Shell/core differences in mu- and delta-opioid receptor modulation of dopamine efflux in nucleus accumbens

The mu- and delta-opioid receptors located at the terminal level in nucleus accumbens are involved in the opiate modulation of dopamine release in this brain area. However, recent studies suggest that the effects of opioid drugs on the core subregion of nucleus accumbens may completely differ from those observed in the shell. We used in vivo microdialysis to simultaneously apply selective mu- and delta-opioid receptor agonists and to measure extracellular levels of dopamine in three subregions of the accumbens, namely shell, core, and the transition zone between them. The regional analysis of these subregions of the accumbens demonstrated that basal levels of dopamine and its metabolites were higher in the core, and decreased from this subregion to the shell. Retrodialysis application to the core of both the selective mu-receptor agonist ([D-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) (1 micromol/L)) and of the selective delta-opioid receptor agonist ([D-Pen(2), D-Pen(5)]-enkephalin (DPDPE) (50 nmol/L)) increased the dialysate levels of dopamine. However, the application of these drugs to the shell significantly reduced the dopamine levels in this subregion. Local application of the same doses of these drugs in the transition zone between the shell and the core did not significantly affect the dopamine levels in dialysates. These results suggest that the opioid circuits modulating dopaminergic activity in the shell could differ from those in the core of the nucleus accumbens.