Mechanisms underlying δ- and μ-opioid receptor agonist-induced increases in extracellular dopamine level in the nucleus accumbens of freely moving rats

The nucleus accumbens is a terminal area of the mesolimbic dopaminergic system that arises in the ventral tegmental area. Opioids are thought to enhance dopaminergic activity in the nucleus accumbens by activating delta-and mu-opioid receptors in the ventral tegmental area. However, delta-and mu-opioid receptor agonists increase extracellular levels of accumbal dopamine when infused directly into the nucleus accumbens of rats. Therefore, the roles of delta-and mu-opioid receptors in regulation of accumbal dopaminergic neural activity have been analyzed by using delta-and mu-opioid receptor ligands. This review describes the mechanisms underlying the stimulatory effects on accumbal dopamine efflux, which are induced by local administration of delta-and mu-opioid receptor agonists into the nucleus accumbens of freely moving rats. The focus of this article is neurochemical studies that use in vivo microdialysis techniques. Taken together, the in vivo neurochemical evidence from these studies indicates that delta-and mu-opioid receptor agonists increase accumbal dopamine efflux by activating naloxone-sensitive opioid receptors, and by mechanisms independent of naloxone-sensitive opioid receptors, in the nucleus accumbens.