Cocaine Effects on Dopaminergic Transmission Depend on a Balance between Sigma-1 and Sigma-2 Receptor Expression

Sigma sigma(1) and sigma(2) receptors are targets of cocaine. Despite sharing a similar name, the two receptors are structurally unrelated and their physiological role is unknown. Cocaine increases the level of dopamine, a key neurotransmitter in CNS motor control and reward areas. While the drug also affects dopaminergic signaling by allosteric modulations exerted by sigma R-1 interacting with dopamine D-1 and D-2 receptors, the potential regulation of dopaminergic transmission by sigma R-2 is also unknown. We here demonstrate that sigma R-2 may form heteroreceptor complexes with D-1 but not with D-2 receptors. Remarkably sigma(1), sigma(2), and D-1 receptors may form heterotrimers with particular signaling properties. Determination of cAMP levels, MAP kinase activation and label-free assays demonstrate allosteric interactions within the trimer. Importantly, the presence of sigma R-2 induces bias in signal transduction as sigma R-2 ligands increase cAMP signaling whereas reduce MAP kinase activation. These effects, which are opposite to those exerted via sigma R-1, suggest that the D-1 receptor-mediated signaling depends on the degree of trimer formation and the differential balance of sigma receptor and heteroreceptor expression in acute versus chronic cocaine consumption. Although the physiological role is unknown, the heteroreceptor complex formed by sigma(1), sigma(2), and D-1 receptors arise as relevant to convey the cocaine actions on motor control and reward circuits and as a key factor in acquisition of the addictive habit.