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Dopamine Receptors: A Classification

Biochemical and pharmacological studies have identified at least two classes of dopamine receptors. D-1 receptors mediate the stimulation of adenylate cyclase activity by micromolar concentrations of dopamine, and they are antagonized by phenothiazines and thioxanthenes with nanomolar potencies and by butyrophenones with micromolar potencies. D-2 receptors mediate either the inhibition of adenylate cyclase activity or they are un-associated with this enzyme. These receptors are stimulated by nanomolar concentrations of dopamine agonists, and they are antagonized by nanomolar concentrations of butyrophenones, phenothiazines, and thioxanthenes. D-1 receptors are labeled by [3H]flupentixol, and in the striatum they are found almost entirely on intrinsic neurons. Their physiological function, as well as their role in mediating dopamine agonist-stimulated behaviors, is unclear. D-2 receptors are labeled by 3H-butyrophenones, 3H-agonists, and [3H]flupentixol. Agonist/3H-butyrophenone competition experiments produce Hill slopes <<1 in both the pituitary and striatum. These data are explained by a model whereby D-2 receptors show both a high affinity and low affinity state for agonists. Guanine nucleotides shift this equilibrium toward the low affinity state, and they likely play an important role in the regulation of dopamine agonist-stimulated physiological responses. Agonist stimulation of D-2 receptors is correlated with antiparkinsonian activity, and antagonist affinities at this receptor correlate with antipsychotic potency. In the striatum D-2 receptors are associated with at least intrinsic neurons and corticostriate afferent terminals. An additional binding site termed D-3 has been identified in striatum. This site is labeled by nanomolar concentrations of 3H-agonists but has micromolar affinity for butyrophenones. These sites may represent both 3H-agonist binding to presynaptic autoreceptors and/or binding to a high agonist affinity state of postsynaptic D-1 receptors.