Potential interactions among dopamine or μ-opioid receptors and RGS2, RGS4, and RGS9-2. Because amphetamine and cocaine increase synaptic levels of dopamine in the striatum, dopaminergic receptors are activated indirectly by these drugs. Opioids act directly on the μ receptor. Arrows indicate positive interactions; blunted arows indicate inhibition. The figure is not meant to imply that all receptors are expressed in the same cell.