Dopamine D-1 receptor and cyclic AMP-dependent phosphorylation in Parkinson's disease

D-1 and D-2 receptor densities, evaluated respectively by [3H]SCH 23390 and [3H]spiperone binding, and DARPP-32 (dopamine and adenosine 3':5'-monophosphate-regulated phosphoprotein-32K) concentrations, were studied in the brains of control and parkinsonian subjects postmortem. D-2 receptor density was unchanged in the putamen of parkinsonian patients. D-1 receptor density was unchanged in the putamen and substantia nigra pars reticulata (SNR) of parkinsonian patients, but decreased by 28% in the substantia nigra pars compacta (SNC). DARPP-32, which is localized in the same structures as D-1 receptors of which it is thought to represent the intracellular messenger, decreased by 45% in the putamen, 66% in the SNR, and 79% in the SNC. The decrease in D-1 receptors in the SNC may be due to degeneration of pallidonigral GABAergic neurons, but some of the D-1 receptors may be on the nigrostriatal dopaminergic neurons themselves. The dissociation between the alteration of D-1 receptor densities and DARPP-32 concentrations in both the striatum and substantia nigra, which are of the same order in the two structures, may be an index of functional hypoactivity of D-1 neurotransmission.