Regional differences in mu-opioid receptor-dependent modulation of basal dopamine transmission in rat striatum

The nigrostriatal dopamine system is implicated in the regulation of reward and motor activity. Dopamine (DA) release in dorsal striatum (DS) is controlled by the firing rate of DA neurons in substantia nigra pars compacta. However, influences at terminal level, such as those involving activation of mu opioid receptors (MORs), can play a key role in determining DA levels in striatum. Nonetheless, published data also suggest that the effect of opioid drugs on DA levels may differ depending on the DS subregion analyzed. In this study, in vivo microdialysis in rats was used to explore this regional dependence. Changes in basal DA levels induced by local retrodialysis application of DAMGO (selective MORs agonist) in three different subregions of DS along the rostro-caudal axis were studied. Our results indicate that whereas administration of 10μM DAMGO into the rostral and caudal DS significantly reduced DA levels, in medial DS an increase in DA levels was observed. These data reveal a regional-dependent MOR modulation of DA release in DS, similar to that described in the ventral striatum. Our findings may lead to a better understanding of the nigrostriatal DA system regulation.