The interaction of neurotransmitters, neuromodulators and neuroactive drugs with receptors localized at the cell surface initiates a chain of molecular events leading to integrated neuronal responses to the triggering stimuli. Major advancements in the characterization and isolation of recep- tor molecules have answered many quest ions regarding the nature of the ele- ments that determine the specificity in these interactions. At the same time, recent studies have provided evidence that delicate regulation by intracellular enzymatic systems determines the efficiency of the stimulus- response...