Complementary distribution of the phosphoproteins DARPP-32 and I-1 in the cerebellar system

DARPP-32 (a dopamine and adenosine 3':5'-monophosphate (cAMP) regulated phosphoprotein of M(r) 32 kDa) and I-1 (phosphatase inhibitor-1) are related phosphoproteins that have distinct regional distributions in some specific neuronal structures. To determine whether they are also expressed in different types of neurons we investigated their distribution in the cerebellum, whose cellular structure is well characterized. In the cerebellar cortex, antibodies to DARPP-32 labeled Purkinje cells and antibodies to I-1 labeled granule cells. The two phosphoproteins were also in synaptic apposition in the deep cerebellar nuclei as well as in nuclei that project to the cerebellum via the climbing fiber and mossy fiber systems. This pattern was consistent in different mammalian species, including the mouse, marmoset, rhesus monkey and the mutant mouse reeler. We suggest that DARPP-32 and I-1 have distinct roles in regulating neuronal excitability in the cerebellum and may play different parts in the phenomenon of long-term depression (LTD).