Acute Cocaine Enhances Dopamine DR Recognition and Signaling and Counteracts DR Internalization in Sigma1R-DR Heteroreceptor Complexes.

The current study was performed to establish the actions of nanomolar concentrations of cocaine, not blocking the dopamine transporter, on dopamine D2 receptor (DR)-sigma 1 receptor (δ1R) heteroreceptor complexes and the DR protomer recognition, signaling and internalization in cellular models. We report the existence of DR-δ1R heteroreceptor complexes in subcortical limbic areas as well as the dorsal striatum, with different distribution patterns using the in situ proximity ligation assay. Also, through BRET, these heteromers were demonstrated in HEK293 cells. Furthermore, saturation binding assay demonstrated that in membrane preparations of HEK293 cells coexpressing DR and δ1R, cocaine (1 nM) significantly increased the DR B values over cells singly expressing DR. CREB reporter luc-gene assay indicated that coexpressed δ1R significantly reduced the potency of the DR-like agonist quinpirole to inhibit via DR activation the forskolin induced increase of the CREB signal. In contrast, the addition of 100 nM cocaine was found to markedly increase the quinpirole potency to inhibit the forskolin-induced increase of the CREB signal in the DR-δ1R cells. These events were associated with a marked reduction of cocaine-induced internalization of DR protomers in DR-δ1R heteromer-containing cells vs DR singly expressing cells as studied by means of confocal analysis of DR-δ1R trafficking and internalization. Overall, the formation of DR-δ1R heteromers enhanced the ability of cocaine to increase the DR protomer function associated with a marked reduction of its internalization. The existence of DR-δ1R heteromers opens up a new understanding of the acute actions of cocaine.