This figure shows how dopamine can either increase or decrease levels of cAMP.  When dopamine acts on a D₁ receptor, it results in stimulation of a stimulatory G protein (G_s).  This, in turn, activates the enzyme adenylate cyclase, which increases levels of cAMP.  Since the D₂ receptor is linked to an inhibitory G protein (G_i), dopamine at that receptor will reduce adenylate cyclase activity and reduce levels of cAMP.
 

Cholera toxin results in continual stimulation of Gs proteins.  When injected bilaterally into the nucleus accumbens of rats, there is a long-lasting increase in locomotor activity which is present for about 5 days after the injection.  Drugs like amphetamine also increase activity when infused into the nucleus accumbens, but these effects are much more short-lived.