receptors play, at least in part, a role in the morphine-induced hyperlocomotion and excitation of mesolimbic DA systems, but not antinociception.