Dopamine D1 and D2 receptors are sensitive to the cationic form of apomorphine.

The protonated form of apomorphine was found to be active at the high affinity state of dopamine D1 and D2 receptors in canine striatum, since pH reduction from 7.4 to 6.4 enhanced the apomorphine potency 3-fold at both these sites without affecting the potency of the permanently charged apomorphine methiodide. It was estimated that the protonated form of apomorphine was approximately 20-fold more potent than the uncharged form of apomorphine at both the D1 and the D2 receptors.