Because D 1 dopamine receptors are known to act through stimulatory heterotrimeric guanosine triphosphate-binding (G s ) proteins and activation of the cAMP pathway, up-regulation of this pathway in the nucleus accumbens could account for the functional supersensitivity of D 1 receptors observed in these neurons--which occurs in the absence of detectable changes in the receptors themselves--after chronic cocaine (or other stimulant) exposure (18 ).