The effects of nanomolar cocaine concentrations, possibly not blocking the dopamine transporter activity, on striatal D-σ heteroreceptor complexes and their inhibitory signaling over Gi/o, have been tested in rat striatal synaptosomes and HEK293T cells. Furthermore, the possible role of σ receptors (σRs) in the cocaine-provoked amplification of D receptor (DR)-induced reduction of K-evoked [H]-DA and glutamate release from rat striatal synaptosomes, has also been investigated. The dopamine D-likeR agonist quinpirole (10nM-1μM), concentration-dependently reduced K-evoked [H]-DA and glutamate release from rat striatal synaptosomes. The σR antagonist BD1063 (100nM), amplified the effects of quinpirole (10 and 100nM) on K-evoked [H]-DA, but not glutamate, release. Nanomolar cocaine concentrations significantly enhanced the quinpirole (100nM)-induced decrease of K-evoked [H]-DA and glutamate release from rat striatal synaptosomes. In the presence of BD1063 (10nM), cocaine failed to amplify the quinpirole (100nM)-induced effects. In cotransfected σR and DR HEK293T cells, quinpirole had a reduced potency to inhibit the CREB signal versus DR singly transfected cells. In the presence of cocaine (100nM), the potency of quinpirole to inhibit the CREB signal was restored. In D singly transfected cells cocaine (100nM and 10μM) exerted no modulatory effects on the inhibitory potency of quinpirole to bring down the CREB signal. These results led us to hypothesize the existence of functional D-σR complexes on the rat striatal DA and glutamate nerve terminals and functional D-σR-DA transporter complexes on the striatal DA terminals. Nanomolar cocaine concentrations appear to alter the allosteric receptor-receptor interactions in such complexes leading to enhancement of Gi/o mediated DR signaling.