Dopamine-D1 and δ-opioid receptors co-exist in rat striatal neurons

Cocaine's enhancement of dopaminergic neurotransmission in the mesolimbic pathway plays a critical role in the initial reinforcing properties of this drug. However, other neurotransmitter systems are also integral to the addiction process. A large body of data indicates that opioids and dopamine together mediate emotional and reinforced behaviors. In support of this, cocaine-mediated increases in activation of dopamine D1 receptors (D1R) results in a desensitization of 8-opioid receptor (DOR) signaling through adenylyl cyclase (AC) in striatal neurons. To further define cellular mechanisms underlying this effect, the subcellular distribution of DOR and DIR was examined in the rat dorsolateral striatum. Dual immunoperoxidase/gold-silver detection combined with electron microscopy was used to identify DOR and D1R immunoreactivities in the same section of tissue. Semi-quantitative analysis revealed that a subset of dendritic cellular profiles exhibited both DOR and DIR immunoreactivities. Of 198 randomly sampled DIR immunoreactive profiles, 43% contained DOR. Similarly of 165 DOR-labeled cellular profiles, 52% contained DIR. The present data provide ultrastructural evidence for co-existence between DOR and D I R in striatal neurons, suggesting a possible mechanism whereby D I R modulation may alter DOR function. (c) 2006 Elsevier Ireland Ltd. All rights reserved.