Clinical importance of D-1 and D-2 receptors.

The existence of subtypes of dopamine receptors as defined by Kebabian and Calne is now well accepted. The D1 receptor is typically associated with stimulation of adenylate cyclase, while the D2 receptor is either independent of adenylate cyclase or mediates its inhibition. Considerable interest has been generated by the potential physiological and clinical roles of these receptor subtypes. Availability of agonists and antagonists specifically acting at the D1 or D2 receptor site has stimulated research to characterize the functional effects of each receptor subtype. This might facilitate the development of effective compounds to control the signs and symptoms of Parkinson's disease and perhaps prevent the induction of debilitating side effects. Recent evidence indicates that D1 receptor stimulation is required to obtain full expression of the D2 receptor site, which has typically been associated with the clinical benefits of dopaminergic therapy. Both pre- and postsynaptic location of the receptors must also be taken into consideration as well as involvement of other neuronal systems. It appears that in PD, progressive involvement of the dopaminergic pathways is the principal pathological course, however, noradrenergic and serotonergic pathways are likewise involved. This multineuronal involvement suggests that drugs acting specifically at receptor sites, located on both pre- and postsynaptic neurons, might be required at different times during the course of the disease process to control its symptoms and/or the complications occurring after long-term treatment with a given drug.