Deltorphin II enhances extracellular levels of dopamine in the nucleus accumbens via opioid receptor-independent mechanisms

Abstract

The effects of the δ 2-opioid receptor agonist, deltorphin II, on extracellular levels of dopamine in the rat nucleus accumbens were investigated in awake animals by in vivo brain microdialysis. In agreement with previous studies, perfusion of deltorphin II (50.0 nmol) into the nucleus accumbens significantly increased the extracellular amount of accumbal dopamine. The effect of deltorphin II (50.0 nmol) was not altered by the selective δ 2-opioid receptor antagonist, naltriben (1.5 nmol), which alone did not significantly affect the basal levels of dopamine. Selective antagonists of neither the μ-opioid receptors, d-Phe–Cys–Tyr- d-Trp–Orn–Thr–Phe–Thr–NH 2 (0.15 nmol), nor the δ 1-opioid receptors, ( E)-7-benzylidenenaltrexone tartrate (0.15 nmol), failed to significantly alter the effects of deltorphin II. The nonselective opioid receptor antagonist, naloxone (0.75 and 1.5 nmol), which alone did not significantly affect the basal levels of dopamine, also failed to affect the effects of deltorphin II. Moreover, under the condition that the sodium channel blocker, tetrodotoxin (0.1 nmol), was perfused continuously into the nucleus accumbens, the deltorphin II-induced increase in extracellular levels of dopamine was reduced by 72%. These results suggest that deltorphin II enhances extracellular dopamine in the nucleus accumbens via opioid receptor-independent, tetrodotoxin-sensitive mechanisms.