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

Sigma σ and σ 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 σR interacting with dopamine D and D receptors, the potential regulation of dopaminergic transmission by σR is also unknown. We here demonstrate that σR may form heteroreceptor complexes with D but not with D receptors. Remarkably σ, σ, and D 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 σR induces bias in signal transduction as σR ligands increase cAMP signaling whereas reduce MAP kinase activation. These effects, which are opposite to those exerted via σR, suggest that the D 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 σ, σ, and D 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.